Documents presented EIFAC TECHNICAL at the symposium PAPER on stock enhancement DOCUMENT in the management TECHNIQUE of freshwater DE LA CECPI 42/Suppl./2 Documents présentés au symposium sur l'amélioration des stocks dans le cadre de l'aménagement des pêcheries d'eau douce

Volume 2: Introductions and Transplantations Introductions et transplantations Held in Budapest, 31 May – 2 June 1982 in conjunction with the Twelfth Session of EIFAC

Tenu à Budapest du 31 mai au 2 juin 1982 à l'occasion de la douzième session de la CECPI

FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS Rome, 1984

ORGANISATION DES NATIONS UNIES POUR L'ALIMENTATION ET L'AGRICULTURE Rome, 1984 The designations employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of the Food and Agriculture Organization of the United Nations concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.

Les appellations employées dans cette publication et la présentation des données qui y figurent n'impliquent de la part de l'Organisation des Nations Unies pour l'alimentation et l'agriculture aucune prise de position quant au statut juridique des pays, territoires, villes ou zones, ou de leurs autorités, ni quant au tracé de leurs frontières ou limites.

M-43 ISBN 92-5-002102-X

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Tous droits réservés. Aucune partie de cette publication ne peut être reproduite, mise en mémoire dans un système de recherche bibliographique ni transmise sous quelque forme ou par quelque procédé que ce soit: électronique, mécanique, par photocopie ou autre, sans autorisation préalable. Adresser une demande motivée au Directeur de la Division des publications. Organisation des Nations Unies pour l'alimentation et l'agriculture, Via delle Terme di Caracalla, 00100 Rome (Italie), en indiquant les passages ou illustrations en cause. PREPARATION OF THIS DOCUMENT This collection of papers represents the Proceedings of the Symposium on Stock Enhancement in the Management of Freshwater Fisheries, held in conjunction with the Twelfth Session of EIFAC (Budapest, Hungary, 31 May – 2 June 1982). The Proceedings are in two volumes, which are supplementary to the Report of the Symposium issued in 1982 as EIFAC Technical Paper No. 42.

PREPARATION DE CE DOCUMENT Cette ensemble de documents represents les actes du Symposium sur l'amélioration des stocks dans le cadre de l'aménagement des pêcheries d'eau douce, tenu à Budapest (Hongrie) du 31 mai au 2 juin 1982 à l'occasion de la douzième session de la CECPI. Les actes sont presentés en deux volumes complémentaires au Rapport du Symposium émis par la CECPI en 1982 sous forme de document technique № 42.

Distribution For biblïographic purposes this document should be cited as follows/La référence bibliographique Authors/Auteurs de ce document doit être donnée ainsi: Participants in the Symposium/ Participants au Symposium European Inland Fisheries Advisory Commission EIFAC Mailing List/Liste de distribution 1984 Commission européenne consul tative de la CECPI pour les pêches dans les eaux FAO Fisheries Departement/Départment intérieures, 1984 Documents des pêches de la FAO presented at the Symposium on stock FAO Regional enhancement in the management of Officers/Fonctionnaires régionaux des freshwater fisheries. Documents pêches de la FAO presentés au Symposium sur l'amélioration des stocks dans le cadre de l'aménagement des pêcheries d'eau douce. Vol.2. Introductions and transplantations Introductions et transplantations. Held in Budapest, Hungary, 31 May – 2 June 1982 in conjunction with the Twelfth session of EIFAC. Tenu à Budapest, Hongrie du 31 mai au 2 juin 1982 à l'occasion de la douzième session de la CECPI. EIFAC Tech.Pap./Doc.Tech.CECPI, (42)Suppl.Vol.2:283–554

ABSTRACT

Volume Two of these Proceedings groups together the 26 papers presented at the Symposium on Stock Enhancement in the Management of Freshwater Fisheries, which dealt with the advantages and disadvantages of introduction of new into European waters.

RESUME

Le volume deux de ces actes regroupe 26 documents presentés au Symposium sur l'amélioration des stocks dans le cadre de l'aménagement des pêcheries d'eau douce, qui traitait des avantages et desavantages de l'introductions et transplantations avec des espèces autochtones des eaux européennes. CONTENTS TABLE DES MATIERES

Page INTRODUCTION THE IMPACT OF A TRANSPLANTED FORAGE , THE ALEWIFE (Alosa pseudoharengus) ON A RESERVOIR FISHERY IN SOUTHEASTERN UNITED STATES C.C. Kohler 283 REVIEW OF EXPERIMENTS WITH INTRODUCTION AND ACCLIMATIZATION OF THE , hucho (LINNAEUS, 1758) () J. Holcik 290 INTRODUCTION OF (Cyprinus carpio (L.)) IN FINLAND P. Ahlfors, P. Kummu and K. Westman 299 HEAVY SILVER CARP (Hypophthalmichthys molitrix (Val.)) STOCKING IN LAKES AND ITS INFLUENCE ON INDIGENOUS FISH STOCKS D. Barthelmes 313 INTRODUCTION OF (Ctenopharyngodon idella (Val.) INTO P.M. Markmann 325 THE INTRODUCTION, SPREAD AND INFLUENCE OF THE (Barbus barbus) IN THE RIVER SEVERN, GREAT BRITAIN A.S. Churchward, P. Hickley and E. North 335 A CENTURY OF PIKEPERCH IN DENMARK J. Dahl 344 THE IMPACT OF ZANDER (Stizostedion lucioperca (L.)) IN THE UNITED KINGDOM AND THE FUTURE MANAGEMENT OF AFFECTED FISHERIES IN THE ANGLIAN REGION R.S.J. Linfield 353 INTRODUCTION AND THE PRESENT STATUS OF (Salvelinus fontinalis Mitchill) IN NORWAY M. Grande 363 RESULTS OF STOCKINGS IN FINLAND 1957–81 A. Mutenia, O. Simola and O. Tuunainen 381 RESULT OF THE INTRODUCTION OF LAKE TROUT (LAKE CHARR, Salvelinus namaycush) INTO SWEDISH LAKES A.P. Gonczi and N.-A. Nilsson 392 INTRODUCTION OF THE NORTH AMERICAN CRAYFISH (Pacifastacus Ieniusculus (DANA)) INTO SWEDEN M. Furst 400 PRESENCE EN FRANCE D'ESPECES EXOTIQUES D'ECREVISSES PROVENANT D'INTRODUCTIONS RECENTES P.J. Laurent, D. Vigneux et E. Vigneux 405 THE RESTORATION OF THE CRAYFISH (Astacus astacus) IN RIVER SIIKAJOKI, FINLAND M. Pursiainen and K. Westman 412 INTRODUCTION OF THE AMERICAN CRAYFISH (Pacifastacus leniusculus) IN FINLAND; IMPACT ON THE NATIVE CRAYFISH (Astacus astacus) K. Westman and M. Pursiainen 422 INTRODUCTION ET ACCLIMATATION DE POISSONS D'EAU DOUCE EN FRANCE; HISTORIQUE ET BILAN J. Allardi 427 A REVIEW OF FISH AND CRAYFISH INTRODUCTIONS MADE IN FINLAND K. Westman and P. Tuunainen 436 THE EFFECTS OF INTRODUCTIONS INTO IRELAND P. Fitzmaurice 449 INTRODUCTION DE NOUVELLES ESPECES DE POISSONS DANS LES PECHERIES D'EAU DOUCE DE LA ROUMANIE N. Bacalbasa-Dobrovici 458 THE EXOTIC ICHTHYOFAUNA OF THE CONTIGUOUS UNITED STATES WITH PRELIMINARY OBSERVATIONS ON INTERANATIONAL TRANSPLANTS W.R. Courtenay, Jr. and J.N. Taylor 466 SOME CONSIDERATIONS ON THE ROLE OF INTRODUCED SPECIES OF FISH IN THE MANAGEMENT OF INLAND FISHERIES J. Holcik 488 THE NICHE CONCEPT AND THE INTRODUCTION OF EXOTICS N.-A. Nilsson 496 REDUCING THE RISK OF FISH INTRODUCTIONS: A RATIONAL APPROACH TO THE MANAGEMENT OF INTEGRATED COLDWATER COMMUNITIES R.A. Ryder and S.R. Kerr 510 PARASITE RANGE EXTENSION BY INTRODUCTION OF FISH TO HUNGARY K. Molnar 534 IMPLEMENTATION OF A REVIEW AND DECISION MODEL FOR EVALUATING PROPOSED INTRODUCTIONS OF AQUATIC ORGANISMS IN EUROPE AND NORTH AMERICA C.C. Kohler and J.G. Stanley 541 EXOTIC FISH SPECIES ACCLIMATIZED IN HUNGARIAN NATURAL WATERS J. Toth and P. Biro 550 INTRODUCTION Following the recommendations of the International Consultation on Fishery Resources Allocation (Vichy, France, 1980) and its own discussions during its Eleventh Session (Stavanger, Norway, May 1980), EIFAC felt that subsequent sessions should examine closely the current situation for managing stocks of fish in European inland waters. Because the whole topic of management is too large for coverage during any one symposium, it was recommended by the Commission that the subject be covered in two symposia - one on biotic aspects of management to be held in conjunction with the Twelfth Session of EIFAC and one on abiotic aspects to be held with the Thirteenth Session of EIFAC. There has been a long-standing concern with the effects of introductions of exotic species into European rivers and lakes. Furthermore, whilst stocking with native or introduced species is common management practice, there is very little evidence for or against its biological or economic effectiveness. Therefore, the first Symposium on management addressed the twin problems of introductions and stockings in an effort to define guidelines for both practices in European waters. The Symposium considered these two specific aspects of stock manipulation in seven sessions as follows: Session 1 Stocking with non-salmonids Session 2 Stocking with brown trout Session 3 Stocking with Session 4 Introductions and transplantations - case histories and experience with some species Session 5 Introductions - country reviews and Lake Kinneret case Session 6 Introductions - ecological and practical aspects Session 7 Summary, conclusions and recommendations This document is a compilation of the papers presented at the Symposium, which are grouped into two volumes as follows: Volume 1: Stocking - contained those papers considered under Sessions 1, 2 and 3 of the Symposium Volume 2: Introductions - contains those papers considered under Sessions 4, 5 and 6 of the Symposium The conclusions and recommendations of the Symposium are presented in EIFAC Technical Paper No. 42 - Report of the Symposium on Stock Enhancement in the Management of Freshwater Fisheries. INTRODUCTION Conformément aux recommandations de la Consultation technique sur la répartition des ressources ichtyologiques (Vichy, France, 1980), la CECPI a estimé, à sa onzième session (Stavanger, Norvège, mai 1980), qu'il était nécessaire d'examiner de près la situation actuelle pour l'aménagement des stocks dans les eaux intérieures européennes. Le sujet était trop vaste pour pouvoir être traité en une seule réunion. La Commission a done recommandé que l'on organise deux symposiums, l'un sur les aspects biotiques de l'aménagement à l'occasion de sa douzième session, l'autre sur les aspects abiotiques à l'occasion de sa treizième session. On s'intéresse depuis longtemps déjà aux conséquences de l'introduction d'espèces exotiques dans les cours d'eau et lacs européens. Par ailleurs, il est courant de ne peupler les eaux intérieures avec des espèces locales ou exotiques mais, en définitive, on sait très peu de choses des avantages et inconvénients de cette pratique sur le plan biologique et économique. Le premier symposium s'est donc intéressé à ces deux problèmes en vue d'établir des directives pour les eaux intérieures européennes. Le Symposium a examiné ces deux aspects de la manipulation des stocks au cours de sept sessions: Session 1 Repeuplement - les non-salmonidés Session 2 Repeuplement - la truite de mer Session 3 Repeuplement - le saumon Session 4 Introductions et transplantations - résultats obtenus avec certaines espèces Session 5 Introduction - examen pays par pays et cas du lac Kinneret Session 6 Introduction - aspects écologiques et pratiques Session 7 Résumé, conclusions et recommandations Ce recueil regroupe les documents presentés au symposium qui sont répartis dans les deux volumes de la façon suivante: Volume 1: Repeuplement - contient les documents traités par les sessions 1, 2 et 3 du symposium. Volume 2: Introductions et transplantations - contient les documents traités par les sessions 4, 5 et 6 du symposium. Les conclusions et recommandations du symposium sont presentées dans le document technique № 42 de la CECPI - Rapport du Symposium sur l'amélioration des stocks dans le cadre de l'aménagement des pêcheries d'eau douce (1983). THE IMPACT OF A TRANSPLANTED , THE ALEWIFE (Alosa pseudoharengus), ON A RESERVOIR FISHERY IN SOUTHEASTERN UNITED STATES

C.C. Kohler Fisheries Research Laboratory and Department of Southern Illinois University Carbondale, Illinois, U.S.A.

CONTENTS

1. INTRODUCTION 2. STUDY AREA 3. ANALYSIS OF ALEWIFE FORAGE BASE 3.1 Abundance 3.2 Utilization by Predators 3.3 Influence on predator growth 3.4 Alewife die-offs 3.5 Interspecific competition 3.6 Emigration 4. PROGNOSIS 5. TRANSPLANTATIONS, IMPACT PREDICTION AND FISHERIES MANAGEMENT 6. REFERENCES ABSTRACT Alewife, Alosa pseudoharengus, is currently being transplanted into large reservoirs in southeastern United States to serve as a pelagic forage species. With regards to landlocked populations, the anadromous clupeid had previously been limited to the Great Lakes and small glacial lakes in northeastern United States. One of the earlier (1968) transplantations of alewife occurred in Claytor Lake, Virginia, and this population is the subject of the present paper. Several trophic and population studies were conducted from 1977 to 1979 to assess the impact of alewife on the Claytor Lake fishery. Alewife were found to have several undesirable characteristics, the most serious of which included predation on larval sportfishes, alteration of the species and size composition by selective predation and rapid growth beyond a size vulnerable to most predators. In terms of growth, only pelagic predators such as white bass (Morone chrysops) and walleye (Stizostedion vitreum vitreum) benefited from the introduction, although recruitment of walleye coincidentally declined following alewife establishment. Alewife have emigrated from Claytor Lake to at least one other reservoir and this, coupled with several recent transplantations, has given them potential access to the Ohio and Mississippi River drainages; in effect, to nearly half of the continental United States. This example demonstrates the need for decision-makers to conduct benefit-risk analyses prior to widespread stocking of not indigenous to a region. RESUME Le gaspareau, Alosa pseudoharengus, est actuellement transplanté dans de grands réservoirs du sud-est des Etats-Unis pour y servir de proie aux poissons pélagiques. Jusqu'à présent, ce clupéidé anadrome ne se rencontrait à l'intérieur des Etats-Unis que dans les Grands Lacs et les petits lacs glaciaires du nord-est. L'une des premières transplantations (1968) a été faite dans le lac Claytor (Virginie); c'est aux conséquences de cette transplantation que s'intéresse l'auteur. Plusleurs études trophiques et études de populations ont été réalisées de 1977 à 1980 pour évaluer l'impact du gaspareau sur les pêcheries du lac Claytor. On a constaté que ce poisson présentait plusieurs défauts; les principaux sont les suivants: il se nourrit de larves de poissons destinés à la pêche sportive; il a une prédilection pour le gros zooplancton, d'où une altération de la composition de la faune zooplanctonique; il a une croissance rapide et atteint en peu de temps une taille qui le met à l'abri de la plupart des prédateurs. En termes de croissance, seuls des prédateurs pélagiques tels que le bar d'Amérique (Morone saxatilis) et le doré jaune (Stizostedion vitreum vitreum) ont bénéficié de l'introduction du gaspareau, on constate toutefois que le recrutement du doré jaune a diminué à la suite de son installation. Du lac Claytor, le gaspareau est passé dans un autre réservoir au moins et, compte tenu de plusieurs autres transplantations récentes, il a désormais accès à tout le bassin de l'Ohio et du Mississippi, soit près de la moitié de la partie continentale des Etats-Unis. L'auteur souligne que les responsables devraient procéder à des analyses advantages risques avant toute introduction sur une grande échelle. 1. INTRODUCTION A common fisheries management objective is to augment growth and production of piscivorous through expansion of the forage base. Expansion is usually attempted by transplantation of forage, often without full consideration of the ecological implications. The introduction of a forage fish may not produce the desired result due to competitive interactions with the resident species complex. Nevertheless, there is often considerable justification for expanding a forage base, particularly in reservoirs. After impoundment, reservoirs rarely contain adequate numbers of forage fish to support large populations of game fish because endemic riverine forage species are often unable to expand in the new lacustrine habitat (Kimsey, 1957; Shields, 1957; Fritz, 1968). Introduction of potential forage species into reservoirs has consequently become common practice in North American fisheries management (Ney, 1981). Unfortunately, most of these stockings have been made on a trial-and-error basis (Lackey, 1974; Pritchard et al., 1978; McCammon and von Geldern, 1979), and as with introductions in general, have often been undertaken without adequate information on the possible consequences (Courtenay and Robins, 1975). In the southeastern United States, the principal reservoir forage species are the gizzard shad (Dorosoma cepedianum) and the threadfin shad (Dorosoma petenense). These clupeids feed primarily upon algae, detritus and benthos (Kutkuhn, 1957; Miller, 1967; Baker and Schmitz, 1971). The trophic status of shad would appear to make them ideal forage fish since they convert underutilized food material into biomass that can be consumed by piscivores. Unfortunately, both shad species have often proved inadequate; gizzard shad rapidly grow beyond a size vulnerable to most predators (Dendy, 1946; Rathum, 1967; Jester and Jensen, 1972), whereas the threadfin shad is limited in its range due to an inability to withstand temperatures below 9°C for prolonged periods (Strawn, 1963). The desire for an ideal forage fish in the southeast has led to the consideration of other species such as the anadromous alewife, Alosa pseudoharengus (Wilson). Much of the impetus for transplanting alewives is due to the highly successful salmonid stocking programme of the Great Lakes; the salmonid populations are almost entirely supported by the existence of an abundant alewife forage base. Despite the apparent benefits of alewife in the Great Lakes, there is substantial evidence that alewives may also have serious deleterious effects. Historically, the alewife has had a major negative impact on the endemic fisheries of several of the Great Lakes. In Lakes Ontario, Huron and Michigan, the shallow-water planktivores declined in the first decade after alewife establishment, the minor piscivores initially increased then declined in the second decade and the deep-water planktivores declined in the third decade (Smith, 1968, 1970). The dominance of alewives resulted in a severe reduction in overall fishery productivity as well as species diversity. Trophic interactions between alewife and young-of-the-year fishes have been suggested as a major cause for the reduction in recruitment of various resident fishes in the Great Lakes (Smith, 1970; Wells and McLain, 1972). Alewife are extremely size-selective zooplanktivores, and have been reported to cause shifts in zooplankton species and size composition toward small forms in several lacustrine systems (Brooks and Dodson, 1965; Brooks, 1968; Wells, 1970; Hutchinson, 1971; Warshaw, 1972). Consequently, alewife are potentially severe competitors with other planktivorous fish species or life stages. Moreover, it has been suggested that alewife directly affected population densities of Great Lakes' fish species by predation on their pelagic young (Smith, 1970; Wells and McLain, 1972). The evidence that alewife have had positive and negative effects on the fisheries of the Great Lakes, makes it imperative that the impacts of alewife establishment be evaluated prior to widescale transplantation in southeastern United States. To date, alewife have been transplanted to several reservoirs in both Virginia and Tennessee. The first of these transplantations occurred in 1968 at Claytor Lake, Viriginia. The work summarized here was designed to assess the consequences of that transplantation in order to evaluate the benefits and risks of alewife establishment in reservoirs throughout the southeastern portion of North America. 2. STUDY AREA Claytor Lake is a mainstream hydro-electric impoundment of the New River located in southwestern Virginia (U.S.A.), which was filled in 1939. It has a surface area of 1 820 ha at a normal pool elevation of 663 m above mean sea level and drains approximately 3 862 km2. The reservoir includes approximately 161 km of shoreline, contains approximately 2.74 × 103 m3 of water and has a maximum depth of 37.5 m (Roseberry, 1950). Claytor Lake contains several shallow coves but with little rooted aquatic vegetation. The littoral (5 m depth) regions are not otherwise extensive. The marginally eutrophic lake is dimictic, containing a distinct thermocline throughout the summer with spring and fall overturns. Anoxic conditions occur in summer in the hypolimnion. Ice commonly occurs from approximately mid-January through mid-March. The fishery of Claytor Lake is similar to other impoundments in the region. The primary angler-exploited species are black basses ( salmoides, M. dolomieui, M. punctulatus); striped bass (Morone saxatilis), which are maintained on a put-grow-take basis; white bass (Morone chrysops); walleye (Stizostedion vitreum vitreum), maintained by natural spawning and supplemented by stocking; (Ictalurus punctatus); (Pomoxis nigromaculatus and P. annularis); bluegill (Lepomis machrochirus), and yellow (Perca flavescens). Although alewife were introduced to provide forage for all angler-exploited species, they were particularly intended to serve as food for the large, pelagic predators (striped bass, white bass and walleye). 3. ANALYSIS OF ALEWIFE FORAGE BASE Several trophic and population ecology studies were conducted in Claytor Lake from November 1977 through August 1979 in order to assess the impact of introduced alewife on the reservoir fishery. In the following sections the major findings of those studies are summarized; for fuller detail the reader is referred to Kohler (1980), Kohler and Ney (1980, 1981, 1982, in press) and Kohler et al. (1979). 3.1 Abundance Alewife established a reproducing population two years after their introduction in 1968 (Boaze, 1972), and subsequently became a major component of the species complex of Claytor Lake (Boaze and Lackey, 1974). The pelagic nature of this clupeid makes it difficult to assess their relative biomass, but it is significant and is roughly estimated to range up to 25 percent of that of the total reservoir. 3.2 Utilization by Predators Alewife were found to be an apparent preferred prey of pelagic predators (striped bass, white bass, walleye) but were of minor importance in diets of littoral-inhabiting black basses and were not found in stomach contents of black and white crappie. Because alewife are primarily pelagic, spatial segregation appears to be the operating mechanism for the disparity in predator utilization. Alternative forage included crayfish, golden shiner (Notemigonus chrysoleucas), sunfishes, and yellow perch. Alewife growth in Claytor Lake exceeds all documented growth rates for other landlocked populations, and the clupeid rapidly reaches a size not vulnerable to most predators. Maximum total length of alewife found in predator stomachs was 165 mm, the approximate size of age-1 alewife. Consequently, it appears that in Claytor Lake, predator utilization is limited to age 1+ and younger alewife; age 2 and older alewife essentially serve only to tie up biomass, similar to the management problem commonly experienced with gizzard shad. 3.3 Influence on predator growth Growth rates of white bass and walleye significantly (P 0.05; Wilcoxon's signed rank test) increased following establishment of the alewife forage base, whereas a general decline was noted for black basses and crappie. Whether alewife were in any way involved in the decline in growth rates of black basses and crappie is not known, but the overall trend is cause for concern. Although no before-and-after growth data were available for sunfishes, interviews with sport fishermen indicated that sunfish growth dramatically declined following alewife introduction. Conceivably, sunfish populations stunted due to reduced predator pressure that resulted when pelagic predators switched to alewife as their primary source of prey. 3.4 Alewife die-offs Severe die-offs of landlocked alewife are a common occurrence and such a major die-off occurred in Claytor Lake during the winter of 1977–78. However, catch-per- unit effort and piscivore stomach analyses indicated that the alewife population rapidly recovered due to highly successful reproduction in 1978. Accordingly, pelagic predators were able to return to an alewife diet within a year of the die-off, probably minimizing the effects of the increased predator pressure that was exerted on the alternative forage supply in the interim. However, a predator may not always be able to successfully utilize alternative prey in the absence of their primary forage. Such a situation could occur if other potential forage species were less available or vulnerable, or if the predator had established an inflexible search image (Krebs et al., 1974) for its primary prey. For example, striped bass in Santee-Cooper Reservoir, South Carolina, did not exploit abundant alternative prey following a clupeid forage base collapse, and many starved (Stevens, 1979). 3.5 Interspecific competition Alewife in Claytor Lake consumed the young (maximum 26 mm total length) of at least five fish species (, white bass Lepomis sp., yellow perch, golden shiner) as well as their own young. As previously noted, alewife predation on larval fish has been suspected as a causative factor for the collapse of several resident fish populations of the Great Lakes. The observance of alewife piscivory in Claytor Lake adds much support to that hypothesis. Direct dietary overlaps (Levins, 1968) were not significant between alewife and most larval sport fish. However, alewife may have an indirect adverse impact on other species and/or life stages of corresponding trophic level by altering the zooplankton species complex. Alewife in Claytor Lake were found to be highly size-selective planktivores, as has been demonstrated in northern lakes of the United States (Brooks, 1968; Hutchinson, 1971; Janssen, 1976; Janssen and Brandt, 1980). Electivity (Ivlev, 1961; Strauss, 1979) indices and statistical analyses indicated that alewife selectively prey on zooplankers 1.0 mm length, and such selective predation apparently exerts a major influence on species and size composition of the zooplankton community. The disparity in alewife abundance between 1978 (following die-off) and 1979 (population recovered) provided the opportunity to assess the response of the zooplankton community to alewife size-selective predation in Claytor Lake. Major limnetic zooplankters (Daphnia, Diaphanosoma, Cyclops, Diaptomus) were significantly (P 0.05; Wilcoxon's rank-sum test) smaller when alewife abundance was high. Alewife-induced shifts in zooplankton composition toward smaller forms represent a potentially significant adverse impact on cohabiting planktivores, including young-of-the-year sportfishes. 3.6 Emigration The potential for inadvertent establishment of alewife populations is exemplified by their rapid and unplanned spread throughout the Great Lakes (Miller, 1957). In a main-stream reservoir situation, alewives obviously have two avenues for escapement, upriver and downriver. Alewife have, in fact, emigrated to and have become established in Bluestone Lake, West Virginia, an impoundment over 100 km downriver from Claytor Lake. From Bluestone Lake, it is possible for alewife to emigrate to the Ohio River and from there into much of the Mississippi drainage. Thus, the transplantation of alewife to Virginia, as well as to Tennessee, has given them potential access to nearly half of the continental United States. 4. PROGNOSIS Alewife in Claytor Lake were found to have several undesirable characteristics for a forage species. These included predation on larval sportfishes, alteration of zooplankton species and size composition by selective predation, and rapid growth beyond a size vulnerable to most predators. Increased growth rates of white bass and walleye following the alewife introduction indicated they benefited from the expanded forage base. However, it must be noted that walleye recruitment has coincidentally declined following alewife establishment, and supplemental stockings of walleye have been necessary to maintain their population levels. The results of studies summarized in this paper raise serious questions about the advisability of further alewife transplantations. Unless future studies can prove otherwise, the risks of alewife establishment in waters of southeastern United States appear to far out-weigh the benefits, and consequently further transplantations should be halted. Unfortunately, the alewife may already have access to most of the southeast, and it may only be a matter of time until they are a common component of the species assemblages of the region. 5. TRANSPLANTATIONS, IMPACT PREDICTION AND FISHERIES MANAGEMENT It is hoped that the case history of alewife in Claytor Lake will serve to create an awareness among decision-makers as to the importance of conducting benefit-risk analyses prior to widespread stockings of non-indigenous fish. It should be noted that alewife were transplanted to Claytor Lake before most of the literature concerning alewife interspecific impacts in northern waters was available. However, such has not been the case for several transplantations recently made elsewhere. Transplantations should be viewed by fisheries managers with the same concern as exotic introductions. A proposed protocol (Kohler and Stanley, two papers in press) for evaluating exotic fish introductions would have equal utility for evaluating transplantations, and this or a similar protocol should be adopted as a standard fisheries management tool. 6. REFERENCES Baker, C.D. and E.H. Schmitz, Food habits of adult gizzard and threadfin shad in two 1971 Ozark reservoirs. Spec.Publ.Am.Fish.Soc., (8):3–11 Boaze, J.L., Effects of landlocked alewife introduction on white bass and walleye 1972 populations, Claytor Lake, Virginia. M.S. Thesis, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, 103 p. Boaze, J.L. and R.T. Lackey, Age, growth and utilization of landlocked alewives in 1974 Claytor Lake, Virginia. Prog.Fish-Cult., 36(3):163–4 Brooks, J.L., The effects of prey size selection by lake plantivores. System.Zool., 1968 17(3):272–91 Brooks, J.L., and S.I. Dodson, Predation, body size and composition of plankton. 1965 Science, Wash., 150(1):28–35 Courtenay, W.R., Jr. and C.R. Robins, Exotic organisms: an unsolved, complex problem. 1975 Bioscience, 25(5):306–13 Dendy, J.S., Food of several species of fish, Norris Reservoir, Tennessee. 1946 J.Tenn.Acad.Sci., 21(1):105–27 Fitz, R.B., Fish habitat and population changes resulting from impoundment of Clinch 1968 River of Melton Hill Dam. J.Tenn.Acad.Sci., 43(1):7–15 Hutchinson, B.P., The effect of fish predation on the zooplankton of ten Adirondack 1971 lakes, with particular reference to the alewife, Alosa pseudoharengus. Trans.Am.Fish.Soc., 100(2):325–35 Ivlev, V.S., Experimental ecology of the feeding of fishes. New Haven Connecticut, Yale 1961 University Press, 302 p. Janssen, J., Feeding modes and prey size selection in the alewife (Alosa 1976 pseudoharengus). J.Fish.Res.Board Can., 33(9):1972–5 Janssen, J. and S.B. Brandt, Feeding ecology and vertical migration of adult alewives 1980 (Alosa pseudoharengus) in Lake Michigan. Can.J.Fish.Aquat.Sci., 37(2): 177–84 Kohler, C.C., Trophic ecology of an introduced, landlocked alewife (Alosa 1980 pseudoharengus) population and assessment of alewife impact on resident sportfish and crustacean zooplankton communities in Claytor Lake, Virginia. Ph.D. Dissertation. Virginia Polytechnic Institute and State University, Blacksburg, Virginia, 208 p. Kohler, C.C. and J.J. Ney, Piscivority in a landlocked alewife (Alosa pseudoharengus) 1980 population. Can.J.Fish.Aquat.Sci., 37(8): 1314–7 , Consequences of an alewife die-off to the fish and zooplankton of a 1981 reservoir. Trans.Am.Fish.Soc., 110(3):360–9 , Suitability of alewife as a pelagic forage fish for southeastern reservoirs. 1982 Proc.Annu.Conf.Southeast.Assoc.Fish.Wildl.Ag., 34:137–40 , A comparison of methods for quantative analysis of feeding selection of fishes. Environ.Biol.Fishes, (in press) Kohler, C.C. and J.G. Stanley, Implementation of a review and decision model for 1984 evaluating proposed exotic fish introductions in Europe and North America. EIFAC Tech.Pap., 42:Vol. 2:541–9 Kohler, C.C. and J.G. Stanley, Implementation of a review and decision model for evaluating proposed exotic fish introductions in Europe and North America. In Distribution, biology and management of exotic fishes, edited by W.R. Courtenay and J.R. Stauffer. (in press) Kohler, C.C. et al., Compact, portable vertical gill net system. Prog.Fish-Cult., 41(1):34– 1979 5 Kutkuhn, J.H., Utilization of plankton by juvenile gizzard shad in a shallow prairie lake. 1957 Trans.Am.Fish.Soc., 87(1):80–103 Lackey, R.T., Seasonal abundance and availability of forage fish and their utilization by 1968 landlocked and brook trout in Echo Lake, Mount Desert Island, Maine. M.S. Thesis. University of Maine, 98 p. , Introductory fisheries science. Blacksburg, Virginia. Sea Grant Extension 1974 Division, Virginia Polytechnic Institute and State University, 275 p. Levins, R., Evolution in changing environments. Princeton, N.J., Princeton University 1968 Press, 120 p. McCammon, G.W. and C. von Geldern, Jr., Predator-prey systems in large reservoirs. 1970 Predator-prey systems in fisheries management, edited by H. Clepper. Washington, D.C., Sport Institute, pp. 431–42 Miller, R.R., Origin and dispersal of the alewife, Alosa pseudoharengus, and the gizzard 1957 shad, Dorosoma cepedianum, in the Great Lakes. Trans.Am.Fish.Soc., 86:97–111 , Food of the threadfin shad, Dorosoma petenense, in Lake Chicot, 1967 Arkansas. Trans.Am.Fish.Soc., 96(3):243–6 Pritchard, D.L. et al., Stocking of predators in the predator-stocking-evaluation 1978 reservoirs. Proc.Southeast.Assoc.Game Fish.Comm., 30:108–13 REVIEW OF EXPERIMENTS WITH INTRODUCTION AND ACCLIMATIZATION OF THE HUCHEN, Hucho hucho (LINNAEUS, 1758) (SALMONIDAE)

J. Holčík Laboratory of Fishery Research and Hydrobiology Bratislava, Czechoslovakia

CONTENTS

1. INTRODUCTION 2. RESULTS OF INTRODUCTIONS 2.1 Czechoslovakia 2.2 England 2.3 Switzerland 2.4 Morocco 2.5 Belgium 2.6 Poland 2.7 France 2.8 Sweden 2.9 Spain 2.10 Other countries 3. IMPACT OF INTRODUCED HUCHEN UPON THE POPULATIONS OF NATIVE FISH 4. REASONS OF FAILURE OF THE INTRODUCTION OF HUCHEN 5. PRINCIPLES FOR SUCCESSFUL INTRODUCTION OF THE HUCHEN 6. TO INTRODUCE OR NOT TO INTRODUCE THE HUCHEN? 7. ACKNOWLEDGEMENTS 8. REFERENCES ABSTRACT The world's biggest salmonid fish, the huchen (Hucho hucho (Linnaeus, 1758)) is now threatened with extinction and has been introduced outside its original area of distribution into several European countries as well as into North Africa and North America. The objectives of the experimental introductions were: (i) to replace disappearing Atlantic salmon (Salmo salar Linnaeus, 1758) or sea trout (Salmo trutta, Linnaeus, 1758); (ii) to introduce a new and interesting game fish; (iii) to control coarse fish populations, and (iv) to save this valuable species from extinction. After reviewing individual experiments, the author concludes that they have mostly failed through ignorance of the ecology of the species, stocking at too young a developmental stage, the short duration of the experiments and the changes in the initial ideas which led to the introduction of this species. The impact of huchen upon the native fish is discussed along with principles for successful introduction and the meaning of the huchen introductions. RESUME Le plus gros salmonidé du monde - le saumon du Danube (Hucho hucho, Linnaeus, 1758) maintenant menacé d'extinction - a été transplanté hors de son aire originale de répartition (le bassin du Danube) et introduit dans plusieurs pays européens ainsi qu'en Afrique du Nord et en Amérique du Nord. Les objectifs des expériences d'introduction et d'acclimatation étaient les suivants: (i) remplacer le saumon atlantique en voie de disparition (Salmo salar, Linnaeus, 1758 -Angleterre, Suisse, Pologne, Tchécoslovaquie); (ii) introduire un nouveau poisson intéressant pour les pêcheurs sportifs (Pologne, Maroc, Belgique, France et Espagne); (iii) limiter les populations de poisson commun (Tchécoslovaquie, Suède); (iv) sauver de l'extinction une espèce précieuse (Tchécoslovaquie, Pologne). Après avoir passé en revue diverses expériences, l'auteur impute l'échec de la plupart d'entre elles aux facteurs suivants: ignorance de l'écologie de l'espèce, utilisation d'individus trop jeunes, durée insuffisante des expériences, dégradation de l'environnement, manque de suite dans les idées. Dans ses recommandation, il indique les principales règles à suivre pour introduire et acclimater le saumon du Danube. 1. INTRODUCTION The biggest salmonid fish still living is the common huchen (Hucho hucho (Linnaeus, 1758)), which is distributed over a very large but disjunctive area. In Europe the species is represented by the nominate form - the Danubian huchen (Hucho hucho hucho (Linnaeus, 1758)) which is confined originally to the basin of the River Danube. An eastern sub-species, the taimen (Hucho hucho taimen (Pallas, 1773)) occurs in the upper part of the Volga River Basin and in the Pechora River Basin in Europe, over the extensive area of Siberia, beginning from the Ob River Basin in the west up to the Yana River in the east and north, in the basin of the Amur River and also in some rivers entering the Okhotsk Sea, respectively. Since ancient time the huchen was appreciated as a valuable food fish. The taimen still has commercial value in some areas while the Danubian huchen is highly appreciated as a game fish. In spite of its great areas of distribution the huchen is now considered to be an endangered species because of the effects on stocks of increasing stream regulation and pollution (Danubian huchen) as well as due to incorrect management taimen). The total world catch of this species is estimated to be about 211–214 t, of which 11–14 t are shared by the nominate form and about 200 t by the taimen. The attempts to establish this species in the new water bodies have been based exclusively on the nominate form (called “huchen” in this paper). Attempts started toward the end of the last century and were closely connected with the first attempts to breed this fish in captivity. This paper presents a short summary of transplantations and introductions carried out in various countries based on data from Holčík et al. (in press). For the sake of briefness, only a few references are quoted below. 2. RESULTS OF INTRODUCTIONS 2.1 Czechoslovakia Apart from those rivers to which huchen was native, the species has been transplanted into several rivers of the Danube River Basin previously not inhabited by it and introduced into rivers flowing into the North Sea (Elbe River Basin) and Baltic Sea (the basins of the Vistula and Oder Rivers) respectively. Nine rivers of the Danube River Basin were stocked with huchen and in one river it was reacclimated. This species established itself in all but four smaller streams but at present it is to be found in four streams only, and has disappeared from all others due to pollution and stream regulation. In the Oder River Basin the huchen was stocked into four streams. In two streams naturalization occurred but subsequently the fish disappeared from these too due to pollution. In two further streams huchen stocks are maintained only by repeated stockings. The Poprad River (Vistula River Basin) was stocked with huchen at the end of the nineteenth and at the beginning of the twentieth century. Full naturalization occurred and now this species forms a permanent part of the fish community. The huchen was introduced into six streams of the Elbe River Basin. However, it presently inhabits only a short stretch of the Vltava River upstream of Prague. However natural breeding has not been observed here. The species was introduced into basins of the Rivers Danube, Vistula and Oder in Czechoslovakia both to enlarge the number of streams inhabited by it and to conserve stocks in the face of threatened decline. It was also thought that huchen might replace the Atlantic salmon in the Elbe River Basin. 2.2 England England was probably the first country outside this original area into which huchen were introduced. In 1905 fertilized eggs were imported from Austria for angling and some of the hatched fry were released into the Thames River (Pölzl, 1910). Other fry were reared successfully in ponds and subsequently reproduced successfully (Pölzl, 1929). There are, however, few reports of specimens being caught by anglers and Marlborough (1963) suggested that some of the large trout caught recently in the Thames River may have been huchen. According to Wheeler and Maitland (1973), however, there is no indication that the huchen ever managed to establish itself. 2.3 Switzerland Attempts were made to introduce huchen into the Rhine and Sarina Rivers in Switzerland in 1949–50 and 1965–67, respectively. Anglers initiated these introductions to obtain compensation for the disappearance of Atlantic salmon and to utilize dense stocks of undesirable Chondrostoma nasus. However, scientific circles were against this introduction (Steinmann, 1948; Brodbeck, 1953). Both rivers were repeatedly stocked with fry hatched from eggs imported from Czechoslovakia. Rhine stockings fully failed because no reproduction was observed and only four sub-adult fish of maximum weight 2 kg each, were caught of the 13 thousand fry planted. Natural reproduction was not observed in the Sarina River either. 2.4 Morocco Morocco imported fertilized eggs of huchen from Czechoslovakia in 1953 and in 1959. The fry were reared in ponds and after some time were introduced into some rivers of the Central Atlas (Rivers Fellate, Oum-er-Rbia and Guigou) hopefully to obtain a valuable game fish. The huchen showed extremely fast growth (15–16 month old fishes reached 46–57 cm total length) but naturalization has not been observed in any of the natural waters nor have attempts to breed this species in captivity met with any success. 2.5 Belgium More than 22 thousand fertilized huchen eggs of Yugoslavia origin were imported into Belgium in 1954, 1960 and 1961. More than six thousand yearlings measuring 10– 18 cm were then planted into the River Lesse and an unknown lesser number into the Semois River (Maas River Basin). The huchen did not acclimatize in either stream although some specimens were observed in the Lesse River and a few specimens have also been caught by anglers (Timmermans, pers.comm.). 2.6 Poland With the exception of the Black Orava River (Danube River Basin) huchen did not formerly occur in Poland. The first attempts to introduce the species were made at the end of the last century when it was planted into the Dunajec and Poprad Rivers in the Vistula River Basin. Here, it has naturalized and has persisted until the present time. Specimens from Polish huchen culture were then used in attempts to introduce this species into other streams of the Vistula and San River Basins as well as into the Drweca River, but until now no fish have become naturalized. The purpose of this introduction was to supplement Atlantic salmon and sea trout whose stocks are now very low (Witkowski and Kowalewski, 1980; Guziur, pers.comm., Witkowski, pers.comm.). 2.7 France An introduction of huchen into the Usses River, France, was attempted in 1957. The stocking site was a left-bank tributary of the Rhone in Haute Savoie, whose upper reaches are inhabited by brown trout (Salmo trutta m. fario) and cyprinids among which Chondrostoma nasus is predominant. One year before introduction, the density of C. nasus was reduced by electro-fishing. A total of 50 thousand fertilized eggs were imported from Yugoslavia in 1957 and 1958 and after hatching the fed fry were released into the river. Stocking was repeated in 1959, and in 1960 when 10 thousand fertilized eggs were imported from Yugoslavia and the fry were released after hatching. Test fishing carried out in subsequent years confirmed the presence of huchen in the river and the young arising from natural reproduction were found in the spring and summer of 1963 (Vivier et al., 1964). Direct observation of natural spawning has been recorded in 1976 about 20 km upstream (Laurent pers.comm.). Angling for huchen was opened in 1965 and each year since several specimens have been caught, among which were some big fish weighing up to 15 and 25 kg (Svetina, 1967). Huchen penetrated into the Rhone from this tributary and one specimen was reported caught 50 km downstream from the confluence of the Usses and Rhone Rivers. According to Laurent (pers.comm.) the huchen is still to be found in the Usses River but only in small numbers because anglers are not interested in this species and the fishing season and size limit are no longer observed. The anglers have come to the conclusion that the huchen is unsuitable for such a small stream as the Usses and they are convinced that it destroys the spawning sites of trout and has thus been responsible for a decline of that species. 2.8 Sweden In 1963 a batch of 8 000 fertilized eggs of Yugoslavian origin was imported into Sweden. Advanced fry have been released into the Rexforsan River and the Indalsälven Reservoir. However, no huchen have either been caught or observed subsequently. In spite of this, there is a continuing interest in the introduction of this species into Swedish waters, especially into man-made lakes where it is hoped that it will be able to control the population of coarse fishes (Nilsson and Svärdson, 1963; Nilsson, pers.comm., Henricson, pers.comm.). 2.9 Spain Twenty thousand fertilized eggs of huchen from Czechoslovakia (not from Yugoslavia as erroneously reported by Anon., 1969; 1973) were imported to Spain in 1968. Half was placed in the hatchery at La Coruña and some of the hatched fry were planted into the surrounding rivers, but no results were recorded. The rest of the fry were stocked into ponds and a brood is produced each year (Lobon-Cervia, pers comm.). The fry resulting from the second half of the import were introduced into the Tormes River near Salamanca (left-bank tributary of the Duero River). Here, four years after the introduction natural spawning was observed in 1972. Apparently the huchen has become naturalized here because it is regularly caught by anglers with records of specimens weighing up to 15 kg (Corredera-Martin, 1981). The growth of the huchen in the Tormes River seems to be very fast, as indicated by the following figures (Mondejar- Reyna, 1981): four-year old fish reaches 6.1 kg; five-year old - 9 kg; six-year old - 10.5 kg; and seven-year old - 11.9 kg. At present, the population density of the huchen is certainly low due to the closing of the spillway in the dam upstream. In 1976 the flow of the river decreased rapidly and mass mortalities of fish downstream were observed. The purpose of the introduction of the huchen to Spain was angling for a valuable trophy game fish. 2.10 Other countries For the sake of completeness, it is necessary to add that huchen has also been introduced into other countries. According to Skácel (1976) fertilized eggs were sent to the German Democratic Republic Finland, Bulgaria, Denmark and Canada. However, there is only information from Canada, where fertilized eggs were imported in 1966 and where fry were stocked into some rivers of Quebec (Crossman, 1968). The fate of the huchen in Canada is unknown (Lachner et al., 1970; Martin, pers.comm.). 3. IMPACT OF INTRODUCED HUCHEN UPON THE POPULATIONS OF NATIVE FISH There is very little direct quantitative information on the rate of predation of the common huchen either from the original area of its distribution or outside it. Pirozhnikov (1955) justified the lower fish yield in the River in comparison with that in the Yenisei by the higher density of the taimen. It is also reported that the absence of some species ( , Rutilus rutilus and Leuciscus idus) in some stretches of the Vilyui River (left-bank tributary of the Lena River) is due to the high density of predators, dominated by the taimen (Kirillov, 1962). The high density of C. nasus, dace (Leuciscus leuciscus) and chub (L. cephalus) in the Turiec River, Czechoslovakia, is reported to be correlated with the decline of the Danubian huchen (Novák, 1971); Bastl et al., 1976). Following the introduction of the huchen into the Usses River the C. nasus populations have declined and changes in the fish community structure have been observed. According to Vivier et al., (1964) and Svetina (1967) electro-fishing carried out in a limited part of this river in 1956 gave 8 t of fish, among which the C. nasus predominated. Four years after the introduction in 1961 2.8 t were caught; in 1962 - 1.9 t and in 1963 - 1.3 t of fish were caught in the same stretch. According to these authors the place formerly occupied by C. nasus and other cyprinid fishes (the chub and the barb) was gradually filled by brown trout, whose population density increased mainly in the weight groups of 100–300 g. According to anglers this species subsequently declined as the density of C. nasus and other cyprinids (which are the main food of the huchen - see Nagy, 1976) decreased and became less available for the huchen. Trout were the more available and predation on them increased accordingly. In one stretch of the Hornád River, Czechoslovakia, a transplanted and naturalized population of huchen had to be eradicated because the abundance of the brown trout and the grayling (Thymallus thymallus) significantly decreased (Skácel, 1976). Another anonymous author (1970) writes that the density of the huchen in the lower part of the Ammer River as well as in the upper part of the Isar River (basin of the Danube River) has had to be limited to protect brown trout and grayling. On the other hand, in the Tormes River, the population of the native Chondrostoma cyclolepis (the only fish inhabiting this stretch of the river) did not display any negative changes after the introduction of the huchen (Lobon-Cervia, pers.comm.). Unfortunately, there are no details on the density of either huchen or other fishes in any of the above cases. 4. REASONS OF FAILURE OF THE INTRODUCTION OF HUCHEN From 38 attempts to introduce the huchen into new water bodies only eight (21 percent of the cases) were successful. According to information from individual sources the main reasons of the introduction failure may be as follows: (i) Ignorance of the ecology of the species with regard to its environmental requirements (Morocco); (ii) Stocking at too early a developmental stage (fry) (e.g., Rhine in Switzerland), i.e., high mortality due to predation; (iii) Insufficient numbers of fishes stocked (almost in all cases); (iv) The short duration of introduction attempts - in almost all cases stocking was performed only once. It is remarkable that attempts to introduce the huchen inside the original area of its distribution (transplantation) were more successful even though in these cases the same shortcomings were in evidence. It is furthermore not in agreement with Burmakin (1963) who writes that higher success of introduction is expected when any species is introduced into a water body outside its original area of distribution. The reasons for the disappearance of the huchen after previously successful acclimatization lie in both the deterioration of the environment (in all cases studied increased pollution was evident) or eventually a change in the management philosophy which had led to the introduction in the first place (France). 5. PRINCIPLES FOR SUCCESSFUL INTRODUCTION OF THE HUCHEN The common huchen can be successfully introduced into waters from which it has been previously absent providing certain generally valid principles for the introduction of exotics (Burmakin, 1963; Karpevich, 1975) and some measures particular to this species are followed. These are: (i) The huchen may be transplanted into streams of the foot-hill zone with stony-gravel or gravel-sandy bottom, where the water temperature in summer months is not higher than 20°C, the dissolved oxygen does not fall below 8–9 mg/1, and the water is not polluted either by industrial, urban or agricultural waste. Those parts of a river where riffles alternate with bigger and deeper pools, where the flow is branched by islands, the banks are overgrown with shrubs and trees and interrupted by the mouths of tributaries are to be preferred. Reaches selected should be sufficiently long and the zone of the of foothill brooks with possible spawning grounds should be accessible. The overall length of a reach should be about 20 km. (ii) Introduction should be preceded by limnological investigations aimed at defining the hydrological regime and hydrochemical properties of a water, and evaluating the bottom fauna and fish fauna quantitatively and qualitatively. Sufficient amounts of and coarse fish should be present in a chosen reach. Streams which are densely populated by the brown trout, grayling or other valuable game fish should be avoided. (iii) Prior to the first introduction of the huchen, it is desirable to reduce the number of native fishes and particularly of some predators (pike, burbot) and abundantly-occurring coarse fishes (in European conditions these are chub, dace, nase, perch). Big specimens of brown trout should also be removed. The reduction should be selective as numbers of big specimens above 100 mm of the total length) only should be restricted. (iv) Only healthy fingerlings (minimum six-months old) should be used for stocking. If only fry are available these should be planted into the side tributaries of the chosen section of stream or into the uppermost part of it. In this eventuality the relevant stream or section should be completely deprived of all native fishes. (v) Stocking should be repeated each year during the first four-six years until specimens from the first introduction reproduce. (vi) Angling or fishing should be prohibited in the stream during the whole course of introduction. Particularly in streams where brown trout occur because here anglers frequently mistake young huchen for brown trout. (vii) The situation in the whole of the stream into which huchen have been introduced should be closely monitored for the presence and density of huchen, growth, health condition, diet and sexual maturation, migrations and also with regard to the time and place of spawning. 6. TO INTRODUCE OR NOT TO INTRODUCE THE HUCHEN? By giving the principles for introduction of huchen, it does not mean that the author is advocating the transfer of this species indiscriminately The huchen should be introduced only into rivers where it occurred formerly (reacclimatization) or into rivers which were originally not inhabited by it, but which are inside the original area of its distribution (transplantation). In these cases introduction is justified as a measure aimed at the rescue of this species before its extinction. It is true that because of its specific environmental requirements the huchen is limited only to some reaches of rivers and it is therefore possible to consider its introduction outside its original area of distribution. However, the benefits of such introduction should be carefully judged. The huchen, as the top predator of the ecosystem of foothill rivers, may be used to control populations of overcrowded forage species, however, it is unable to form dense populations and, therefore, is unable to replace diadromous salmonids such as the Atlantic salmon or the sea trout. Furthermore, because of the present state of the water quality in European rivers the huchen is able to populate only relatively short portions of any river. The contribution of huchen introduced outside its natural range will probably be dubious with high costs and a possible negative effect upon the populations of native fishes. 7. ACKNOWLEDGEMENTS I have a great many acknowledgements to make. In addition to many friends in Czechoslovakia, I have to mention only a few of the foreign colleagues who sent me information and data on the introduction of the huchen in their countries. They are as follows: Mr. J. Bél, Seenforschungslaboratorium der EAWAG/ETH Kastanienbaum, Switzerland; Dr. J. Guziur, Instytut Ichtiobiologii i Rybactwa ART Olsztyn, Poland; Dr. P. Laurent, Station d'Hydrobiologie Lacustre INRA, Thonon-les Baines, France; Dr. J. Lobon-Cervia, Centro de Zoologia Aplicada INCN, Madrid, Spain; Mr. N.V. Martin, Ontario Ministry of Natural Resources, Maple, Canada; Ing. C. Mondejar-Reyna, Seccion de Hidrologia INCN, Madrid, Spain; Dr. N.A. Nilsson, Institute of Freshwater Research, Drottningholm, Sweden; Dr. S.U. Qadri, University of Ottawa, Ottawa, Canada; Dr. J.A. Timmermans, Station de Recherches des Eaux et Forets, Greonendaal, Belgium, and Dr. A. Witkowski, Uniwersytet Wroclawski, Wroclaw, Poland. 8. REFERENCES Bastl, I. et al., Niekolko poznámok k príčinám úbytku hlavátky v Turci a k problémon jej 1976 ochrany všeobecne. In Hlavatka podunajáská - súbor referátov. Bratislava, Príroda, pp. 65–74 Brodbeck, W., Woher auch “Donaulachse” im Oberrhein? Schweiz.Ztg.Sportfisch., 1953 4/5:152–6 Burmakin, E.V., Akklimatizacija presnovodnykh ryb v SSSR. 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Fishing-Club du Moyen Atlas (Nouv.Ser.), 6:1–15 Skácel, L., Súčasný stav a rozscaron;írenie hlavátky podunajskej (Hucho hucho L.) 1976 a perspektíva aklimatizačných pokusov an Slovensku a v zahraničí. In Hlavátka podunajská (Hucho hucho L.) - súbor referátov. Bratislava, Príroda, pp. 11–21 Steinmann, P., Sollen wir in der Schweiz Huchen einsetzen? Schweiz.Fischereiztg., 1948 56(2):31–3 Svetina, M., Der Huchen hat sich in Frankreich gut eingeburdet. Allg.Fischereiztg., 1967 92(11):337–8 Vivier, P. et al., Le huchon et son acclimation en Haute-Savoie. Bull.Fr.Piscicult., 1964 36(212):77–85 Wheeler, A. and P.S. Maitland, The scarcer freshwaters fishes of the British Isles. 1. 1973 Introduced species. J.Fish Biol., 5(1):49–68 Witkowski, A. and M. Kowalewski, Aklimatizacija i rozsiedlenie głowacicy w Polsce. 1980 Gospod.Rybn., 32(1): 6–9 Anon., Recent introductions of fish, shrimps and oysters. FAO Fish.Cult.Bull., 2:15 1969 , Zum Huchenvorkommen in Oberbayern. Allg.Fisch.ereiztg., 1970 95(23):791 , Sulec v Spaniji. Ribič, 32(9):255 1973

Fig. 1 Introductions of the huchen in Europe and North Africa: 1 - Introductions; 2 - Autochthonous distribution of Danubian huchen; 3 - Autochthonous distribution of taimen/its western part only. According to Holčík et al., 1981 (slightly modified) INTRODUCTION OF CARP (Cyprinus carpio (L.)) IN FINLAND

P. Ahlfors, P. Kummu and K. Westman Finnish Game and Fisheries Research Institute Helsinki, Finland

CONTENTS

1. INTRODUCTION 1.1 Early attempts at carp cultivation in Finland 1.2 Importation of present population to Finland 2. METHODS USED FOR PRODUCING JUVENILES FOR STOCKING 2.1 General 2.2 Spawning and first summer 2.3 First winter 2.4 Second summer and winter 2.5 Brood fish 2.6 Accompanying fish in carp cultivation 3. STOCKINGS 4. THE SUCCESS OF CARP STOCKINGS 4.1 Tagging experiments 4.2 Loss of tags 4.3 Other factors affecting tag return 4.4 Reproduction 4.5 Growth and age 4.6 Predators, diseases, parasites 4.7 Behaviour, nutrition 5. FACTORS AFFECTING THE SUCCESS OF CARP STOCKING 5.1 Age and size of stocked juveniles 5.2 Quality and latitude of stocking waters 6. ECONOMIC SIGNIFICANCE 6.1 Fishing 6.2 The valuation of carp 6.3 Profitability of carp stockings 6.4 Impact and compound effects of carp stockings 7. THE EFFECT OF CARP ON THE STOCKING WATERS 8. CONCLUSIONS AND RECOMMENDATIONS 9. REFERENCES

ABSTRACT The earliest attempts to introduce carp to Finland were made in 1861, but results were not obtained until farming and stocking were begun again in the fifties. Juveniles are raised in earthen ponds, primarily on natural feed. A total of about 170 000 juvenile carp of one year and older were stocked from 1956 to 1981 in inland and brackish waters between 60° and 68°N, and 13 000 juveniles of two years and older were marked with Carlin tags. Catch varied between 0 and 336 kg/1 000 stocked individuals. Stocking was not economically profitable. As these estimates were made from returned tags, they appear low because of tag loss or failure to report a proportion of tags recaptured. Furthermore, Finnish fishermen are unaccustomed to fishing for carp. The best results were obtained with two or three year old juveniles, at least 18– 20 cm long and weighing 150 g or more. In eutrophicated waters with a great deal of vegetation, carp had an incremental weight gain during the year of stocking of up to 1 000–1 500 g. The largest tagged carp caught weighed 5 100 g and the largest untagged 11 900 g. Under Finnish conditions, carp do not appear to be able to form self-reproductive populations, since the juveniles die during the first winter in natural waters. Carp has not been observed to have any harmful effects on the water systems. Interest in carp stocking is increasing, especially in eutrophicated waters in southern Finland. RESUME Les premiers essais d'introduction de la carpe en Finlande remontent à 1 861. Toutefois, il a fallu attendre, pour obtenir des résultats, que l'élevage et le repeuplement reprennent au cours des années cinquante. Des juvéniles sont élevés dans des étangs de terre avec, le plus souvent, une alimentation naturelle. De 1957 à 1980, on a utilisé environ 170 000 individus d'un an ou plus pour repeupler les eaux intérieures et les eaux saumâtres entre 60° et 68°30'N. Des juvéniles de deux ans et plus servant au repeuplement ont été marqués (marques Carlin, 13 000 individus au total, 46 groupes de marquage dans 33 sites de repeuplement). Les captures ont varié entre 0 et 340 kg pour 1 000 individus mis à l'eau. Le repeuplement n'a eu aucun avantage économique. Toutefois, le taux de survie des carpes mises à l'eau n'a pas été aussi faible qu'on pourrait le penser compte tenu du nombre de marques récupérées (certains se sont perdues, d'autres n'ont pas été renvoyées). De plus, les pêcheurs finlandais n'ont pas l'habitude de pêcher la carpe. Les meilleurs résultats ont été obtenus avec des juvéniles de deux ou trois ans d'au moins 18–20 cm et 100–150 g et, lorsque les populations de brochets étaient importantes, avec des juvéniles de 25 cm et 250 g. Dans les eaux eutrophes à forte végétation, le poids des carpes pouvait augmenter de 1 000–1 500 g durant l'année de repeuplement. La plus grosse carpe marquée que l'on a capturée pesait 5 100 g; la plus grosse carpe non marquée 11 900 g. Compte tenu des conditions qui règnent en Finlande, la carpe ne semble pas à même de former une population reproductrice; les juvéniles meurent durant leur premier hiver dans les eaux naturelles. Pour autant que l'on sache, la carpe n'a eu aucun effet nuisible dans les réseaux hydrographiques. L'intérêt s'accroît pour le repeuplement en carpes, notamment dans les eaux eutrophes du sud de la Finlande. 1. INTRODUCTION Carp (Cyprinus carpio) has been cultivated in its native Asia for thousands of years. Because of its cultivation by Man, carp very early on spread beyond its original range of distribution. It was brought to Europe quite possibly as early as the time of ancient Rome, but at the very latest by the 13th or 14th century. In many areas carp is frequently either deliberately stocked or individuals have escaped from a fish farm and formed a natural population. Today carp is found in all parts of the world with the exception of Antarctica (cf. Huet, 1971). 1.1 Early attempts at carp cultivation in Finland Carp was brought to Finland fairly late, in 1861. The earliest attempts at carp cultivation were made in the latter half of the 19th century in southern Finland (Malmgren, 1883; Sandman, 1892; Wuorentaus, 1938). These stocks did not become established because of a lack of proper facilities for overwintering. In addition, at that time sharp variations occurred in the climatic conditions, for example, a sudden return to harsh winter conditions after the spring thaw. The next attempt in the thirties in central Finland also failed. In this case, the suspected reason was a summer too cold to allow spawning (Saari, 1937, 1939; Wuorentaus, 1938). 1.2 Importation of present population to Finland In 1951, 40 one and two-summer old carp were introduced into Finland from the Aneboda Fish Culture Station in Sweden to the Porla Fish Culture Station in southern Finland (60°15'N). Initially the aim was simply to see if carp could be cultivated so far north. The following spring 18 carp were left, which thrived in the Station, and a further 300 one-year old individuals were imported in 1955 to increase the stock. At this point the original aim was extended to see if carp could succeed in natural waters and to study its possibilities in the management of eutrophicated waters. In 1955, the brood fish in Porla also reproduced for the first time thus making continuos cultivation possible (Kajosaari, 1980; Sormunen and Kajosaari, 1975). Carp were imported into Finland for a third time during the fifties. In 1958 450 one-summer old, third-generation offspring of a cross between Galician mirror carp and Amurian “sazan” carp were imported from the Ropsha Experimental Station near Leningrad (Kajosaari, 1959). The aim was to find a race of carp which would be able to withstand the winter better than ordinary carp. It was thought that such carp would be easier to cultivate and would be suitable for stocking in waters in northern Finland, as well as possibly forming a self-reproductive population in natural waters. The first crossbred carp were hatched in Porla in 1963. The last brood fish in this population were taken out of the experiment in 1977. No significant advantage of the crossbred carp over the ordinary carp could be demonstrated (Sormunen and Kajosaari, 1975). This may also have been affected by the narrow genetic base of the crossbred carp fingerlings imported to Finland. Under hatchery conditions, the crossbred fingerlings overwintered slightly better than did the ordinary carp fingerlings. As mature adults, the crossbred carp exhibited a slow rate of growth. In the following we will deal primarily with the experiences obtained with the offspring of carp imported from Aneboda, Sweden, to Finland. 2. METHODS USED FOR PRODUCING JUVENILES FOR STOCKING 2.1 General Carp fingerlings cannot survive winter conditions in Finnish natural waters during their first year. For this reason, juveniles are raised in a hatchery until they are two- summers or two-years old before stocking. With the exception of the last two or three years, the carp raised in Finland have been cultivated in the Porla Fish Culture Station in Lohja. Cultivation has been run in one phase using the “uncontrolled natural reproduction in ponds” method described by Huet (1971). This cultivation has been described by Sormunen and Kajosaari (1975) and Kajosaari (1980). 2.2 Spawning and first summer For spawning brood carp are moved from the winter ponds to spawning ponds with earth bottoms and considerable vegetation at the beginning of May. The maximum depth of these ponds is 1.5 m and their surface area is generally 0.5–0.75 ha. The number of brood fish is between 30 and 34 individuals/ha; and the sex ratio is 2:1 females:males. The carp freely in the ponds. The time of spawning is regulated mainly by the weather. In warm springs spawning may be as early as May but in cool springs as late as the end of June. An even, rapid, warming spell is most certain to bigger spawning and no spawning was observed at temperatures below 20°C. In Porla, conditions are such that the brood fish cannot be moved out of the spawning ponds during the summer, so that these ponds are also raising ponds for juveniles during their first summer. The brood fish and juveniles feed on natural food in the ponds. As a supplementary feed, the juveniles are given cracked wheat at the end of summer. In the autumn, the ponds are emptied, the brood fish moved to winter ponds and the fingerlings taken indoors. Because of the natural method used and the variations in the weather during Finnish summers, the results of cultivation vary considerably from year to year, both as to number of fingerlings and their size. In the best case, the density of one-summer old fingerlings obtained was 2 individuals/m2 (Kajosaari, 1966). The average size of juveniles varied from below 10 g to nearly 50 g. 2.3 First winter The first winter is critical to the adaptation of carp fingerlings to Finnish climatic conditions. The loss of juveniles during this period has been attributed to the insufficient amount of stored nutrients in one-summer old fingerlings. This means that they do not have enough energy to maintain basal metabolism during the period when the water is too cold for them to feed (Kirpitschnikov, 1957). During the first winter in the Porla Fish Culture Station, spring water at an even temperature of +6.3°C is used for rearing. The fingerlings are divided into two size classes and placed in 1–1.5 m2 wintering basins made of wood, fibreglass or aluminium. Spring water is circulated at the rate of 1 1/minute/2–3 kg of fish. The depth of the water in the basin is about 15 cm, and the density of fingerlings generally 15–30 kg/m2. As the winter progresses, the juveniles are fed with dry salmon feed once or twice a week. According to Huet (1971), carp stop eating when the temperature falls below +5°C. In Porla, however, carp in an experiment stopped eating only when the temperature fell to +2.5°C (Kajosaari, 1962). It is possible that the carp population in Porla is slowly becoming adapted and acclimatized to local conditions. In the Porla Station the carp fingerlings are given an NaCl bath (1.5 percent, 15 minutes, no aeration) as needed during the winter to prevent gill and skin parasites. With these methods, the losses during the first winter at Porla were only about 10 percent and were highest with the smaller fingerlings. 2.4 Second summer and winter For the second summer, the fingerlings are moved to earthern ponds with lush vegetation when the water in the ponds has reached the same temperature as that in the wintering basins (+ 6°C). The surface area of the ponds is 0.1–0.75 ha and their average depth about 1 m. The juveniles feed on natural food and are given small amounts of cracked wheat as a supplement. Boiled potatoes, ground fresh fish and dry salmon feed have also been found to be good supplemental feed. A good carp pond in southern Finland produces at least 100–300 kg of carp/ha. If a weight of 150 g is desired for two-summer old carp, then between 1 000 and 2 000 one-year old fingerlings per hectare should be stocked into the growing ponds at the beginning of the second summer (Ilmarinen, 1982). During the second winter, the carp of two summers are placed in the same wintering ponds as the mature brood fish. The depth in the pond used is fairly even, 1–1.5 m, with a maximum depth of 2 m and a surface area of about 0.1 ha. There is a slight current of water, which cuts across only a narrow section of the water mass in the pond. The advantage of the simple cultivation method used in Porla is that the fish require very little handling, transfer or care. The greatest drawback is the short, first growing season for fingerlings. The Finnish Game and Fisheries Research Institute intends to begin research and development of controlled reproduction methods suitable for Finnish conditions, intensive rearing of carp fingerlings during their first summer with dry feed developed especially for carp, and the rearing of carp in the warmwater effluents of power plants. 2.5 Brood fish The carp imported from Aneboda first spawned when they were five years old and the largest individual weighed 4.5 kg. To maintain genetic variability, the stock of brood fish is kept larger than production requires and supplemented from time to time by leaving a few young individuals to grow with the brood fish in the spawning ponds. The last imported carp in 1951 died during the winter of 1980–81 and was thought to have still spawned in the spring of 1980. 2.6 Accompanying fish in carp cultivation A cultivated population of (Tinca tinca (L.)) originating from Galicia in Poland, is also present in Porla. Its method of cultivation is the same as for carp and the tench use the same ponds at the same time as the carp. However, tench fingerlings cannot withstand wintering over in spring water in indoor basins. The carp do not appear to have suffered any ill effects as the result of this simultaneous cultivation. 3. STOCKINGS The first carp were stocked into natural waters in 1956. Initially the numbers stocked were only 20–400 individuals per year. The first crossbred carp were stocked in 1960. Since 1961, the numbers stocked have increased noticeably. In the best years, Porla has supplied about 16 000 juveniles per year for stocking purposes. The method of cultivation and weather conditions have combined to exaggerate the annual variations in numbers; in some years there were no juveniles at all available for stocking. This also illustrates the difficulties of cultivating carp in Finnish conditions. The Porla Fish Culture Station has supplied between the year 1956 and 1981 a total of 138 000 one-three year old individuals for stocking (Table 1), the main purpose of which has been to determine whether carp could be used in the management of eutrophicated waters. Other fish culture stations supplied in 1979 27 500 one-summer to two-year old carp for stocking (Anon., 1980b) and in 1980 5 000 one and two-summer old carp (Eskelinen and Sumari, 1981; Westman et al., 1982; Westman and Tuunainen, 1982). The majority of the stockings were made in southern Finland and the southernmost inland waters of central Finland (60–62°N). A few stockings were also made in the brackish bays (S °/00 3–5) on the coast of the Gulf of Finland. The northernmost carp stocking was done in the Kajaani district (c 64°N) and the northernmost crossbred carp stockings in the lijoki River water system (65–66°N), in Lake Kemijarvi (66°30'N) and the southern part of the municipality of Inari (c 68°N) (Map 1). 4. THE SUCCESS OF CARP STOCKINGS 4.1 Tagging experiments In order to study the profitability of carp stocking, some of the stocked juveniles were tagged. By the end of 1981 some 13 000 two-year and older carp had been individually marked with metal wire Carlin tags and more recently with the plastic wire form of Carlin tags. Over 1 200 of the tags have been returned. A summary report has been made of all taggings done up to 1973 with spring-stocked carp of at least two years of age (Sormunen et al., 1976). In inland waters the rate of return of carp tags has varied between 0 and 32 percent, which in terms of a catch means 0–336 kg/1 000 stocked fish. The best result in brackish water has been 40 percent, or 257 kg/1 000 stocked juveniles. Of the 18 marked stockings, 12 had a rate of return of 10 percent or more, 10 stockings had a calculated catch of over 100 kg/1 000 stocked of these 10 stockings 7 had a catch of over 150 kg/1 000 stocked fish (Sormunen et al., 1976). Some of the results of tagged stockings are shown in Tables II–IV. However, as 1 000 20–21 cm long carp weigh about 150 kg in the spring, and 1 000 23-cm long carp weigh about 200 kg, it seems that the catch calculated from returned tags often does not equal even the total weight of the juveniles stocked. 4.2 Loss of tags It is often suspected that the Carlin tags work loose and drop off the carp, perhaps in large numbers, and for this reason the tag return gives too pessimistic a picture of the success of stocking (Sormunen and Kajosaari, 1975). This view is supported by the fact that the largest carp whose tag was returned weighed 5.1 kg and only a few tagged carp of over 3 kg have been reported, while untagged carp of over 6 kg are being caught constantly as well as a number of 10-kg carp, including the current Finnish record carp of 11.9 kg (Anon., 1981). In addition, in some lakes in which only tagged carp have been stocked, carp with no tags have been caught. These often have clear scars where the tag should be. The tag could come loose if the fish catches it on vegetation or a fish trap and pulls itself free or the tag may be rejected by the body of the fish. 4.3 Other factors affecting tag return Along with tag loss the results of tagging trials are affected by the willingness of fishermen to return the tags. It has been estimated that the number of unreturned tags varies between 10 and 70 percent of the number of returned tags depending on species and fishing ground. The results from tagging carp are also affected by the fact that the behaviour of the fish is not well known to Finnish fishermen who rarely catch it. 4.4 Reproduction Although it is likely that carp spawn in some of the waters in which they are stocked the offspring would appear unable to survive the first winter. There has not been a single observed instance of the successful self reproduction of carp in Finland. 4.5 Growth and age The growth of carp in eutrophicated waters is good compared with other fish. The additional growth of two-year old stocked juveniles, which weigh about 200 g at stocking, is often over 1 000 g during the first year. After stocking, carp may be fished for a long time. Some tagged carp have been caught ten years after stocking. In one instance carp (untagged) have been caught 21 years after stocking (Kajosaari, 1980). All in all, it is an appreciable benefit that once stocked, carp remain in the waters for a long time without forming a stunted population. 4.6 Predators, diseases, parasites Carp stockings are threatened by the same enemies as other fish. The worst predator is pike; several carp tags have been found in pike stomachs. In practice, the only predator of carp over 300 g is Man. No predator, disease or parasite specific to carp alone has been observed. The introduction of carp to Finland has not introduced any new fish parasites or diseases. 4.7 Behaviour, nutrition Carp swim in schools, particularly during the fingerling stage. A frequent observation has also been of schools of carp at the surface of the water, sexually mature fish basking in the sun. In the fish culture stations, however, large carp appear to be content alone or in pairs during the summer. Several of the Finnish manuals have stated that carp hibernate in the winter, remaining motionless on the bottom or even dug into the silt. As far as we know, this behaviour has not actually been observed. In the Porla Fish Culture Station, carp remain in motion throughout the winter, even in ponds which have frozen over. During the winter the fish collect in very dense schools and move slowly all the time even though the school as a whole may remain in the same place. Sometimes the eddies caused by such schools melt the ice above them completely. In natural waters, large carp have been caught by stationary traps below the ice, which also means they must be moving about in the winter. There are no observations on the behaviour of fingerlings in natural waters. The diet of carp is broad and they are capable of efficient feeding even under competitive conditions. Carp thrive even in those waters with large, slow-growing bream (Abramis brama (L.)) and roach (Rutilus rutilus (L.)) populations where there is great competition for food. Carp is able to utilize the larger molluscs and to syphon food from deeper in the bottom than any of our domestic fish species. It prefers to search for food in warm, shallow waters, among vegetation, and for this reason seeks the littoral and when stocked in brackish water, shallow inlets. Carp has not been observed as having an impact on vegetation in any of the waters in which it has been stocked. Nowhere in Finland has carp displaced another species of fish; of course, the number of juveniles stocked has not in any case been large enough to cause such displacement. In oligotrophic waters, the growth of carp has been slow in some cases and as a rule becomes slower after sexual maturity. There have been no observations of stocked populations forming stunted populations. 5. FACTORS AFFECTING THE SUCCESS OF CARP STOCKING 5.1 Age and size of stocked juveniles One year old fingerlings have not been tagged as even during a good growing season, these are so small that the added stress caused by a Carlin tag would undoubtedly have proved fatal. Untagged, one-year old carp have been stocked in tens of thousands in many places (Table 1, Map 1). For the moment there is no confirmed data on whether or not fish from these stocking have survived to reach fishing size. As expected, the tagging trials have shown that the larger the average size of the fish, the better the average rate of tag return. The results for stocked juveniles under 20 cm length have been below 100 kg/1 000 stocked individuals. The poor results may be due to stress and related phenomena caused by the tags themselves, predators and winter deaths, all of which most strongly affect the smaller individuals. This is supported by the comparative stocking study carried out by the authors on marked carp juveniles of the same age, but different size, in a lake in southern Finland (Lake Valkeajarvi, c. 61°N). Two different size classes (average lengths 14 and 21 cm) of two-year old carp were stocked in the lake. There were otherwise no differences between the two classes. After three summers, the return of 14 cm juvenile tags was 1.1 percent, and that of 21 cm juveniles 11.1 percent. On the basis of some findings, it appears that some of the under 14 cm fish are not able to grow enough to withstand the next winter and some of them are caught by predators, usually pike (Esox lucius L.). The rate of return in both cases was affected by the Carlin tags themselves. The tags' slow growth decrease resistence to infection and attract predators because of their visibility. 5.2 Quality and latitude of stocking waters After size at stocking the most important factor in the success and growth of carps is the biotic potential of the water stocked, at least in southern and central Finland. The best results have been obtained in relatively eutrophicated lakes, with lush aquatic vegetation and bottom fauna. In at least one instance, the lake in question was one in which bream remained undersized because of competition for food but in which the carp nonetheless thrived. On the other hand, some data also exists on instances of lakes with minimal biotic potential in which carp grew to a relatively large size (Seppovaara, 1965; Kajosaari, 1980). The northernmost limit for successful carp stockings has proved difficult to define. Carp appeared to do sufficiently well in Lake Sokajarvi (64°15'N) and a few of the best results were obtained near Kuopio (63°N). 6. ECONOMIC SIGNIFICANCE 6.1 Fishing There is no fish which can compare with carp for size and strength, behaviour and caution in the natural waters in Finland. The closest comparison is with bream (Abramis brama) but bream weighing over 2 kg are rare. In Finland, however, fishermen are not accustomed to catching carp and no fishing gear designed for them is in use. Furthermore, the fishermen lack knowledge of the habits and behaviour of carp and do not know when or where it is best to fish for this species. In many of the stocked waters, most of the carp probably remain uncaught and die of old age. On the basis of the tag return data, carp have been caught by rod and line with worms as bait, in trawl lines, fyke nets, fish traps and various size gill nets. Usually the nets have had a mesh which is too small for carp, since people are used to catching pike, pike perch (Stizostedion lucioperca (L.)) and small fish. The most effective have been wide-mesh trammel nets. These have been used for a long time in areas where there are fast-growing bream. Another particularly effective method has been the old fishing trick of torch fishing at night in shallow water with artificial light and a multipronged, long-handled, fishing spear. The largest carp caught so far in natural waters in Finland weighed 11.9 kg and was caught with a fishing spear (Anon., 1981). However, torch fishing is prohibited in many places. 6.2 The valuation of carp Carp generally has a rather high value as a game fish. The reason for this is the large size attained: large individual fish have a recreational value, which does not depend very much on the food value of the catch. The rarity of the species increases the significance of the catch. The largest carp caught are usually published in the popular press. However, organized sportsfishing specifically for carp is very rare in Finland. More carp stockings have been requested in almost every body of water in which trial stockings have produced large-size game fish. The appreciation of carp as food is very variable. It should be pointed out that in Finland carp is not available as food even as an import item, so that methods for its preparation and consumption are unknown. In warm waters, the flesh of carp, like that of all cyprinid fish, may take on an unpleasant flavour but the poor reputation of carp in Finland comes from fish caught in polluted waters in the summer or from small-sized, newly stocked individuals. The taste of individual carp of 1 kg caught during the cold season or kept in fish pens in fresh, clean waters, has been described as good or even excellent. Usually the flesh of carp weighing several kilos is reddish. The taste of properly treated carp is praised no matter how the fish is prepared and served. Since large-size food fish are valued in Finland, we expect interest in carp to increase considerably. For the moment, carp is not commercially available. 6.3 Profitability of carp stockings On the basis of the tagging trials carried out so far, it would appear that the value of the catch of carp is only in rare cases enough to cover the costs of stocking and fishing. Even though the true catch might be almost double that calculated on the basis of the returned tags, carp stocking in Finland has been either run at a loss or in the best cases at a very small profit, because Finnish fishermen are not used to fishing for carp. The development of more versatile catching methods, increased carp fishing and directing fishing at more economic size-classes, could combine to increase the catch from carp stockings. 6.4 Impact and compound effects of carp stockings As Sormunen and Kajosaari (1975) have pointed out, the results of stocking with carp or those of any other stocking for that matter, cannot be judged solely on the basis of how many of the stocked fish are actually caught. In evaluating the results, all factors should be considered, including the extent to which stocking has improved fishing and recovery for all species in a fish community, the profitability of fishing and its recreational benefit. Carp appears to be comparable to salmonids as “prize fish”. The catch of an exceptional individual weighing several kilogrammes increases interest in the management of fishing waters as well as fishing in general, so that other fish are more efficiently utilized and the value of all fishing as a whole increases. In some cases the catch appears to have influenced fishing habits very quickly. For example, at Lake Valkjarvi, formerly ordinary nets with a mesh size of 45–50 mm were in general use; but since carp were stocked, the use of 70–80 mm mesh trammel nets is more common. 7. THE EFFECT OF CARP ON THE STOCKING WATERS Carp have not been observed to affect other fish, although they can be considered as competing with other bottom-feeding species. Equally, carp have not been observed to affect aquatic vegetation in the waters in which it has been stocked. No observed new disease or parasite has been observed as a result of the importation of carp into Finland, neither have cultivated or stocked carp been observed to carry or spread any harmful disease. The carp appears to occupy an otherwise free ecological niche. Its most likely competitors are the other cyprinid fish such as bream, white bream (Blicca bjoerkna (L.)), crucian carp (Carassius carassius (L.)), (Leuciscus idus (L.)), roach (Rutilus rutilus (L.)) and rudd (Scardinius erythrophthalmus (L.)). Of these, only bream and, to some extent, ide have any value as a catch. Carp does not appear to have undergone any changes for the worse in its new environment but the species has developed during the 30 years it has been cultivated in Finland in that it is better able to tolerate cold water. 8. CONCLUSIONS AND RECOMMENDATIONS The data collected on carp in Finland has provided the basis for the following conclusions and recommendations: 1. Carp stockings have given economically significant results only in rare cases. This is mainly due to the unfamiliarity of fishermen with the behaviour of the species: they do not know where and when they should fish for carp. In Finland there is no tradition of fishing for fish like carp so the gear used is rarely suitable for catching it. 2. Carp evidently does not reproduce successfully under natural conditions. Stockings have succeeded in many different types of waters, even in northern Finland. The best results have been obtained in eutrophicated lakes. Because carp support waters with rather little dissolved oxygen they seem to be of significance particularly in the management of lakes in southern Finland which are already eutrophicated or in the process of eutrophication. The effective utilization of these waters has proved to be a problem because of the lack of domestic fish species suitable for stocking into these waters. 3. On the basis of 30 years experience with carp in Finland, it would appear that carp has not caused any observed damage in the waters stocked to other fish species, to vegetation or to the ecosystem in general. The species also does not appear to be a serious competitor of domestic fish species or to replace or hybridize with them. 4. No dangerous fish diseases or parasites have been observed in carp, neither has it been observed to carry or spread such or to serve as an intermediate host for them. 5. Carp does not appear to have undergone any changes since introduction except that the Finnish population is perhaps more cold-tolerant. 6. Carp stockings have achieved their intended objectives to some extent; although lack of interest in catching carp limits the success. 7. There is reason to continue carp stockings, especially in eutrophicated or eutrophicating lakes. Juveniles for stocking should be over 20 cm in length; and stocking should be made in early summer. 8. Fishing for carp should be increased in the waters in which it is stocked; fishing should be aimed at that size-class which gives the best economic result, yield, and value of catch; and fishing methods should be developed and made more versatile. This requires that research and trial stockings be emphasized and strengthened and that more information be given to fishermen on the behaviour and life-cycle of carp, their growth and mortality, and the results of studies and experiments on catching carp. 9. Methods for the controlled reproduction of carp and the cultivation of juveniles for stocking purposes should be developed so that the short growing season in Finland may be utilized to the full. New cultivation methods, such as the use of the warm effluents of power plants, should be developed. 10. REFERENCES Eskelinen, U. and O. Sumari, Kalanviljely Suomessa vuonna 1980. 1981 Suom.Kalankasvattaja, 1981(4):21–3 Huet, M., Textbook of fish culture, breeding and cultivation of fish. Farnham, Surrey, 1971 Fishing News (Books) Ltd., 454 p. Ilmarinen, P., Porlan kalanviljelylaitoksen toiminnan kehittamisesta. 5p. (mimeo) 1982 Kajosaari, H., Karpinpoikasia Neuvostoliitosta. Suom. Kalestuslehti, 66(1): 21–2 1959 , Ulkolaisten kalalajien kasvatuskokeilut, valiaikatietoja. Metsastys 1962 Kalastus, 51(4):139–42 , Karppikokeista myonteisia tuloksia. Metsastys Kalastus, 55(7–8):331– 1966 3 , Karpin viljely ja istutus. Suom.Kalastuslehti, 87(3):74–6 1980 Kalataloussaatio (Fisheries Foundation), Toimintakertomukset vuosilta 1960–77 (mimeo 1961–78 reports) Kirpitschnikov, V.S., Karpfenzucht im Norden der UdSSR. Dtsch.Fischereiztg., 4:213– 1957 6 Malmgren, A.J., Lausunto, jonka keinotekoisen kalankasvatuksen soveliaisuudesta 1883 Suomeen on keisarillisen senatin maanviljelystoimikuntaan antanut And. Joh. Malmgren, kalastuksen tarkastaja, ylim. prof. ym. Hameenlinna, Hamalaisen osakeyhtio, 86 p. Saari, L., Karpin istutusta kokeiltu Keski-Suomessa. Suom.Kalastuslehti, 44(9):192 1937 , Karppi ei ole kutenut Mantassa. Suom. Kalastuslehti, 46(1):22 1939 Sandman, J.A., Karpin kasvattamisesta pohjoismaissa. Suom.Kalastuslehti, 1(5):69– 1892 76 Seppovaara, O., Karpista. Urheilukalastaja, 1965 (4):21–4 1965 Sormunen, T. and H. Kajosaari, Karpin viljely ja istutus Suomessa. In Eramaailma, 1975 edited by H. Sarrala. Hameenlinna, Karisto, pp. 41–50 Sormunen, T. et al., Yhdistelma Kalataloussaation suorittamien kalamerkintojen 1976 merkkipalautusten yhteydessa saaduista saalistiedoista. Kalataloussaation monist.julk., 55:1–468 Westman, K. and P. Tuunainen, A review of fish and crayfish introductions made in 1984 Finland. EIFAC Tech.Pap., (42) Vol.2:436–48 Westman, K. et al., A review of fishing stockings in Finland. EIFAC Tech. Pap., (42)Vol. 1984 1:252–68 Wuorentaus, Y., Ulkomaisten kalaiajien koteuttamisyrityksia. Luonnon Ystava, 42(3):87– 1938 92 Anon., Karppiennatys kasvoi kilolla. Suom.Kalastuslehti, 87(5):151 1980 , Kalanviljely vuonna 1979. Suom.Kalatalous, 49:1–3 1980a , Riista- ja kalatalouden tutkimuslaitoksen toimintakertomus vuodelta 1979. 1980b 144 p. + 6 tables (mimeo) , Karppiennatys parani. Suom.Kalastuslehti, 88(8):251 1981 , Riista- ja kalatalouden tutkimuslaitoksen kalantutkimusosaston tutkimus- 1982 palvelu-, tiedotus- ja julkaisutoiminta vuonna 1980. (Research, service and information work, and publications of the Fisheries Division of the Finnish Game and Fisheries Research Institute in 1980.) Riista- ja kalatalouden tutkimuslaitos kalantutkimusosasto tiedonantoja, 19:1–118 Table 1 The number of various aged carp and crossbred carp supplied by the Porla Fish Culture Station for stocking purposes in the years 1956–81 (Kajosaari, personal communication, Fisheries Foundation Reports 1961– 78; Anon., 1979, 1980c, 1982) Year one-year 2-summer 2-year 3- 3-year 2–4 Total summer year?

1956 - - - - 160 - - - - 160 1957 ------20 - 20 1958 ------400 400 1959 ------200 200 1960 - - - - 225 - - 125 - 350 1961 5 900 ------5 900 1962 9 250 ------9 250 1963 ------0 1964 4 900 ** ------4 900 1965 3 700 ** - - - 220** - - - - 3 920 1966 4 900 ** ------4 900 1967 325 7 450* - - 150 520* - - - 8 445 1968 6 880 - - - 750 1 020* - - - 8 650 1969 - - - - 1 490 420* - - - 1 910 1970 14 040 - 30 20* 850 - - - - 14 940 1971 2 200 - 275 - 900 1 770* 1 450 - - 6 595 1972 500 - - - 760 970 - 600 - 2 830 1973 10 770 - 2 100 1 070* 2 070 - - - - 16 010 1974 8 115 - 1 140 - 2 830 310* - - - 12 395 1975 80 - 325 - 2 190 1 785* 830 - - 5 210 1976 6 130 - 1 920 - - 1 445* - 490 - 9 985 1977 2 900 - 1 640 - 1 220 - - - - 5 760 1978 ------0 1979 2 880 - - - 630 - - - - 3 510 1980 3 500 - 1 120 - 2 050 - - - - 6 640 1981 2 300 - 500 - 940 - - - - 3 740

137 650 * crossbred carp ** exact number of crossbred carp unknown Table 2 Carp stocking in brackish water: Helsinki, Suomenlinna - Kaivopuisto (60°10'N) 10/6/68 - 500 individuals stocked, average length 23.5 cm, average weight 247 g. Return result 257 kg/1 000 stocked (Sormunen et al., 1976) Catch year Catch Average Largest weight individual Ind. % kg g g

1 = 1968 172 34.4 104 602 1 400 2 = 1969 22 4.4 20 894 1 600 3 = 1970 4 0.8 5 1 325 2 100

TOTAL 198 39.6 129 673 Table 3 Carp stocking in a lake in southern Finland: Tuusula, Lake Tuusulanjarvi (60°20'N) 1/6/67 - 50 individuals stocked, average length 26.6 cm, average weight 300 g. Return result 336 kg/1 000 stocked (Sormunen et al., 1976) Catch year Catch Average Largest weight individual Ind. % kg g g

1 = 1967 13 26.0 9 690 1 100 2 = 1968 0 3 = 1969 1 2.0 2 (2 000) (2 000) 4 = 1970 1 2.0 2 (2 400) (2 400) 5 = 1971 1 2.0 3 (3 450) (3 450)

TOTAL 16 32.0 17 1 052 Table 4 Carp stocking in a lake in central Finland: Siilinjarvi, Lake Sulkavanjarvi (63°10'N) 10/6/70 - 200 individuals stocked, average length 20.9cm. Return result 230 kg/1 000 stocked fingerlings (Sormunen et al., 1976) Catch year Catch Average Largest weight individual Ind. % kg g g

1 = 1970 17 8.5 13 757 1 300 2 = 1971 18 9.0 18 1 006 1 500 3 = 1972 7 3.5 12 1 689 2 200 4 = 1973 1 0.5 3 (3 150) (3 150)

TOTAL 43 21.5 46 1 076

HEAVY SILVER CARP (Hypophthalmichthys molitrix(Val.)) STOCKING IN LAKES AND ITS INFLUENCE ON INDIGENOUS FISH STOCKS

D. Barthelmes Institut fur Binnenfischerei Berlin, German Democratic Republic

CONTENTS

1. INTRODUCTION 2. MAIN RESULTS FROM CARP POND EXPERIMENTS 3. PRELIMINARY RESULTS FROM LAKES 4. SUMMARY DISCUSSION 5. REFERENCES

ABSTRACT On the basis of stocking experiments with up to 10 000 silver carp/ha in ponds and eutrophic lakes, heavy stocking may be defined as that level required to establish metabolic silver carp biomasses of more than 1 t/ha. There have been very few experiments in lakes to date. Ther was no evidence for massive alteration of phytoplankton biomass by a stock of 10 000 individuals of age 2/ha in a highly eutrophied, weakly stratified lake nor did H2S accumulation in the “hypolimnion” alter significantly. Conditions for bream (Abramis brama (L.)) and eel (Anguilla anguilla L.) remained relatively good. In contrast zooplankton declined sharply, experimental stocking of pike-perch (Stizostedion lucioperca (L.)) fry was unsuccessful and survival of perch fry was reduced (Perca fluviatilis L.)). Since there was high zooplankton production in the littoral, growth and probably survival of cyprinid fry was good. As heavy silver carp stocking renders seine fishing impossible overcrowding with indigenous cyprinids may result with a consequent slowing of growth of individual silver carp. At present optimization of total yield in small, highly eutrophic lakes in the German Democratic Republic is attempted by stocking 1 000 two-year old individuals/ha with total harvesting every four years. An alternative might be stocking smaller numbers each year and catching the silver carp by gill netting at greater age. The long-term effects of both management strategies are still obscure. RESUME Sur la base des expériences de repeuplement de lacs eutrophes et d'étangs à carpes avec des quantités de carpes argentées pouvant aller jusqu'à 10 000 individus à l'hectare, le repeuplement intensif en carpes argentées peut être défini comme l'établissement de biomasses métaboliques supérieures à l t à l'hectare environ. Jusqu'à présent, on a fait très peu d'expériences dans les lacs et rien ne permet de dire qu'un stock de 10 000 individus à l'hectare dans un lac faiblement stratifié et très eutrophe entraîne une altération massive de la biomasse du phytoplancton. De même, l'accumulation de H2S dans l'hypolimnion ne change guère. Les conditions sont restées assez favorables aux brèmes (Abramis brama L.) et aux anguilles (Anguilla anguilla L.), comme elles l'étaient déjà auparavant. En revanche, on a observé une nette diminution du zooplancton en cas de repeuplement au taux de 10 000 individus à l'hectare; par ailleurs, les essais de repeuplement avec du frai de sandre (Stizostedion lucioperca (L.)) ont échoué et le frai de perche (Perca fluviatilis L.) n'a pas survécu. Etant donné que la production de zooplancton était importante dans le littoral, la croissance du frai des cyprinidés a été bonne et il en a probablement été de même de leur taux de survie. Comme le repeuplement intensif en carpes argentées rend impossible la pêche à la senne, on risque d'aboutir à un surpeuplement de cyprinidés indigènes, d'où un ralentissement considérable de la croissance des carpes argentées. On essaie d'optimiser le rendement total des petits lacs très eutrophes de la République démocratique allemande en utilisant 1 000 individus à l'hectare et en procédant à une pêche complète tous les quatre ans. Une autre solution serait de déverser de petites quantités de carpes argentées chaque année et de capturer ce poisson au filet maillant lorsqu'il a atteint un stade de développement plus avancé. On connait encore mal les effets à long terme de ces deux stratégies d'aménagement. 1. INTRODUCTION When rearing of silver carp spread over European countries in the fifties the main objectives were to increase fishery yields from eutrophic waters and to improve water quality in the stocked ecosystems. The practicality of these objectives have been tested mainly in carp ponds and in similar waters. The results from the experiments in ponds are taken as the starting point for the experiments in lakes described below. In spite of the limitations of one-year class of fish only and well mixed water column, carp pond experiments with silver carp have yielded valuable basic information on how the fish interact with their environment. 2. MAIN RESULTS FROM CARP POND EXPERIMENTS Silver carp induce alterations in the environment at a threshold value of feeding pressure somewhat below 1 000 3–4 year old individuals/ha for zooplankton and between 1 000 and 2 000 3–4 year old individuals/ha for phytoplankton (Barthelmes and Kleibs, 1978). Zooplankton declines, the smaller sized elements being most vulnerable, while phytoplankton increases, as does primary production. Related alterations take place in phytoplankton-dependent variables such as nutrient and oxygen concentrations or pH (Opuszynski, 1979, 1979a). When these thresholds are converted to the more informative figures of metabolic silver carp biomass (mean individual fish weight for the growing season multiplied by the exponent 0.8 according to Vinberg (1956) and then further multiplied by the number of fish present) zooplankton decline begins at about 0.5 t/ha and phytoplankton increase begins at about 1 t/ha. There is some evidence that temperature (Barthelmes and Kleibs, 1978) and trophic state (Adamek, pers.comm.) influence these thresholds which seems reasonable since temperature accelerates nutrient recycling by the fish. Furthermore, a given amount of nutrients will produce more pronounced effects on primary production if nutrients are in shorter supply. Metabolic silver carp biomasses in excess of these thresholds due to heavy stocking have only slow, predictable effects on phytoplankton as opposed to effects on fish yield and the rapid decrease of zooplankton. Indeed, phytoplankton may increase (e.g., Tujutjunik et al., 1976, Henderson, 1977, Januszko, 1978) decrease (e.g., Milanovski, 1974; Kajak et al., 1975, Barthelmes, 1975; Schroeder, 1978; Bednadz et al., 1978) and/or change in community structure (e.g., Grygierek, 1973; Januszko, 1974 and the papers cited above). These effects do not always correlate well with observations on the composition of the food of the fish implying indirect and complex relationships (Kajak, 1977). Fish yield increases continuously with stocking density above the threshold values, but the increment per unit stocking reaches a maximum and then decreases. The maximum may possibly reach 1 to 1.5 t of fish flesh increment per hectare under mid-European conditions in carp ponds (Opuszynski, 1976; Barthelmes and Kleibs, 1978). In warmer regions of the world yields are much higher. For instance, Liang et al. (1981) report maximum yields in China (31°N, 114°E) of up to 14 t/ha, though the stocked waters seems to have trophic conditions similar to carp ponds in Europe. It is not clear from Liang et al. (1981) what changes in phytoplankton and zooplankton underlie these figures and, therefore, what would be the possible effect on other indigenous fish species. In any case, the apparent influence of temperature on fish and on the thresholds strongly suggests that comparisons of results can only be made between waters with identical or similar climatic conditions. For this reason, in the following experiment 10 000 two-year old individuals/ha were stocked into a small lake of 5 ha (L. Grunz) approximately 100 km NNE of Berlin. In the same area and at the same time additional experiments were performed in other lakes, stocking densities were only 1 000 2-year old individuals/ha. However, the results are only quoted exceptionally, since the experiment with the greatest stocking density may be expected to give the strongest effects on the environment and the resident fish fauna. 3. PRELIMINARY RESULTS FROM LAKES The experiment in Lake Grunz was only one of several experiments involving lesser stocking rates. Thus, the results demonstrating effects on the indigenous fish stocks should be considered somewhat more than preliminary. A detailed description of the experiment in Lake Grunz is given in Barthelmes et al. (1982) and only the main background information and the results are presented here. The lake area is 5 ha with a maximum depth of 6 m. The water mass is weakly stratified in summer, and a real hypolimnion is missing. Each year, soon after formation of the thermocline, H2S appears at depths greater than 3 m. The lake is hypertrophic with primary production greater than 500 g C/m2/year; total phosphorus - 1 493 mg/m3; chlorophyll a - 127 mg/m3 and Secchi depth of 0.44 m (mean values for the growing seasons from 1978 to 1981). Silver carp were stocked in the autumn of 1977 and harvested in early spring 1981. Results are shown in Fig. 1. The return was 79.3 percent of the number stocked. One year later the lake was fished again and the number of returns rose to over 90 percent (not included in Fig. 1). Fig. 1 shows that individual gain in weight decreases with time which suggests that the food of silver carp becomes poorer in quality. The quantity of food available did not change, however, if the first sample with its predominance of Microcystis is excluded (Fig. 2). This interpretation is justified since there was no trend in phytoplankton abundance, parallelling the decline in growth of the fish. Zooplankton reacts more to the growing silver carp biomass as was expected from the carp-pond experiments (Fig. 3). This implies that zooplankton represents an important component of high quality food. Apparently feeding pressure by silver carp and indigenous cyprinids together is sufficient to reduce zooplankton population to low levels, as may be seen to some degree in the experiments with the lower stocking density of 1 000 fish/ha. Since the indigenous cyprinid stocks are only thinned out by common fishing gear and the small, zooplankton- eating individuals especially are likely to increase in response to strong fishing pressure on the adults these compete with silver carp. Because the trophic state reached nearly maximum values and primary production was highly light-limited, the silver carp yields of 377 kg/ha as in Lake Grunz, might represent the maximum possible yield from cyprinid lakes in mid-Europe, however, this figure should be kept under continuous review. The decline of zooplankton is of importance especially to the indigenous fish stocks but as in Lake Grunz littoral zooplankton compensates to some degree for losses in the open waters of the lake and provides a very good food base for fish fry spawned in the littoral. This was confirmed by special investigations on littoral zooplankton production as opposed to zooplankton production in the limnion (Wellner, 1980) and by investigations on the growth of fish fry in the littoral. Zoobenthos was present in only moderate numbers and biomass and its area of occurrence was not reduced by an assumed increase of the part of the bottom influenced by H2S. The most important bottom-feeding fish species (by number) of the lake developed as well as before. These include white bream (Abramis bjorkna (L.)), roach (Rutilus rutilus (L.)) and eel (Anguilla anguilla L.), the latter being stocked in 1977 with approximately 400 specimens/ha and 19 g individual weight. It was also found by mark and recapture that the nearly closed lake still contained practically all the stocked eel in 1979, the individual weight of which had increased to 110 g. In 1979 the stock of white bream amounted to 2 425 ± 80 percent individuals/ha of greater than 15 cm total length. Growth of this species was near the mean growth in northern Germany (Bauch, 1953) both before and after stocking with silver carp (Fig. 4). No signs of massive growth alteration between years were found by scale analysis. In 1979 the number of roach of more than 15 cm long was 1 652± 43 percent/ha. Its growth was also near the mean growth in the region investigated (Fig. 5). Bream (Abramis brama (L.)) was nearly eradicated by very strong fishing pressure 5–10 years prior to our stocking experiment in connexion with carp management of Lake Grunz but was re-established by the time of our investigations. When the silver carp were harvested in 1980/81 the yield of roach, bream and white bream was 192 kg/ha with bream amounting to 35 percent in weight. The growth of bream was very good and scale analysis revealed a growth increase during the years when silver carp were present, but this was probably connected with the shift in feeding from zooplankton to zoobenthos (Fig. 6). In contrast to the littoral spawning cyprinids which eat zoobenthos as adults, these species that spawn in the sub-littoral and have pelagic zooplankton-eating fry seem to be adversely affected. In 1979 Lake Grunz was stocked with 1 000 pike perch (Stizostedion lucioperca (L.)) fingerlings approximately 2 cm long. Only a few of these fish were found in 1980/81 and their growth was only moderate. The very small return contrasts sharply with the spawning success of these few survivors in 1981 when the silver carp stock was strongly reduced by harvesting and zooplankton began to recover. Today there is a multitude of young, well-grown pike-perch in Lake Grunz. While silver carp were present in the lake survival of perch fry was also very low. There were two other species which were at least partly planktonophage and which were initially abundant in the lake, Leucaspius delineatus (Heckel) and Alburnus alburnus (L.) which were not investigated. However, from the results of several electrofishings for eel it appeared that the number of these fish did not change much in response to silver carp stocking, possibly because they fed more in the littoral. 4. SUMMARY DISCUSSION It seems advisable to manage suitable small lakes by stocking with 1 000 two- year old fish/ha for periods of four years. Under these conditions some competition for zooplankton apparently exists, which in severe cases may lead to stunting of both indigenous fish and silver carp, but this effect appears to be within tolerable limits. If the full growth potential of individual silver carp is desired however, it seems necessary to further reduce the numbers stocked within the temperature limits prevailing in central Europe to only 100 fish/ha or so. In any case, the niche occupied by silver carp seems to overlap with that of indigenous fish more than was thought initially. Moreover, the influence of silver carp on water quality is less than was assumed earlier. In Lake Grunz much of the phytoplankton (Oscillatoria) was not eaten or digested so this genus became predominant in the phytoplankton. Thus, the only effect of silver carp was to shift the phytoplankton structure from Microcystis-dominated to Oscillatoria-dominated associations. This slight effect of silver carp on water quality was expected because of the rapid recycling of nutrients in non- or weakly-stratified water bodies and because of heavy loading with nutrients from the surrounding land. This should not be taken as an argument against further heavy silver carp stocking investigations in strongly stratified lakes. Indeed such work is badly needed. 5. REFERENCES Barthelmes, D., Elemente der Sauerstoffbilanz in Karpfenteichen, ihre Wirkungsweise 1975 sowie die Optimierungemoglichkeiten durch Silberkarpfen (Hypophthalmichthys molitrix). Z.Binnenfisch.D.D.R., 22:325–33, 355–63 Barthelmes, D. and K. Kleibs, Wirkungen von Silberkarpfen (Hypophthalmichthys 1978 molitrix) auf das Plankton in Flachgewassern nach Untersuchungen in Karpfenteichen. Int.Rev.Gesamt.Hydrobiol., 63:411–9 Barthelmes, D. et al., Ein Besatzexperiment mit 10 000 zweisommerigen Silberkarpfen 1982 (Hypophthalmichthys molitrix) pro Hektar in einem stark eutrophen See unter fischereilichen und wasserwirtschaftlichen Gesichtspunkten. Teil 1. Versuchsgewasser und Ergebnisse. Z.Binnenfiach.D.D.R., 29(7):207–15 , Ein Besatzexperiment mit 10 000 zweisommerigen Silberkarpfen 1982a (Hypophthalmichthys molitrix) pro Hektar in einem stark eutrophen See unter fischereilichen und wasserwirtschaftlichen Gesichtspunkten. Teil 2. Diskussion der Ergebnisse und Zusammen-fassung. Z.Binnenfisch.D.D.R., 29(8):246–8 Bauch, G., Die einheimischen Susswasserfische. Berlin, Neumann, 187 p. 1953 Bednarz, T. et al., Rozwoj planktonu oraz ocena produkcji karpia w polikulturze z tolpyga 1978 biala. Rocz.Nauk Roln.(Ser.H.), 99:153–76 (in Polish) Grygierek, E., The influence of phytophagous fish on pond zooplankton. Aquaculture, 1973 2:197–208 Henderson, S., An evaluation of filter feeding fishes for water quality improvement. Little 1977 Rock, Arkansas, Arkansas Game and Fish Commission, 17 p. Januszko, M., The effect of three species of phytophagous fish on algae development. 1974 Pol.Arch.Hydrobiol., 21:431–54 , The influence of silver carp (Hypophthalmichthys molitrix (Val.)) on 1978 eutrophication of the environment of carp ponds. Part 3. Phytoplankton. Rocz.Nauk Roln.(Ser.H), 99:55–79 Kajak, Z., Feeding habits of silver carp Hypophthalmichthys molitrix (Val.) and the 1977 problem of clean water. Wiad.Ekol., 23:258–68 Kajak, Z. et al., Influence of the planktonivorous fish Hypophthalmichthys molitrix (Val.) 1975 on the plankton and benthos of the eutrophic lake. Pol.Arch.Hydrobiol., 22:301–10 Liang, Y. et al., Primary production and fish yields in Chinese ponds and lakes. 1981 Trans.Am.Fish Soc., 110:346–50 Milanovski, Yu.E., The exploitation of herbivorous fishes in pond polyculture. In Zoology 1974 of invertebrates. Moscow, vol. 5:95–135 (in Russian) Opuszynski, K., Possibilities of increasing production in carp ponds by means of raising 1976 silver carp. Gospod.Rybn., 28(8):3–5 (in Polish) , Opuszynski, K., Weed control and fish production. In Proceedings of the 1979 Grass carp conference, edited by J.V. Shireman. Gainesville, Aquatic Weeds Research Center, University of , Institute of Food and Agricultural Science, pp. 103–38 , Silver carp, Hypophthalmichthys molitrix (Val.) in carp ponds. 3. Influence 1979a on ecosystem. Ekol.Pol., 27:117–33 Schroeder, G.L., Autotrophic and heterotrophic production of micro-organisms in 1978 intensely-manured fish ponds, and related fish yields. Aquaculture, 14:303– 25 Tjutjunik, S.N. et al., The joint rearing of carps and herbivores of different ages in ponds 1976 in Moldavia. In Third Congress of the All Union Society of Hydrobiology, Riga, pp. 113–6 (in Russian) Vinberg, G.G., Intensity of metabolism and food requirements of fishes. Minsk, Izdat. 1956 Byelorussian University, 253 p. (in Russian) Wellner, E., Zur Nutzung der Zooplanktonproduktion eutropher Gewasser unter 1980 besonderer Berucksichtigung der Silberkarpfenwirtschaft. Dissertation, Humboldt-Universitat, Berlin, 116 p.

Fig. 1 Increase in mean weight and biomass of one batch of silver carp stocked into Lake Grünz

Fig. 2 Abundance of phytoplankton in Lake Grünz from 1977 to 1981

Fig. 3 Abundance of zooplankton in Lake Grünz from 1977 to 1981

Fig. 4 Growth of white bream (Abramis bjoerkna (L.)) in Lake Grünz ( ² ) in relation to mean growth in the area according to Bauch, 1953 (x). Density of white bream > age 6 in 1979, 2 425 sp/ha ± 80 percent (mark and recapture, P = 95 percent)

Fig. 5 Growth of roach (Rutilus rutilus (L.)) in Lake Grünz ( ² ) in relation to mean growth in the area according to Bauch, 1953 ( x ). Density of roach > age 4 in 1979, 1 652 sp/ha ± 43 percent (mark and recapture, P = 95 percent)

Fig. 6 Growth of bream (Abramis brama (L.)) in Lake Grünz ( ² ) in relation to mean growth in the area according to Bauch, 1953 ( x ). Density of bream of age 4 and 5 ca. 300–500 sp/ha beach seining) INTRODUCTION OF GRASS CARP (Ctenopharyngodon idella Val.) INTO DENMARK

P.N. Markmann National Agency of Environmental Protection , Denmark

CONTENTS

1. INTRODUCTION 2. DANISH PROVISIONS CONCERNING IMPORT AND STOCKING 3. DANISH IMPORTS AND STOCKINGS 1978–82 3.1 Control programme 3.2 Stocking localities and densities 3.3 Growth, overwinter survival and weed reduction 3.4 Concluding remarks 4. ACKNOWLEDGEMENTS 5. REFERENCES

ABSTRACT Grass carp (Ctenopharyngodon idella Val.) was stocked into Danish natural waters in Denmark for the first time in 1968 but only in very few numbers in a single locality. In 1978 a second and larger series of stocking experiments were begun. Stockings were for weed control and only closed water bodies or localities situated within water systems with no fish farms have been stocked. Stocking material has been imported from Malaysia, the Netherlands, Sweden and Austria. Until now, grass carp has been stocked in 14 separate ponds and lakes which range in area from .04 to 35 ha in densities ranging from 2 to 500 kg/ha. Overwinter survival has generally been high in several localities, nearly 100 percent. Growth has mostly been fast with yearly weight increments of up to 150 percent. Weed density has been markedly reduced in 4 of the 14 localities. No spreading of fish diseases has occurred. Undesirable side effects have only been reported from two localities where a reduction in light penetration due to increased phytoplankton growth has been observed. This has lead to supplementary stocking of silver carp (Hypophthalmichthys molitrix Val.) in one locality. This first stocking of silver carp into a Danish lake was made in December 1981. RESUME En 1968, on a pour la première fois déversé des carpes de roseau (Ctenopharyngodon idella Val.) dans les eaux libres du Danemark mais en très petites quantités et en un seul endroit. En 1978, des expériences ont été entreprises sur une plus grande échelle. Ces opérations visaient uniquement à limiter la prolifération des herbes aquatiques. Le repeuplement de même que les importations restent soumis à des restrictions. On n'a repeuplé que des pièces d'eau fermés ou des endroits situés sur des réseaux hydrographiques où il n'y a pas d'établissement piscicole. Le matériel utilisé à été importé de Malaisie, des Pays-Bas, de Suéde et d'Autriche. Jusqu'ici, on a repeuplé avec la carpe de roseau 14 étangs et lacs de 0,04 à 35 hectares à des densités initiales de 2 à 500 kg/ha. La survie à l'hiver a en général été bonne, avoisinant 100 pour cent en plusieurs endroits. La croissance a la plupart du temps été rapide avec des augmentations annuelles de poids pouvant atteindre 150 pour cent. La densité des herbes aquatiques s'est déjà nettement réduite dans 4 des 14 sites. Aucune maladie transmissible aux poissons n'a été propagée. Des conséquences néfastes n'ont été observées que dans deux endroits où l'accélération de la croissance du phytoplancton a entraîné une réduction de la visibilité. Dans l'un de ces endroits, on a procédé à un repeuplement supplémentaire avec des carpes argentées (Hypophthalmichthys molitrix Val.). Ce premier repeuplement d'un lac danois avec des carpes argentées a eu lieu en décembre 1981. 1. INTRODUCTION Grass carp (Ctenopharyngodon idella Val.) was introduced into Denmark for the first time in 1965 with 12 adult fish from U.S.S.R. Only one year later the potential of grass carp for weed control in Denmark was reviewed by Christensen (1966). In June 1968 a 2-km long, 5-m wide reach of the River Esrum a in northern was stocked with about 100 small grass carp (mean length 9 cm) originating from Malaysia. (pike and perch) were removed from the reach by electro- fishing shortly before stocking. In November only about 10 percent of the stocked fish were still alive. These fish had reached a mean length of only 15 cm and no weed reduction could be found (Denmark, County of Frederiksborg, 1966, 1971; Hansen, 1969). As a result of these somewhat discouraging results no new stocking experiments with grass carp were attempted until 1978. 2. DANISH PROVISIONS CONCERNING IMPORT AND STOCKING The introduction of fish species not indigenous to the Danish fauna implies certain risks including the spreading of fish diseases, unforeseen ecosystem responses and uncontrolled spreading of the new species due to natural reproduction. Although natural reproduction is not likely in Denmark, where the June isotherm is below 20°C, import and stocking into Danish waters have been highly restricted. Permission for import and stocking is granted by the Danish Veterinary Service and the Ministry of Fisheries. Permission to import is only granted for fish showing no clinical signs of diseases when dispatched and which originate from fish farms which are subjected to official sanitary control. Furthermore, the fish farm must be guaranteed free of spring viraemia of carp for a period of at least two years prior to shipment. The consignment should be accompanied by a certificate from the veterinary authorities conducting the official sanitary control in the exporting country. Until now stocking has only been permitted into closed water bodies or localities situated within water systems with no fish farms. In certain cases permission to stock has had to wait for the results of Danish veterinary spot tests on the imported fish and the total consignment has been kept in quarantine until the results are available. The present import regulations and stocking procedures are overly bureaucratic and are to be changed during 1982 on the basis of results given in this paper and in Mortensen (1982). 3. DANISH IMPORTS AND STOCKINGS 1978–82 In 1978 a series of rather comprehensive stocking experiments was started when the Inland Fisheries Laboratory, the Water Quality Institute, the County of northern Jutland and the municipalities of and Pandrup cooperated in a project to stock grass carp into Lake Nols So in northern Jutland and into four artificial waters in west of Copenhagen (Markmann, 1979; Markmann and Warrer- Hansen, 1980; Denmark, County of Northern Jutland, 1980). The activities in Lake Nols So came to an end during the spring of 1979 and since then both monitoring and new stockings have been managed solely by the Water Quality Institute. Since 1978 a total of about 3 600 kg of grass carp has been imported into Denmark from the Netherlands, Sweden and Austria (Table 1). The stocking material has generally consisted of fish with mean weights of about 0.5 kg but in 1979 three different size groups with mean weights of about 0.2, 0.4 and 0.8 kg, respectively, were stocked. 2 900 kg of the imported fish were stocked before the spring of 1982. About 225 kg still remain at the Water Quality Institute awaiting stocking during the spring of 1982 into two new localities (compare Table 2) where the status of the ecosystem just before stocking has been investigated in more detail than in previous localities. The rest of the imported fish have been used in aquacultural experiments and veterinary investigations or died during transport or quarantine. 3.1 Control programme Due to lack of money an intensive control programme for periodically measuring water quality parameters as pH, alkalinity, N- and P-compounds, phytoplantkon primary production and weeddensity has only been conducted in one locality (Nols So). In this locality, however, the experiments came to a sudden end after a few months as the subject fish died during the winter 1978/79. Generally the increase in biomass of the stocked grass carp has been used as a measure of the weed reduction. Rough estimates of fish biomass have been calculated from growth data and from mortalities reported in the period between two estimates. The fish were sampled once or twice a year to check growth in length and weight as well as general health. 3.2 Stocking localities and densities Grass carp have been stocked into 14 separate localities between 1978 and April 1982 and two additional localities are to be stocked during this spring with fish imported from Austria later in 1981. Initial stocking densities have ranged from 2 to 500 kg/ha but densities of about 100 kg/ha have been more common (Table 2). Generally stockings have been one-time plantings of densities believed to ensure significant weed reduction shortly after stocking (i.e., in the second or third summer). In one locality (Vallensbaek Mose) the numbers of grass carp have been built up slowly during successive stockings (Table 2). 3.3 Growth, overwinter survival and weed reduction All available data on growth are given in Table 3. More detailed information on growth, survival and weed reduction are given below for individual localities. 3.3.1. Nols So In May 1978 about 830 grass carp were stocked to control dense aquatic vegetation dominated by Myriophyllum spicatum L., Ceratophyllum demersum L. and filamentous green algae. The fish did well during summer and autumn and increased their weight by about 50 percent until November 1978. The winter 1978/79 was very severe and the lake was covered with ice for more than three months. Four hundred and eighty grass carp were found dead but fresh in March when the ice disappeared and these fish had a mean weight increase of 51 percent indicating no loss in weight through the winter. No grass carp were found despite all the intensive fishings and it is assumed that fish stocked died during this period. Although this experiment was very short-lived there were signs of effective grazing on the filamentous algae but no significant trends in weed density could be monitored. 3.3.2 Albertslund Kanal sections I and II This canal runs through a dense urban district and is surrounded on both sides by pavement, bicycle tracks and low buildings. Sections I and II were originally separated by wire netting barriers, which broke down shortly after stocking. The results from these sections are, therefore, treated together. There were heavy losses through the whole summer and autumn of 1978, presumably due to stress caused by the heavy traffic in the surroundings. All the remaining fish died during the winter of 1978/79 when this stretch of the canal was nearly frozen solid leaving only 3–5 cm of muddy anoxic water below. No substantial weed reduction could be observed although the weed was dominated by Myriophyllum spicatum L. and Potamogeton natans L., which are both soft plants known to be esteemed by grass carp elsewhere. 3.3.3 Albertslund Kanal section III Section III is upstream of sections I and II, with similar surroundings but with less traffic. The ice-cover of section III during the winter 1978/79 was not as thick as that of the downstream sections due to a groundwater inlet at the upstream end of the section. Some fish died during the summer and autumn but losses were significantly smaller than in the downstream sections and the remaining fish survived the winter. The growth during the first growing season was however slow, only 19 percent, presumably due to stress caused by the traffic along the banks of the canal. In September 1979 the remaining grass carp were transferred to locality 4 (Albertslund Radhusbassin). When transferred, the grass carp had reached a mean weight of 1 400 g. Any weed reduction was difficult to quantity as the grass carp were found in small shoals confined to totally grazed areas, leaving the rest of the vegetation nearly untouched. 3.3.4 Albertslund Radhusbassin This artificial lake constitutes an integrated part of the town hall of Albertslund. It has concrete walls and is completely surrounded by pavements and low buildings. The water is up to 2.5 m deep. The bottom consists of muddy sand as the lake receives urban storm water run-off as well as pumped ground water. The grass carp stocked in May 1978 increased their weight by 129 percent during the first growing season. This rapid growth continued in the following years and the total weight increase during the 3.5 growing seasons from stocking until September 1981 was 320 percent. These growth data reflect rapid growth which decreases with increasing size. The decrease could be due to normal physiological growth-regulating mechanisms although shortness of food seems to be the more likely explanation as intense grazing during 1981 resulted in the extermination of the submersed macrophytic vegetation in the late summer of 1981. As a consequence the Water Quality Institute plans to transfer these grass carp to the nearby locality 5 (Vallensbaek Mose) where food is abundant. Before stocking in May 1978, the lake water had high transparency and a dense macrophyte vegetation dominated by Myriophyllum spicatum L., Potamogeton pectinatus L. and macro algae belonging to the family Characeae. During 1981 the transparency declined markedly due to phytoplanktonic blooms and silver carp (Hypophthalmichthys molitrix Val.) were stocked in December 1981 for the first time in Denmark. One hundred kilogrammes (about 350 fish with a mean weight of about 300 g) was stocked and observations in April 1982 indicate that the silver carp have had a very limited weight-loss during the winter of 1981/82. 3.3.5 Vallensbaek Mose Vallensbaek Mose is part of a recreational area southeast of Copenhagen. Vallensbaek Mose consists of two lakes of nearly equal size connected by a narrow canal. The lakes are used for sailing, canoeing and waterskiing, which uses were impeded by dense aquatic vegetation dominated by Elodea canadensis Rich., Potamogeton natans L. and Potamogeton pectinatus L. It was decided to try to reduce the weeds by means of herbivorous fish and in May 1978 about 115 grass carp were stocked. These were supplemented in August 1980 with about 475 additional fish. The fish stocked in 1978 and 1980 increased their mean weights by 114 and 55 percent, respectively, during the first growing season. As the fish stocked in 1980 had a short growing season due to the late stocking, the growth of the two groups are of the same order of magnitude. Dead grass carp have not been reported even during or after this latest very severe winter of 1981/82. It has been impossible to quantify the extent of weed reduction as the weed was reduced mechanically in the summer of 1980. The weed reduction caused by grass carp must, however, have been considerable as indicated by the very fast growth reported for the fish from this locality, where the mean weight of the earliest fish stocked is expected to exceed 3 kg in the spring of 1982. 3.3.6 Avnso Eleven grass carp were stocked in 1979 and were all found dead in the spring of 1980. This shallow lake receives road surface run-off and has a thick organic sediment. Although the ice-cover lasted for only about 1.5 months in the winter of 1979/80 the water has presumably become anoxic due to the high oxygen demand of the sediment. The 3–5 cm thick layer of Lemna minor L., which covered all the pond surface when the fish were stocked showed no signs at all of being grazed by the grass carp. 3.3.7 Pennesoen Four of eleven grass carp stocked in 1979 were found dead in the spring 1980. The owners of this shallow pond believe that the fish have been of some use but growth of the surviving fish is uncertain and cannot confirm any possible reduction in weed. 3.3.8 Duevej So Five of eleven grass carp stocked in 1979 were found dead in the spring 1980. The owners believe that the fish had been of some use but growth of the surviving fish is uncertain and cannot confirm the presumed reduction in weeds. 3.3.9 Krenkerup Voldgrav In September 1979 this rather shallow most was stocked with about 245 grass carp of medium size (mean weight about 412 g). The growth has been very fast. In the first and second year after stocking the fish increased their mean weight by 73 and 106 percent, respectively. No dead grass carp have been reported during this period. But in the winter of 1981/82, when the ice-cover lasted for about 3.5 months, approximately 150 grass carp died showing clear signs of asphyxiation as they all crowded around a little opening in the ice at the water inlet to the most. Whether some of the originally stocked 245 grass carp are still alive is yet uncertain. The reduction in weeds in this locality is reported to be very satisfactory. When the grass carp were stocked, Ceratophyllum demersum L. made up nearly 100 percent of the dense weed, which covered about 70 percent of the lake surface. In the autumn of 1981, however, only a thin layer of this species was left on the lake bottom. 3.3.10 Fuglsang Park Voldgrav This moat was stocked in September 1979 with about 200 grass carp of the smallest size group used (mean weight 174 g). Due to a very early ice-covering the first winter after stocking (1979/80) the growth has not been measured. More than 50 percent of the stocked fish were found dead when the ice melted, and presumably all grass carp must have died during this winter, as no grass carp at all were caught during two experimental fishings in 1980 and 1981, respectively. The grass carp had no obvious effect on the dense vegetation of mainly Ceratophyllum demersum L. and Elodea canadensis Rich., which covered the total lake surface. 3.3.11 Glorup Park Fontainedam In this artificial fountain lake about 160 grass carp of the largest size group used (mean weight 815 g) were stocked in September 1979. The lake had at this time clear water (visibility more than 2 m (to the bottom)). The vegetation was very dense. A thick layer of Ceratophyllum demersum L. covered the bottom nearly totally, and Potamogeton crispus L., Potamogeton natans L. and Lemna minor L. were abundant together with green epiphytic filamentous algae. Growth of fish has been rather slow in this locality - only 23 percent during the first growing season. As food seems abundant the slow growth could be a result of low summer temperatures resulting from shading by trees which surround the lake on nearly all sides. 3.3.12 Holstenshuus Voldgrav In September 1979 about 105 grass carp with a mean weight of 815 g were stocked to graze Phragmites communis L. and Typha latifolia L. The lake had no submersed vegetation and only a narrow zone of other macrophytes along the banks. The growth has been rather slow in this locality which could be a consequence of the food items present. It is uncertain whether the grass carp have grazed on the Phragmites but as the soft species present has not been significantly reduced this is a possibility. Only two dead grass carp have been reported from this locality. These fish have been injured during transport or stocking. 3.3.13 Shell Reservoiredam In September 1979 about 75 grass carp with a mean weight of 412 g were stocked in this artificial reservoir. The water was clear (visibility about 3 m) and the vegetation very sparse consisting mainly of spots of Potamogeton natans L. and Potamogeton pectinatus L. Intensive fishings in 1981 showed no grass carp at all which was quite inexplicable as only a limited number of fish were reported dead - mainly shortly after stocking. The grass carp must either have died during the winter of 1979/80 when the lake was ice-covered for more than 2.5 months or have served as prey for a resident population of (Salmo gairdneri Rich.) which had been stocked for angling. Very large scales have been found in the stomachs of large rainbow trout (2–3 kg). The grass carp have had no effect on the sparse macrophytic vegetation. 3.3.14 Oster Molle Soen In this shallow lake about 475 grass carp were stocked in August 1980 where the water surface was nearly covered by dense coherent vegetation consisting of Elodea canadensis Rich. with epiphytic filamentous green algae. The transparency was about 1 m. The grass carp have grown very fast in this locality as the increase in mean weight during the first growing season has been 146 percent. In spite of this, no significant trend in the density of weeds could be detected. On the other hand there seemed to be a slight decrease in the transparency presumably caused by an increased phytoplankton growth. 3.4 Concluding remarks In 10 of 14 localities weed density has been reduced to a varying extent, from marked reductions to reductions appearing to be only just perceptible. The localities where no weed reduction have been recorded include waters where the stocked population has been completely exterminated during the first year or winter after stocking (localities 3, 6, 10 and 13). The localities where the grass carp have had only limited effect include some localities where the stock has been depleted or completely exterminated during the first year or winter after stocking (localities 1, 2, 7 and 8) and localities where growth due to other reasons has been slow to moderate (localities 11 and 12). The localities where the grass carp have had a substantial effect on the density of aquatic weeds include waters where the stocked fish have displayed yearly weight increments between 70 and 150 percent (localities 4, 5, 9 and 14). There have been no reports of any outbreak of fish disease and the stocked fish have shown no signs of being more vulnerable to normally occurring fish pathogens than the indigenous Danish fish species. 4. ACKNOWLEDGEMENTS This paper could not have been written without the help of several persons. I, therefore, wish to express my deep gratitude to my old colleagues Ivar Warrer-Hansen and Sverre Mohr Mortensen at the Water Quality Institute, Horsholm for supplying parts of the latest data, to Hans Heidemann-Lassen at the Water Authority of the County of Northern Jutland for supplying data concerning weed density in Lake Oster Molle Soen. Further, I am indebted to the staff of the Inland Fisheries Laboratory, , especially to Jorgen Dahl (Head of the Laboratory) and Gorm Rasmussen for supplying investigational equipment. Finally, I wish to express deepest gratitude to my wife, Susse Mohr, for invaluable practical help during stocking and monitoring activities and linguistic and professional criticism of this paper. 5. REFERENCES Christensen, J., Are grass carp the solving of our weed problems? Fiskeriundersogelser 1966 1965. Pap.Dan.Inst.Fish.Mar.Res., (26): 72–4 (in Danish) Denmark, County of Frederiksborg, Recipient waters in the County of Frederiksborg 1966 1963–66. Report. County of Frederiksborg, Water Authority, pp. 8–9 (in Danish) , Report from the water authority of the County of Frederiksborg 1971 concerning the period 1967–71. County of Frederiksborg, Water Authority, 4 p. (in Danish) Denmark, County of Northern Jutland, Report concerning experimental weed-control 1980 with grass carp in Lake Nols So 1978/79. Report. County of Northern Jutland, Water Authority, June 1980, 52 p. (in Danish) Hansen, A., A note concerning grass carp. Yearb.Soc.Water Auth.Inspectors, (41): 19 p. 1969 (in Danish) Markmann, P.N., Grass carp and other herbivorous fishes. Vand, 1979 (3): 64–74 (in 1979 Danish) Markmann, P.N. and I. Warrer-Hansen, Experimental stocking of grass carp in small 1980 water-bodies in Albertslund Municipality, May 1978. Report. Horsholm, Denmark, Water Quality Institute, January 1980, 30 p. (in Danish) Mortensen, S.M., State of affairs report to the Danish Veterinary Services and the 1982 Ministry of Fisheries concerning grass carp in Danish lakes. Report. Horsholm, Denmark, Water Quality Institute, May 1982 (in Danish) Table 1 Danish grass carp imports 1978–81 and their origin Year Biomass Origin

1978 1 000 kg The Netherlands 1979 400 kg Sweden 1980 500 kg Sweden 1981 1 700 kg Austria

1978–82 3 600 kg

Table 2 Stocking localities in Denmark 1978–82 together with data on stocking densities and size of stocked fish. Data in brackets represent estimates of total biomass after supplementary stockings

Stocking Biomass Mean Stocking Locality date Stocking localities Area stocked weight of density fish No. ha kg g kg/ha

1 5/5/78 Nols So 2.80 440 530 160 2 5/5/78 Albertslund Kanal I 0.26 80 498 300 2 5/5/78 Albertslund Kanal II 0.34 170 498 500 3 5/5/78 Albertslund Kanal III 0.65 64 498 100 4 5/5/78 Albertslund Radhusbas. 1.00 100 498 100 5 5/5/78 Vallensbaek Mose 35.00 57 498 2 5 29/8/80 Vallensbaek Mose - 280 587 8(15) 5 21/12/81 Vallensbaek Mose - 1 200 645 35(60) 6 13/9/79 Avnso 0.05 5 446 100 7 13/9/79 Pennsoen 0.04 5 446 125 8 14/9/79 Duevej So 0.04 5 446 140 9 18/9/79 Krenkerup Voldgrav 1.00 100 412 100 10 18/9/79 Fuglsang Park Voldgrav 0.40 35 174 90 11 22/9/79 Glorup Fontainedam 1.30 130 815 100 12 24/9/79 Holstenshuus Voldgrav 0.85 85 815 100 13 24/9/79 Shell Reservoiredam 1.00 30 412 30 14 30/8/80 Oster Molle Soen 2.20 220 587 100 15 Spring 82 Agerso 2.00 150 - - 16 Spring 82 Naesbyholm Voldgrav 0.70 75 - - Table 3 Mean weight in g of grass carp on stocking date and biomass-estimate days. In brackets the weight increase in percentage of the weight at stocking are given Year of Mean weighting and in brackets weight increase in percentage since stocking Locality stocking 1978 1979 1980 1981 No. May Nov. May Sept. Aug. Oct. Apr. Jul. Aug. Sept. Oct. Dec. 1 520 8001 Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx 2 498 - 5972 xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

(19) 3 498 1 xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx 1978 4 xxxxxxxxxxxxxxx - - - - - 1 400 - - (180) 4 498 - 1 143 ------2 100 - - (129) (320) 5 498 - 1 068 - - - 2 225 - - - - - (114) (347) 6 xxxxxxxxxxxxxxx 4463 xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx 7 xxxxxxxxxxxxxxx 446 ------8 xxxxxxxxxxxxxxx 446 ------9 xxxxxxxxxxxxxxx 412 - 714 - - - 1 472 - - (73) (257) 1979 10 xxxxxxxxxxxxxxx 1743 xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx 11 xxxxxxxxxxxxxxx 815 - 1 004 - 1 417 - - - - (23) (73) 12 xxxxxxxxxxxxxxx 815 - 1 068 - 1 276 - - - - (31) (56) 13 xxxxxxxxxxxxxxx 4123 xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx 5 xxxxxxxxxxxxxxxxxxxx 587 - 910 - - - - - (55) 1980 14 xxxxxxxxxxxxxxxxxxxx 587 - - - 1 449 - 1 690 - (146) (187) 1981 5 xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx 645

1 All fish died during the winter 1978/79 2 All fish transferred to locality 4 in autumn 1979 3 All fish died during the winter 1979/80 THE INTRODUCTION, SPREAD AND INFLUENCE OF THE BARBEL (Barbus barbus) IN THE RIVER SEVERN, GREAT BRITAIN

A.S. Churchward, P. Hickley and E. North Severn-Trent Water Authority Malvern, Worcestershire, England

CONTENTS

1. INTRODUCTION 1.1 Geography of the Severn Basin 1.2 Fisheries 2. BIOLOGY OF THE SEVERN BARBEL 2.1 Growth 2.2 Movement 2.3 Reproductive ecology 3. THE INFLUENCE OF BARBEL ON ANGLERS' CATCHES 3.1 Development of the fishery 3.2 Present status of the fishery 4. EVALUATION OF STOCKINGS 5. REFERENCES

ABSTRACT The barbel (Barbus barbus L.) were introduced into the River Severn, the longest river in Great Britain, in order to improve sport for anglers. From an initial stocking of only 509 fish in 1956, the barbel has become the predominant species in the 75 km-long middle reach of the river, whilst sparser populations exist throughout the whole system. The biology of the barbel is discussed with particular reference to an observed decline in relative growth rate. The present status of the barbel as a sport fish, and how its importance in catches varies with river temperatures is assessed from analysis of angling returns. Results, based on a Relative Importance Value, show that the barbel now dominates fish catches in the middle reach of the river and is steadily increasing in importance elsewhere. Implications are that the barbel does not have a deleterious effect on resident populations of other fish but behaves as a supplementary species. RESUME Le barbeau (Barbus barbus L.) a été introduit dans la Severn, le plus long fleuve de Grande-Bretagne, pour améliorer la pêche sportive. La première introduction remonte à 1956; elle ne portait que sur 509 individus. Aujourd'hui, le barbeau est l'espèce dominante du cours moyen (75 km de long) et il est présent, mais à des densités plus faibles, dans l'ensemble du réseau. La colonisation du fleuve est examinée sous l'angle de la structure, de la densité et des mouvements des populations; l'auteur fournit également des données de base sur la croissance et sur l'écologie de la reproduction. A partir de l'analyse des captures, l'auteur évalue la situation actuelle du barbeau en tant que poisson de pêche sportive et montre comment son importance dans les prises varie en fonction de la température des eaux. L'examen de l'importance relative des poissons pêchés dans la Severn montre que le barbeau est l'espèce dominante sur le cours moyen et qu'il ne cesse de gagner en importance dans le reste du fleuve. L'auteur conclut que le barbeau n'a pas d'effets néfastes sur les autres populations de poissons auxquels il ne fait que s'ajouter. 1. INTRODUCTION 1.1 Geography of the Severn Basin The River Severn is the longest river in Great Britain, measuring 354 km from its source in the Welsh hills to its estuary within the Bristol Channel. It drains a basin of approximately 11 500 km2. Fig. 1 indicates the position of the basin together with a map of the river. Below point BZ1, the river is impounded. The long-term mean flow at Bewdley is 5 300 M1/d.

1.2 Fisheries As the Severn is of high water quality throughout its length, it supports almost every kind of freshwater fish found in the British Isles including salmon (Salmo salar), trout (Salmo trutta), grayling (Thymallus thymallus), chub (Leuciscus cephalus), dace (Leuciscus leuciscus), roach (Rutilus rutilus), bream (Abramis brama), pike (Esox lucius), barbel (Barbus barbus) and perch (Perca fluviatilis). The barbel population in the Severn derives entirely from a stock of 509 fish from the River Kennet, England, introduced in September 1956 between Shrewsbury and Bewdley in a reach noted principally as a chub and dace fishery (see Fig. 1). An additional introduction of 102 barbel from the River Swale, Yorkshire, was made to the River Avon in 1964. Both barbel and chub populations are heavily infested with the acanthocephalan Pomphorhynchus laevis. Fishing for freshwater fish is very popular, particularly between Bewdley and Bridgnorth with thousands of anglers using the river during the season.

2. BIOLOGY OF THE SEVERN BARBEL 2.1 Growth Hunt and Jones (1974a, b; 1975) and Hancock (1976) used back calculated lengths to study the seasonal and life growth patterns of Severn barbel between 1961 and 1975. A reappraisal of their data has been made in order to assess the change in relative growth rate between 1963 and 1975 taking female barbel as the example. Relative growth rates given in Table 1 were derived using the method of Kempe (1962) as interpreted by Hickley (1980). The very clear trend of declining relative growth rates is evident and may be associated with substantial increases in density of the species following their introduction in 1956.

2.2 Movement Both Hunt and Jones (1974b) and Hancock (1976) studied the movements of barbel in the Severn. From their recapture data Hunt and Jones suggest two discrete components of the population, one static and the other mobile. Hancock, who continued the mark-recapture work but used different analysis techniques confirmed the two components of the population and convincingly suggested a home range of only 2 km for the static component. Both groups of workers present evidence indicating that the mobile populations are composed of larger individuals and that the main movements occur in the spawning period of May–July with a maximum recorded upstream movement of 40 km. 0+ barbel fry were found predominantly in the extensive gravel reaches between Ironbridge and Shrewsbury (Sites S1 to S4, Fig. 1) well upstream of the principal home ranges of the adults (Sites S5 to S8, Fig. 1). 2.3 Reproductive ecology From his studies on the reproductive biology of Severn barbel, Hancock (1976) infers that spawning is likely to occur once river temperatures rise to 14°C and that it is temperature rather than day length or flow that controls its onset. Laboratory experiments showed that temperature significantly affected egg survival with an optimum range between 12°C to 16°C and zero survival at 8°C and 22°C. Minimal larval abnormalities were recorded between 13° and 16°C. 3. THE INFLUENCE OF BARBEL ON ANGLERS' CATCHES

3.1 Development of the fishery Barbel first featured in catches in 1958, two years after their release in the river. There was then a steady increase in the number of reports received and by 1965 it seems that barbel were appearing regularly in catches taken between Shrewsbury and Worcester, with most fish being caught between Bridgnorth and Bewdley (Fig. 1). The first incidence of a fishing contest winner having an all barbel catch followed two years later in 1967. In the same year angler interest in the species started to improve after two anglers publicised their catching of over 300 barbel in six months. In 1970, barbel were deemed to be the most important species for many stretches of river with contest winners on the middle reach of the river usually having barbel-dominated catches. The fish continued to spread throughout the catchment with reports of captures in the last five years in all the major tributaries including the Teme, Vyrnwy, Banwy, Tanat and Avon (Fig. 1).

3.2 Present status of the fishery An angling census commenced in 1975, and data from six fishing seasons have been processed. Catches of coarse fish made during fishing competitions held in three river sections, spaced between Shrewsbury and Tewkesbury (Fig. 1), were monitored by a postal questionnaire system (North, 1980). Sections A and B are in the predicted barbel reach of the river whereas Section C is outside it. A comparison of river temperature and catchability (percentage of anglers catching any species of fish) shows a remarkable correlation with a statistically valid parabolic regression (North, 1980). Barbel are particularly sensitive to temperature and virtually disappear from anglers catches below 8°C. A measure of relative abundance of the various species was derived using a points system whereby, for each contest result, individual species were awarded 3, 2 or 1 point in relation to their observed contribution to the catch. Percentage Abundance of a species was then the sum of its awarded points expressed as a percentage of the sum for all species combined. Percentage Occurrence of a species was calculated as the percentage number of competitions in which that species was caught. Histograms for these abundance and occurrence values are given in Fig. 2. Mean weights for different fish in the three river sections were determined from field measurements and then converted to relative weights by dividing the mean weights of the different species by that of the smallest species. This data was then used to calculate a Relative Importance Value (Hickley and North, 1981) for each of fish where: RIV = (% Abundance + % Occurrence) × Relative Weight The RIV results are plotted in Fig. 3, and show how catch composition varied during the six fishing seasons monitored so far. It is clear that at sites A and B barbel are predominant and can readily be considered to be the most important species in this part of the River Severn. Even at site C barbel are starting to make an important overall contribution to catches although actual abundance is low at present (Fig. 3).

4. EVALUATION OF STOCKINGS Clearly the barbel, stocked to improve anglers' sport, has successfully established itself in a vacant niche in the Severn Basin but its range is still expanding 25 years after its introduction in 1956. It has not, however, expanded in the same way in the River Avon due mainly to the impounded nature of most of the river and the comparatively short stretch of spawning gravel. Its success on the Severn is due to the physical compatibility of the habitat with the species requirements, in terms of temperature tolerance, spawning facilities and bed gradient. Before the introduction of barbel, chub was the dominant species in anglers' catches, but it is believed that the barbel has not competed with it in any way and may be considered a bonus to anglers rather than a substitute. The fact that Relative Importance Values for the Mid Severn for chub and barbel have remained static, supports this. The success of the barbel was not foreseen and it is fortunate that it has proved to be an additional species without detrimental effects.

5. REFERENCES Hancock, R.S., Aspects of the reproductive ecology of the barbel Barbus barbus in 1976 the Rivers Severn and Hull. Ph.D. Thesis. Liverpool University Hickley, P., An ecological investigation of benthic invertebrates and fish in a small 1980 lowland river. Ph.D. Thesis. Chelsea College, London University Hickley, P. and E. North, An appraisal of anglers' catch composition in the barbel reach 1981 of the River Severn. In Proceedings of the Second British Freshwater Fish Conference. University of Liverpool, pp. 94–100 Huet, M., Apercu des relations entre la pente et les populations piscicoles des eaux 1949 courantes. Schweiz.Z.Hydrol., 11:333–51 Hunt, P.C. and J.W. Jones, A population study of Barbus barbus L. in the River Severn, 1974 England. 1. Density. J.Fish Biol., 6:255–67 , A population study of Barbus barbus L. in the River Severn, England. 2. 1974a Movement. J.Fish Biol., 6:269–78 , A population study of Barbus barbus L. in the River Severn, England. 3. 1975 Growth. J.Fish Biol., 7:361–76 Kempe, O., The growth of roach (Leusiscus rutilus L.) in some Swedish lakes. 1962 Rep.Inst.Freshwat.Res., Drottningholm, (44):142–204 North, E., The effects of water temperature and flow upon angling success in the River 1980 Severn. Fish.Manage, 11(1):1–9 Table 1 Relative growth rates of female barbel (Barbus barbus) during the years 1963– 75 where a value of 100 is the standard (0 group excluded)

Season Relative Season Relative Season Relative of growth rate of growth rate of growth rate growth growth growth 1963 167 1967 148 1971 102 1964 196 1968 147 1972 91 1965 189 1969 122 1973 105 1966 157 1970 116 1974 115 1975 95

Fig. 1 Map of the Severn Basin showing barbel distribution, initial sites of introduction, and angling census sites

Fig. 2 Percentage abundance and occurrence of major species in the three angling census sites, R. Severn

Fig. 3 Relative Importance Values 1975–80 A CENTURY OF PIKEPERCH IN DENMARK

J.Dahl Inland Fisheries Laboratory Silkeborg, Denmark

CONTENTS

1. INTRODUCTION 2. HISTORY OF INTRODUCTIONS 3. STOCKING PRACTICE 4. YIELD 5. THE FUTURE

ABSTRACT Pikeperch (Stizostedion lucioperca (L.)) was first introduced into Denmark in 1879. Since then, an increasing number of introductions - most of them successful - have been made and the pikeperch is now well established in about 70 lakes and larger river systems in Denmark. Today, pikeperch occupies second place in the total catch of commercial species in Danish inland fisheries and is only surpassed by eel. The first introductions were made with eyed ova and fingerlings from Germany but during and just after the first world war, fingerlings were imported from Sweden. Since about 1930 pikeperchless lakes have been stocked with undersized fish from well established national populations. Annual commercial catches of pikeperch amount to 49–90 t. Average yield in commercially-exploited lakes range between 3–9 kg/ha. Pikeperch has become a useful addition to the Danish inland fish fauna. It is an effective predator of valueless species which compete with eel in turbid, eutrophic lakes, and it has added invaluably to previous non-profitable commercial lake fisheries. Furthermore, being originally regarded with suspicion by anglers it is today a highly praised game fish. RESUME Le sandre (Stizostedion lucioperca (L.)) a été introduit pour la première fois au Danemark en 1879. Les introductions, de plus en plus nombreuses au fil des ans, ont en général été couronnées de succès. Le sandre est aujourd'hui bien établie dans une soixantaine de lacs et de grands systèmes hydrographiques du Danemark. Il vient en deuxième position, juste après l'anguille, dans les captures d'espèces commerciales dans les eaux intérieures danoises. Les premières introductions ont été faites avec des oeufs embryonnés et des alevins importés d'Allemagne. Durant la première guerre mondiale et dans les années qui ont immédiatement suivi, des alevins ont été importés de Suède. Depuis 1930 environ, on introduit le sandre dans des lacs dont il est absent en utilisant des individus d'une taille inférieure à la normale que l'on prélève dans des populations bien établies dans le pays. Les captures commerciales de sandre sont de 49–90 t par an. Le rendement moyen des lacs qui font l'objet d'une exploitation commerciale est compris entre 3 et 9 kg/ha. Le sandre complète avec bonheur la faune des eaux intérieures danoises. C'est un prédateur efficace des poissons sans valeur qui font concurrence à l'anguille dans les lacs eutrophes aux eaux turbides. Il a rendu la pêche commerciale rentable dans plusieurs lacs. Enfin, il est aujourd'hui très apprécié des pêcheurs sportifs qui s'étaient pourtant montrés à l'origine assez réservés à son égard. 1. INTRODUCTION Pikeperch (Stizostedion lucioperca (L.)) is widespread on the European continent, having invaded most of northern and western Europe, including Sweden and eastern Norway, during its westward migration after the last glaciation. But it never succeeded in reaching Danish inland waters. Therefore, pikeperch is regarded as an exotic species in Denmark, even if today pikeperch is present in 70 Danish lakes. These populations have been created by active introductions over the last 100 years. Most Danish lakes are by nature good pikeperch waters. As the majority are situated in the part of the country which was covered by ice during the last glaciation, they rest on morainic deposits rich in minerals. To this must be added a constant supply of nutrients from intensely fertilized fields and sewage from urbanized areas. The typical Danish lake is therefore generally highly eutrophic, having a high primary production, a high degree of turbidity during summer and a correspondingly low visibility. The fish population is dominated by cyprinids and eel, with only a few predatory species of which the pike is the most important. As lake eutrophication has been increasing during the past 50 years, the pike has been badly affected and is often unable to effectively fulfil its predatory functions. In the absence of an indigenous pelagic predator, like the pikeperch, this has caused an unbalanced fish population dominated by species which both from a commercial and a recreational point of view are regarded as valueless or even noxious (competitors with eel). During the past century the efforts to introduce pikeperch into Danish lakes have primarily aimed at supplementing the indigenous fish fauna with a predatory fish to fill an empty niche in the ecological lake community and, secondly, to add to the Danish inland fishery a valuable fish species to benefit both commercial and recreational interests. The following paper describes the history of pikeperch introduction in Denmark, the stocking practice and the significance of this important addition to the indigenous Danish freshwater fish fauna. 2. HISTORY OF INTRODUCTIONS The first attempt to introduce pikeperch into Danish inland waters was made in 1879 when a group of enthusiastic fishermen transferred a few (20) fish of about 1.5 kg each from Lake Borgdorf in south Schleswig (Germany) to the River in Funen. However, mainly because of the very primitive means of transport available, this first initiative failed and another attempt was not made until 20 years later, when in 1898 200 pikeperch (weighing between 125 g and 1 kg) were transferred from the Jels lakes in north Schleswig (which at that time was under German government) to a lake close to the then Danish-German border. This time the introduction succeeded, and pikeperch still occur in this lake. From that year on pikeperch introductions were made with increasing intensity. In the years up to the first world war all stocking material was imported from Germany, both as fry, fingerlings or larger fish and as eyed ova. However, due to a lack of knowledge of proper transportation techniques and the ecological requirements of the pikeperch, many of these first attempts failed. Out of 16 introductions in nine inland waters made up to 1914 only three were successful resulting in lasting, self-reproducing populations. In two of these cases stocking was made with two-year old fish or spawners, in the third case with eyed ova (200 000–300 000 ova per year over a five- year period). It could be said that in these years the success of a pikeperch introduction depended on sheer luck. During and after the first world war, pikeperch was stocked into an increasing number of lakes, the main part of the stocking material now being provided by Swedish fish farms, where in these years artificial propagation of pikeperch was rapidly developing. Gradually, the majority of transfers were of one and two-year old fish and spawners from national stocks which by that time were well established. Since the middle thirties introductions have been made exclusively with two-year old or older fish from national stocks. With a few exceptions an increasing number of liberations were successfully undertaken, and since the beginning of the fifties all introductions have been successful. Today pikeperch occur in 70 Danish fresh- and brackish-water areas. Out of these 51 have been established by direct stocking, whereas 15 other lakes have been populated by invasion from stocked populations in other lakes within the same river systems. In four additional areas, stocking has been undertaken within the last few years so it is too early to evaluate success. Pikeperch are present to this day in two lakes in South Jutland, most probably as a result of liberations undertaken when this part of the country was under German government (1864–1920). We do not know when these liberations took place, most probably though before 1898. Table 1 shows the different types of Danish inland waters where pikeperch has been introduced during the last 100 years, both with success and failure. The diagrams in Figs. 1a-c summarize the history of pikeperch introductions into Danish inland waters. In Fig. la the columns represent the total number of introductions undertaken (128). The hatched part of the columns represents the first introduction made, thus showing the total number of waters areas stocked. The diagram illustrates the stocking activity throughout the period, showing three peaks, one before 1914, a second between 1915–34 and a third from about 1950 onward. In Fig. 1b the total number of introductions is separated into those which failed and those which have proved successful. Fig. 1c again shows the total number of introductions, but here illustrating the different origin of the stocking material throughout the years. 3. STOCKING PRACTICE During the first half of the century most pikeperch introductions were undertaken uncritically and unprofessionally, often with no regard to whether the water in question was suitable for pikeperch or whether the stocking material at hand was appropriate, hence the many unsuccessful introductions in these years (Table 2, Fig. 1b). Since 1951, however, the Danish Inland Fisheries Act states that any new introduction of pikeperch requires permission from the Ministry of Fisheries. This means that from this year on all pikeperch introductions have been performed under control. Also the catch and sale of undersized pikeperch for stocking requires permission from the fishery authorities. Nowadays almost all new introductions are undertaken with two-year old (20–30 cm) fish as stocking material. Normally a stocking rate of 2 kg/ha (4–5 fish per kg) is recommended. But for economical reasons lower rates are often used. It is recommended that stockings be repeated at least through a three-year period and the introduced fish be protected from fishing until natural reproduction has been verified. Experience has shown that a newly-created pikeperch population can be exploited between five and eight years after the first introduction. 4. YIELD Since 1925, when the total catch of pikeperch exceeded 1 000 kg for the first time, the yield increased up to 1935, since when it has stabilized on a level of between 5 and 8 percent of the total catch in Danish inland waters (40–90 t/year). Of the commercial species, pikeperch is today second both in weight of catch as well as in value in the total inland water yield, and is only surpassed by eel. The yield per ha varies between different lakes, mainly depending on the type of exploitation. Table 3 gives a few examples of lake yields in kg/ha in relation to lake type, area, depth and type of fishery. The table shows that commercially-exploited, deep, freshwater lakes give the highest yield, the lowest yields being obtained in shallow, brackishwater lakes. The great variation in annual yields through the nine-year period may be due partly to variations in catch per unit effort and/or to climatic changes resulting in strong or weak year-classes. An average yield of about 5 kg/ha should be regarded as normal for Danish pikeperch lakes. Originally, pikeperch was introduced in order to supplement the indigenous fish fauna with a valuable commercial species. This aim has been successfully achieved. As regards the recreational value of the pikeperch it is interesting to note that for many years anglers regarded the pikeperch with great suspicion. With a few exceptions this attitude prevailed up to the fifties, and in some manuals for anglers it was even stated that the pikeperch is a fish which should not be tolerated in a good fishing water. Nowadays, this situation has completely changed and today it is a highly praised game fish, even if angling for pikeperch must be considered a fishery for specialists. The preferred fishing method is trolling, either using live or artificial bait. Having first found the right method, a few sport fishermen can be responsible for quite a considerable catch. This may be seen from the diagram in Fig. 2, which illustrates the development of the pikeperch fishery in Lake Furesö, the deepest Danish lake (36 m), situated a few kilometres north of Copenhagen, Zealand. The lake is exploited both commercially and recreationally. Since the beginning of the century, at a time when the lake was exploited only by anglers, several attempts were made to create a pikeperch population, the first attempt as early as 1903. The angling club in question tried several times to stock pikeperch-imported from Germany - in the lake, but the attempts all failed. Not until 1964, when new and more professional introductions were made, using two-year old fish as stocking material, the creation of a good pikeperch population in the lake eventually succeeded. The diagram shows the development of the pikeperch fishery in the lake since its introduction in 1964. The catches from this particular lake can be separated into a commercial portion and a recreational portion. It is seen that pikeperch entered the fishery in 1970, six years after the introduction, first appearing in the anglers' catches. In 1973 pikeperch for the first time appeared in the commercial catch, due to the fact that the commercial fisherman has protected the newly created population, partly until a reasonable catch could be obtained and partly until the time when the first young pikeperch were found, proving that pikeperch had completed its first natural reproduction in the lake. From 1973 onward, pikeperch catches have increased, until 1979, the greatest portion being in the commercial catches. But from 1980, the situation has changed so that the anglers' catches now surpass the commercial catches. It is remarkable, however, that this high catch is obtained only by two or three anglers - to the great annoyance of the commercial fisherman who condemns it as overfishing and fears that this heavy fishery will eventually be detrimental to the pikeperch population. The catches illustrated in the diagram, however, only represent about 1 kg/ha so it is considered that the population is quite able to withstand the fishery. 5. THE FUTURE Even if in a zoogeographical sense pikeperch is still regarded as an exotic species in Denmark the creation of a great number of well established pikeperch populations throughout the past century makes it reasonable today to regard the pikeperch as a true member of the indigenous Danish freshwater fish fauna, especially as, since the middle thirties, all new introductions have been made with stocking material from well established national stocks. In view of the still increasing eutrophication of Danish lakes, which adversely affects the profitability of commercial lake fisheries, and also from the angler's point of view, the pikeperch must be regarded as a fortunate addition to the fish population in the lakes. It is, therefore, to be expected that in the years to come pikeperch introductions into lakes from which the species is still absent will continue unabated. Table 1 Number and type of waterbody where pikeperch were introduced Introduction successful Introduction Success After only one After two or failed even yet too Type of water stocking more after two or Total early to stockings more assess stockings

Lakes: less than 10 ha 20 19 8 2 49 more than 10 ha 3 4 1 1 9

Estuaries, brackish waters 1 3 1 5

Rivers 2 2

Total 24 23 14 4 65

Table 2 Type of stocking material used in Danish pikeperch introductions

Only one introduction Two or more introductions Type of stocking material Success Successful Failed yet Successful Failed unknown Ova 1 1 2 Fry 1 2 Half-yearlings 1 3 8 Yearlings 3 2 Two-year old 11 3 12 1 Spawners 3 4 1 5

Total 19 8 4 33 3 Table 3 Yield of pikeperch (kg/ha) from six Danish lakes

LAKE Mossö Saltbaek Arresö Haraldsted Julsö Jels söer vig sö AREA (ha) 1 689 1 418 4 056 199 564 88 MAX. DEPTH (m) 32 3.5 6.5 9.0 17.5 11.5 TYPE OF FISHING commerci commercial commercia commercial commercial recreativ al l + recreative + recreative e TYPE OF WATER fresh brackish fresh fresh fresh fresh 1970 5.9 7.9 0.6 7.2 10.2 3.4 1971 5.9 3.6 4.3 2.1 6.5 ? 1972 5.0 1.4 4.4 3.0 2.4 2.3 1973 11.9 1.3 6.4 1.0 3.1 3.4 1974 18.5 1.6 3.2 10.0 2.8 3.4 1975 8.8 1.6 6.5 1.0 4.2 4.0 1976 9.0 2.0 6.0 7.8 2.4 3.4 1977 6.4 1.5 4.7 1.2 2.9 3.4 1978 7.9 1.8 3.2 7.2 6.3 2.8 AVERAGE 8.8 2.5 4.4 4.6 4.5 3.2

Fig. 1 Danish stocking activity with pikeperch

Fig. 2 Lake Fure sø. Development of the pikeperch fishery THE IMPACT OF ZANDER (Stizostedion lucioperca (L.)) IN THE UNITED KINGDOM AND THE FUTURE MANAGEMENT OF AFFECTED FISHERIES IN THE ANGLIAN REGION

R.S.J. Linfield Anglian Water Authority Huntingdon, Cambs., England

CONTENTS

1. INTRODUCTION 2. CURRENT DISTRIBUTION IN BRITAIN 3. IMPACT OF THE SPECIES 4. ZANDER CONTROL IN ANGLIA 4.1 Measures to restrict the further spread of the species 4.2 Future management of fisheries containing zander 4.3 Restoration of fisheries already adversely affected 5. MONITORING THE OUTCOME OF THE MIDDLE LEVEL CULL 6. DISCUSSION 7. ACKNOWLEDGEMENTS 8. REFERENCES ABSTRACT The history of zander introductions within the United Kingdom is briefly reviewed and the current distribution of the species updated to the end of 1981. Fisheries survey data is presented to demonstrate that a severe decline has occurred in the quality of natural coarse fish populations in many of the rivers and drainage channels which have been colonized by the species during the past two decades. Reasons are advanced for attributing this decline primarily to predation by zander and an account is given of an exercise aimed at restoring a satisfactory predator-prey balance in an extensive network of drainage channel fisheries in eastern England. Policies are discussed for the future management of these and other recreational fisheries in the Anglian region which have acquired zander since 1963 and measures are considered for limiting the further spread of the species. RESUME L'auteur fait un bref historique des introductions de sandres au Royaume-Uni et indique la répartition de cette espèce à la fin de 1981. Un certain nombre d'enquêtes montrent que la qualité des populations naturelles de poissons communs a nettement diminué dans beaucoup de cours d'eau et de canaux de drainage colonisés par le sandre au cours des vingt dernières années. L'auteur explique pourquoi le prédatisme du sandre est considéré comme la cause principale de cette diminution et rend compte des efforts visant à rétablir un équilibre satisfaisant entre les prédateurs et les proies dans un vaste réseau de drainage de l'est de l'Angleterre. II s'intéresse à l'aménagement futur des pêcheries dans ce réseau ainsi que dans d'autres parties de l'Anglia où se pratique la pêche récréative et où le sandre a commencé à apparaître en 1963. II préconise aussi un certain nombre de mesures pour limiter la propagation de cette espèce. 1. INTRODUCTION The zander (Stizostedion lucioperca) was first introduced to the U.K. in 1878 when 24 zander averaging 0.9 kg in weight were netted from Bothkamper Lake in Schleswig-Holstein, Germany, and transferred successfully to enclosed waters at Woburn Park, Bedfordshire (Cawkwell and McAngus, 1976). Further importations occurred in 1910 (Germany to Woburn) and 1960 (Sweden to Mepal Pit, Cambridgeshire) and a number of successful stockings from Woburn to enclosed waters in southeast England also took place between the end of the second world war and 1962. Introductions to the Grand Union Canal and the River Ouzel were carried out in circa 1950 but the fish apparently perished. Thus the species remained very limited in its distribution within the U.K. for the greater part of a century and the preliminary evidence seemed to indicate that it was only in certain favourable years that the species was able to breed successfully (Maitland, 1969). However, the introduction of zander to Norfolk's Great Ouse Relief Channel in 1963 changed the situation dramatically and the species subsequently bred very successfully in the wild and rapidly spread through the adjoining rivers of East Anglia. Currently the colonization of the extensively interconnected rivers of this region of England is still only partial but the species is steadily extending its range and has been illegally introduced to other parts of the U.K. by anglers. 2. CURRENT DISTRIBUTION IN BRITAIN The historical sequence of events concerning the original importations of zander into Britain and the subsequent establishment and spread of the species has already been described in some detail by Cawkwell and McAngus (1976) and by Wheeler and Maitland (1973). The earlier accounts were updated to 1978 in respect of the Anglian region by Linfield and Rickards (1979) and to 1980 by Klee (1981). The distribution at 1 January 1982 is now summarized in Fig. 1. The main advances since 1978, based largely on fisheries surveys and reported captures by rod and line, have involved further progression upstream on the River Cam (including invasion of Reach and Burwell Lodes), completion of the colonization process within the major channels of the Middle Level Drainage System (and also many small interconnecting drains not shown in Fig. 1 for reasons of clarity) and confirmation of successful spawning of the species in the Suffolk Stour. The Stour is believed to have acquired zander as a result of a water transfer scheme (Cawkwell and McAngus, 1976; Linfield and Rickards, 1979) and the first ripe females (three fish), the largest weighing 2.04 kg (4.5 1b) were captured from the river by Anglian Water Authority fisheries staff in May 1977 during a routine fishery operation. Mature fish have subsequently been caught on many occasions and a breeding population has become fully established. In 1980 large numbers of zander fry were also taken from Abberton Reservoir, a 490-ha pumped storage reservoir which draws water from the lower Stour. The Reservoir is listed as being of international conservation importance in the Ramsar Convention. While the distribution of the species within Anglia has been largely a natural or accidental process since the 1963 stocking, the distribution outside Anglia has resulted from a number of illegal stockings by anglers. Each illegal stocking has provided a new locus for natural expansion of the species through successful reproduction of the stocked fish and this process is now ensuing in many parts of England. For purposes of water cycle management and fisheries administration, England and Wales has been divided into the ten water authority regions illustrated in Fig. 2. A recent enquiry to each of these authorities revealed that officially recognized populations of zander were still restricted to the Anglian, Severn-Trent and Thames Water Authorities. However, in a recent book on the species, Rickards and Fickling (1979) listed 15 counties considered from angling reports to support zander, including some in three further water authority regions. Whatever the true position in these areas may be, the confirmed occurrence of populations of the species in the Midlands Canal system around Coventry, and occasional specimens from the River Severn, suggest that widespread colonization will occur throughout the greater part of England in a manner not dissimilar to that which has already occurred in a large part of Anglia. Unconfirmed reports of the species in the River Thames also give rise to fears of further extensions of the species range over a considerable area in the near future. 3. IMPACT OF THE SPECIES Between 1963 and 1980 reactions to the zander in the U.K. varied greatly from fanatical enthusiasm to absolute horror. Views on their likely fisheries impact have thus been strongly conflicting and until 1980 were largely based on angling experiences and subjective assessment. Since Linfield and Rickards (1979) attempt at reconciling the various views against a background of known biological information on the species, a programme of fisheries survey work by Anglian Water Authority fisheries scientists has provided a considerable amount of quantitative data on Anglian river fisheries which has never been previously available (Anglian Water Authority, 1981). The first point to emerge from this work was that the early seventies produced relatively poor recruitment to the cyprinid stocks in the Anglian region (Linfield, 1981) with the result that biomass levels as determined during 1978, 1979 and 1980 were generally lower than might have been expected from the traditionally high standard of the Anglian coarse fisheries. The second major point was that the stocks of cyprinids in river fisheries containing zander had generally fallen to a lower level than fisheries not containing zander and the extent of the deterioration appeared loosely related to the length of time zander had been present. The first open waters to be colonized, the Relief Channel and Ely Ouse, had crashed to very low total biomass levels of circa 2 g/m2 (Klee, 1979, 1979a). Comparison of the structure of the depressed stocks in fisheries containing zander with those not yet colonized immediately suggested a mechanism for the decline. A very strong 1975 year- class for most cyprinid species in the region, particularly for roach, was suppressed in the waters containing zander and a high level of mortality appeared to be acting on young fish in the population. A predator-prey imbalance was clearly evident from a comparison of pike plus zander biomass with cyprinid biomass and the extent of the imbalance appeared loosely correlated both with the length of time zander had been present and the extent to which cyprinid biomass had become reduced. Data to support these conclusions, and other evidence indicating the role of zander in the decline of the fisheries concerned, is given by Klee (1981). The relationship between zander, predator- prey ratios and total biomass levels for 35 Anglian fisheries is summarized in Fig. 3. 4. ZANDER CONTROL IN ANGLIA 4.1 Measures to restrict the further spread of the species It is possible that the Lower Ouse/Middle Level fisheries are exceptionally suitable for zander in the U.K. and that the problems encountered will not occur so extensively in other waters. However, the potential for zander to bring about predator- prey imbalances, particularly during periods of poor prey recruitment, has been shown from the Lower Ouse experience to be such a serious threat that a policy of protection of open fisheries from the species has been firmly adopted throughout'the Anglian region. While colonization of the remainder of the Great Ouse and its tributaries cannot be prevented, the likelihood of the species colonizing the River Nene from King's Dyke (see Fig. 1) and hence reaching adjoining catchments, has been considerably reduced by repeatedly electro-fishing and removing all zander from the last section of the Dyke between two lock structures which themselves provide a partial barrier to emigration. Also chlorination and dechlorination of water passing through the Wixoe Pumping Station on the River Stour has so far prevented the species from traversing the second stage of the Ely Ouse to Essex Water Transfer Scheme and hence colonizing other Essex rivers. The major threat as far as extension of the species range into entirely new catchment areas is concerned appears to come from illegal transfer by anglers, a form of vandalism which has successfully spread the species into other parts of the country as described earlier in this paper. Unfortunately, very little can be done to prevent this activity. 4.2 Future management of fisheries containing zander In spite of the U.K. experiences the zander appears to be a much respected species in other European countries, causing little or no damage in sport fisheries, and pike/perch species are also considered highly desirable in North America and Canada. When considering why zander should behave so differently in the U.K., the most pertinent point appears to be that until recently British anglers have generally returned all zander caught alive to the water and thus fishing mortality has not been a significant factor affecting population levels. In Canadian situations experience has shown that reduction of the mean age of a walleye population to a critical point through fishing mortality can result in a collapse of stocks (P.J. Colby, pers.comm.). Similarly, it is understood that in many European rod and line fisheries the problem with zander appears to be more related to maintaining adequate zander stocks for anglers to catch and remove for eating than in limiting their natural expansion. In the U.K. pike removal by netting and electro-fishing has been tried many times to reduce predator levels in certain river fisheries but with little or no lasting impact in productive lowland waters. While there seems little hope of the same methods proving any more successful with zander the reported impact on stocks by anglers in other countries seems to offer a means of zander control in the U.K. if anglers could be persuaded to change their traditional habit of returning fish alive to the water. A policy has therefore been adopted of encouraging the culling by anglers of all zander caught and this will be given an extended trial to determine its level of success in preventing further fishery damage by the species.

4.3 Restoration of fisheries already adversely affected When considering what steps might be taken to restore those fisheries in which cyprinid stocks have already been damaged a number of practical difficulties immediately arise. The size of the waters involved is such that instant restoration based on restocking is not feasible and reestablishment of adequate breeding stock levels for cyprinids appears the best that can be attempted. Even where this can be carried out the right balance between predators and prey has to be established otherwise the cycle of over-predation would probably be repeated. An overall assessment of the problem areas in Anglia indicated that the waters requiring restoration fell into two categories: (a) those where the biomass of both prey and predators had crashed to an extremely low level and (b) those where a decline appeared to be rapidly progressing but where an adequate head of prey for breeding purposes was still present and predators were still at a high level. Waters in the first category comprised the Relief Channel, which received the 1963 zander stocking, and the Ely Ouse, the next river to be colonized. In both of these it was possible to recommend restocking with cyprinids to provide the right balance and provide a prey species breeding stock. This action was pursued and 12 g/m2 of cyprinids (60 000 fish of total weight approximately 14.5 t) were stocked to the Ely Ouse during 1980/81. The Relief Channel stocking was commenced in 1981 and is continuing. Waters in the second category were predominantly to be found in the Middle Level system of drains, which is a discrete catchment (bounded by a dotted line in Fig. 1) separated from neighbouring waters by pumps at the downstream end and two lock structures with restricted operation at the upstream end. These waters have traditionally offered some of the best pike fishing in the United Kingdom as well as excellent sport for other species. With the demise of the prey species through enhanced predation pressure coupled with relatively low recruitment in the early seventies, the ration of pike alone to residual prey had become unbalanced over much of the drain network and thus any restorative action in these waters necessarily involved both pike and zander. Upon considering all these factors it was ultimately recommended that an attempt be made at restoring the fisheries in the whole Middle Level system by treating all those waters in direct communication with one another as a single fishery (Anglian Water Authority, 1980). Altogether biomass estimates had been made for fish 10 cm fork length in 30 sections (length 60–380 m each) distributed along representative waters in the system and this data was pooled to provide an approximate estimate of total stocks in the 145 km of drains comprising the whole system (278.3 ha). This procedure smoothed out the extreme imbalances identified in some areas and the following results were obtained: Species Total stock g/m2 (t)

Prey 6.1 2.2 Pike 3.1 1.1 Zander 0.8 0.3

Total 10.0 3.6 A decision was taken to aim for a 9:1 ratio of cyprinid biomass to predator biomass (pike plus zander) as a basis for subsequent natural expansion of the cyprinid population. A series of options to achieve this within a single angling season was offered to anglers' representatives based on various combinations of predator culling by the anglers themselves and restocking with cyprinids by the Anglian Water Authority. An option leaning heavily toward predator culling was recommended because of the uncertainties involved in both the acquisition of roach and bream for restocking and their likely survival rate in their new environment. In August 1980 it was agreed between the Authority and the angling bodies involved that 3 117 kg of pike and zander would be removed (80 percent of estimated total available) by anglers followed by the introduction of 891 kg of roach and bream by the Authority. A compromise had to be reached with the Pike Anglers' Club of Great Britain, however, whereby all pike in excess of 10 lb weight (4.54 kg) would be selectively preserved. An extensive system was set up for receiving and scrutinizing cull returns and it was agreed that the situation would be reviewed at the end of the year when additional survey data from field work in July (eight further sections of total length approximately 1.5 km) would be available and the rate of culling achieved in practice could be assessed. The review document (Anglian Water Authority, 1981a), produced in January 1981, revealed the following revised stock levels for the system at the pre-cull stage. Species Total stock g/m2 (t)

Prey 6.3 2.3 Pike 2.7 1.0 Zander 0.6 0.2

Total 9.6 3.5 The implications of these revised figures were considered in the light of the progress of the cull at that stage and crude estimates that were attempted of likely levels of production of predator flesh within the fishery during the period of the cull. Following this assessment, during which it was evident that the zander stock level had been substantially underestimated, it was concluded that the original cull target should be retained but should be broken down into 639 kg of zander (the total weight estimated to have been present) plus 2 478 kg of pike. Zander culling would continue indefinitely beyond the target (shown to be low) whereas pike culling would be called to a halt as soon as the target figure was reached. This was achieved before the end of February and, when all outstanding returns had been collected and verified as far as possible, the final figures stood at 1 415 kg for zander (984 fish) and 2 971 kg for pike (1 447 fish). The roach and bream to be restocked were provided during April and May 1981 and released at various points within the system. This stocking actually involved 1 188 kg of fish and a further 187 kg were added in February 1982. 5. MONITORING THE OUTCOME OF THE MIDDLE LEVEL CULL The Middle Level cull has been viewed as an experiment in the management of lowland fisheries which have incurred predation problems from zander and arrangements have accordingly been made for monitoring the outcome. A consortium of sponsors, namely the Angling Foundation, the Water Space Amenity Commission, Sports Council and the Anglian Water Authority have collectively financed a three-year research project on predator/prey relationships in one of the major drainage channels within the Middle Level system with a view to assessing the effectiveness of the cull and monitoring subsequent developments within the fishery. The work is being carried out by Liverpool University and will continue until 31 December 1983. In addition, the Anglian Water Authority will repeat routine survey coverage within the system during the next few years to provide broadly based quantitative data for various parts of the drainage network. 6. DISCUSSION Angling results during 1981 and survey catches by workers from the University suggest that the Middle level cull has been very successful in reducing the zander biomass to a low level. However, young zander from 1980 and 1981 spawnings are well in evidence within the fishery and young pike from these year-classes are also fairly numerous. For the pike a good cross section of age groups and size classes has also survived the cull leaving a good breeding population still active within the fishery to ensure a rapid recovery of this species. While preliminary indications suggest the management action has been successful in achieving the initial objective, good spawning and a rapid re-expansion of the cyprinid stocks is essential during the period of reduced predation pressure (probably only three to four years) if the ultimate goal of a high quality fishery for both cyprinids and pike is to be re-established. It now remains to be determined what the subsequent course of events will prove to be. If the management action is successful, and zander stocks can subsequently be kept at a low level by persistent angler culling in the future, then the species may yet come to be viewed more favourably in the U.K. than has so far been the general case. However, if the action should fail and zander should come to enjoy a further rapid expansion in numbers in the Middle Level system with subsequent damage to cyprinid stocks, then fishery managers in the U.K. will have a persistent problem of some magnitude on their hands and the popularity of zander in the U.K. will remain restricted. 7. ACKNOWLEDGEMENTS The author wishes to thank Mr. A.W. Davies, Director of Scientific Services, Anglian Water Authority, for his support in the preparation and subsequent presentation of this paper to the Symposium. Any views expressed are those of the author and not necessarily of the Anglian Water Authority. 8. REFERENCES Anglian Water Authority, Regional policy for zander and proposals for the Middle Level 1980 system. Committee report. Anglian Water Auth.Comm.Rep.Huntingdon, (F.26/80):14 p. , List of internal reports on fisheries surveys and investigations, 1978–80 1981 inclusive. Huntingdon, Anglian Water Authority, 4 p. (mimeo) , Progress with management action and research in the Middle Level 1981a system. Anglian Water Auth.Comm.Rep., Huntingdon, (F.7/81):5 p. Cawkwell, C. and J. McAngus, Spread of the zander. Angler's Mail, March 3 issue:12– 1976 3 Klee, C., Report on fish survey of the Ely Ouse from Little Thetford to Denver. March and 1979 July 1979. Report. Huntingdon, Anglian Water Authority, 27 p. , Report on fish survey of the Relief Channel, October 1979. Report. 1979a Huntingdon, Anglian Water Authority, 21 p. , An assessment of the contribution made by zander to the decline of 1981 fisheries in the lower Great Ouse area. In Proceedings of the Second British Freshwater Fisheries Conference. Liverpool, University of Liverpool, pp. 80 9 Linfield, R.S.J., The current status of the major coarse fisheries in Anglia. In Proceedings 1981 of the Second British Freshwater Fisheries Conference. Liverpool, University of Liverpool, pp. 67–79 Linfield, R.S.J. and R.B. Rickards, The zander in perspective. Fish Manage., 10(1):1– 1979 16 Maitland, P.S., A preliminary account of the mapping of the distribution of freshwater fish 1969 in the British Isles. J.Fish Biol., 1(1):45–58 National Water Council (England and Wales), Water industry review. London, National 1982 Water Council, 23 p. Rickards, R.B. and N. Fickling, Zander. London, A. and C. Black, 174 p. 1979 Wheeler, A. and P.S. Maitland, The scarcer freshwater fishes of the British Isles. 1. 1973 Introduced species. J.Fish Biol., 5(1):49–68

zander present zander not recorded The colonised sections of the River Stour (bottom right corner) continue downstream to Cattawade barrage, at the head of the estuary

* Modified with permission from Linfield and Rickards, 1979.

Fig. 1 The distribution of zander in Anglian watercourses at 1 January 1982

REPRODUCED WITH PERMISSION FROM THE NATIONAL WATER COUNCIL, 1982

Fig. 2 The areas of the ten Regional Water Authorities in England and Wales (a) Relationship between total fish biomass and the presence of zander.

Zander present≥ 10 years at the time of the survey.

Zander present 4–9 years at the time of the survey.

Zander present < 4 years or not recorded at the time of the survey.

(b) Relationship between total fish biomass and the percentage of predator biomass (pike + zander) in the population.

Pike + zander occupy > 30% total fish biomass.

Pike + zander occupy 20–30% total fish biomass.

Pike + zander occupy < 20% total fish biomass.

Fig. 3 The relationship between zander, predator: prey ratios and total fish biomass (fish ≥ 10 cm fork length) in 35 Anglian river fisheries. The data was collected during 1978–80 and the fisheries are identified in Linfield (1981) INTRODUCTION AND THE PRESENT STATUS OF BROOK TROUT (Salvelinus fontinalis Mitchill) IN NORWAY

M. Grande Norwegian Institute for Water Research Oslo, Norway

CONTENTS

1. INTRODUCTION 2. INTRODUCTION OF BROOK TROUT IN NORWAY 3. NATURALIZED POPULATIONS OF BROOK TROUT IN NORWAY 3.1 The Oyfjell populations 3.2 Other populations 4. STOCKING OF BROOK TROUT 4.1 Experimental stocking in the Langtjern area 4.2 Experimental stocking in areas seriously altered by acid precipitation 4.3 Experimental stocking in sea water 4.4 Stocking for fishery purposes in acid waters 4.5 Stocking for sport fishery purposes in urban areas 5. TOLERANCE OF BROOK TROUT TO ACID WATERS 5.1 Tests in acid stream water 5.2 Physiological tests 5.3 Information from the literature 6. SOME CONCLUSIONS AND TENTATIVE PRACTICAL GUIDELINES FOR THE MANAGEMENT OF BROOK TROUT IN ACID LAKES 6.1 Naturalized populations 6.2 Life cycle, food, age and growth 6.3 Water quality 6.4 Stocking strategy 6.5 Migrations 6.6 Fishing for brook trout 6.7 Further investigations 7. REFERENCES ABSTRACT The brook trout was first introduced to Norway in 1876. Since then the fish has been stocked in many parts of Norway, but only a few naturalized populations occur. These are found in the upper parts of creeks and small rivers. Stocking experiments have shown that the brook trout may survive and thrive in rather acid waters (pH down to about 4.5) and seems to be more tolerant than the native species and the rainbow trout (Salmo gairdnerii Richardson). Survival tests in acid river water and physiological studies seem to confirm these results. The brook trout may grow faster than the brown trout (Salmo trutta L.) in acid lakes and obtain a better condition and quality. On the other hand it is a more short-lived species than the brown trout and does not attain the same size. It usually migrates out of lakes in the autumn upon reaching sexual maturity after one to three years. It is advisable to catch most of the brook trout in the second year after stocking due to high natural mortality and migration. The number of the fish stocked therefore must not exceed that at which the fish reach a catchable size within two years. RESUME Le saumon de fontaine a éte introduit en Norvège pour la première fois en 1876. Depuis lors, ce poisson a servi à empoissonner de nombreuses régions du pays, mais il ne subsiste que quelques populations acclimatées. On le trouve dans les parties amont des petits cours d'eau. Les expériences d'empoissonnement ont montré que le saumon de fontaine peut survivre et se développer dans des eaux relativement acides (pH descendant jusqu'à environ 4,5) et qu'il semble être plus tolérant que les espèces indigènes et la truite arc-en-ciel (Salmo gairdnerii Richardson). Des tests de survie dans des eaux de rivière acides et des études physiologiques semblent confirmer ces résultats. Le saumon de fontaine peut grossir plus vite que la truite de rivière (Salmo trutta L.) dans les lacs acides et acquérir une condition et une qualité supérieures. Cependant, il vit moins que cette truite et n'atteint pas la même taille. En général, il quitte les lacs en automne après avoir atteint sa maturité sexuelle en un à trois ans. Il est recommandable de capturer la plupart des saumons de fontaine au cours de la deuxième année suivant le repeuplement, étant donné la haute mortalité naturelle et les migrations. Le nombre des poissons stockés ne doit donc pas dépasser celui des poissons de taille capturable au cours d'une période de deux années. 1. INTRODUCTION In Norway there has been only a moderate interest in introducing and stocking exotic fish species. Today a special licence is required to introduce new fish species to waters in which they are not already found. The two salmonids rainbow trout (Salmo gairdnerii) and brook trout (Salvelinus fontinalis), however, have been introduced and stocked more or less intensively for the last 100 years. The interest in brook trout has increased in the last decade due to the fact that it may be an alternative to the native brown trout in acid lakes (Grande et al., 1978). In Norway more than 1 500 lakes and a large number of running waters have lost their fish populations due to acid precipitation (Jensen and Snekvik, 1972; Sevaldrud et al., 1980). Several thousands of native self- reproducing fish stocks have thus been eliminated. It is, therefore, of great importance to find valuable fish species which may be equally or even more tolerant to acid stress as native species. With this background the following presentation on the brook trout in Norway is made. 2. INTRODUCTION OF BROOK TROUT IN NORWAY The first introduction of brook trout to Norway was made in the winter 1876–77. Eggs were imported directly from North America. The larvae of these eggs were stocked in some localities around Oslo, but only a few fishes were recaptured and no reproduction occurred (Collett, 1905). In 1883 a new batch of eggs was imported from North America and in the following years (to 1890) fry of brook trout were stocked several places in Norway. In most places stocking apparently did not succeed but there were also reports of recaptures of fish up to a size of 1.5 kg. In 1909–10 there was a new series of stockings in central Norway of fish originating from the introduction in 1883 (Landmark, 1894, 1910). From all these stockings probably only one self-reproducing stock was established in a small river, and this did not become officially known until 1969. It is still not known exactly when stocking of this population took place. In 1918 about 1 000 fry of brook trout were imported from Denmark and stocked into a lake in Oyfjell in Norway. This resulted in self-reproducing stocks in several small brooks and rivers (Huitfeldt-Kaas, 1924; Huitfeldt, 1947 and Grande, 1964). In the years 1966–69 some stocking experiments with brook trout of the Oyfjell (Norway) population and other salmonids in acid lakes indicated that brook trout was relatively tolerant to acid water (Grande et al., 1978). This led to increased interest in this fish and in the seventies the fish was stocked extensively in Norway, especially in areas where acid precipitation has led to fish mortalities. Some of these recent stockings have led to naturalized populations, probably only in creeks and small rivers. 3. NATURALIZED POPULATIONS OF BROOK TROUT IN NORWAY As mentioned in the previous chapter a few naturalized populations of brook trout have become established in creeks and small rivers. Some comments shall be given to these populations and especially those in the Oyfjell area in Telemark which was investigated in 1958–60 (Grande, 1964). 3.1 The Oyfjell populations The population in Oyfjell was in 1960 the only officially known population of brook trout in Norway. It was studied with regard to distribution, population size, age and growth, reproduction and feeding habits. A comparison with the brown trout (Salmo trutta L.) in the same water system was also made. 3.1.1. Distribution It was found that the brook trout was distributed in five oligotrophic creeks and rivers (pH 5.8–6.5, conductivity 13–25 uS/cm at 20°C) about 600–1 000 m above sea level. In the upper parts of the water system it was found alone; there was also an area where it coexisted with the brown trout. Further downstream the brown trout was alone and brook trout occurred only sporadically. The mean annual water flow was 40–280 1/s in the localities where brook trout were dominant (Fig. 1). 3.1.2 Population size The brook trout were small, and most of the fish caught weighed 30–80 g. The brown trout caught in the same localities usually reached a higher maximum size. By means of a marking technique the population in two creeks with brook trout was estimated to 650 and 800 g/100 m2, respectively. 3.1.3 Age and growth Examination of and scales indicated that the oldest brook trout sampled were eight winters old. However, few fish in the catches were older than 2–4 years. The brown trout caught were generally older than the brook trout. The growth rate of the brook trout was calculated to be about 6.0, 4.5, 4.0 and 3.0 cm, respectively, in the first four years of life. From the fourth year of life stagnation of growth was remarkable. The brook trout seemed to grow somewhat faster than the brown trout in the first year of life, but after this the growth rate was about the same for the two species. 3.1.4 Sexual maturity and spawning Sexual maturity occurred after one winter for fast-growing males of brook trout. Usually the first spawning took place after two to three winters for the males and after three winters for the females. The brown trout reached maturity later than the brook trout, and the first spawning usually seemed to take place after three to four winters for the males and after three to five winters for the females. 3.1.5 Feeding habits The brook trout as well as the brown trout lived mainly on insects and there was a marked competition for food between the two species. 3.1.6 Competition It was concluded from the Oyfjell investigation that the brook trout was able to compete and maintain its numbers in creeks and small rivers in the area. 3.2 Other populations Further investigations in other localities where brook trout is present have shown that the picture from Oyfjell is typical. In Overnbekken in Buskerud, for example, the brook trout has maintained its numbers in the upper part of a small river for approximately 80 years. Also here it is found alone in the upper part while there is a short stretch where it is found together with the brown trout. Further down the brown trout and also other species completely dominate and brook trout are caught only sporadically. In the years 1962–76 brook trout fry was stocked by the author in three fishless oligotrophic creeks (200–600 m above sea level). In all cases the brook trout grew up, reproduced and established itself as self-reproducing, naturalized populations. Remarkable in these cases is the stationary tendency of the fish. There is little migration down the creeks from the place where they are stocked, and the fish is practically unknown in the lower part of the water system. Length distribution of fish in catches from two creeks are shown in Fig. 2. This typical distribution of naturalized brook trout populations in Norway seems to be in concordance with the occurrence of brook trout in Denmark (Ernst and Nielse, 1981), Sweden (Kjellberg, 1969) and other countries in Europe (MacCrimmon et al., 1969 and 1971). 4. STOCKING OF BROOK TROUT 4.1 Experimental stocking in the Langtjern area Since 1966 different strains of brook trout, brown trout, rainbow trout and hybrids between brook trout and Arctic charr (Salvelinus alpinus L.) have been stocked in an experimental area with 11 small acid lakes (0.5–25 ha). The Langtjern area is situated in central Norway about 500–600 m above sea level. The lakes have a pH varying from 4.7–5.5 and a chemistry rather similar to that for Langtjern (Table 1). Most of the lakes have no natural fish populations partly due to lack of spawning areas and party due to the low pH. One example from these stockings is illustrated in Fig. 3. The following conclusions can be made of the results obtained hitherto. 4.1.1 Recaptures When brook trout, brown trout and rainbow trout were stocked together the recapture of brook trout was generally higher than for brown trout. There were few or no recaptures of rainbow trout. Stocking of the hybrids between brook trout and sometimes gave good results. There was relatively little difference between strains of brook trout (Danish domestic, Oyfjell and Temiscamie strain). There are indications that stocking of brown trout alone gives better results than when it is stocked together with brook trout (Fig. 4). 4.1.2 Age Most brook trout were caught after 1–3 years of life with a peak in the second year. Very few, if any, caught are older than three years (3+). The brown trout grows older and may be six years or older in the same lake (Fig. 4). 4.1.3 Growth, condition and quality Under similar conditions brook trout may grow much faster than brown trout, but brown trout may grow larger due to a greater age. Some typical growth rates are shown in Fig. 5. Usually brook trout have good condition, red colour in the flesh and delicious taste even in rather acid lakes with low pH and a high content of humic matter. Its quality in such lakes often seems to be better than the brown trout. 4.1.4 Migrations The brook trout has a great tendency to migrate downstream out of a lake after 1–2 years of life (Fig. 6). This may, to a great extent, be due to sexual maturity and be regarded as spawning migrations. The tendency to migrate seemed in this case to be much less for the brown trout even if it were sexually mature. 4.1.5 Feeding habits The brook trout and brown trout have the same mainly insectivorous diet. Corixidae, Chaeborus sp., Sialis spp., Ephemeroptera, Trichoptera and Coleoptera and Odonata are the most important groups. As would be expected, there is competition for food between the two species in the lakes. 4.2 Experimental stocking in areas seriously altered by acid precipitation These results showed that the brook trout may be regarded as an alternative or supplement to the brown trout in acid lakes. By contrast, the rainbow trout has been a complete failure for stocking into such lakes. It was, therefore, decided to do some experimental stockings of brook trout in lakes in the southern part of Norway which are seriously altered by acid precipitation. In 1975 17 400 fingerlings (0+) of brook trout were stocked in 13 small and medium sized acid lakes in southern Norway. The purpose was to study how this species would thrive in these lakes where stockings of the native brown trout in the last years had failed. Through the period 1975–79 experimental net fishing and water sampling were made in the lakes. The following summarizes the results (Grande et al., 1980). Stocking was carried out in lakes where the mean pH varied between 4.48–4.98. In three of the lakes stocking resulted in few or no recaptures. Mean pH for these lakes were 4.48, 4.68 and 4.84. Acceptable results were obtained in the other lakes where the most acid had a pH of 4.58. The results are discussed with respect to the water quality in the lakes, and it is concluded that other factors as well as low pH may explain the results. In the other lakes the mean weight of brook trout for the years 1976 (1+) and 1977 (2+) was 171 and 312 g, respectively. The condition and quality of the fish was good or very good. The stomach content consisted mainly of insects with Corixidae, Chaeborus spp., Sialis spp., Ephemeroptera and Trichoptera as the most important groups. Nearly all recaptures were made in 1976 and 1977. A great number of the fish migrated out of the lakes. Brook trout were also caught in 1978 and 1979 in those places where recaptures were registered further down the water system. The migration is probably mainly related to the development of sexual maturity after one or two years. The mean yield in the lakes was 0.9 kg/ha. This is, however, only a minimum number because some recaptures have not been registered.

4.3 Experimental stocking in sea water The increased availability of stocking material has also led to stocking in other biotopes. Per Aass (pers. comm.) has experimented with stocking of marked fish (1+ and 2+) in sea water of the inner Oslo fjord. Typical for these stockings is that brook trout migrate rather rapidly up nearby rivers and creeks where recaptures are made. It is remarkable that some of these rivers are very polluted and have no self-reproducing fish populations. This brook trout originates from the Oyfjell strain mentioned earlier in this article and is an inland, river-dwelling form.

4.4 Stocking for fishery purposes in acid waters The results from these stockings have led to increased stocking activity of brook trout in acidified areas of Norway. Introduction of exotic species to a water system is forbidden in Norway, but allowances have been made in many cases. Few of these stocking were followed up by investigations, but the general impression from scattered information is as follows. Typical problems are the short life and mass migrations out of the lakes and down the water system. On the other hand, positive aspects are better survival than for the earlier stockings of brown trout and thus a higher recapture rate; also the fish caught are of good quality. There are reports of good results from lakes in which pH at least periodically goes down to 4.3–4.5 4.5 Stocking for sport fishery purposes in urban areas In the last ten years brook trout has been stocked in oligotrophic lakes (pH 5.5– 6.5) in the surroundings of Oslo together with brown trout and rainbow trout. The stockings have resulted in high numbers of recaptures but many of the fish are caught before they reach an acceptable size. This is also the case with the rainbow trout. These species are more easily caught than the brown trout in the first year of life. To get an optimal catch fishing for brook trout ought to be highest in the second summer after stocking in such lakes as well as in acid lakes (Qvenild and Holt, 1981). 5. TOLERANCE OF BROOK TROUT TO ACID WATERS

5.1 Tests in acid stream water The results obtained in the stocking experiments indicated differences in tolerance to acid water between the salmonids involved. Therefore some experiments were made to study the tolerance of several salmonids to waters with low pH (Grande et al., 1978). One experiment was carried out at a field station in southern Norway. Under- yearlings (0+) of Atlantic salmon (Salmo salar L.), rainbow trout and brook trout were placed in tanks with throughflow of untreated and limed water (temperature 1–4°C) from a nearby acid brook. Twenty-five fish of each species were used. The survival of the fish was then inspected daily during a six-week period. Results of this experiment are shown in Fig. 7. Table 2 gives the concentrations of some chemical components in the brook water. The variation in pH of untreated and limed water is illustrated in Fig. 7. This experiment indicates that the tolerance of fish to the stream water can be ranked in the order rainbow trout, Atlantic salmon, brown trout and brook trout, with the latter as the most tolerant. Treatment of water with lime to pH 5.4–6.3 exerted a significantly positive effect on the survival time, but there was still complete mortality among rainbow trout and Atlantic salmon. Recent findings suggest that this may be due to incompletely precipitated aluminium on the gills of the fish (Muniz et al., 1980).

5.2 Physiological tests Rosseland (1980) studied the effects of acid water on metabolism and gill ventilation in brown trout and brook trout and found differences in metabolic response in the two species. These differences indicated that the brook trout was more tolerant than brown trout to acid water. Several of his experiments were made at the same field station mentioned above.

5.3 Information from the literature Practical experience and experiments in Norway (Jensen and Snekvik, 1972) have shown that the Atlantic salmon is more sensitive to acid water than brown trout. On the basis of Swedish experience Berzins (1960) maintains that the thresholds for mortality are pH 5.5, 5.0 and 4.8 for rainbow trout, brown trout and brook trout, respectively. Trojnar (1977) concludes from studying the results of Daye and Garside (1975) and Johnson (1975) on brook trout and Lloyd and Jordan (1964) and Kwain (1975) on rainbow trout that brook trout are clearly more tolerant than rainbow trout. Investigations of the distribution of fish in hydrogen-ion gradients in a creek indicated that brook trout was more tolerant than brown trout (Dunson and Martin, 1973). Power (1980) has given a thorough discussion of some abiotic factors controlling the distribution of brook trout, among them the hydrogen-ion concentration. From this it can also be concluded that brook trout are more tolerant to acid water than the rainbow trout. However, more research on the relative tolerance of different species and strains of salmonids to acid waters is evidently needed. 6. SOME CONCLUSIONS AND TENTATIVE PRACTICAL GUIDELINES FOR THE MANAGEMENT OF BROOK TROUT IN ACID LAKES Some practical conclusions from the results obtained hitherto follow. These conclusions and tentative guidelines for management apply to Norwegian conditions and may not, of course, be valid under other circumstances (Grande et al., 1980).

6.1 Naturalized populations Evidently the brook trout has difficulty in reproducing and establishing itself in larger water bodies in Norway. If brown trout are able to thrive, it will usually displace brook trout. Similar results have been obtained from other countries in Europe and from U.S.A. where the brown trout has been introduced from Europe. Self-reproducing populations of brook trout will probably only be established in small rivers with a good water quality. In some of these localities it may be able to displace the brown trout. It is not known how it will be able to compete in elevated mountain lakes.

6.2 Life cycle, food, age and growth Brook trout in Norway usually spawn in running water in autumn, at the same time as brown trout. The eggs are hatched in spring. The fry and large fish seem to have approximately the same food habits as brown trout with preference for insects, crustaceans and mussels. The fish are usually short-lived (3–5 years) but can grow older under special circumstances. They are sexually mature after one-three years, the males usually one year before the females. Brook trout may reach a weight of up to 1 000 g or even larger under favourable conditions. In Norway good growth after stocking in acid lakes has been 300–600–900 g after one, two and three years, respectively.

6.3 Water quality Experience in Norway has shown that stocking of under-yearlings (0+) of brook trout and older fish may give good results in acid lakes. In some cases the fish may live well in lakes with pH values of about 4.6–4.7 and even lower (down to 4.3–4.5) if the other physical/chemical conditions are good (e.g., rich in humic matter, no oxygen deficiency in late winter). Good results imply that the fish survive and have good growth and that stocking gives good recaptures by the common fishing procedures. Reproduction will usually not occur in localities with pH below about 5.0. 6.4 Stocking strategy Growth depends, among other things, on the number of fish stocked in relation to the productivity of the water. If the lake has been fishless for some years, it may have accumulated a considerable amount of food and growth may be good. During continuous stocking growth may be poorer due to reduced quantity of available food items. This ought to be taken into consideration, and the number of fish regulated accordingly. About 20–50 fish (0+) per hectare has proved to give good results in many occasions by the first stocking. Experience indicates that it might be preferable to stock brook trout in a greater number every three or four years instead of each year - the usual practice for the brown trout. In this way it is easier to control the population by fishing the right year classes and to protect the newly stocked fish from being eaten by larger fish. It should then also be possible to re-establish the population of food organisms if the lake is more or less empty of fish for one summer. It is possibly important that there is a good supply of food in the stocking period so that the fish will have a better chance to withstand stress due to changes in water quality (low pH-aluminium). If possible, it might be preferable to stock the fish in the summer, when the water temperature is higher, and when the water quality often is best and the population of small food animals (e.g., crustaceans) is high. Since the brook trout was first introduced into Norway in 1876 the fish has not created any problems in Norwegian waters. This seems to be the case also in other countries in Europe. There does not seem to be any risk in stocking brook trout in forest areas in southern Norway. Here, it should primarily be stocked in fishless or nearly fishless lakes where the brown trout does not thrive due to high acidity. It might be best suited to relatively high-elevation lakes (300–700 m above sea level). In high mountain lakes and the northern part of Norway uncontrolled stockings should await further experiments. This is because the fish may spawn directly in the lake which becomes overpopulated as has occurred in some parts of U.S.A. (Moyle, 1976). 6.5 Migrations The brook trout has a great tendency to migrate out of the lake one to three years after stocking. It is also a short-lived species. The number of fish stocked therefore must not exceed that at which the fish reach a catchable size (at least 100–200 g) after one year. Closing of the outlet of the lake by a net or similar arrangement may reduce migration. In Norway, however, this requires a special licence. Migration is usually downstream. If the circumstances are suitable, considerable recaptures might be possible in those areas. This may be taken into consideration in stocking programmes in which the whole water system may be looked upon as a unit.

6.6 Fishing for brook trout The brook trout is less shy than the brown trout and is easier to catch in nets as well as on lines. This means that the fish can be caught with less effort, but might also be wrongly harvested. If there is intense and uncontrolled fishery (near urban areas, e.g., Oslo) there will often be overfishing of small and unvaluable fish the first year and the efficiency of stocking is rather doubtful. It is advisable to catch most of the brook trout in the second year after stocking due to high natural mortality and migration.

6.7 Further investigations The brook trout has proved to have many valuable properties relative to the brown trout and warrants more attention in the future. More research is needed, and it is important to obtain more knowledge about its ecology and the practical advantages and disadvantages in relation to other salmonids in Norwegian water types. Management of the brook trout in acid lakes should be studied more intensively. It is also of interest to carry out more experiments with hybrids of brook trout and other species such as the Arctic charr and lake trout (Salvelinus namaycush). 7. REFERENCES Berzins, B., Kalkning av sjoar. Sodra Sver Fiskeriforen., (1959–60): 28–35 1960 Collett. R., Meddelelser om Norges fiske i Aarene 1884–1901. Christiana, Bd. 3, 173 p. 1905 (in Norwegian) Daye, P.G. and E.T. Garside, Lethal levels of pH for brook trout, Salvelinus fontinalis 1975 (Mitchill). Can.J.Zool., Dunson, W.A. and R. Martin, Survival of brook trout in a bog-derived acidity gradient. 1973 Ecology, 54:1370–6 Ernst, N.E. and J. Nielsen Sjaeldne og truede ferskvandsfisk i Danmark (Rare and 1981 threatened freshwater fish in Denmark). Danmarks Fiskeri- og Havundersogelser. Medd.Ferskvandsfiskerilab., (1):70 p. Grande, M., En undersokelse av bekkeroya i Oyfjell i Telemark (A study of the brook 1964 trout, Salvelinus fontinalis Mitchill in Telemark. Fauna, 17:17–33 (in Norwegian with English summary) , Age determination from scale and obliths in the brook trout (Salvelinus 1964 fontinalis Mitchill). Nytt.Mag.Zool., 12:35–7 Grande, M., J.P. Muniz and S. Andersen, Relative tolerance of some salmonids to acid 1978 waters. Verh.Int.Ver.Theor.Angew.Limnol., 20:2076–84 Grande, M., S. Andersen and I. Sevaldrud, Forsok med utsetting av av bekkeroye 1980 (Salvelinus fontinalis Mitchill) i sure innsjoer 1975–78. (Brook charr stocking in acid lakes 1975–78). Oslo, SNSF-project, Norway, IR 66–80:88 p. (in Norwegian) Henriksen, A. and R.F. Wright, Effects of acid precipitation on a small lake in southern 1977 Norway. Nord.Hydrol., 8:1–10 Huitfeldt, E., Bekkeroye i norske vannlop. Nor.Jeger- og Fiskerforbunds Tidsskr., 1947 (76):210–2 Huitfeldt-Kaas, H., Vellykket indforelse av bekkeroye. Nor.Jaeger- og Fisker- 1924 Foren.Tidsskr., 54:401–3 Jensen, K.W. and E. Snekvik, Low pH levels wipe out salmon and trout populations in 1972 southernmost Norway. Ambio, 1:223–5 Johnson, D.W., Spawning behaviour and strain tolerance of brook trout (Salvelinus 1975 fontinalis Mitchill) in acidified water. M.S. Thesis Cornell University. 100 p. Kjellberg, G., Nagra data om backradingen ISL. Drottningholm, Sverige, No. 4:12 p. 1969 Landmark, A., Indforelse av Amerikansk orret (Salmo fontinalis) Fisk.Insp.Indberet.Oslo, 1894 (1891–94):76–7 in Norwegian) , Amerikansk baekkeroye. Fisk.Insp.Indberet.Oslo, (1909–10):46–7 (in 1910 Norwegian) Lloyd, R. and D.H.M. Jordan, Some factors affecting the resistance of rainbow trout 1964 (Salmo giardnerii Richardson) to acid waters. Int.J.Air.Water Pollution, 8:393–403 MacCrimmon, H.R. and J.S. Campbell World distribution of brook trout, Salvelinus 1969 fontinalis. J.Fish.Res.Board Can., 26(7):1699–725 MacCrimmon, H.R., B.L. Gots and J.S. Campbell, World distribution of brook trout, 1971 Salvelinus fontinalis: further observations. J.Fish.Res.Board Can., 28(4):452–6 Moyle, P.B., Inland fishes of California. Berkeley, University of California Press. 405 p. 1976 Muniz, I.P. and H. Leivestad, Toxic effects of aluminium on the brown trout. In Ecological 1980 impact of acid precipitation. Proceedings of an International Conference, Sandefjord, Norway, March 11–14, 1980, edited by D. Drablos and A. Tollan. Oslo, Oslo-As, SNSF project, pp. 320–1 Power, G., The brook charr. In Charrs: Salmonids fishes of the Genus Salvelinus, edited 1980 by E.K. Balon. The Hague, Dr. W. Junk bv. Publishers, pp. 141–203 Qvenild, T. and T. Holt, Fiskeproduksjonen i Sondre Trollvann i Ostmarka, Oslo, 1976– 1981 77. Oslo, Rapporta fra Fiskerikonsultenten i Ost Norge, 18 p. (in Norwegian) Rosseland, B.O., Physiological responses to acid water in fish. 2. Effects of acid water 1980 on metabolism and gill ventilation in brown trout, Salmo trutta L. and brook trout, Salvelinus fontinalis Mitchill In Ecological impact of acid precipitation. Proceedings of an International Conference, Sandefjord, Norway, March 11–14, 1980, edited by D. Drablos and A. Tollan. Oslo, Oslo-As, SNSF project. pp. 348–9 Sevaldrud, I.H., I.P. Muniz and S. Kalvenes, Loss of fish populations in southern 1980 Norway, dynamics and magnitude of the problem. In Ecological impact of acid precipitation. Proceedings of an International Conference, Sandefjord, Norway, March 11–14 1980, edited by D. Drablos and A. Tollan. Oslo, Oslo-As, SNSF project, pp. 350–1 Trojnar, J.R., Egg hatchability and tolerance of brook trout (Salvelinus fontinalis) fry 1977 under low pH. J.Fish.Res.Board Can., 34(5):574–9 Table 1 Chemical data, Langtjern. Samples collected at 1 m depth over the period 23 January 1974 to 10 Octobrer 1976. (From Henriksen and Wright, 1977) Wright, 1977) No. of Components Mean Standard deviation observations Ph 90 4.95 0.23 Conductivity uS/cm 20°C 90 17.70 3.10 Colour mg Pt/l 86 100.00 30.00 Ca mg/l 88 1.38 0.27 Mg mg/l 89 0.76 0.04 Na mg/l 74 0.70 0.18 Cl mg/l 86 0.68 0.16 Al ug/l 73 218.00 46.00

Table 2 Physical and chemical data from Ramse Brook, Tovdal. Samples collected in experimental period 19 December 1973 to 31 January 1974 (Grande et al., 1978)

Components Mean Range pH 4.57 4.40–4.60 Conductivity uS/cm 20°C 23.10 11.50–37.70 Colour mg Pt/l 9.70 0 –19.50 Ca mg/l 0.85 0.73–0.97 Mg mg/l 0.36 0.31–0.41 K mg/l 0.08 0.04–0.12 Cl mg/l 2.00 1.60–2.60

SO4mg/l 6.90 5.00–8.90 Temp. °C 2.50 1.00–4.00

Fig. 1 Distribution of brook charr in the Øyfjell area (Grande, 1964). The brook charr occupies the upper parts of the water system, while it is found together with brown trout in a medium zone. In the lower parts the brown trout dominates completely, and brook charr is only found sporadically.

Fig. 2 Typical length distribution of naturalized brook charr sampled by electro-fishing in two small rivers in late autumn. The low number of 0+ year-class is due to selectivity of the fishing method.

Fig. 3 Recaptures from stocking of brook charr, brown trout and rainbow trout (0+) in an acid lake, Langtjern. pH in outlet (Grande et al., 1978)

Fig. 4 Recaptures of brown trout and “Temiscamie” and “Øyfjell” strain of brook charr from Langtjern when brown trout was stocked alone (1975) and the two brook charr strains together (1977). Total recaptures brown trout: 30.3%, Temiscamie strain 32.6%, Øyfjell strain 27.6%. (The Temiscamie strain comes from the southeast James Bay area, Quebec and was provided in 1977 by Dwight A. Webster and Carl Schofield, Adirondach Fishery Research Program of Cornell University, U.S.A.)

Fig. 5 Growth of brook charr stocked in some acid lakes in southern Norway 1975 (Grande et al., 1980)

Fig. 6 Recaptures in outlet-trap of Langtjern. Migration of brook charr usually occurs the second or third autumn after stockings of 0+ (Grande et al., 1978)

Fig. 7 Mortality of salmonids (0+) in tanks supplied with acid stream water (Grande et al., 1978) RESULTS OF LAKE TROUT STOCKINGS IN FINLAND 1957–81

A. Mutenia Finnish Game and Fisheries Research Institute Ivalo, Finland

O. Simola Finnish Game and Fisheries Research Institute Taivalkoski, Finland

O. Tuunainen The Federation of Finnish Recreational Fishermen's Associations Helsinki, Finland

CONTENTS

1. INTRODUCTION 2. CHARACTERISTICS OF LAKE TROUT 3. CULTIVATION OF LAKE TROUT 4. TAGGED STOCKINGS CARRIED OUT IN 1957–68 4.1 General 4.2 Good lake results 4.3 Inconclusive or poor lake results 4.4 Results of sea stocking 5. SAMPLE LAKE: LAKE INARI 5.1 General 5.2 Effect of water regulation on the char population in Lake Inari 5.3 Management of char population 5.4 Exploitation of lake trout 6. REFERENCES

ABSTRACT In 1955, lake trout eggs from Lake Superior were imported to Finland from the U.S.A. In addition to fish for stockings, the eggs were used to cultivate brood fish at the two fish farms of the Fisheries foundation. Stockings in lakes began in 1958 with two- year old juveniles. The Fisheries Foundation stocked circa 100 000 marked (Carlin tags) 2–3 year old, 17–22 cm long lake trout in eight different lakes as well as in the sea in various parts of Finland, especially in the lakes in Lapland. In the seventies, the Finnish Game and Fisheries Research Institute continued introductory stockings of lake trout. Lake trout prefer cold water. For the most part they were stocked in large, deep clear lakes which have a large volume of hypolimnion in the summer. The results of the trials indicate that lake trout should be stocked at a length of 20–25 cm in the spring or autumn when the water is cold. The best results were obtained in Lake Pallasjarvi in Lapland. The catch there was 237 kg/1 000 individuals stocked on 14 October 1966. Good results were also obtained in Lake Inari and Lake Iijarvi. The only good results in small but deep and clear lakes (2 km2) were obtained in southern Finland. Brown water lakes in general gave poor results. Stockings in the sea and in brackish waters also gave poor results. Stockings in Lake Inari (1 000 km2) were carried out over a period of ten years, with an average of 100 000 individuals/year of 2–3 year old juveniles. The present annual catch of lake trout is circa 10 000 kg. The fish weigh 1.0–1.5 kg, with the largest individuals weighing 6–7 kg. Most of the catch. is taken in nets. Reproduction of lake trout has taken place in Lake Pallasjarvi and apparently in Lake Inari as well. Trout generally feed on vendace and whitefish. Cultivation of lake trout brood fish and juveniles in the fish farms of northern Finland is easier than, for example, that of salmon or sea trout. RESUME En 1955, la Finlande a importé des Etats-Unis (Lac supérieur) des oeufs de truites de lac, non seulement à des fins de repeuplement, mais aussi pour élever des géniteurs dans les deux établissements piscicoles de la Fondation des pêches. Le repeuplement des lacs a commencé en 1957 avec des juvéniles de deux ans. La Fondation a déversé environ 100 000 truites marquées (marques Carlin) de 2–3 ans et de 17–22 cm dans huit lacs différents ainsi que dans la mer dans plusieurs parties de la Finlands, notamment en Laponie. Au cours des années soixante-dix, l'Institut finlandais de recherche sur les pêches et le gibier a poursuivi cet effort de repeuplement. Ces truites préfèrent l'eau froide. La plupart d'entre elles ont été déversées dans de grands lacs, profonds et clairs, où l'hypolimnion est important en été. Les résultats des essais montrent que le repeuplement devrait avoir lieu au printemps ou en automne, lorsque l'eau est froide, avec des truites de 20–25 cm. Les meilleurs résultats ont été obtenus dans la lac de Pallasjarvi (Laponie) avec des captures de 237 kg/1 000 individus mis à l'eau le 14 octobre 1966. Les résultats ont également été satisfaisants dans les lac d'Inari et d'Iijarvi. En ce qui concerne les lacs de petites dimensions (2 km2) mais profonds et clàirs, on n'a obtenus de bons résultats que dans le sud de la Finlande. Les résultats ont en général été médiocres dans les lacs aux eaux brunes, dans la mer et dans les eaux saumâtres. Le repeuplement du lac Inari (1 000 km2) a duré 10 ans à raison de 100 000 juvéniles de 2–3 ans chaque année en moyenne. Les captures de truites de lac sont actuellement de l'ordre de 10 000 kg par an. Ces truites pèsent de 1 à 1,5 kg (maximum: 6–7 kg). Elles sont le plus souvent capturées au filet. La truite s'est reproduite dans le lac de Pallasjarvi et, semble-t-il, aussi dans le lac d'Inari. Elle se nourrit en général de vendace et de poisson blanc. Des géniteurs et des juvéniles sont élevés dans les établissements piscicoles du nord de la Finlande; leur élevage est plus facile que celui du saumon ou de la truite de mer par exemple. 1. INTRODUCTION Lake trout (Salvelinus namaycush) spawn originating from a population in Lake Superior were imported into Finland from the U.S.A. in 1955. The spawn was used to rear broodfish as well as juveniles for stocking in two hatcheries belonging to the Finnish Fisheries Foundation. The foundation stocked nearly 100 000 individual, tagged (Carlin- tags), two-three year old, 17–22 cm long, lake trout in eight waters and in the sea in different parts of Finland, particularly in lakes in Lapland (see Nilsson and Dahlstrom, 1968). During the seventies, the Finnish Game and Fisheries Research Institute continued a programme of stockings aimed at the domestication of lake trout. Lake trout is known to be a cold-water species and, therefore, the rearing of juveniles was begun in those two fish culture and research stations where spring water was used during the hottest part of the summer. The rearing of broodfish commenced in the Porla Fish Culture and Research Station in Lohja, and until the mid-seventies, broodfish were also reared in the Hatsina Central Fish Culture and Research Station in Hollola in southern Finland. The majority of juveniles and broodfish are now reared in the State Central Fish Culture and Research Stations in northern Finland. The rearing of broodfish and juveniles was successful in Porla and Hatsina, and the first stockings of two-summer old juveniles were carried out in the autumn of 1957. The objective of the lake trout stocking trials was to find out if this lake-spawning species could compensate for the destruction of predatory salminoid, river-spawning species, caused by construction in the rivers and by pollution. That same autumn, stockings of lake trout were also begun in the brackish waters of the Perameri, the northernmost end of the Gulf of Bothnia. These stockings, those made in Lake Pallasjarvi in the autumn of 1957 of untagged, two-summer old juveniles, and the stocking of two-year old juvenile lake trout in Lake Maarjarvi in Kisko gave optimistic returns. It was, therefore, decided that a programme of widespread stocking trials should be carried out. An investigation of the results from the tagging of lake trout was carried out in support of this programme, to discover those characteristics which are most important for the survival of lake trout in Finland. In the evaluation of the tagging results for lake trout which are presented here, it has particularly been noted that often the relatively long distances over which fish have been transported have caused losses. A good example of this is at Lake Pallasjarvi, which has given one of the best catches of tagged lake trout but which has also been the site of one complete failure. 2. CHARACTERISTICS OF LAKE TROUT The most promising characteristics of lake trout were the fact that it spawns in lakes, its long life and its relatively large final size. In addition, lake trout begins consuming at a noticeably smaller size than does domestic Finnish char and trout. This was felt to be significant in the management of the fish stock in regulated lakes because the regulation of the water level decreases benthic production and thus also the survival of migratory lake trout smolts beginning their river migration on the outward flow of water. In Finland, there have been reports of lake trout beginning to consume fish as food when they have attained a size of only circa 17 cm. The fact that lake trout is a coldwater fish, particularly as an immature juvenile, was from the very beginning one of the limiting factors in the usefulness of lake trout in Finland. Positive results in Finland have been obtained in those exceptional, deep, clear lakes which, because of their large volume, have a relatively large, cold hypolimnion. Another limiting factor was that lake trout require very clear, almost transparent, water. In those deep lakes in which the cold hypolimnion was dark, i.e., in which light did not penetrate to the depth at which the lake trout lived, the results have generally been poor. The taste of the flesh of lake trout is considered comparable to that of migratory trout but the flesh of individuals weighing under 0.5 kg is considered “watery”. This characteristic of small individuals would, on its own, be sufficient to keep people from fishing for undersized lake trout but because whitefish nets are generally used for fishing purposes in good lake trout waters, a very large percentage of the lake trout stock is undersized when caught. This is also partly caused by the slow initial growth of the species and its habit of moving extensively in the lakes. 3. CULTIVATION OF LAKE TROUT The cultivation of lake trout in spring water is very successful. Growth depends on cultivation density and the volume of water. Lake trout juveniles utilize the volume of water in growing ponds better than brown trout juveniles. The size of lake trout juveniles has been as follows:

Age Length 1 year 6–9 cm 2 years 14–18 cm 3 years 18–24 cm The juveniles have been reared using dry feed developed for salmon. Cultivation in natural food ponds during the first summer generally failed due to the high temperature of the water in the ponds. The production costs, excluding investments for construction, were the following:

Age Cost per individual

1 summer 0.80 FIM 1 year 1.00 FIM 2 years 2.00 FIM 3 years 3.00 FIM Transportation and stocking costs were of the order of 10–20 percent of the production costs for juveniles, which costs must be added to the costs of production, as must the construction costs, which almost doubles the price above. 4. TAGGED STOCKINGS CARRIED OUT IN 1957–68 4.1 General The slow growth of lake trout also arises from the long recovery time from stresses caused by tagging (Carlin tags) and a fish is often caught by the tag in a net while still undersized when it could have escaped through the mesh without the tag. Data on unmarked lake trout being caught has come from many lakes, which leads to the conclusion that tagging appears to decrease the size of the catch to a large extent and evidently also slows the rate of growth. Lake trout juveniles which have been reared to tagging size may be stocked either in the spring or autumn, even late autumn, as the best tag-return result shows. This was obtained from a stocking made in Lake Pallasjarvi on 14 October 1966, where the catch was 43.3 (70.8) percent of stocked individuals, i.e., 237 (300 kg/1 000 stocked individuals). On the basis of the catch obtained, the stocking was judged economically very profitable. The relatively promising catch results and the possibility that once domesticated, lake trout populations might become self-reproducing, argued for the continuation of widespread stocking trials, with the stocking sites limited to some extent, but in other respects following a well planned and sufficiently detailed programme. 4.2 Good lake results The best results for stocked lake trout were obtained from Lake Iijarvi and Lake Inari, both in the Inari system, by the Fisheries Division of the Finnish Game and Fisheries Research Institute; and by the Finnish Fisheries Foundation in Lake Pallasjarvi in Muonio. Good results were otherwise only obtained in deep, clear lakes: Keski-Pirinjarvi, Suvasvesi, Vuohijarvi and Iso Simijarvi. Lake also Simijarvi (1.9 km2) is a clear and very deep small lake. It and Lake Keski-Pirinjarvi in the Iijoki River system are the only small lakes in which lake trout have given positive results. For a number of years, lake trout has been the main species of fish stocked in lake Iso Simijarvi. 4.3 Inconclusive or poor lake results There was a rather important group of experimental waters, with relatively clear water, but which did not stratify thermally every summer. Results of stocking in this type of lake were so bad that they could be considered more advantageous than stocking of another species only in those cases where the lake trout formed a naturally reproducing population. In some regulated lakes (Hoytiainen, Juojarvi, Saarijarvi and Pyhajarvi) results have been inconclusive or poor, which indicates that lake trout cannot be used for compensation stocking in all clear, large, regulated lakes. The result in Lake Paijanne and in Lake Kallavesi was below 30 kg/1 000 stocked individuals; and in the deep Lakes Lammin Paajarvi and Toisvesi, the result was even worse. The result of 38 kg/1 000 stocked individuals in Lake Maarajarvi in Kiska, the null result obtained in Lake Vesijarvi in Lahti, and the above illustrations of the poor or inconclusive results obtained in relatively shallow, clear waters in regulated lakes, all strengthen the conception that lake trout can be adopted only in relatively rare cases for management of lakes in Finland, unless the species slowly adapts to Finnish conditions. We have not yet studied the reasons for the differences in the results of stockings of lake trout stocked in similar lakes. Incidental data on the flexibility of feeding on a wide variety of fish (inter alia, vendace, stickleback, stunted whitefish and burbot juveniles) in the species indicates that the quality of the fish available as food is probably not a limiting factor to the success of lake trout. The extent to which differences in the depth at which lake trout and the prey species are found affect the results of stocking, has not been studied in Finland. The null result obtained in the reservoirs of power plants located at the Petajakoski Falls in the Kemijoki River and at the Pahkakoski Falls in the Iijoki River, indicates that lake trout is not suitable in the management of reservoirs. 4.4 Results of sea stocking The objective of stocking lake trout in the brackish waters of the Baltic Sea was to investigate whether lake trout would be suitable as a local marine fish to compensate for declining salmon and migratory sea trout stocks. In particular, the aim was to study whether this lake-spawning species would be able to reproduce in coastal waters and in the brackish water of river mouths. The first sea stockings of untagged lake trout were made in 1957, and the first stockings of tagged fish in 1965. The lake trout stockings carried out in the bays and inlets and off the coast of the Gulf of Finland have given such poor results that the species should be considered unsuitable for Finnish marine conditions. Stockings made near Merenkurkku and in Perameri have resulted in returns of some tagged fish, on the basis of which, the catch is estimated to be so poor that the stocking of lake trout in the Gulf of Finland appears to be uneconomical; much better results can be obtained by stocking, inter alia, migratory sea trout. The tagging trials carried out in the northern part of Perameri have produced very little yield in the stocking area. At least part of the lake trout stocked have migrated southward to the deeper waters of the central and southern parts of the Gulf of Finland but even here growth of lake trout, with few exceptions, has been slow (Sormunen, 1968). 5. SAMPLE LAKE: LAKE INARI 5.1 General Lake Inari covers an area of 1 153 km2, is situated on the borderline between the sub-Arctic and the temperate zones (28°E; 68°N) at an altitude of 118 m above sea level, and is one of the northernmost great lakes in Finland and indeed in the world. It is covered by ice during the winter and open water in summer. Lake Inari has remained unpolluted because it lies far from industrialized areas and its drainage basin is very sparsely populated. However, the natural state of the water system has been changed by water-level regulation for power production. Lake Inari is the central lake in the Paatsjoki River system and is a typical oligotrophic, barren lake. 5.2 Effect of water regulation on the char population in Lake Inari The regulation of Lake Inari began in its present form in 1948 with the construction of the Paatsjoki River Hydropower Plant. The water level in Lake Inari is raised above its natural level and during the winter the rate of fall of the water surface is considerably greater than the natural rate. The maximum allowed amplitude of regulation is 2.36 m. The regulation has caused serious erosion, which has led to a decrease in littoral vegetation and bottom life, which has resulted in deterioration of habitats suitable for reproduction and feeding for the fish species in the lake. Fishing has declined in the years since the lake has been regulated, because the catches of many of the species in the lake have decreased sharply from the level of catches obtained before regulation. The development of catches of Arctic char (Salvelinus alpinus (L.)) in Lake Inari, is shown in Table 3. The char catch in 1977 was only 12 percent that of earlier char catches taken from the lake in its natural state. Factors in the decline in the size of the char population could not be retroactively determined in detail. 5.3 Management of char population In order to compensate for the drastic decline in char catches, a new species of char, lake trout (Salvelinus namaycush (Walbaum)), has been stocked in Lake Inari since 1972. The legal obligation to restock is for an annual stocking of 250 000 one- summer old, or a monetary equivalent number of three-year old, Arctic char or lake trout juveniles. This obligation has been fulfilled in its entirety every year since 1976 (Toivonen et al., 1981). For these stockings, lake trout juveniles have been transported to Lake Inari in 2.4 m3 tank trucks. At the lake, the juveniles are carried in transportation containers (2.0 m3) and stocked in the currents in backwaters or near the deep water of a strait. The fish were transported to North Inari (Vasikkaselka) and Central Inari (Kasarinselka). The stockings in river mouths and in West Inari (Ukonselka) were for net fishing for trout and whitefish combined. In 1972–80 a total of some 500 000 two-three year old lake trout were stocked in Lake Inari. In 1972–79 another 500 000 one-summer or one-year old juveniles and 3.6 million lake trout fry were stocked. The stockings have been monitored annually by tagging some of each year's juveniles with Carlin-type fish tags in north, central and west Inari. The compilation of the results of the tagging trials is still in progress. During the time that stockings have been carried out, the factors that would have a decisive effect on the results have not been verified. The best results, however, have come from stockings made during the end of winter and the cool-water period at the beginning of summer and in the autumn. The stocking results improved with growth in the size of the fish stocked. More tagged than untagged fish were caught but the structure of the annual rate of tag-return depended on the type of fishing carried out in the stocking site, in this case net fishing. There were no limits on the mesh size of the nets. The rate of tag return with age is shown in Table 4. The average growth and unadjusted catch are presented in Table 5. From the typically good growth rate and the good catch per number of individuals stocked, it would appear that the new species has adjusted well. The results deteriorated as the number of stockings increased, as efforts to improve the profitability of fishing increased, and as a result of the lack of size limits on fishing. There were no minimum size requirements for lake trout. On the basis of the stocking statistics and the tag return data from various districts, the estimated development of the lake trout catch in north, central and west Inari has been calculated. The results of this calculation are shown in Table 6. Due to errors often associated with tag returns, the estimated catch based on tag returns is about half that of the actual catch recorded in the statistics. According to the tag return data, lake trout has not migrated out of Lake Inari. Lake trout have not gone upriver or down the Paatajoki River. Migration does take place between the different inlets and the larger straits in Lake Inari. The future of lake trout in Lake Inari depends on the continuing abundance of its food species and on the development of fishing. The catch can be improved by transporting fish to areas in north Inari which are difficult to fish. With the development of fishing techniques and means of transportation, the choice of stocking site no longer sufficiently protects lake trout. 5.4 Exploitation of lake trout Lake trout are primarily exploited by net fishing, which is concentrated in west and east Inari and in the larger straits of central Inari. In the same areas, stocking primarily with lake trout is carried out. The nets recorded as being used by professional fishermen and those fishing for home consumption were mainly of under 45-mm mesh. This means that lake trout are presently caught while still very small, as a secondary catch along with whitefish (Coregonus). The average size of lake trout caught in 1979 was 0.7 kg, with the largest individuals caught weighing about 5 kg. The results of lake trout stockings have also been studied by compiling the annual fishing statistics for Lake Inari (Table 7). Fishing techniques have not yet been developed for catching large lake trout, which would make it possible for lake trout to successfully spawn and reproduce naturally in Lake Inari before capture. The lake trout catch has improved yearly and presently accounts for about 10 percent of the total catch taken from the lake. The fish caught are primarily consumed by the fishermen themselves at home, since only about 25 percent of the catch is sold in fish markets. The value of the lake trout catch was 15 percent of the value of the total catch from Lake Inari in 1979. Net fishing specifically for lake trout is rare. Because stocking efforts have only recently begun, the number of young individuals in the population and in the catch is very large. With the passing of the years, we may expect that the size of the catch will increase; because lake trout grow slowly and live a long time. The profitability of lake trout stockings could be improved with better organized and regulated fishing. 6. REFERENCES Airaksinen, E. and P. Heinonen, Inarinjarven tutkimus vuonna 1974. Vesihallituksen 1976 Tiedotus, 103:1–99 Hydrobiologinen vuosikirja 1976–77. Vesientutkimuslaitoksen Julkaisuja, 35:1–193 1980 Nilsson, N.-A. and H. Dahlstrom, Harmaanieria (Lake trout). In Kalat, kalavesien hoito ja 1968 kalanviljely (Fish, management of fishing waters and fish culture), edited by G. Svardson et al. Helsinki. 302 p. Sormunen, T., Kalataloussaation suorittamat harmaanierian (Salvelinus namaycush 1968 Walbaum) viljelyja istutustutkimukset. Suom.Kalastuslehti, 75(8):216–25 Toivonen, J., Veden saannostelyn vaikutus Inarijarven kalastukseen. Helsinki, 4 p. 1979 (mimeo) Toivonen, J. et al., Verkkojen alimman silmakoon maarittaminen Inarijarven 1979 kalastuksessa. Riista-ja Kalatalouden Tutkimuslaltos Kalantutklmusosasto Tiedonantoja, 17:12–30 Table 1 Stockings of lake trout carried out by the Finnish Fisheries Foundation in 1957– 68 Stocking site Number of individuals

Perameri 17 991 Merenkurkku 3 985 Gulf of Finland 10 997 Tornionjoki River 3 000 Kemijoki River 11 259 Iijoki River 15 668 Kokemaki River 4 000 Lake Iso Simijarvi and Lake Maarjarvi (municipalities of Pohja and Kisko) 972 Karjaajoki River 50 Kymijoki River 15 110 Vuoksi waterway 15 815 98 847

Table 2 Lake Inari: Salient hydrological data (Airaksinen and Heinonen, 1976; Hydrological Yearbook, 1980) Surface area, km2 1 153 Amplitude of regulation, m 2.36 Length of shoreline, km 2 776 Maximum depth, m 96 Thickness of ice, avg.cm. ps 1961–75 78 Average date of complete ice cover, 1961–75 30 Oct. Average date of completely open water, 1961–75 3 June Oxygen saturation value, % 87–101 pH 6.8–7.0

Table 3 Average annual char catch in Lake Inari, taken by professional fishermen and for home consumption, in tonnes (t) for various periods (Toivonen, 1979) Period in years Catch in tonnes

1935–40 20.5 1950–55 15.3 1960–64 4.4 1966–70 3.3 1977 2.5 Table 4 The rate of tag-return (%) of three-year old tagged lake trout stocked in Lake Inari Growth period Total % 1 2 3 4 5 6 7 8 Central Inari 28/6/72 3.3 4.1 10.8 7.4 1.7 0.5 0.3 - 28.1 25/8/72 4.6 7.8 11.8 3.4 0.5 0.3 0.2 - 28.5 28/4/73 3.5 10.3 11.9 7.2 3.3 1.8 0.3 0.3 38.6 21/6/73 3.5 9.2 11.6 8.2 4.5 0.8 0.4 0.1 38.9 21/6/73 3.1 6.8 7.8 6.0 3.8 0.9 0.5 0.3 29.2

North Inari

25/7/72 - 2.3 7.1 1.3 0.5 0.3 - - 11.3 5/5/73 0.2 0.9 5.9 5.8 2.2 0.5 - - 15.5 2/7/76 1.2 1.0 2.8 3.4 4.6 - - - 13.1

Table 5 Average weight in grammes (g) and catch as kg/1 000 stocked individuals of three-year old tagged lake trout stocked in Lake Inari; based on tag-return data Average weight in grammes Catch kg/1 000

ind. Central Inari 28/6/72 115 259 520 844 1 345 - - - 155 25/7/72 225 424 861 1 085 1 866 4 250 - - 205 18/4/73 168 402 736 980 1 475 1 611 2 335 - 290 21/6/73 155 386 639 948 1 309 1 546 2 266 - 276 21/6/73 136 375 651 1 153 1 337 1 810 1 475 1 700 229

North Inari

25/7/72 - 605 960 1 290 2 225 - - - 109 5/5/73 200 354 769 1 228 1 690 2 142 - - 168 2/7/76 270 173 344 558 1 091 - - - 84 Table 6 The lake trout catch in Lake Inari; estimated on the basis of tag-returns and the actual catch as reported in fishing statistics

Catch/year (kg) Stocking area 1975 1976 1977 1978 1979 1980 Based on tag returns West Inari (Ukonselka) 1 950 1 400 750 723 676 686 Central Inari (Kasariselka) 7 990 9 385 7 885 4 363 2 947 2 362 North Inari (Vasikkaselka) 458 239 293 387 597 1 788 Total 10 398 11 024 8 928 5 473 4 220 4 836 Statistically-known catch 6 900 7 300 10 200 10 200

Table 7 Lake trout catch (t) of professional fishermen and those fishing for home consumption, and its share (%) of the total catch, in Lake Inari in 1977– 80 Lake trout Percentage of total Year catch (t) catch 1977 6.9 7.1 1978 7.3 7.9 1979 10.2 9.4 1980 10.2 9.6 RESULTS OF THE INTRODUCTION OF LAKE TROUT (LAKE CHARR, Salvelinus namaycush) INTO SWEDISH LAKES

A.P. Gonczi and N.-A. Nilsson The Fishery Superintendency Harnosand, Sweden

CONTENTS

1. INTRODUCTION 2. RESULTS 3. REFERENCES ABSTRACT Lake trout have been introduced into Swedish lakes since 1959 with very variable results. The percentage recapture ranges between 0 and 60 percent. Growth rates also vary considerably, depending on the availability of suitable prey, which mainly consists of stunted whitefish, cisco, smelt and Mysis relicta. No harm to the native fish populations has been observed. On the other hand many valuable introductions have been recorded. RESUME L'introduction depuis 1959 de truites (Salvelinus namaycush) dans les lacs suédois a donné des résultats très variables, le pourcentage de recapture étant compris entre 0 et 60 pour cent. Le taux de croissance de ces poissons est également très variable; il dépend avant tout de la présence de proies appropriées, lesquelles se composent essentiellement de formes naines de poisson blanc, de cisco, d'éperlan et de Mysis relicta. Ces introductions ne semblent pas avoir été préjudiciables aux populations de poissons indigènes; dans de nombreux cas, elles ont été bénéfiques. 1. INTRODUCTION Lake trout (Salvelinus namaycush) have been introduced into some 70 Swedish waters since 1959 (Nilsson and Svardson, 1968) and before that introductions had been made into Switzerland and Finland. The primary aim of these introductions was to enhance fisheries in lakes subjected to the adverse influences of hydro-electric development, but has since also been seen as an attempt to fill a vacant niche in large lakes with cold-water hypolimnia. Lake trout were chosen as a possible candidate for these purposes, because: (a) It is a cold-stenothermal species, adapted to the Arctic-temperate conditions prevailing in the waters to be stocked; (b) It is known as the most piscivorous of Salvelinus species, which would be desirable in reservoirs where dwarfed coregonids often dominate the fish communities; (c) It does not depend on streams or shallow waters for spawning; (d) It is an esteemed food and game fish in North America. 2. RESULTS The introductions have in most cases not been successful; recaptures of released fish vary from 0 to 60 percent. This variation is supposed to depend mainly on biotic variables, such as predation on young released fish and competition from native fish species. Four lakes have been chosen to illustrate in some detail the reactions of lake trout to different habitats and fish communities. Lake Storsjon: a large (456 km2) oligotrophic lake in the boreal region of the northern Swedish highland, with a complex fish community, was first stocked with S. namaycush in 1962, and since 1964 has been continuously stocked with eggs and fingerlings. 500 to 1 000 fish per year were tagged to secure data on recapture and growth rates. Captures of young wild fish have possibly been results of natural reproduction or of fertilized eggs introduced in boxes at presumed future spawning sites. The growth rate (Fig. 1) was initially very good, probably because of a super- abundance of food (ten-spined stickleback, stunted whitefish (mainly Coregonus wartmanni) and the introduced Mysis relicta). Later growth rate declined, possibly because of overgrazing of prey species (Fig. 2). After the introduction of smelt (Osmerus eperlanus), this species became the most important food of lake trout as well as of Arctic charr (Fig. 2), which probably explains the subsequent increase in growth rate (Fig. 1). Figs. 1 and 2 thus suggest three phases in interaction between the three exotic species and the indigenous fish community: (a) During the first phase lake trout mainly fed on suitable native species, whitefish (Coregonus wartmanni) and ten-spined stickleback (Pungitius pungitius), with a consequent high rate of growth. (b) After having grazed down the stickleback population, which until 1970 was very dense, the introduced Mysis relicta became an important food item for lake trout, as well as for Arctic charr. The growth rate of lake trout, however, decreased, possibly because of over-exploitation of the stunted Coregonus wartmanni. (c) After 1977 the introduced smelt (Osmerus eperlanus) became more and more important as food for both lake trout and Arctic charr. The growth rate of lake trout consequently increased. The recaptures of lake trout have been good, ranging between 11 and 67 percent with yields of between 60 and 460 kg per 1 000 released fingerlings. Lake Kallsjon: a large (155 km2) oligotrophic lake in the sub-Arctic region upstream of Lake Storsjon, with a simple fish community, was first stocked in 1964, and since then has been continuously stocked with fingerlings. The growth rate of tagged fish has been very poor as compared to Lake Storsjon (Fig. 1) probably because the one species of whitefish found in Lake Kallsjon has not been available in sizes suitable for food. The predominant food has been the introduced Mysis relicta even for relatively big specimens of lake trout. Natural reproduction has not been observed for certain, although some wild fish with deviant colour and behaviour have been caught (biggest fish recorded - 7kg). In Lake Kallsjon recaptures of tagged fish were calculated to range between 50 and 60 percent. Most of the recaptured fish, however, were very small in size and in poor condition. Lake Stora Tjultrasket: an oligotrophic lake in the sub-Arctic region of the north Swedish highland, with a very simple fish community, consisting of brown trout, Arctic charr and burbot) was stocked with lake trout in 1966 (6 000 one-year old fingerlings) and in 1972 (2 000 two-year old fingerlings). The growth rate characteristically has been poor for the first few years after introduction. However, since then has increased to around 2 kg after five years (biggest fish on record - 5 kg). The main food has appeared to be burbot and Arctic charr (up to 20 cm in length). Interestingly enough the growth rate of the native Arctic charr has also increased, very probably because of the predation of lake trout on young charr (Fig. 3). Natural reproduction has probably occurred, judging from the capture of young wild fish (Filipsson, pers.comm.). Lake Ivosjon: a eutrophic lake in south Sweden, with very complicated fish community, was stocked yearly since April 1972 with 1 000 fingerlings. The introduced fish have grown well (Fig. 4), feeding mainly on small smelt. Recaptures have been good, up to around 300 kg per 1 000 released fingerlings (Almer, 1978). It may be assumed that introductions of lake trout into Sweden have not been harmful to the native fish community. On the contrary, the species has added to the general value of the fishery in several lakes. 3. REFERENCES Almer, G., Fish in the offshore region of Lake Ivosjon. Inf.Inst.Freshwat.Res., 1978 Drottningholm, 1978:49 p. Nilsson, N.-A. and G. Svardson, Some results of the introduction of lake trout (Salvelinus 1968 alpinus Walbaum) into Swedish lakes. Rep.Inst.Freshwat.Res., Drottningholm, (48):5–16

Fig. 1 The growth of tagged lake trout in the Lakes Storsjön and Kallsjön

Fig. 2 The food of lake trout in Lake Storsjön

Fig. 3 The growth rate of Arctic charr in Lake Tjulträsket, before and after lake trout predation (from Filipsson, 1982)

Fig. 4 The growth of lake trout in Lake Ivösjön as compared to some other lakes (after Almer, 1978) INTRODUCTION OF THE NORTH AMERICAN CRAYFISH (Pacifastacus leniusculus (DANA)) INTO SWEDEN

M. Fürst Institute of Freshwater Research Drottningholm, Sweden

CONTENTS

1. INTRODUCTION 2. STOCKING, REPRODUCTION AND VALUE 3. THE CRAYFISH PLAGUE IN Pacifastacus AND Astacus 4. REFERENCES

ABSTRACT Crayfish are in great demand by the Swedish people but the crayfish plague Aphanomyces astaci has decreased the yield to 20 percent of its previous level. Nearly all attempts to restore old populations by using the native species Astacus astacus have failed. In many cases the reason for this is probably that the plague is kept alive in the lakes and streams by a few surviving crayfish. These crayfish infect each other and stocked crayfish continuously. At least some North American crayfish species are resistant to the plague, e.g., Pacifastacus leniusculus. This species was first introduced into Sweden in 1960. Up to 1982 it had spread to 260 lakes and rivers in this country. Most introductions have been very successful, and the catches have in several cases been reported to exceed the former catches of Astacus in the same lakes. The two species are very similar from an ecological and taste point of view. During moulting Pacifastacus is highly susceptible to the plague spread by an infected sympatric Astacus population. During experimental conditions in aquaria or in farms it was found that Pacifastacus had lost its resistance and died. It is also possible that other diseases, heavy metal ions or biocides cause reduction of resistance. No other harmful disease or parasite has as yet been recorded on Pacifastacus in Sweden. RESUME Les écrevisses sont très recherchées par les Suédois mais comme elles sont victimes d'une maladie Aphanomyces astaci les rendements ont baissé de 20 pour cent. Presque toutes les tentatives pour régénérer les anciennes populations avec l'espèce indigène Astacus astacus ont échoué. Dans de nombreux cas, cela tient probablement à ce que la maladie continue à être propagée par les quelques écrevisses survivantes dans les lacs et les cours d'eau. Les écrevisses s'infectent les unes les autres ainsi que les nouveaux sujets. Tout au moins quelques espèces d'écrevisses d'Amérique du Nord notamment Pacifastacus leniusculus résistent à la maladie. Cette espèce a été introduite pour la première fois en Suède en 1960. Jusqu'en 1982, elle a été propagée dans 260 lacs et cours d'eau du pays. La plupart de ces peuplements ont réussi et dans plusieurs cas on a indiqué que les captures de cette espèce dépassaient celles d'Astacus effectuées dans les mêmes lacs. Les deux espèces ont pratiquement les mêmes exisgences du point de vue écologique et la même saveur. Pendant la mue, Pacifastacus est fortement exposée à la maladie propagée par une population sympatrique contaminée d'Astacus. Au cours des expériences effectuées en ou dans les fermes d'élevage on a constaté que Pacifastacus avait perdu sa résistance et mourait. Il est également possible que d'autres maladies, la présence d'ions métalliques louds ou de biocides aient affaibli cette résistance. On n'a encore enregistré aucune maladie ou parasites qui nuisent à Pacifastacus en Suède. 1. INTRODUCTION The yearly catch of crayfish in natural waters in Sweden was estimated as being about 1 000 t before the crayfish plague started to spread in 1907. Today the yield is reduced to 100 t in spite of a much greater interest in the utilization of this natural resource. The plague is a fungus, Aphanomyces astaci Shikora, and was probably introduced to Europe from North America in 1860 when it first appeared in Italy. Crayfish were probably introduced or transported in the ballast of ships from the Mississippi River in Louisiana. This is a deduction arrived at from recent research on the biology of the crayfish plague (Unestam, 1969, Unestam and Weiss, 1970, Unestam, 1972) which has shown that several (probably all) of the North American crayfish species are resistant to Aphanomyces but are frequently carriers of it. Apart from the economic loss to man the ecosystem received a considerable blow because crayfish interacted directly in most trophic levels, e.g., on periphyton, macrophytes, zooplankton, bottom fauna and fish. If the nature of the bottom is good for crayfish the littoral zone has less periphyton and macrophytes and such lakes seem to be less influenced by eutrophication. The crayfish Orconectes limosus Rafinesque was introduced from North America to Germany in 1890 (Pieplow, 1938) and spread over part of northwestern Europe (Müller, 1954) without being affected by the plague. It was presumed that other American crayfish might also be resistant, since it was probably not just coincidental that this particular species was resistant. In 1960 Svärdson (1965) therefore introduced Pacifastacus leniusculus from California to Sweden for experimental purposes. He found that Pacifastacus might possibly fulfil the demands required of a new species and compensate for the losses of the old one. 2. STOCKING, REPRODUCTION AND VALUE In 1981, 21 years after the first introduction, Pacifastacus was stocked in 260 Swedish lakes and rivers. (Each river was stocked in several places but this was counted as only one stocking.) Permission to stock a new water is granted by the National Board of Fishery and, in principle, those parts of Sweden are excluded where no plague is registered. The aim is to try to preserve areas for Astacus astacus so that populations of this species will have a chance to survive in the future. Stocking is normally undertaken by using newly-hatched fry after the second moult. This method is found to involve a high mortality probably during the first summer and only 5–10 percent of the stocked material reach sexual maturity (Fürst, 1977). No other material has been available until recently when adult crayfish caught in lakes and rivers are also moved from one water to another. Reproduction has been registered in most of the stocked waters. As yet probably no population has reached the carrying capacity in the whole lake or river but locally the populations seem to be even higher than for the earlier Astacus populations. A local yield of 55 kg/ha has been registered and a maximum of 90 crayfish in one trap at one emptying. The taste is comparable to Astacus and the appearance is pleasing to the crayfish lover. The price is as high as for Astacus, about 80 Swedish crowns/kg to the fishermen and about 190 over the counter. 3. THE CRAYFISH PLAGUE IN Pacifastacus AND Astacus Pacifastacus is resistant to crayfish plague under natural conditions and is principally a carrier of the plague. Most of the brown-blackish spots between 1 and 5 mm in diameter which may be registered on all parts of the body contain hyphs of Aphanomyces astaci. The frequency and intensity of visible symptoms vary depending on age, length, sex and locality (Fürst and Boström, 1978). In populations with low fishing intensity both frequency and intensity are higher. Frequencies are found to be between 0 and 35 percent, intensities are from 0 to 4 spots per individual. “Normal” frequency for Pacifastacus caught by traps seems to be about 5 percent and intensity 1– 4. In one case there were higher numbers of larger spots and this was suspected to be due to abnormal concentrations of some heavy metals in the sediment. It has been shown in experiments that Pacifastacus is susceptible during moulting to infestation of zoospores of Aphanomyces arising from dead or dying Astacus (Appelberg and Fürst, 1978). In these cases Pacifastacus die. Astacus has such a low resistance to the plague that all infested animals die. Purely by chance a few escape infestation and survive. Sometimes they build up new populations which are found very locally in most of the old area of dispersal. In less than 5 percent of the cases, however, do populations reach their former densities and that only for a limited time. There are very few examples of such populations having survived the last four decades. In most cases Astacus survives only in scattered populations which no longer have any economic value. Aphanomyces is said not to produce any resistant spores (Unestam, 1969a) but it spreads zoospores from infested crayfish some days after they have succumbed. It seems that Aphanomyces is kept alive by continual infestation of only a few specimens of Astacus. This, in turn, prevents the development of permanent and productive Astacus populations. This theory does not exclude the possibility that Aphanomyces could also be reintroduced from other waters. In several cases the Astacus population finally disappeared when Aphanomyces- carrying Pacifastacus were introduced. In a few cases, however, the stocked fry obviously did not carry the Aphanomyces. No visible symptoms were found and the start of a parallel development of the two species took place. In ten cases more than ten specimens of Astacus were trapped together with Pacifastacus. In one lake the two populations developed as follows:

Year Fishing effort No. No. No. traps/night Pacifastacus Astacus

1972 150 7 5 1973 300 0 2 1974 410 9 63 1975 300 9 36 1976 286 0 0 1977 250 0 0 1978 200 0 0 1979–80 200 0 0

In 1976 crayfish plague was recorded in a nearby lake and since then no crayfish have been trapped. One specimen was observed in 1977 indicating that at least some Pacifastacus have survived. This example indicates that the presence of a population of Astacus may imply a risk that, for example, the plague carrying Pacifastacus infests Astacus which, in its turn, dies and spreads zoospores to such an extent that most of the moulting Pacifastacus die. If this happens during the peak of the moulting period the result is devastating. To avoid the risk of such a situation crayfish plague must be spread to the Astacus population before stocking with Pacifastacus. During experimental conditions in aquaria or in farms it has happened that Pacifastacus has lost its resistance to the plague and died. In aquaria especially the intensity of the plague increases and often legs or chilipeds are lost. Too much handling seems to stress crayfish so that they lose their resistance. It is also possible that other diseases or the influence of heavy metals or biocides causes reduction of the resistance. The above example is the only recorded case of an introduction of Pacifastacus failing because of a disease or a parasite. However, the development of most of the populations of Pacifastacus is being followed and great interest is being shown by the local people and any unusual situation has, therefore, a great likelihood of being recorded. There have been fears that some known or unknown disease or parasite native to Europe might affect Pacifastacus very hard because of a lower resistance than that of the native Astacus. The imported Pacifastacus were reported to carry a great many small or microscopic organisms. Xironogiton instabilis (Branchiobdellidae) was found in great quantities on the chilipeds. Neither this species nor others have been recorded on Swedish Pacifastacus during the past 21 years. This seems very hopeful and if we can learn how to handle Pacifastacus so that it does not lose its resistance against plague there seems to be a real possibility of restoring the old crayfish waters and building up farms producing Pacifastacus. 4. REFERENCES Appelberg, M. and M. Fürst, Försök att smitta signalkräfta under skalömsningen med 1978 hjälp av pestsmittad flodkräfta. In Frekvens av en skalsvamp (kräftpest) pa signalkräftor, edited by M. Fürst and U. Boström. English summary: Frequency of visible symptoms of crayfish plague in populations of Pacifastacus leniusculus Dana. Inf.Inst.Freshwat.Res., Drottningholm, (1):24 p. Fürst, M., Introduction of Pacifastacus leniusculus into Sweden: methods, results and 1977 management. In Freshwater crayfish, edited by O.V. Lindquist. Kuopio, Finland, University of Kuopio, pp. 229–47 Fürst, M. and U. Boström, Frekvens av en skalsvamp (kräftpest) pa signalkräftor. 1978 English summary: Frequency of visibile symptoms of crayfish plague in populations of Pacifastacus leniusculus Dana. Inf.Inst.Freshwat.Res., Drottningholm, (1):24 p. Müller, H., Die Flusskrebse. In Die neue Brehm-Bucherei, 121. Wittenberg, A. Ziemsen 1954 Verlag, 40 p. Pieplow, U., Fischereiwissenschaftliche Monographie von Cambarus affinis. Z.Fisch., 1938 36(3) Svärdson, G., The American crayfish Pacifastacus leniusculus (Dana) introduced into 1965 Sweden. Rep.Inst.Freshwat.Res., Drottningholm, (46):90–4 Unestam, T., Resistance to the crayfish plague in some American, Japanese and 1969 European crayfishes. Rep.Inst.Freshwat.Res., Drottningholm, (49):202–9 , On the adaptation of Aphanomyces astaci as a parasite. Physiol.Plant., 1969a 22:221–35 , On the host range and origin of the crayfish plague fungus. 1972 Rep.Inst.Freshwat.Res., Drottningholm, (52):192–8 , and D.W. Weiss, The host-parasite relationship between freshwater 1970 crayfish and the crayfish disease fungus Aphanomyces astaci: responses to infection by a susceptible and a resistant species. J.Gen.Microbiol., 60:77–90 PRESENCE EN FRANCE D'ESPECES EXOTIQUES D'ECREVISSES PROVENANT D'INTRODUCTIONS RECENTES

P.J. Laurent Institut National de la Recherche Agronomique Thonon les Bains, France et D. Vigneux et E. Vigneux Conseil Supérieur de la Pêche Boves, France

TABLE DES MATIERES

1. INTRODUCTION 2. EVOLUTION DE LA LEGISLATION FRANCAISE EN MATIERE D'ACCLIMATATION 3. LES ECREVISSES EN COURS D'ACCLIMATATION EN FRANCE 3.1 Ecrevisse à pattes grêles (Astacus leptodactylus Escholtz) 3.2 Ecrevisse de Californie (Pacifastacus leniusculus Dana) 3.3 L'écrevisse rouge de Louisiane (Procambarus clarkii) 4. REFERENCES

RESUME Les résultats d'un inventaire national terminé en 1979 permettent de montrer l'existence, sur le territoire français de quelques points d'apparition d'écrevisses exotiques. Quelques informations plus récentes révèlent l'acclimatation certaine ou très probable d'écrevisses totalement étrangères à la faune française jusqu'à ces derniers temps. ABSTRACT The national inventory completed in 1979 has shown that exotic crayfish species have appeared within French territory. Recent information shows that certain crayfish foreign to the French fauna have either certainly or very probably become acclimatized. 1. INTRODUCTION Deux enquêtes réalisées sur tout le territoire français, l'une en 1959–60 (2) et l'autre en 1977 (5) ont montré que la faune astacicole s'était profondément modifiée par rapport à ce qu'on pouvait en savoir à la lumière de travaux plus anciens (1). Les traits essentiels de cette évolution concernent la régression des écrevisses autochtones et au contraire l'extension, à pratiquement toutes les régions du territoire français, de l'écrevisse américaine Orconectes limosus Rafinssque introduite en Europe à la fin du siècle dernier et en France à partir de 1911 (1). L'écrevisse à pied rouge Astacus astacus L. naguère largement répandue subsiste sur une trentaine de sites notamment dans le Centre Est, l'Est et le Nord Est du pays (5). L'écrevisse à pied blanc Austropotamobius pallipes Lereboullet reste l'espèce indigène la plus fréquente mais son aire d'extension est en constante diminution. Elle subsiste le plus souvent en tête des réseaux hydrographiques mais sa rareté a amené l'Administration à instaurer des périodes prolongées d'interdiction de capture qui peuvent atteindre cinq ans. L'écrevisses de torrent Astacus torrentium pourtant mentionne dans les faunes anciennes n'a pas été retrouvée ni en 1959–60 ni en 1977. Alors que les écrevisses se raréfient en France, à l'exclusion de l'américaine qui pullule mais qui ne jouit ni d'une bonne réputation gastronomique ni d'une cotation commerciale élevée, un nombre de plus en plus élevé d'amateurs souhaite assurer à notre pays une production nationale capable de remplacer les importations dont nous sommes tributaires en totalité et pour une quantité de l'ordre de 2 000 t/an. Des tentatives de repeuplement sont faites avec des juvéniles d'écrevisses à pied rouge, offertes aux amateurs par un nombre restreint d'astaciculteurs. Aucune publication ne rend compts, pour l'instant, des résultats obtenus par cette méthode. Des transplantations d'adultes d'écrevisses à pied blanc et à pied rouge ont été également réalisées soit avec des animaux récoltés en France, soit avec des sujets importés, sans qu'on connaisse encore les suties de ces opérations. La production ou l'importation de juvéniles et d'adultes d'espèces non autochtones ont conduit à des acclimatations parfois volontaires mais le plus souvent accidentelles dont nous allons tenter de faire brièvement le point. 2. EVOLUTION DE LA LEGISLATION FRANCAISE EN MATIERE D'ACCLIMATATION La loi française interdit la propagation, dans les eaux libres, d'espèces étrangères à notre faune. Par contre dans les eaux closes, tout propriétaire peut légalement procéder à des immersions d'espèces nouvelles. La seule condition requise pour qu'une eau soit close et qu'elle soit totalement isolée du réseau hydrographique. Cette législation a permis la dissémination incontrôlée d'écrevisses étrangères à notre faune en de nombreux points du territoire français. Les résultats des enquêtes n'ont permis de déceler d'éventuelles acclimatations que là où les animaux introduits ont franchi les limites de la propriété privés et ont commencé à coloniser les eaux libres, ou là où les propriétaires ont consenti à donner des informations. De nouvelles dispositions plus restrictives sont à l'étude et seront sans doute adoptées prochainement. Les eaux continentales seront sans doute mieux protégées que par le passé des introductions intempestives, mais tant que le commerce alimentaire des écrevisses se fera à partir d'animaux vivants, il sera loisible à quiconque de libérer dans la nature n'importe quel importé. Certes un tel acte serait répréhensible, mais quel contrôle pourra être assez efficace pour démasquer à temps tous les délinquants et éviter des acclimations préjudiciables à l'avenir astacicole de notre pays? Devant un tel risque la seuie solution envisageable semble résider dans l'interdicition de livrer à la consommation des écrevisses autrement que cuites ou congelées. Cette réglementation peut être contraignante sur le plan économique et commercial est appliqués avec succès par des pays comme la Suède. La France devrait rapidement se doter de dispositions réglementaires similaires de façon à pouvoir arrêter, quand il en est encore temps, une propagation anarchique d'écrevisses indésirables ou malades et jouant alors le rôle de vecteurs d'épizooties aussi dangereuses que l'aphanomycose. Ces mesures ne viseraient pas à interdire à tout jamais les acclimatations, mais à assurer un meilleur contrôle quant à la nature des espèces introduites, à leur état de santé et aux lieux où les immersions seraient faites. 3. LES ECREVISSES EN COURS D'ACCLIMATATION EN FRANCE La France n'a adopté aucun plan officiel d'acclimatation d'écrevisses étrangères. Les organismes responsables de la pêche ont simplement entrepris des observations ou des expérimentations limitées dont les conclusions, quand elles seront tirées, permettront de définir en connaissance de cause une politique astacicole rationnelle. A côté de la prudence des organismes officiels, de nombreux particuliers intéressés aux écrevisses se sont procuré des juvéniles ou des adultes d'espèces étrangères et ont procédé à des essais totalement incontrôlés. Il est résulté de cette activité des acclimatations occules, illégales pour la plupart. Trois espèces sont en cours d'acclimatation actuellement en France, nous les énumérerons en fonction de la probabilité décroissante de leur installation sur notre territoire. 3.1 Ecrevisse à pattes grêles (Astacus leptodactylus Escholtz) Plusieurs astaciculteurs de l'ouest de la France se sont spécialisés dans la production de juvéniles de cette espèce. D'autre part cet animal fournit plus de 90 pourcent des importations d'écrevisses de consommation, de sorte que de très nombreuses tentatives illicites d'acclimatations ont pu être réalisées à partir d'adultes achetés dans des magasins de comestibles. L'écrevisse à pattes grêles est un crustacé d'eaux stagnantes et sa rencontre en France se limite, pour l'instant, à des lacs naturels ou artificiels. Elle existe sans doute en de nombreux points du territoire français mais elle est notablement plus fréquente dans le sud-ouest et le Languedoc (Fig. 1). Un lac collinaire d'une dizaine d'hectares de département du Gers s'est trouvé peuplé depuis une dizaine d'années par migration d'adultes d'un ruisseau amont où elles avalent été déposées dans l'espoir de les acclimater. Outre le plan d'eau de 10 ha de Barbotan les Bains dans les Landes, l'écrevisse à pattes grêles serait implantée dans plusieurs barrages collinaires du Gers et du Tarn et Garonne et des Pyrénées Atlantiques ainsi que dans le Volp, cours d'eau ariégeois. Ce nouveau crustacé pour le sud-ouest jouit déjà d'une excellente réputation auprès des bordelais qui le pêchent pour en garnir leurs congélateurs. Il ne fait aucun doute que l'aire de répartition d'A. leptodactylus va s'étendre dans le sud-ouest. Des informations plus précises indiquent que deux plans d'eau languedociens du système hydrographique du Vidourle ont été peuplés à titre expérimental, et avec les autorisations administratives officielles, l'un en 1975 et 1978 avec 13 400 adultes de 10 à 12 centimètres et l'autre en 1979 avec 4 500 juvéniles élevés par un astaciculteur français. Des acclimatations vérifiées ont également été signalées dans l'est. La plus ancienne concerne une ballastière peuplée vers 1945–46 de quelques sujets techécoslovaques ramenés par des troupes américaines et qui ont laissé une descendance assez abondante pour permettre une pêche régulière depuis 1967. En Meurthe et Moselle, 50 adults lachés dans une eau close en 1970 se sont multipllés et ont donné naissance à une population abondant en 1980. Dans le Léman enfin, en août 1979 nous avons observé la première écrevisse à pattes grêles qui était capturée au filet par des pêcheurs professionnels dans les environs de Thonon. L'hiver suivant, plusieurs sujets se faisaient prendre par 80 m de fond avec des ombles chevaliers (Salvelinus alpinus) occupés à frayer. Le peuplement du Léman résulte sans aucun doute de la fuite d'écrevisses entreposées dans des viviers de stockage par des riverains grossistes en poissons. 3.2 Ecrevisse de Californie (Pacifastacus leniusculus Dana) L'importation de cette écrevisse adulte reste exceptionnelle, par contre, il est possible de se procurer des juvéniles auprès de producteurs étrangers, suédois notamment, ou même français. Il est impossible de connaître le détail des achats de juvéniles mais il y a certainement eu de nombreuses tentatives d'acclimatations illégales dont le résultat reste encore inconnu. Des informations précises sont toutefois disponibles dans deux régions: l'Indre et la Savoie. Dans le sud du département de l'Indre, un peuplement limité à 50 juvéniles a été déposé dans trois ruisseaux à faune piscicole salmonicole dominante. Ce déversement pourtant très réduit a été le point de départ d'une population qui est devenue apparente trois ans plus tard et qui semble se développer sans avoir encore migré à plus de 500 m de son point de déversement. Les plus gros sujets, trois ans après leur introduction, atteignaient le poids de 110 g. Plusieurs publications ont déjà rendu compte des résultats de tentatives expérimentales d'acclimatation de Pacifastacus leniusculus dans des plans d'eau close de Savoie (3–4). Le résultat le plus remarquable a été obtenu sur un gravière de 43 ha alimentée par de l'eau de nappe phréatique. Du déversement de 1 500 juvéniles en 1973 et de 1 000 nouveaux juvéniles en 1976 est née une population florissante qui fait l'objet d'une exploitation intéressante. La pêche dans le plan d'eau dont il s'agit est soumise à un règlement particulier établi par la Société détentrice du droit de pêche. L'exploitation des écrevisses n'a été autorisée qu'en 1980, sept ans après leur introduction. Des sondages avaient montré que la population était florissante et qu'elle supporterait un prélèvement raisonnable. Il fut donc décidé de n'ouvrir la pêche aux écrevisses que 15 jours, de limiter les prises à 10 sujets par jour, de fixer la taille légale de capture à 13 cm et surtout d'en autoriser que la ligne pour exercer cette pêche, redevable d'une taxe spéciale. Le succès fut total, il fut vendu plusieurs centaines de permis de pêche aux écrevisses et des ressources affluèrent dans les caisses de la Société. En 1981, l'état de la population semblant inchangé malgré la pression de capture, la durée de la pêche fut portée à deux mois, la taille légale des écrevisses fut abaissée à 12 cm, les autres conditions d'exploitation restant identiques. Un nombre encore plus grand de permis fut vendu tandis que la réputation des écrevisses s'affirmait. Par précaution, chaque pêcheur reçut un permis sur lequel il est indiqué sans ambiguité, que la transplantation des écrevisses de Californie est illégale et peut être nocive. Il semble que ce rappel n'ait pas été bien compris puisque la rumeur publique donne pour probable le déversement de Pacifastacus leniusculus dans le Léman par des pêcheurs d'écrevisses. Sur un autre plan d'eau close peuplé en 1975 de 500 juvéniles et en 1976 de 600 juvéniles une population très dense s'est également développée sans que son exploitation soit encore autorisée. La pêche va être ouverte dans des conditions analogues à celles du cas précédent. Il existe actuellement en Savoie (5) eaux closes occupées par des écrevisses de Californie, ce sont les résultats d'introduction expérimentales qui se sont révélées jusqu'à présent favorables. Sans aucun doute, Pacifastacus leniusculus doit exister en d'autres points du territoire français mais sa présence n'est pas connue de manière officielle. 3.3 L'écrevisse rouge de Louisiane (Procambarus clarkii) La France a reçu des écrevisses rouges de Louisiane des U.S.A., en faible quantité du Kenya à partir de 1976, et plus récemment d'Espagne. La couleur de cette écrevisse et surtout l'appellation commune de “patte rouge” sous laquelle elle est présentée au public, risquent d'inciter des amateurs à l'introduire en France. Nous n'avons encore vu aucun exemplaire de ce crustacé sur notre territoire, cependant son acclimatation peut être considérée comme hautement probable en divers points. Dans l'ouest tout d'abord, où elle a fait l'objet de tentatives privées d'élevage, elle s'est échappée, suivant son habitude, des plans d'eau où elle avait été consignée. Sa présence serait signalée dans la région de Redon. Un peuplement existerait également dans une gravière close de la Crau non loin de Marseille. Enfin des pisciculteurs de Charente et de l'Hérault ont stocké des animaux avant de les vendre et il n'est pas impossible de penser que des sujets échappés sont en train de s'acclimater. Un plan de développement astacicole était envisagé en Camargue avec Procambarus clarkii pour tenter d'apporter à la riziculture française en détresse une ressource nouvelle. Ce projet semble avoir été abandonné avant que les premières implantations n'aient donné à Procambarus clarkii l'occasion d'envahir le delta du Rhône comme elle l'a déjà fait sur la Guadalquivir en Espagne. En l'absence de précautions particulières que nous recommandions plus haut, il y a tout lieu de craindre que l'écrevisse rouge de Louisiane libérée inconsidérément s'installe et compromette toute tentative ultérieure d'acclimatation d'espèces plus intéressantes. 4. REFERENCES Andre, M., Les écrevisses françaises. Paris, Lechevalier, 293 p. 1960 Laurent, P.J., Premiers résultats des introductions expérimentales en eaux closes de 1979 Pacifastacus leniusculus Dana. Piscicult.Fr., (56):51–7 , Stocking of lakes with American signal crayfish. In The two lakes twelfth 1980 fishery management training course report. Chislehurst, Kent, Janssen Services, pp. 139–48 Laurent, P.J. et M. Suscillon, Les écrevisses en France. Ann.St.Cent.Hydrobiol.Appl., 1962 9:335–95 Vigneux, D., Enquête sur les écrevisses en France. Paris, Conseil Supérieur de la 1980 Pêche, 152 p.

Fig. 1 Département où ont été repérés des espèces nouvelles d'écrevisses: A. 1. = Astacus leptodactylus, P. 1. = Pacifastacus leniusculus, P. c. = Procambarus clarkii THE RESTORATION OF THE CRAYFISH (Astacus astacus) IN RIVER SIIKAJOKI, FINLAND

M. Pursiainen Evo Inland Fisheries and Aquaculture Research Station Evo, Finland and K. Westman Finnish Game and Fisheries Research Institute, Fisheries Division Helsinki, Finland

CONTENTS

1. INTRODUCTION 2. RIVER SIIKAJOKI AS AN ENVIRONMENT FOR CRAYFISH 2.1 Water quality 2.2 Discharge and water level fluctuations 2.3 Crayfish habitats 3. CRAYFISH IN THE RIVER BEFORE STOCKING 4. STOCKING TRIALS 5. TEST TRAPPING AND RESULTS 5.1 Catches 5.2 The condition of the crayfish 6. DISCUSSION AND CONCLUSIONS 7. REFERENCES ABSTRACT The Siikajoki River flows into the Baltic Gulf of Bothnia. Previous to changes in the natural state of the water system it was one of Finland's most productive crayfish waters. In the late sixties, construction of a 28 km2 reservoir on the upper river and intensive dredging of the main river and its tributaries combined to reduce the quality of the water to the point that the crayfish population was almost completely destroyed. The lost crayfish catch was calculated at 0.5 million individuals per year. The water quality below the reservoir remained rather bad (low oxygen and pH) for many years. Short-term fluctations in the water level (because the reservoir power plant discharges only during the day) strongly limit suitable areas for crayfish. Since the water quality had improved, however, introductions of crayfish were begun. The restoration possibilities were tested by stocking mature crayfish in some selected areas. Test fishings in the following years showed that crayfish survived well in areas with low fluctuations in the water level. In shallow areas and areas having a soft bottom, where erosion is strong, the results were poor. After the test stockings it was concluded that the water quality was good enough for crayfish to survive, but the strong fluctuations in water level due to discharges would reduce the areas suitable for crayfish to 50 percent of the natural situation. RESUME Le Siikajoki se jette dans le golfe de Botnie. Avant que son état physique ne soit modifié, c'était l'une des principales sources d'écrevisses de la Finlande. A la fin des années soixante, la construction d'un réservoir de 28 km2 sur le cours supérieur du fleuve et le dragage intensif de l'ensemble du réseau ont entraîné une dégradation de la qualité de l'eau, dégradation telle que les populations d'écrevisses ont été presque complètement détruites. Les captures ainsi perdues ont été estlmées à 0,5 million d'individus par an. La qualité de l'eau en aval du réservoir (oxygénation et pH) est restée assez médiocre pendant plusieurs années. Les fluctuations fréquentes du niveau des eaux (l'eau du réservoir n'est déversée dans le fleuve que durant la journée) font que les zones où pourraient s'lnstaller les écrevisses sont très limitées. Néanmoins, on a commencé à en introduire lorsque la qualité de l'eau s'est améliorée. Les possibilités de reconstitution des stocks ont été étudiées en repeuplant un certain nombre de zones avec des écrevisses matures. Les essais de pêche effectués les années suivantes ont montré que le taux de survie des écrevisses était bon dans les zones où les fluctuations du niveau des eaux étaient faibles. En revanche, les résultats ont été médiocres dans les eaux peu profondes et sur les fonds friables où l'érosion est forte. On a conclu de ces expériences de repeuplement que la qualité de l'eau était suffisamment bonne pour la survie des écrevisses mais que les fortes fluctuations de niveau dues à la centrale hydro-électrique réduiraient à 50 pour cent de la superficie originale les zones adaptées aux populations d'écrevisses. 1. INTRODUCTION Astacus astacus L. is the only native freshwater crayfish species in Finland. Its original area of distribution has extended to latitude 62°N. In order to improve and extend the crayfish fisheries, crayfish have been introduced to many waters where the species has not earlier occurred. Nowadays there exist self-reproducing crayfish stocks up to latitude 65°N in eastern parts of the country and to 67°N in western parts (Westman, 1973). One of the water courses in which crayfish were introduced at the beginning of this century is the River Siikajoki, which is one of the large rivers flowing into the Baltic Gulf of Bothnia (see Fig. 1). Crayfish thrived there so well that the river was one of the most important fishing areas for the species in the country before hydraulic construction in the late sixties. The annual crayfish catch in the approximately 96-km long main river was assessed at about 460 000 individuals (Pursiainen et al., 1981). In connexion with the construction of the 28 km2 Uljua Reservoir in 1969 the whole crayfish stock disappeared almost completely in the main river below the reservoir. As a reason of the destruction, the sudden worsening of the water quality was apparent during the construction period (Westman, 1974). Two smaller reservoirs were built and intensive dredging operations carried out in connexion with the construction of the big reservoir. The water quality in the Uljua Reservoir and also in the river below it remained bad for many years. In particular, oxygen level (min. 2 mg/l) and pH (5.8–6.0) were low and suspended solids high (Alasaarela, 1979), which are the most important factors adversely affecting crayfish survival (Lindroth, 1950; Niemi, 1977; Jarvenpaa et al., 1981). Short-term fluctuations in the water level below the reservoir (the reservoir power plant discharges only during the day) have in addition strongly changed the environment of the crayfish (see also Fig. 1). About 10 years after the construction operations the water quality was so much improved that it was considered feasible to reintroduce crayfish in the river. This report supplies information concerning the three years period 1978–80, when test stockings were made and the development of the new crayfish stocks monitored. 2. RIVER SIIKAJOKI AS AN ENVIRONMENT FOR CRAYFISH 2.1 Water quality The Uljua Reservoir remains the most important factor affecting water quality in the River Siikajoki. Although a part of the negative effects on water quality have decreased, the oxygen level during the late winter was very low even at the end of the seventies (2 mg/1). On the lower part of the main river small tributaries and rapids aerate the water and so improve the oxygen situation somewhat (Alasaarela and Salmela, 1980). Other factors (e.g., the pH value and suspended solids) affecting the life of crayfish are slowly improving even though they still continue to be worse than they were before the reservoir was constructed. 2.2 Discharge and water level fluctuations The water regime of Finland's western river systems comprises high spring floods, a slow decrease in water level during summer, autumn flooding and minimum levels again during the winter. The natural changes in the discharge and water level are slow and do not adversely affect the life of the crayfish. Since the construction of the Uljua Reservoir the situation has completely changed. The reservoir itself is regulated to an annual amplitude of 8 m by collecting the top of the spring flood in the reservoir and then discharging the water during the low water periods. The discharge from the reservoir power plant must be arranged such that the daily mean current is not less than 1 m3/s in the main river and the maximum current through the power plant does not exceed a daily mean of 50 m3/s (Vesihallitus, 1978). Neither the daily mean current nor discharge describe the river environment sufficiently clearly, because the power plant discharges water according to the peaks in electrical consumption. This means, of course, that during low water periods (summer and winter) the discharge occurs during the day, while during nights and weekends the discharge is at its minimum. The impact of this kind of regulation is clearly seen in Fig. 2, which presents the fluctuations of the water level 10 km below the reservoir and the daily mean discharge from the power plant at the end of the spring flood and during the low water period in the summer. The daily fluctuations in the water level are not so great further down the river. 2.3 Crayfish habitats Crayfish prefer hard and relatively sloping bottoms with a lot of refuges and hiding places (Westman and Pursiainen, 1978) and in general this kind of habitat exists in abundance in river environments (Niemi, 1977). This was the situation, too, in the River Siikajoki before the construction. However, the river has been dredged for timber floating and flood protection and this has resulted in great changes in the habitats for crayfish and also increased the effects of short-term regulation. It must, however, be noted that a proportion of the negative effects of the dredging operations diminished slowly through so-called “naturalization”. A great part of previous crayfish habitats have been destroyed ultimately through the loss of all refuges and hiding places. Short-term regulation also causes sliming, which fills all possible refuges and prevents to a large extent the production of zoobenthos and vegetation which are the source of crayfish nutrition. 3. CRAYFISH IN THE RIVER BEFORE STOCKING In order to discover what was the status of crayfish populations in the River Siikajoki before stockings, some trial catches were made in 1978 in areas 1, 2, 3, and 6 in the main river and in areas 4 and 7 in tributaries (see Fig. 1). Test catches have been made at the end of July, when crayfish are most active, with an Evo-trap (Westman et al., 1978) developed specially to research purposes. 25–50 traps were used overnight in each area at 5-m intervals. The catch amounted to only two crayfish on area 1 and one crayfish on area 7, on the Lamujoki tributary. Some crayfish stocking was carried out prior to 1978 by local fishermen and some of them have been successful, as can be seen in area 1. Test catches, however, clearly showed that the crayfish stock in 1978 was very weak in selected stocked areas. 4. STOCKING TRIALS Crayfish for stocking purposes were caught from the nearest available river, the River Oulujoki, about 80 km north of the River Siikajoki, in order to obtain crayfish which were as far as possible adapted to similar conditions and latitude as in the River Siikajoki. In Finland it is thought that only crayfish originating from running waters should be used for stocking rivers, and “lake” crayfish for stocking lakes. All stocked crayfish were marked by cutting the left uropod in 1978 and the right one in 1979. Only crayfish with a hard carapace and in good condition were employed for stocking purposes. Transportation was carried out dry in wooden or plastic boxes. At the stocking sites crayfish were kept in net cages for one day prior to release since it has been observed that as a result of this method crayfish remain more attached to the stocking site than they do after having been released immediately after transportation. Crayfish were spread over an area of 100–150 m in the littoral zone. In stocking, the density aimed at was the same as observed some natural populations in rivers (2.5 individuals per m2, adults) (Westman and Pursiainen, 1982). In 1978 about 3 500 crayfish were released at six different sites. The mean carapace length was 52.6 ± 0.37 mm. In 1979 stockings were made in five areas with 2 300 crayfish having a mean carapace length of 53.0±0.25 mm. Different stocking areas and the numbers released are shown in Table 1. 5. TEST TRAPPING AND RESULTS To study the success of stocking test trapping was carried out in 1979 and 1980 in selected stocking areas by the same method as in 1978 prior to stocking, described earlier. 5.1 Catches In 1979 the total catch was 249 crayfish, which corresponds to 0.83 crayfish per trap night. In 1980 the catch was 200 crayfish which is 1.00 crayfish per trap night. Catches from different areas are listed in Table 2. In connexion with catches from different areas, the following points are of interest: − In area 1 no crayfish were caught in 1979 from the 1978 stocking area, which was a shallow rapids, but the re-stocking made in 1979 200 m below the rapids was successful. The reason for this is probably the ice situation in the rapids, which was very adverse in the winter of 1978–79. − In area 2 only a few crayfish were caught in 1979 and therefore no fishing was done in 1980. It seems obvious that shallow muddy areas like area 2 are too difficult for crayfish to move along because of the erosion of the bottoms due to regulation. − In area 3 the catch was rather good in both years and in 1980 stocked individuals both from 1978 and 1979 were caught, although the 1979 stocking was done 300 m downstream from the previous stocking area which served as a test fishing area in both years. This clearly shows how crayfish can move rather far from their stocking site. − On tributary area 4 the catch was good in the first year after stocking, but the reduction in 1980 probably shows how crayfish have become distributed over a larger area. − In area 5, where stocking was carried out in 1979, the catch was one of the best, which is a good sign for high survival over the first winter. − In area 6 and in tributary and control areas 7 and 8 a certain proportion (7–20 percent of the catch) of crayfish were unmarked, indicating that there existed crayfish either as survivers over a longer period or as the result of previous stockings. It was surprising especially in area 6, which was fished in 1978 without captures being made. It seems obvious that these unmarked individuals have moved into the area during the research period. 5.2 The condition of the crayfish Crayfish had moulted in both test fishing summers before trapping, which is also clearly apparent from the mean size of specimens captured. For example, the carapace mean length of stocked crayfish in 1978 was 52.6±0.37 mm and that of trapped individuals (excluding crayfish without uropod cutting) varied from 55.4±2.52 to 58.9±0.79 mm in different areas. This means that the carapace length had grown by 3–6 mm, which is the expected normal growth in one year (Abrahamsson, 1972). All crayfish caught by test trapping were in good condition and almost all females were ready to reproduce, which might be considered a sign of suitable enviornmental conditions. No information about reproductive success could be obtained, however, because electric fishing was impossible in turbid waters and so newly-hatched juveniles could not be caught. 6. DISCUSSION AND CONCLUSIONS According to the test trapping it seemed obvious that in the River Siikajoki there were only few crayfish left prior to stocking. These crayfish originated from some previous stocking done by local fishermen. However, the population was very weak. Test trapping also showed that on the upper part of the river and in some tributaries a part of the crayfish population may have survived in spite of dredging and suspected crayfish plague. Stocking of crayfish proved rather successful except in two areas, the rapids area (1) and shallow soft bottom area (2). It seems obvious that this kind of biotope offers relatively poor possibilities for supporting productive crayfish stocks. In the winter, short- term fluctuations in discharge lead to the freezing of the rapids from the bottom during heavy frost, which naturally can destroy a crayfish population during the winter. On areas with soft bottoms crayfish do not seem to thrive, probably because all shelters will be filled with loose mud due to the fluctuations in water level. On such areas, where regulation is not so considerable, i.e., on deep areas with relatively hard bottoms, the crayfish can survive well. It is, of course, clear that the upper part of the river and also the tributaries, where no regulation exists and where the negative effects of dredging have diminished, offer rather good possibilities for crayfish production. The stocking density used (about five individuals per metre) seemed successful, because the density of the population one-two years after stocking was sufficient for reproduction, though crayfish seemed to spread slowly in larger areas. No information about the success of reproduction could be gathered because juveniles born in the river were not caught. All females in the catch of the test fishings were, however, in reproductive condition, which can be seen from the cement glands under the abdomen. The results allow the conclusion that crayfish production can be restored at least in part in the main river below the Reservoir Uljua. According to the profile of the river (rapids, other shallow areas, deep areas) and the extent of regulation, it was estimated that crayfish production may reach half the level of that occurring in the natural state before any construction. This would mean an annual production of 230 000 legal-sized (total length over 10 cm) crayfish. In order to obtain the expected production an effective stocking programme is needed. Stocking areas should have hard bottoms where the fluctuation of the water level due to the short-term regulation are less than 10 cm. Stocking should be concentrated only in the best biotopes and the number of crayfish should be the same as that shown to give results in test stockings, i.e., 400–500 individuals for each area and 4–5 specimens per metre. If rapid results are required, mature crayfish exceeding 8 cm in length should be used. The stocking of large-sized males should be avoided as they terrorize smaller specimens and occupy the best biotopes. The sex-ration of the stocking material should be 3 females to 1 male. Only specimens with a hard exoskeleton should be released. The most suitable time for stocking would be the end of the summer when all crayfish have already moulted. They still have enough time to become adapted to the new environment before mating and the onset of winter. To avoid migration from the stocking areas the crayfish should be kept in net-cages, etc., in the new water area for a couple of days before they are released. The prerequisite for the success of stocking is, however, that there be no new hydraulic constructions and that the water quality is good enough for crayfish to reproduce. The development of the new crayfish population should be monitored for several years along with the water quality and other conditions on the whole river so as to organize the crayfish fisheries in the best way. 7. REFERENCES Abrahamsson, S., Fecundity and growth of some populations of Astacus astacus Linne 1972 in Sweden. Rep.Inst.Freshwat.Res., Drottningholm, (52):23–37 Alasaarela, E., Siikajoen yhteistarkkailu: osa II. Siikajoen vesistotarkkailun tulokset v. 1979 1978 ja vesiston veden laadun kehittyminen v. 1963–78. Pohjois- Suom.Vesitutkimustoimisto, 22 p. (mimeo) Alasaarela, E. and K. Salmela, Siikajoen yhteistarkkailu. Siikajoen vesistotarkkailun 1980 tulokset v. 1979 ja Uljuan altaan vaikutus Siikajoen veden laatuun ja ainetaseisiin v. 1969–1979. Pohjois-Suom.Vesitutkimustoimisto, 33 p. (mimeo) Jarvenpaa, T. et al., Effects of hypoxia on the haemolymph of the freshwater crayfish, 1981 Astacus astacus L., in neutral and acid water during the intermoult period Lindroth, Reactions of crayfish on low oxygen pressure. Rep.Inst.Freshwat.Res., 1950 Drottningholm. (31):110–2 Niemi, A., Population studies on the crayfish Astacus astacus L. in the River Pyhajoki, 1977 Finland. In Freshwater crayfish, edited by O.V. Lindquist, Kuopio, Finland, University of Kuopio, vol. 3:81–94 Pursiainen, M. et al., Ravun elinmahdollisuudet Siikajoessa ja rapukantojen 1981 hoitosuunnitelma. 40 p. (MS) Vesihallitus, Pohjanmaan pohjoisosien vesien kayton kokonaissuunnitelma. 1978 Vesihallituksen asettaman tyoryhman ehdotus. l. osa. Yleiskuva suunnittelualueesta, vesivarat ja vesien nykyinen kaytto. Vesihallitus, tiedotus, 137:1–327 Westman, K., The population of the crayfish, Astacus astacus L. in Finland and the 1973 introduction of the American crayfish Pasifastacus leniusculus Dana. In Freshwater crayfish, edited by S. Abrahamsson. Lund, Studentlitteratur, vol. 1:42–55 , Uljuan tekoaltaan vaikutukset alapuolisen Siikajoen rapukantoihin. Riista- 1974 ja Kalatalouden Tutkimuslaitos, Kalantutkimusosasto. Tiedonantoja, 1:37– 55 Westman, K. and M. Pursiainen, Development of the European crayfish Astacus astacus 1978 L. and the American crayfish Pacifastacus leniusculus Dana populations in a small Finnish lake. In Freshwater crayfish, edited by P.J. Laurent. Thonon-les-Bains, Institut National de la Recherche Agronomique, pp. 243–50 Westman, K. and M. Pursiainen, Size and structure of crayfish (Astacus astacus) 1982 populations on different habitats in Finland. Hydrobiologia, 86:67–72 Table 1 Crayfish stockings in various areas in the River Siikajoki (Numbers of individuals released)

Area 1978 1979 Total 11 570 600 1 170 2 595 - 595 32 594 600 1 194 4 570 - 570 5 - 600 600 6 592 - 592 7 582 - 582 8 - 519 519 Total 3 503 2 319 5 822

1 In 1978 stocking was done in the rapids in area 1 and in 1979 200 m below the rapids 2 The stocking sites are about 300 m apart

Table 2 Test trapping and the crayfish catches in the River Siikajoki in 1979 and 1980 Number of individuals, individuals per trap night and mean length of carapace 1979 1980 Area Total Catch/ Carapace length Total Catch/ Carapace catch trap catch trap length 1 0 - - 22 0.44 58.6 ± 0.96 2 5 0.10 55.4 ± 0.52 3 50 1.00 58.9 ± 0.79 35 0.70 59.8 ± 0.83 4 56 1.12 57.5 ± 0.62 12 0.48 63.3 ± 1.78 5 42 1.68 59.2 ± 0.65 6 61 1.22 60.4 ± 0.83 46 1.84 62.9 ± 0.89 7 77 1.54 56.8 ± 0.62 8 43 1.72 57.1 ± 0.59

Fig. 1 The River Siikajoki water course and the test areas

Fig. 2 Fluctuation of the water level 10 km below the Uljua Reservoir in centimetres (curved line) and the mean daily discharge of the Uljua Power Plant in cubic metres per second (straight line) INTRODUCTION OF THE AMERICAN CRAYFISH (Pacifastacus leniusculus) IN FINLAND; IMPACT ON THE NATIVE CRAYFISH (Astacus astacus)

K. Westman Finnish Game and Fisheries Research Institute Helsinki, Finland and M. Pursiainen Evo Inland Fisheries and Aquaculture Research Station Evo, Finland

CONTENTS

1. INTRODUCTION 2. INTRODUCTION OF SIGNAL CRAYFISH 3. PRELIMINARY ASSESSMENT OF THE INTRODUCTION OF SIGNAL CRAYFISH 4. REFERENCES

ABSTRACT Since 1893, numerous unsuccessful attempts have been made in Finland to control the disastrous crayfish plague. No resistant strains of the only native crayfish, Astacus astacus, have developed. In order to restore crayfish production, the plague- resistant American crayfish Pacifastacus leniusculus was introduced to Finland in 1967. In 1967–74 a total of approximately 40 000 juvenile and adult Pacifastacus were stocked in 53 crayfish lakes devastated by the plague. In a few lakes, both Astacus and Pacifastacus occur simultaneously, offering excellent possibilities for comparative studies of both species. Pacifastacus seems to be able to survive and reproduce under Finnish condition at least up to 63°30'N and to tolerate fishing pressure. Yearly removal of legal-size (10 cm) Pacifastacus from some of the experimental lakes seems not to have caused any harm to the populations or their renewal. According to studies run in the same lake, Pacifastacus seems to grow faster and become sexually mature earlier than Astacus. Both species seem to prefer similar biotopes and have similar bionomics and life histories. No signs of hybridization have been observed. No harmful effects of the Pacifastacus population have so far been observed, either to the Astacus population or to the ecosystem of the lake. However, the niches of both species seem to overlap to such a great degree that competition for living space will probably occur. RESUME Dès 1893, la Finlande a commencé à combattre la maladie des écrevisses, aux conséquences catastrophiques. Ses efforts sont restés vains. Aucun type résistant d'Astacus astacus, seule écrevisse indigène. n'est apparu. Pour relancer la production, la Finlande a décidé d'introduire en 1967 l'écrevisse américaine Pacifastacus leniusculus qui, elle, résiste à la maladie. Entre 1967 et 1974, on a utilisé environ 40 000 juvéniles et adultes de cette espèce pour repeupler 53 lacs dévastés. Astacus et Pacifastacus coexistent dans certains lacs, ce qui permet des études comparatives. Le second cité paraît à même de survivre et de se reproduire en Finlande, du moins jusqu'à 63°30'N, et de supporter la pêche. Le prélèvement annuel de Pacifastacus de taille légale (10 cm) dans certains des lacs expérimentaux ne semble pas avoir nui aux populations ni à leur renouvellement. Par ailleurs, Pacifastacus semble avoir une croissance plus rapide qu'Astacus et atteindre plus vite la maturité sexuelle. Les deux espèces ont apparemment une préférence pour les mêmes biotopes et ont un cycle biologique et une bionomie identiques. Aucun signe d'hybridation n'a été observé. Pour l'instant, la population de Pacifastacus ne paraît pas avoir eu d'effets néfastes sur la population d'Astacus ni sur l'écosystème. Toutefois, les niches de ces deux espèces semblent se recouvrir à tel point qu'il y aura vraisemblablement concurrence entre Astacus et Pacifastacus. 1. INTRODUCTION There are some 60 000 lakes in Finland, with a total area of 31 613 km2, and owing to their irregular shores, their total shoreline is very long, measuring about 130 000 km. The total length of the numerous Finnish rivers exceeds 20 000 km. Consequently, the extent of the littoral zone, which is the habitat of the crayfish, is remarkably great, and Finland has particularly well-suited natural conditions for large- scale crayfish production. The greatest obstacle for the utilization of these possibilities has been the disastrous crayfish plague, Aphanomyces astaci, which has caused great losses to the populations of Astacus astacus, the only endemic crayfish species occurring in Finland. The crayfish plague reached Finland in 1893 and of the 74 major watercourses, excluding the seven flowing north in which crayfish have never lived, only 20 remain uninfected. New cases have appeared yearly, partly in waters which have previously escaped the disease, and partly in waters which have new crayfish populations derived from stockings (Westman, 1973; Westman and Nylund, 1979). Along with the plague, the increasing manipulation of our watercourses, dam construction, draining, dredging etc., have contributed to the disappearance of the crayfish (see Pursiainen and Westman, 1983). In Finland, many unsuccessful attempts have been made to control the crayfish plague. No resistant strains of Astacus astacus have developed (Unestam, 1969) and the crayfish seems to be incapable of re-establishing itself in chronically infected water courses. In order to restore crayfish production and to improve the earlier important crayfish fisheries, it was decided to start research and stocking experiments with the plague-resistant signal crayfish Pacifastacus leniusculus. 2. INTRODUCTION OF SIGNAL CRAYFISH The first specimens of signal crayfish were imported as adults from California, U.S.A. in 1968–69. Due to the danger of inadvertently introducing fish or crayfish diseases or parasites, we abandoned the direct transfer of adult signal crayfish from North America and continued stocking experiments with newly-hatched juveniles, produced from broodstock from controlled cultivation systems in Sweden. In 1967–74, a total of about 40 000 juvenile and adult signal crayfish were stocked in 53 lakes. The greater part, about 35 500, were newly-hatched juveniles from Sweden, the rest were larger juveniles cultivated in Finland, or adults from the U.S.A. Since 1974, no new introductions have been made, as we wished to obtain the results of previous stockings before continuing. Preferred sites for stocking signal crayfish were small lakes with no inlets or outlets. Such sites ensured that stocked signal crayfish would not spread, and it also made it easier to follow the development of the populations. Moreover, all the selected lakes had previously supported native A. astacus populations which had been devastated by the crayfish plague. However, it was later revealed that there were also Astacus in some lakes in which Pacifastacus juveniles had been stocked. It would appear that some Astacus escaped the attacks of the plague fungus and gave rise to new Astacus populations. Lakes in which both species occur simultaneously offer excellent possibilities for making comparative studies of the native and the exotic crayfish species. Since 1977, the Finnish Game and Fisheries Research Institute has conducted extensive research of this type, especially in one of the small lakes with self-reproductive Astacus and Pacifastacus populations. The aim of the research is to investigate and compare the populations of Astacus and Pacifastacus: their structure and densities in different biotopes, the biology of both species, e.g., growth, activity, reproduction, etc. Special attention has been paid to the relationships between both species, especially to possible competition on biotopes, for example (Westman and Puralainen, 1979). 3. PRELIMINARY ASSESSMENT OF THE INTRODUCTION OF SIGNAL CRAYFISH Signal crayfish greatly resemble the native crayfish in appearance. The body weight in relation to length is considerably higher in Pacifastacus than in Astacus. The difference is connected with the greater development of the chelipeds in Pacifastacus, which is an advantage for utilization. The taste of Pacifastacus is as good as that of Astacus, when traditionally prepared. Signal crayfish seem to be able to survive and reproduce under Finnish conditions at least up to 63°30'N. According to studies made on the same small lake, signal crayfish seem to grow faster and become sexually mature earlier than the native crayfish. According to Abrahamsson (1973), the egg production of Pacifastacus is approximately 90 percent higher than that of Astacus. Although egg production is higher and maturity occurs earlier in the signal crayfish than in the native crayfish, the development of Pacifastacus populations has been very slow in all of the experimental lakes. No signs of uncontrolled, vigorous reproduction and spread of the signal crayfish have been observed (Westman, 1973; Westman and Pursiainen, 1979). Signal crayfish seem to tolerate fishing pressure. Yearly removal of legal-size (10 cm) Pacifastacus from some of the experimental lakes seems not to have caused any harm to the populations or to their renewal. This seems to indicate that in Finnish conditions signal crayfish would be able to develop populations that are economically viable, i.e., populations which are able to support fishing and give profitable catches. Signal crayfish seem to be more active than the native crayfish, and the species is easy to catch with traditional methods and traps. Both crayfish species seem to prefer habitats of the same type, as the catches of both have been greatest in the same areas. The preferred biotopes are those with hard bottoms of rock, stones or gravel; and those with bottoms suitable for digging, covered with sunken tree trunks, twigs, litter and vegetation which provide shelter for the crayfish. No signs of hybridization between Astacus and Pacifastacus have been observed in the specimens examined. There is, however, the possibility that Astacus males copulate with Pacifastacus females and Pacifastacus males with Astacus females, which might cause some disturbances in the “normal” copulation, thus slowing the increase of the populations in the same water bodies. No harmful effects of the signal crayfish populations have so far been observed, either to the Astacus populations or to the ecosystem of the experimental lakes. However, the niches of both species seem to overlap to such a great degree that competition for living space will probably occur (Westman and Pursiainen, 1979). Signal crayfish show high, but not complete, resistance against the crayfish plague fungus (Unestam, 1973) and infected specimens, if stocked or migrating to new water areas, may spread the fungus, thus causing harm to the highly susceptible native Astacus astacus. The cultivated juveniles may be free of the plague fungus as indicated by the simultaneous presence of Astacus and Pacifastacus in some lakes for several years. Survival and reproduction of signal crayfish in some selected lakes does not necessarily prove that it would be a good substitute for the native crayfish in all the chronically infected crayfish waters. Introductions of new animal species often have a number of consequences that are not sufficiently well known in advance (see Westman and Tuunainen, 1983). Accordingly, caution must be exercised in expanding the introduction of Pacifastacus leniusculus, and special care must be taken to ensure that stocking does not out-distance research on the subject, as happens very easily with a new and interesting species. The results are very promising and further stocking will be made in the near future. The importation of large numbers of living crayfish is expensive, and there is also the constant danger of introducing communicable fish or crayfish diseases or parasites not earlier found in Finland. For example, two Branchiobdellidae previously unknown in Finland, Xironogiton instabilius Moore and Cambarincola sp. were found on the exoskeletons of Pacifastacus imported from California. Accordingly, it seems to be safest and most advantageous in the long run to produce the crayfish needed for restoration of the devastated crayfish waters in Finland itself. Hatching and rearing investigations with Pacifastacus have been made in Finland since 1967 (Westman, 1973a). The main purpose is to develop cultivation techniques for producing crayfish suitable for stocking on a large scale. 4. REFERENCES Abrahamsson, S., The crayfish Astacus astacus in Sweden and the introduction of the 1973 American crayfish Pacifastacus leniusculus. In Freshwater crayfish, edited by S. Abrahamsson. Lund, Studentlitteratur, Vol. 1:27–40 Pursiainen, M. and K. Westman, The restoration of the crayfish (Astacus astacus) stock 1984 in River Siikajoki, Finland. EIFAC Tech.Pap., (42)Vol.2:412–21 Unestam, T., Resistance to the crayfish plague in some American, Japanese and 1969 European crayfishes. Rep.Inst.Freshwat.Res., Drottningholm, (49):202–9 , Significance of diseases on freshwater crayfish. In Freshwater crayfish, 1973 edited by S. Abrahamsson. Lund, Studentlitterature, vol.1:136–50 Westman, K., The population of the crayfish, Astacus astacus L. in Finland and the 1973 introduction of the American crayfish Pacifastacus leniusculus Dana. In Freshwater crayfish, edited by S. Abrahamsson. Lund, Studentlitteratur, vol.1:41–55 , Cultivation of the American crayfish Pacifastacus leniusculus. In 1973a Freshwater crayfish, edited by S. Abrahamsson. Lund, Studentlitteratur, vol.1:211–20 Westman, K. and M. Pursiainen, Development of the European crayfish, Astacus 1979 astacus (L.), and the American crayfish Pacifastacus leniusculus (Dana), populations in a small Finnish lake. In Freshwater crayfish, edited by P.J. Laurent. Thonon-les-Bains. Institut National de la Recherche Agronomique, vol.4:243–50 Westman, K. and P. Tuunainen, A review of fish and crayfish introductions made in 1984 Finland. EIFAC Tech.Pap., (42)Vol.2:436–47 INTRODUCTION ET ACCLIMATATION DE POISSONS D'EAU DOUCE EN FRANCE; HISTORIQUE ET BILAN

J. Allardi Ministère de l'Agriculture, CEMAGREF Paris, France

TABLE DE MATIERS

1. INTRODUCTION 2. ANALYSE BIBLIOGRAPHIQUE 2.1 Famille des Acipenseridae 2.2 Famille des Salmonidae 2.3 Famille des Cyprinidae 2.4 Famille des Ictaluridae 2.5 Famille des Poecilidae 2.6 Famille des 2.7 Famille des 3. DISCUSSION 4. REFERENCES

RESUME Parmi les dix-sept espèces passées en revue dans cette analyse bibliographique, neuf sont considérées comme acclimatées (parmi celles-ci, deux espèces ont un statut très incertain) et huit autres espèces sont présentes mais peu nombreuses. Deux espèces sont classées comme nuisibles. ABSTRACT Seventeen exotic species new to the French fauna are reviewed in this bibliographic analysis. Of these, nine are considered acclimatized (of which two are of uncertain status) and eight are present but in very small quantities. Two introduced species are classified as pests. 1. INTRODUCTION L'introduction d'animaux et de végétaux exotiques a été favorisée en France, au cours du siècle dernier. L'existence de la Société Nationale d'Acclimatation en relation étroite avec le Muséum National d'Histoire Naturelle de Paris est à l'origine de nombreuses introductions. Dès 1874, cette Société offrait un prix de F.F. 500 à la personne qui acclimaterait dans notre pays le saumon californien ou Quinnat (Mazeaud, 1981). L'évolution des idées et les échecs consécutifs à l'introduction de plusieurs espèces ont conduit le législateur à réglementer ces activités, et en particulier en ce qui concerne le poisson d'eau douce en eaux libres. Le Code Rural qui rassemble les textes législatifs concernant la police de la pêche fluviale, fixe d'une part la liste des espèces de poissons actuellement représentées dans les eaux libres du territoire et interdit d'autre part l'introduction des espèces non encore représentées, sauf autorisation du Ministre chargé de la Police de la Pêche. Cette liste comprend actuellement 63 espèces dont neuf ont été introduites plus ou moins récemment, et mériterait d'être complétée et remise à jour en tenant compte des données nouvelles, bien qu'encore insuffisantes, comme nous le montrerons dans l'analyse bibliographique succincte présentée ci-dessous. L'ordre dans lequel les espèces sont passées en revue est celui retenu par Spillmann (1961) dans son ouvrage de la Faune de France consacré aux poissons d'eau douce, qui fait actuellement référence. Pour chaque espèce, nous essaierons de donner les conditions d'introduction et la situation actuelle. Ces données pourront être confrontées aux informations déjà anciennes publiées par Vivier (1951) et, plus récemment, Vooren (1972), Wheeler et Maitland (1973). 2. ANALYSE BIBLIOGRAPHIQUE 2.1 Famille des Acipenseridae Les esturgeons sont actuellement représentées dans les eaux douces françaises par l'espèce Acipenser sturio L. étudiée par Magnin (1962). Celle-ci est actuellement très menacée. Elle a disparu du bassin de la Seine (environ 1920), du Rhône (1950–60), elle n'est pas citée dans le bassin de la Loire. Elle n'est plus capturée que dans le bassin de la Gironde et de la Garonne (CTGREF, 1979). Elle fait l'objet d'une interdiction totale de pêche pour une période de cinq ans. Parallèlement à ces mesures de protection et de conservation, des introductions d'esturgeons ont été réalisées en 1976 dans le but de valoriser les eaux de refroidissement des centrales thermiques situées le long de la Garonne et de la Gironde. Ces introductions concernent deux espèces: Acipenser baeri, originaire de Sibérie, ainsi que A. ruthenus. La reproduction artificielle de A. sturio et A. baeri a été réalisée pour la première fois en France en 1981 (Benoit, 1982), et il est encore trop tôt pour pouvoir dresser un bilan de ces introductions. 2.2 Famille des Salmonidae C'est dans la famille des Salmonidés que nous comptons le plus grand nombre d'introductions. Celle-ci concernent des espèces originaires des Etats-Unis mais, également, une espèce originaire d'Europe Centrale. 2.2.1 La truite arc-en-ciel (Salmo gairdneri Rich.) MacCrimmon (1971) trace l'historique de la dispersion de la truite arc-en-ciel à travers le monde. Cette espèce a été introduite en 1882 en Europe. Elle est élevée dans de nombreuses piscicultures, et les sociétés de pêche pratiquent des repeuplement de plus en plus nombreux à partir de sujets d'élevage, ce qui a entraîné une large distribution de l'espèce sur l'ensemble du territoire. Depuis l'enquête de Vivier (1955), il est toujours aussi difficile de connaître les secteurs, peu nombreux, où cette espèce se reproduit naturellement. 2.2.2 Les saumons du Pacifique Saumon chinook (Oncorhynchus tschawytscha (Walbaum)): Dans un article récent, Mazeau (1981) retrace l'introduction du saumon chinook dans nos eaux douces, qui date de 1877. A la fin du siècle dernier, cette espèce a été largement répandue sur l'ensemble du territoire (Seine, Aude, Ain, Saône, Isère, Dordogne, etc.). La reproduction naturelle n'a jamais pu être observée, et c'est vers 1905 que s'arrête l'expérience d'acclimatation de cette espèce. Ces dernières années des introductions non confirmées de saumons chinook auraient été réalisées. Saumon coho (Oncorhynchus kisutch (Walbaum)): L'introduction de saumon coho en France est le résultats d'initiatives privées (pisciculteurs) et d'organismes publics de recherche (Euzenat et Fournel, 1981). Dans leur travail, ces deux auteurs retracent avec précision les conditions d'introduction de cette espèce qui date de 1971. Les premières captures en rivière ont été effectuées en 1974 à la suite d'évasions des bassins d'élevage et ce n'est qu'à partir de 1977 que sont mis en place les moyens de suivre le devenir de ces poissons dans le milieu naturel (Fournel et Euzenat, 1977–79). Dans les cours d'eau de Normandie, une partie des saumons coho libérée est revenue en rivière après un séjour en mer, et il y a eu reproduction naturelle au moins une année. Par la suite, aucune autre remontée n'a été observée dans ces rivières. L'espèce est considérée comme disparue de ce bassin depuis 1977. Euzenat et Fournel (1981) donnent des explications concernant la non-acclimatation de cette espèce en Normandie (condition de salinité et surtout température trop élevée en mer). C'est à la suite de ces introductions et de mortalités en pisciculture que De Kinkelin (1974) a pu diagnostiquer pour la première fois en France la Corynebactériose plus connue sous le nom de “kidney disease” sur le saumon coho. Saumon du Danube, ou huchon (Hucho hucho L.): Cette espèce a fait l'objet d'une introduction en 1957 dans la rivière les Usses, affluent de la rive gauche du Rhône, en aval du barrage de Génissiat (Ain) (Vivier, Blanc et Svetina, 1964). Le but de cette introduction était de réduire les populations de hotus (Chondrostoma nasus) qui avaient tendance à proliférer dans cette rivière. Les introductions ont été poursuivies jusqu'en 1960; après cette date, la présence de jeunes individus accrédite la reproduction naturelle de cette espèce dans la rivière et en 1964 les auteurs de l'introduction du huchon semblaient assez optimistes quant au développement de cette espèce. Depuis, aucune information précise ne permet d'affirmer la présence du huchon dans les Usses. Des sondages effectués dans le Rhône au niveau de la confluence de la rivière n'ont past permis de capturer un seul individu de cette espèce. Bien que ne figurant pas sur la liste des espèces représentées dans les eaux libres françaises, le huchon est protégé par une taille légale de capture de 0,70 m. Saumon de fontaine (Salvelinus fontinalis Mitchill): Le saumon de fontaine a été introduit en France au siècle dernier (Vivier, 1955). Peu d'informations sont disponibles sun sa répartition et l'importance de ces populations. L'espèce est élevée en pisciculture et présente dans de nombreux cours d'eau alpins et pyrénéens. Cristivomer (Salvelinus namaycush Walbaum): Nous possédons peu d'informations sur la répartition du cristivomer. La multiplication des grands barrages hydroélectriques a favorisé l'introduction de cette espèce dans les lacs artificiels de retenues dans les Alpes, les Pyrénées et le Jura. 2.3 Famille des Cyprinidae Les carpes chinoises: sous le terme général de “carpes chinoises” sont regroupées essentiellement trois espèces phytophages ou zooplanctonophages dont l'introduction est souvent proposée pour lutter contre la végétation aquatique ou améliorer les rendements en aquaculture: Ctenopharyngodon idella Val., Hypophthalmichthys molitrix Val.; Aristichthys nobilis Rich. La première demande d'introduction de carpes chinoises en France date de 1964, époque à laquelle C. idella a été introduite dans les canaux du Bas Rhône- Languedoc. Depuis cette date de nombreux pisciculteurs élèvent ces espèces et les proposent dans leur catalogue. Des captures de carpes chinoises ont été signalées dans le Rhin et surtout la Loire au niveau de la centrale nucléaire de Saint-Laurent-des- Eaux. II s'agit surtout de C. idella, mais également récemment de H. molitrix. On peut penser que ces espèces doivent être largement répandues dans les nombreux plans d'eau créés au cours de ces dernières années. Actuellement, la reproduction naturelle de ces espèces n'a jamais été signalée dans notre pays, mais la modification de l'environnement et, en particulier, les rejets d'eau chaude des centrales thermiques dans les fleuves comme le Rhône et la Loire peuvent créer des conditions favorables à cet égard. Ces espèces ne sont pas considérées comme représentées dans nos eaux libres. 2.4 Famille des Ictaluridae Le poissons chat (Ictalurus melas Rafinesque): Les conditions de l'introduction et de la dispersion du poisson chat en France sont relativement bien connues (Vivier, 1951; Boët, 1982). L'espèce ramenée des Etats-Unis a été stockée dans les bassins du Muséum National d'Histoire Naturelle à Paris en 1871. A la suite d'une erreur de manipulation, quelques individus ont pu gagner la Seine toute proche par l'intermédiaire du réseau d'égout. A partir de 1894, plusieurs spécimens ont été capturés dans le fleuve. Cette espèce avait été primitivement décrite sous le nom de Amiurus nebolusus et ce n'est qu'en 1966, à la suite d'un échange de collections avec le British Museum de Londres, que l'on a pu déterminer l'appartenance exacte des poissons-chats introduits en France un siècle plus tôt (Spillmann, 1967). Au début de ce siècle, le poisson-chat a été largement distribué à partir d'élevages en étangs. L'engouement des Sociétés de Pêche pour cette espèce était lié à la bonne qualité de sa chair et à une certaine aptitude à résister à la pollution. La réussite de l'acclimatation et la grande dispersion de cette espéce ont provoqué un changement radical du comportement des pêcheurs, trouvant ce poisson de plus en plus gênant pour la pratique de la pêche. A tel point, qu'actuellement cette espèce est considérée comme particulièrement nuisible, sans qu'aucune étude ne confirme cet aspect particulier de son comportement (Boët, 1982). 2.5 Famille des Poecilidae Gambusia affinis Girard: La gambusie a été introduite en France en 1927 dans la région méditeranéenne, pour lutter contre la prolifération des moustiques dans les étangs littoraux. Espèce thermophile, elle est limitée à cette région et au littoral sud- atlantique. 2.6 Famille des Centrarchidae La famille des Centrarchidae est originaire du nord des Etats-Unis. Elle est représentée en France par quatre espèces dont deux ont un statut particulier. 2.6.1 Les blackbass Le genre Micropterus comprend dans son pays d'origine six espèces auxquelles Robbins et MacCrimmon (1974) ont consacré une mise au point importante. Le blackbass à grande bouche (Micropterus salmoides Lacépède): Le blackbass à grande bouche a été étudié en France par J. Wurtz-Arlet (1952). C'est en 1877 que l'on signale pour la première fois la présence de cette espèce. Ce n'est qu'à partir de 1889–90 que les premiers succès d'élevage ont été obtenus dans la région de Versailles, les premiers géniteurs semblant provenir d'une pisciculture allemande. Ensuite c'est en Sologne qu'il faut rechercher le centre de dispersion de cette espèce. Actuellement elle est surtout représentée dans la moitié sud de la France au sud d'une ligne Brest-Strasbourg. En plaine ses représentants sont souvent capturés dans les annexes hydrauliques des grands cours d'eau et on peut craindre que les grands travaux d'aménagement ne soient fatals à cette espèce dans de nombreux secteurs (Rhône, Doubs, Seine). Cette espèce est encore élevée dans quelques piscicultures et environ une dizaine de départements pratiquent des réempoissonnements. Elle présente un intérêt halieutique certain dans un environnement occupé aujourd'hui par le brochet, le sandre et la perche. La destruction des zones de frayères du brochet, et les problèmes posés par la prolifération du sandre, laissent une place importante pour le développement du blackbass en France. Actuellement, les populations sont toujours marcelées et souvent en voie de régression. Le blackbass à petite bouche (Micropterus dolomieu Lacépède): Dans son pays d'origine cette espèce se développe dans les secteurs moyens des cours d'eau. En 1951 Vivier signale l'espèce dans le département des Pyrénées Atlantiques et dans les étangs landais, alors qu'en 1925 Roule déclarait que cette espèce n'avait jamais été signalée dans nos eaux douces. En 1952, Huet la signale comme bien acclimatée dans les eaux de la Basse Semois, secteur où la population est en forte régression faute d'une reproduction suffisante. Actuellement, même dans ce secteur, il ne semble plus que l'espèce soit présente (Timmermans, Phillipart, comm.pers.). En 1953, Petit signale l'espèce dans le département de l'Hérault, dans le midi de la France. Toutefois, l'absence de collection et de capture récente ne permet pas d'affirmer actuellement la présence de l'espèce en France (Wurtz-Arlet, 1952). 2.6.2 Les sunfish ou crapet La perche soleil (Lepomis gibbosus Lacépède): Les premiers essais d'introduction de la perche soleil datent de 1886 et c'est à partir du sud-ouest de la France que s'est effectuée la dispersion de l'espèce. Vivier (1951) signale la capture en 1907 de spécimens pesant 1 300 g. Ce poids représente une valeur jamais atteinte par les sujets actuellement capturés, ce qui laisse supposer une erreur d'identification ou une réduction considérable de la taille atteinte par cette espèce, suite à son acclimatation. La perche soleil a colonisé un grand nombre de cours d'eau et en 1926 roule pouvait écrire, “… on peut la considérer comme répandue dans notre pays presque entier où elle se reproduit et se maintient d'elle-même …”. Cette espèce, comme le poisson-chat, est considérée comme particulièrement nuisible. Le crapet des roches (Ambloplites rupestris Rafinesque): C'est en 1976 que des pêcheurs du département de Saône et Loire nous ont fait parvenir un exemplaire de crapet des roches capturé dans la Loire en amont de Digoin (Allardi, 1978–80). Cette espèce ne figure pas dans la liste des espèces acclimatées en France ni dans l'ouvrage de Spillmann (1961). Après enquête, nous avons pu retrouver l'origine de l'introduction de cette espèce dans la Saône en 1907 où son acclimatation n'avait pas réussi. Pourtant, en 1910, Gensoul considérait cette espèce comme acclimatée dans la Saône et effectuait son transfert vers des étangs situés sur le Sornin, affluent de la Loire, où l'espéce a proliféré au point de coloniser plusieurs affluents de la Loire: Sornin, Aronce et Loire. Jusqu'à ces dernières années, l'espèce est passée totalement inaperçue, confondue par les pêcheurs avec le “blackbass” sans autre précision. Actuellement, elle ne semble pas étendre son aire de répartition et n'est pas considérée comme représentée dans les eaux libres françaises. 2.7 Famille des Percidae Le sandre (Stizostedion lucioperca L.): L'introduction et la dispersion du sandre sont parfaitement connues (Vivier, 1951) et surtout Goubier (1975) auquel nous empruntons le maximum d'informations concernant cette espèce. En 1889, dans son traité sur la pisciculture en eau douce, Gobin cite le sandre, “… comme un poisson étranger dont l'acclimatation est proposée …”. La première enquête sur la répartition du sandre date de 1925 (Chevey) qui signale l'espèce en 1912–13 dans le canal du Rhône au Rhin en Alsace, dans le Doubs (1915–20) et enfin la Saône (1920). Kreitmann l'a signalé en 1932 pour la première fois dans le Rhône. En 1940, la colonisation du Rhône inférieur est pratiquement terminée. A partir de 1958, la création d'une pisciculture en Camargue a favorisé la dispersion et la prolifération du sandre sur l'ensemble du réseau hydrographique de plaine. Cette prolifération a été caractérisée par une implantation “silencieuse” les pêcheurs ne sachant pas le capturer. A la suite de cette période d'implantation des captures très importantes ont été observées mais souvent de courtes durées (quelques années au plus). Ce phénomène a provoqué deux types de réactions: satisfaction chez les uns, eu égard à la qualité de la chair de ce poisson, inquiétude chez les autres, ce poisson étant accusé de “tuer pour le plaisir de tuer!…” ou de faire disparaître le brochet par exemple. L'intérêt nouveau pour le sandre est surtout lié à l'apparition d'une épizootie provoquant des mortalités de poissons très importantes (De Kinkelin et al., 1969). Cette maladie parasitaire est provoquée par un trématode digénien Bucéphalus polymorphus, dont le ver adulte se développe dans le sandre. Les hôtes intermédiaires étant Dressenia polymorpha, petit lamellibranche parfois très abondant, et des poissons infestés par les métacercaires. Cette maladie mise en évidence dans le bassin de la Seine en 1964, s'est étendue au bassin de l'Yonne, de la Saône, de la Marne, où elle provoque des mortalités importantes (Tuffery, 1977). Dans les cours d'eau menacés par cette maladie, l'introduction du sandre peut être interdite. 3. DISCUSSION Parmi les dix-sept espèces passées en revue dans cette analyse bibliographique, neuf sont considérées comme acclimatées. Nous avons vu que parmi celles-ci, certaines ont un statu très incertain (Blackbass à petite bouche et huchon). Deux espèces sont classées comme nuisibles (poisson-chat et perche soleil). Parmi les huit autres espèces, la situation du rock bass, ou crapet des roches, est très particulière; présent dans des affluents de la Loire depuis le début du siècle, il a pu passer inaperçu jusqu'à ces dernières années. L'introduction de la truite arc-en-ciel est sans doute celle qui a donné lieu au développement le plus important (pisciculture et repeuplement) alors que la situation des autres salmonidés est très mal connue. La dispersion et la prolifération du sandre sont à l'origine d'une grave épizootie qui décime les peuplements ichtyologiques des plus grandes rivières de plaine. Ces introductions ont souvent été réalisées dans un but d'élevage en eau close, mais de nombreuses “fuites” sont à l'origine de la colonisation des milieux naturels (eau libre). Ceci montre bien la limite de la législation actuelle difficilement applicable. La protection des peuplements ichtyologiques autochtones nécessite un contrôle renforcé des introductions d'espèces nouvelles y compris en aquaculture. Toute introduction devrait faire l'objet d'une étude préalable dans le pays d'origine et d'une étude approfondie du bassin hydrographique récepteur. En cas d'introduction, un suivi attentif de l'espèce devrait être obligatoire. Une meilleure connaissance de la répartition et de l'importance des stocks, ainsi qu'une gestion plus rationnelle des ressources locales devraient limiter les tentations de certains gestionnaires qui veulent pallier la dégradation générale des milieux par l'introduction “d'espèces miracle”. 4. REFERENCES Allardi, J., Ambloplites rupestris (Rafinesque, 1817) (Poisson centrarchidae) espèce 1978 nouvelle pour la faune de France. Cybium, (3e Sér.), 1978(3):53–7 , Note complémentaire sur la présence dans la Loire d'Ambloplites upestris 1980 (Poisson centrarchidae). Cybium (3e Sér.), 1980(9):18 Benoit, G., Le plan esturgeon. Communication Entretien Ecologique de Dijon (sous presse) Boet, P., Eléments d'écologie du poisson-chat Ictalurus melas (Rafinesque, 1820), du 1982 lac de Créteil. Structure et dynamique de la population. Exploitation des ressources alimentaires et production. Thèse, Doctorat Université de Paris VI, 123 p. Chevey, P., Sur l'extension de l'aire de répartition géographique du sandre (Lucioperca 1925 lucioperca L.) poisson d'Europe centrale et orientale. Bull.Soc.Cent.Aquicult.Pêche, 32:18–20 CTGREF (Centre Technique du Genie Rural des Eaux et Forêts), Etude halieutique de 1979 l'estuaire de la Gironde, Bordeaux, Rapport EDF/Division Aménagements Littoraux et Aquaculture, 189 p. Euzenat, G., et F. Fournel, L'introduction des saumons du Pacifique en France. 1981 Ministère d'Environnement et Cadre de Vie, 111 p. Fournel, F. et G. Euzenat, Etude des salmonidés migrateurs du bassin de l'Arques 1977 (Seine-Maritime). Paris, Conseil Supérieur de la Pêche, Rapport internal, 53 p. (mimeo) , Etude des salmonidés migrateurs du bassin de l'Arques 1979 (Seine-Maritime). Paris, Conseil Supérieur Pêche, Rapport internal, 54 p. (mimeo) Gensoul, J., Le Rock-bass. Bull.Soc.Nat.Acclim.Fr., 57:112–6 1910 Goubier, J., Biogéographie, biométrie et biologie du sandre Lucioperca lucioperca (L.) 1975 Osteichthyen percidé. Thèse Doctorat Université Cl. Bernard Lyon, 259 p. , Acclimatation du sandre ( lucioperca L.) dans les eaux françaises. 1972 Verh.Int.Ver.Theor.Angew.Limnol., 18:1147–54 Huet, M., Traité de pisciculture. Bruxelles, Editions La vie rustique, 369 p. 1952 Kinkelin, P. de, Corynebactériose des salmonidés: première observation en France. 1974 Bull.Fr.Piscic., (254):4–7 Kinkelin, P. de, et al., La Bucéphalose larvaire à Bucephalus polymorphus (Baer, 1827). 1969 Pathogénie, epizotiologie, possibilités d'intervention. Bull.Fr.Piscic., (234): 5–20? Kreitmann, L., Les grandes lignes de l'économie piscicole du bassin français du Rhône. 1932 Trav.Lab.Hydrobiol.Piscic.Univ.Grenoble, (24):127–51 MacCrimmon, H.R., World distribution of rainbow trout (Salmo gairdneri). 1971 J.Fish.Res.Board Can., 28(5):663–704 Magnin, E., Recherche sur la systématique et la biologie des Acipenseridés. Acipenser 1962 sturio L., Acipenser oxyrhynchus Mitchill et Acipenser fulvescens Raf. Ann.Stn.Cent.Hydrobiol.Appl., 9:9–242 Mazeaud, F., La Société Nationale d'Acclimatation et le saumon californien au 19ème 1981 siècle. Saumons, 35:3–6 Petit, G., Présence de Micropterus dolomieu Lac. dans le canal des étangs (Hérault). 1953 Vie Milieu, 4:758–9 Robbins, W.H. et H.R. MacCrimmon, The black-bass in America and overseas. Sault 1974 Ste-Marie, Ontario, 196 p. Roule, L., Les poissons des eaux douces de la France. Paris, Presses Universitaire. 1925 Spillmann, C.J., Les poissons d'eau douce. Faune Fr., 65:303 p. 1961 , Sur l'identité spécifique des poissons chats importés d'Amérique du 1967 Nord et répandus actuellement dans les eaux douces françaises. Bull.Mus.Paris (2e Sér.), 39(2):288–92 Tuffery, G., Recherche sur la Bucéphalose à Bucephalus polymorphys Baer 1827. 1977 Introduction à l'écopathologie des systèmes piscicoles. Thèse Doctorat Université de Paris VI, 137 p. Vivier, P., Poissons et crustacés d'eau douce acclimatés en France en eaux libres 1951 depuis le début du siècle. Terre Vie, (2):57–82 , Sur l'introduction des salmonidés exotiques en France. 1955 Verh.Int.Ver.Theor.Angew.Limnol., 12:527–35 Vivier, P., L. Blanc et M. Svetina, Le huchon et son acclimatation en Haute-Savoie. 1964 Bull.Fr.Piscic., 212:77–85 Vooren, C.M., Ecological aspects of the introduction of fish species into natural habitats, 1972 in Europe, with special reference to the Netherlands. A literature survey. J.Fish Biol., 4:565–83 Wheeler, A. et P.S. Maitland, The scarcer freshwater fishes of the British Isles. l. 1973 Introduced species. J.Fish Biol., 5:49–68 Wurtz-Arlet, J., Le black-bass en France. Esquisse monographique. 1952 Ann.Stn.Cent.Hydrobiol.Appl., 4:203–86 A REVIEW OF FISH AND CRAYFISH INTRODUCTIONS MADE IN FINLAND

K. Westman and P. Tuunainen Finnish Game and Fisheries Research Institute Helsinki, Finland

CONTENTS

1. INTRODUCTION 2. NEW FISH AND CRAYFISH SPECIES 2.1 General 2.2 Siberian whitefish (Coregonus ) 2.3 Rainbow trout (Salmo gairdneri) 2.4 (Oncorhynchus tsawytscha) 2.5 Sockeye salmon (Oncorhynchus nerka) 2.6 Brook trout (Salvelinus fontinalis) 2.7 Lake trout (Salvelinus namaycush) 2.8 Splake (Salvelinus namaycush × S. fontinalis) 2.9 Carp (Cyprinus carpio) 2.10 “Galician” tench (Tinca tinca spp.) 2.11 Smallmouth black bass (Micropterus dolomieu) 2.12 Bigmouth black bass (Micropterus salmoides) 2.13 Signal crayfish (Pacifastacus leniusculus) 3. RISK FACTORS CONNECTED WITH INTRODUCTIONS OF EXOTIC SPECIES 4. PROCEDURES TO BE FOLLOWED IN THE INTRODUCTION OF NEW SPECIES 5. REFERENCES

ABSTRACT Since the middle of the last century, at least 15 new fish species and one new crayfish species have been introduced to Finland. In addition, a number of fish strains and hybrids have been imported. The imports of new species have been made in an attempt to increase the production of fishing waters and to create entirely new fishing opportunities; and from the fifties onward to help in solving the particularly difficult problems in fisheries management caused by hydro-construction, water-level regulation and water pollution. Fast-growing species were transferred to promote fish cultivation and hardy, resistant species were introduced in an attempt to prevent diseases. The following species are known or are thought to be of importance in fisheries management in Finland: the Siberian whitefish (Coregonus peled), lake trout (Salvelinus namaycush), brook trout (Salvelinus fontinalis), splake (S. namaycush × S. fontinalis), carp (Cyprinus carpio), tench (Tinca tinca) and the signal crayfish (Pacifastacus leniusculus). Experimental stockings with peled whitefish and lake trout have given such promising results that extensive stockings for the improvement of stocks have begun. A total of 6 105 000 one-summer and older whitefish juveniles and 53 000 two-year and older lake trout juveniles were stocked in 1980. Trial stockings with the other species continue. For the moment, the species introduced into Finnish waters do not appear to have caused any harmful effects. The greatest problem in stockings has been that the new species have done poorly in our harsh conditions. The long winter and short growing season has severely limited their growth, reproduction and spread. In the best circumstances some species have reproduced in some stocking waters but no signs of strong increase or spreading have been observed. This paper deals with the risks connected with the introduction of new species and means of reducing these risks. RESUME Depuis le milieu du siècle dernier, au moins Il espèces nouvelles de poissons et une espèce nouvelle d'écrevisse ont été introduites en Finlande. En outre, un certain nombre d'hybrides et de races de poissons ont été importés. L'importation de nouvelles espèces visait à accroître la production des eaux et à créer des possibilités de pêche jusque-là inconnues. A partir des années cinquante, on a surtout cherché par ce moyen à résoudre les difficiles problèmes d'aménagement causés par la construction d'ouvrages hydro-électriques, par la régulation du niveau des eaux et par la pollution aquatique. On a également transplanté des espèces à croissance rapide en vue de promouvoir la pisciculture et des espèces résistantes afin de prévenir les maladies. Les espèces suivantes sont considérées comme importantes dans l'aménagement des pêches finlandaises: poisson blanc sibérien (Coregonus peled), truite de lac (Salvelinus namaycush), omble de fontaine (Salvelinus fontinalis), splake (S. namaycush × S. fontinalis), carpe (Cyprinus carpio), tanche (Tinca tinca) et écrevisse américaine (Pacifastacus leniusculus). Les résultats des essais de repeuplement avec Coregonus peled et Salvelinus namaycush ont été si prometteurs que l'on a décidé d'utiliser ces deux espèces sur une grande échelle. On a ainsi mis à l'eau en 1980 6 105 000 poissons blancs d'un été ou plus et 53 000 truites de lac de deux ans ou plus. Les essais de repeuplement se poursuivent pour les autres espèces. Pour le moment, les espèces introduites dans les eaux finlandaises ne semblent pas avoir eu d'effets préjudiciables. Le plus grand problème est qu'elles donnent des résultats assez médiocres dans les conditions particulièrement rudes qui règnent en Finlande. L'hiver est long et la saison de croissance courte, ce qui limite beaucoup leur croissance, leur reproduction et leur propagation. Dans le meilleur des cas, certaines de ces espèces ont réussi à se reproduire mais aucun signe d'accroissement important ou de propagation n'a été signalé. L'auteur indique quels sont les risques entraînés par l'introduction de nouvelles espèces et quels sont les moyens de les limiter. 1. INTRODUCTION In Finland, attempts began more than 100 years ago, through introductions of exotic species, at increasing the production of fish beyond that occurring naturally or at the creation of entirely new fishing possibilities alongside those provided by the original fauna. The fact that environments in their natural state are becoming increasingly rare, due to construction and to pollution in the aquatic systems, also has resulted in increased interest in those fish species which appear to be capable of surviving better than the original species in the altered waters as in reservoirs, regulated lakes or eutrophicated water systems, etc. The introduction of fast-growing, disease-restistant fish and crayfish species has also been motivated by the promotion of aquaculture and the prevention of disease. Many attempts have been made at enriching Finland's natural fish stocks with exotic fish species and strains. Carp (Cyprinus carpio) was brought to our waters for the first time in the 1860s, and even rainbow trout (Salmo gairdneri) will soon have over a century of history in Finland. Experimental activity increased appreciably when the Evo Inland Fisheries and Aquaculture Research Station, located at Lammi in southern Finland, began operations in 1982 (Seligo, 1897; Brofeldt, 1920). From the very beginning, one of the most important functions of the Station was considered to be the acquisition of “the more valuable fish species” for the waters of the Evo State Park. Hatching, raising and stocking trials were carried out with rainbow trout, brook trout (Salvelinus fontinalis), Lake Peipusjärvi whitefish (Coregonus sp.), tench (Tinca tinca) and big and little mouth black bass (Micropterus salmoides and M. dolomieu) (Brofeldt, 1920). These and other stocking trials with new species at the beginning of this century did not produce any results to speak of. Just as poor were the results of large-scale attempts in the thirties at domesticating the king (i.e., chinook) salmon (Oncorhynchus tsawytscha). By the beginning of the sixties, only two new species of fish, the Galician tench (Tinca tinca spp.) and the brown bullhead (Ictalurus nebulosus) had been successfully acclimatized in a few of the waters in southern Finland. Through continuous stocking, it has also been possible to maintain populations of rainbow trout and carp in natural waters. The main reason for the almost complete failure of most of the stockings of the exotic species attempted was probably poor planning and haphazard introduction. The characteristics and environmental requirements of species have not generally been known beforehand and were not investigated. Some species have evidently been unable to adapt to our harsh climate. Other reasons for the poor results may be that the stocking water was unsuitable for the species, stockings were on too small a scale, or juveniles of too small a size were used as stocking material. Since the mid-fifties there has been considerably renewed interest in the possibilities offered by foreign fish species for developing Finnish fisheries. In particular, an attempt has been made to find new species suitable for managing waters which have proved problematic for fisheries owing to construction, water level regulation or pollution. Instead of the “trial and error” method, an attempt has been made to thoroughly investigate the characteristics of new species and their chances of survival in our waters before commencing stocking trials. Promising species have been used for planned trial stockings in carefully selected waters. These stockings have been used to test the possibilities of a species suitable for management purposes. Trials with many species are still underway but some species have proved so promising that it has been possible to begin widespread management stockings. 2. NEW FISH AND CRAYFISH SPECIES 2.1 General Since the middle of the last century, at least 15 new fish species and one new crayfish species have been imported into Finland for introduction. Table 1 presents selected data on the species used and the success of the stockings. In addition to new species, a large number of fish strains and hybrids have been introduced into Finland, including hybrid carp (cross between Amur River wild carp and Galician mirror carp), Neva salmon, Dahl River sea trout, Gullspangsälven landlocked salmon, Lake Peipusjärvi whitefish and a number of Swedish salmon strains. A number of different stocks of the exotic species have also been imported, for example, stocks of kamloops, American and Danish rainbow salmon. Stockings of splake (Salvelinus namaycush × S. fontinalis) have also been made in Finland. Pink (humpback) salmon (Oncorhynchus gorbusha) and chum salmon (O. keta) have appeared in some of our northern rivers (Tenojoki and Näätämöjoki) as well as along the coast of the Gulf of Bothnia and the Gulf of Finland, originating from stockings in the U.S.S.R. The purpose of this review is to describe briefly the history and results of stockings of exotic fish and crayfish species carried out in Finland. 2.2 Siberian whitefish (Coregonus peled) The Siberian whitefish or peled whitefish originates from Siberian lakes and rivers. Its eggs were imported into Finland in 1965, and broodfish stocks were established in fish farms as well as in some natural lakes (Tuunainen, 1968). One- summer old juveniles have usually been used for stocking and introduction (Westman et al., 1983) and 5.6 million fish of this age were produced in 1979. Results from the stockings have usually been good in polyhumous and in eutrophicated waters as well as in Lokka Reservoir (maximum area 417 km2) in Finnish Lapland. The reservoir is polyhumous. When the reservoir was young (in the seventies) stockings with newly-hatched fry also gave good results. Later, one-summer old young were used for stocking. The catch of peled whitefish has been as high as 12 percent (about 10 tons) of the total catch in Lokka Reservoir. In addition to C.peled, C. lavaretus, Salmo trutta and splake, were used for introductions in this reservoir. The zooplankton- feeding C.peled proved to be the only one of these species which has been of any real importance to the fishery. The potential importance of this species in Finland seems to be quite high, but its importance to the fishery in practice is still mainly unknown. Naturally reproducing stocks have been found in two cases. Comparative studies of the growth, production and food supply of the introduced C. peled and the native whitefish C. muksun, stocked in the same small forest lakes in southern Finland have been made recently (Hakkari et al., 1983; Pruuki, Pursiainen and Westman, 1983). 2.3 Rainbow trout (Salmo gairdneri) The first stockings of rainbow trout were made with eggs of rainbow trout imported from Germany in 1897. Fry and older fish were released into small lakes and ponds of the Evo district, Lammi, as well as into Lake Saimaa, at the beginning of the 20th century. In the Evo lakes the fish grew to a size of 300–400 g and in the Saimaa area, near Savonlinna, a rainbow trout weighing 3.8 kg was caught. No self-reproducing stocks originated from these stockings and the fish disappeared some years after the stocking (Tuunainen, 1970). In the sixties, attention was again paid to the rainbow trout. Rainbow trout were introduced into watercourses all over Finland and also into the sea. Introductions were also made in southern and central Finland into small forest lakes, which were first treated with rotenone to eliminate other fish. The purpose of these experiments was to study the ecology of rainbow trout in such conditions, and to investigate their importance in sport fishing (Tuunainen, 1970). A large cultivation programme was also started for food fish production with fish originating from Denmark and later some stocks were also imported from the U.S.A. To investigate the profitability of the stockings, 17 817 one and two-year old rainbow trout were tagged with Carlin tags in 1963–66. The recapture rate varied remarkably, from 0.3 to 61.1 percent, the average being 4.3 percent. About half of these taggings was carried out in southern Finland in lakes, as well as in the sea area. Of the 7 047 rainbow trout of 13.5–29.5 cm, only 117 were recaptured during the same year and not more than 16 tags were returned during the second year after tagging. During the third year after tagging only one tag was returned. The recapture rate varied from 0.3 to 8.6 percent in the different water areas, higher recapture rates indicating high fishing intensity. The total average catch per one thousand introduced fish was not more than 22 kg. Thus, the life of the stocked fish in this case is not very long and, because of the low recapture rate, the profitability of the stocking remains low. No natural reproduction was found nor have any possibly unfavourable effects on other fish been reported. Later on, rainbow trout gained popularity for put-and-take sports fisheries in many areas (Svärdson and Tuunainen, 1968). In small Finnish forest lakes, empty of other fish, the results with rainbow trout were fairly good. They were introduced together with brown trout in eight lakes to study competition between the two species (Tuunainen, 1970). In the first and second years, the mean weight of rainbow trout was greater than, and in some cases double, that of the brown trout, but after the third growing season the brown trout were usually heavier than the rainbow trout. The total catches of rainbow trout and brown trout were 1.3–10.6 kg/ha/year. The maximum size of rainbow trout in these lakes was 1.0–1.2 kg, which was reached in 2–4 years. The scarcity of food and small size of the food organisms were the most important factors limiting the growth of trout in the small forest lakes treated with rotenone. The similarity of food taken by rainbow trout and brown trout in the eight lakes was 54–63 percent in July–September, and 73–100 percent at other times. In some other lakes in which additional food was provided as dry pellets, the fish did better and their growth was also better than in the lakes with natural food only. However, eutrophication of the lakes and oxygen deficiency in the winter was found to be the most important factor limiting this cultivation method. Lakes with additional feeding were established for sport fishing purposes. Cultivation of rainbow trout as food fish in fish farms and in net cages has been very successful. During the last fifteen years, this type of culture has been developed more than any other branch of fish culture in Finland. The production of rainbow trout for the market in 1979 was 794 t from sea farms and 2 481 t from inland farms, and the production is still growing (Westman et al., 1983). 2.4 Chinook salmon (Oncorhynchus tsawytscha) Great efforts to introduce chinook salmon into Finland were made in the thirties. Over a four-year period, some 0.65 million eggs were imported from the west coast of the U.S.A. Reared juveniles were stocked in inland waters and in rivers flowing to the Baltic. The results of this activity were poor and the species did not become acclimatized in the waters selected for experiments. 2.5 Sockeye salmon (Oncorhynchus nerka) Because of the lack of success with chinook salmon, no further Oncorhynchus were imported into Finland for stocking prior to 1966, when a batch of eggs of sockeye salmon was imported from the U.S.A. The species was cultivated at a few fish hatcheries and juveniles were released during the sixties and seventies in small lakes in central Finland. The species is not known to have reproduced in waters in which it has been released. Random small introductions have not met with any remarkable results and stocks of the species are no longer maintained at fish hatcheries in Finland. It remains possible that stocking trials will be reattempted within the next few years. 2.6 Brook trout (Salvelinus fontinalis) The North American brook trout was brought to Finland for the first time in the last century. The importation of the species from Germany in 1895 was aimed at establishing farming and for stocking trials. Fresh stocking trials were initiated in 1965, this time using imports from the U.S.A. The species has been reared at several fish farms and introductions have been made specifically in small waters. With a few exceptions, stocking in running waters has met with little success and current interest in the farming of this species is slight. It has been suggested that the stock imported into Finland has become too well adapted to fish hatchery conditions and for this reason is unable to succeed in nature. It is also possible that the environmental requirements of the species are too poorly known and the habitats selected have not been suitable. It is not known for certain whether the species has established self-perpetuating populations in natural waters. Although goals respecting brook trout have not been attained, it is obvious that stocking trials will be continued. In connexion with further attempts, the objectives of introduction, selection of suitable waters and catching recommendations must be given special attention. 2.7 Lake trout (Salvelinus namaycush) Eyed eggs of lake trout were imported from the U.S.A. to Finland for the first time in 1955. The eggs originated from the Lake Superior stock. The second import took place in 1966 with eggs originating from Lake Opeongo, Canada. The juveniles of lake trout were introduced into lakes all over Finland, and also since 1957 into certain sea areas (Mutenia, Simola and Tuunainen, 1983). Broodfish stocks were established in fish farms from both of these introductions. In 1963, the first young originating from the broodstock were introduced into natural waters, mostly at two and three years of age. Fish from two months to five years old have also been introduced (Nilsson and Dahlström, 1968; Westman et al., 1983). The positive results of introductions have led to quite extensive stocking of Lake Inari in Finnish Lapland with lake trout (Mutenia, Simola and Tuunainen, 1983). Fish of 5 kg and over have been caught from Lake Inari and from two other places at least. Natural reproduction may have taken place in two lakes in Finnish Lapland but so far has not been of any practical importance. In Lake Inari, the lake trout catch (10 t in 1980) was about 10 percent of the total catch, and these fish have proved even more important to the fishery because they eat the stunted and low-valued whitefish, which are thereby converted to highly appreciated food fish. 2.8 Splake (Salvelinus namaycush × S. fontinalis) The first splake cross was made in Finland by the Finnish Fisheries Foundation in 1968. Interest in the species was deepened by the suitability of the species for stocking into regulated lakes as shown in North America. The species has been farmed at a few fish hatcheries. Introductions have been made over recent years using a few thousand two-year or older juveniles, especially in northern Finnish regulated lakes. The species has not reproduced in our waters and the introductions made so far have not produced any remarkable results. The splake farming and stocking trials have been abandoned for the time being. 2.9 Carp (Cyprinus carpio) The earliest attempts to introduce carp into Finland were made in 1861, but positive results were not obtained until farming and stocking were begun again in the fifties. The present stock of carp originates from the Aneboda Fish Culture Station in Sweden. At first, the aim of the introduction was to see if carp could be cultivated so far north but due to the promising results obtained, the original aim was extended to see if carp would succeed in natural waters and to study its possibilities especially for the management of eutrophicated waters. Juveniles are raised in earthen ponds, primarily on natural feed. Cultivation has been run in one phase, using the uncontrolled reproduction method (Ahlfors, Kummu and Westman, 1983). A total of about 138 000 one-year and older juveniles were stocked from 1957 to 1980 in inland and brackish waters between 60° and 68°N. Most of the stocking has been made in southern Finland. In order to study the profitability of carp stockings, two- year and older stocked juveniles were marked with Carlin tags for a total of 13 000 individuals. In inland waters, the rate of return of carp tags has varied between 0 and 32 percent, which, in terms of catch, means 0–336 kg/1 000 individuals stocked (Sormunen et al., 1976; Ahlfors, Kummu and Westman, 1983). Stockings were not economically profitable although the picture of the survival of stocked juveniles obtained from returned tags is worse than is actually the case, since tags are lost and not all tags were returned. In addition, Finnish fishermen are unaccustomed to fishing for carp. The best results were obtained with two or three-year old juveniles at least 20 cm in length and weigh 150 g or more. In eutrophicated waters with a great deal of vegetation, carp had an incremental weight gain during the stocking year of up to 1 000– 1 500 g. The largest tagged carp caught weighed 5 100 g and the largest untagged 11 900 g. Under Finnish conditions, carp would evidently not appear to be able to form a reproductive population, since the juveniles do not survive the first winter in natural waters. The best stocking results have been obtained in eutrophicated lakes, where carp, which are able to survive in waters suffering from lack of oxygen, would be of use for the management of eutrophic and eutrophicated waters in south Finland, which have proved difficult to develop for fisheries. On the basis of trials carried out in Finland over a 30-year period, carp does not appear to cause any damage to the fish stocks, vegetation or general ecosystem of lakes in which it is introduced. Neither does the species appear to detrimentally compete with indigenous fishes by displacing them or by forming hybrids. No dangerous fish diseases or parasites are known to be associated with carp. With carp introductions, the objectives set have been partially realized, the biggest problem having been the lack of adequate catching operations (Ahlfors, Kummu and Westman, 1983). 2.10 “Galician” tench (Tinca tinca spp.) In the mid-thirties tench of a fast-growing strain related to the tench indigenous to south Finland were imported into Finland from the Aneboda Fish Culture Station in Sweden. The strain originated from Galicia, Poland. The Porla Fish Culture Station raised one to two-summer juveniles for stocking purposes. The culture methods and ponds employed were the same as those used for carp, which were raised simultaneously with the tench (Ahlfors, Kummu and Westman, 1983). Numerous introductions of tench have been made in numerous natural water bodies as far north as latitude 66°30'. The largest specimens recaptured have weighed over one kilogramme. The strain would appear to be of significance especially in nutrient-rich waters in the south suffering from oxygen lack during the winter. No damage by tench either to other fish in the stocked areas or to other forms of aquatic life has been observed. The greatest difficulty in tench stocking seems to be the low value placed on the species which has resulted in a lack of interest in catching and utilizing this fish. 2.11 Smallmouth black bass (Micropterus dolomieu) A number of attempts have been made to acclimatize this species in Finland, but with poor results. The first importations of smallmouth black bass fry were made in the 1890s from Germany to the Evo district in southern Finland. These stockings made in small forest lakes failed to establish self-reproducing populations. Smallmouth black bass imported from Canada to the Porla Fish Culture Station in 1958 grew well during the first summer but died in the subsequent winter. A small stock of smallmouth black bass was again imported to Finland from Sweden in 1966. The fish were stocked in a small lake in northern Finland, but did not form a self-reproducing population and the stocked specimens disappeared within a few years. 2.12 Bigmouth black bass (Micropterus salmoides) This species was introduced from Germany in 1890s to the lake area at Evo, southern Finland, but without any success. No further introductions have been made. 2.13 Signal crayfish (Pacifastacus leniusculus) Since 1893 numerous but unsuccessful attempts have been made in Finland to control the disastrous crayfish plague (Aphanomyces astaci). No resistant strains of the only native crayfish, Astacus astacus, have developed. In order to restore crayfish production and to improve the earlier important crayfish fisheries, the plague-resistant signal crayfish, Pacifastacus leniusculus, was introduced into Finland for research and trial stockings in 1967 (Westman, 1973). In 1967–74 a total of circa 40 000 juvenile and adult signal crayfish were stocked in 53 crayfish lakes devastated by the plague. The greater part, about 35 500, were newly-hatched juveniles imported from Sweden; the rest were further cultivated, larger juveniles and adults. Adult signal crayfish were imported from the U.S.A. Since 1974, no new introductions have been made, as we wished to have the results of previous stockings before continuing. Signal crayfish seem to be able to survive and reproduce under Finnish conditions at least up to 63°30'N and to tolerate fishing pressure. Yearly removal of legal-size (10 cm) signal crayfish from some of the experimental lakes does not seem to have caused any harm to the populations or to their renewal. In a few lakes both Pacifastacus and Astacus occur simultaneously, offering excellent possibilities for comparative studies of both species (Westman and Pursiainen, 1979). According to studies made on the same small lake, signal crayfish seem to grow faster and become sexually mature earlier than the native crayfish. Both species seem to prefer similar biotopes and have similar bionomics and life histories. No signs of hybridization have been observed. No harmful effects of the signal crayfish population have so far been observed, either on the Astacus population or on the ecosystem of the lake. However, the niches of both species seem to overlap to such a great degree that competition for living space will probably occur (Westman and Pursiainen, 1983). Signal crayfish show high, but not complete, resistance against the crayfish plague fungus (Unestam, 1973) and if infected specimens are stocked into or migrate to new water areas, they may spread the fungus, thus causing harm to the highly susceptible native Astacus astacus. 3. RISK FACTORS CONNECTED WITH INTRODUCTIONS OF EXOTIC SPECIES In the last few years the introduction of exotic species has been approached, both in Finland and elsewhere, with increased caution. Several international organizations, including the International Council for the Exploration of the Sea (ICES), the FAO European Inland Fisheries Advisory Commission (EIFAC) and the International Union for the Conservation of Nature and Natural Resources (IUCN), have made recommendations concerning the introductions of new species which advise that tighter controls be adopted for stockings. Registers and reviews of fish and crayfish stockings and their results and effects have also been drawn up under the auspices of, for instance, FAO and EIFAC (Rosenthal, 1976; Welcomme, 1981). The most intense opposition to introductions has come from those who hold the opinion that stocking with exotic species amounts to the “falsification” of the fauna. This judgement is based on the damage which the transfer of exotic species may cause, such as that which occurred when sparrows were imported to North America or rabbits to Australia. Perhaps the best known examples in Finland have been the spread of the damage caused by mink and muskrat. What detrimental effects may result from the introduction of exotic fish or crayfish species? The most important may be the following, modified from Rosenthal (1976) and Shafland (1979): − uncontrolled, vigorous reproduction and spread of the introduced species, leading to direct or indirect competition with, and eventual elimination of, native species; − introduction of new pests, diseases and parasites, harmful to native species and possibly humans; − change in behaviour by the exotic species following introduction, thus allowing it to occupy an unexpected ecological niche or even effect unexpected changes in the behaviour of native species; − hybridization with closely related native or previously established foreign species; − other possible detrimental effects caused by the introduced species to the aquatic flora or fauna, or to such economic interests as fisheries; − the growth and development of the introduced species may be reduced because of less favourable environmental conditions than those found in their indigenous area, resulting in worthless, stunted populations which, even if not always directly detrimental, may nonetheless be a great nuisance in the management and utilization of other natural aquatic resources. Rosenthal (1976), Momot (1978) and Shafland (1979) have reviewed and discussed examples of the deleterious side-effects of introductions (see also Welcomme, 1981). The exotic fish and crayfish species introduced in Finnish waters do not at the moment appear to have caused any damage. The greatest problem with introductions has been that new species have generally not thrived in our harsh climatic conditions. The long winter and short growing season seem to limit too severely the growth, production and spread of fish and crayfish. Of the species introduced in Finland, so far evidently only the signal crayfish (Pacifastacus leniusculus) has been able to form self- reproducing populations in at least some of the stocking waters; but there has been no observed sign whatever of too vigorous reproduction and spread even of this species. Under Finnish conditions there seems to be a clear distinction between the detrimental effects of exotic warm-blooded animals living on land and cold-blooded creatures living in water. Mammals are active throughout the year and usually move freely both on land and in the water; while the cold-blooded aquatic fish and crayfish are very passive during the long cold season, and their environment is always limited. In addition, because of their spawning behaviour and habit of swimming in schools, the abundance of fish and crayfish populations can be controlled by fishing and other methods much more easily and effectively than can the abundance of mammal populations. In warm countries, considerable damage has been caused by the transfer of new aquatic species. These instances should be kept in mind as warnings but, as such, they should not be applied under our much more severe conditions. 4. PROCEDURES TO BE FOLLOWED IN THE INTRODUCTION OF NEW SPECIES In Finland there is still a great deal of interest in the introduction of exotic species. In the management for fisheries of modified waters or those in which construction has taken place, as well as in fish cultivation, there is adequate reason, to search for and conduct trials with other fish and crayfish species than those which are indigenous to the country. Of central importance here is the question of finding procedures which will reduce the risks attached to the introduction of new species to the minimum. In planning for the introduction of exotic species, it is of primary importance to remember that it is impossible to predict with any certainty the results of these introductions. Foreign organisms introduced into new ecosystems do not necessarily behave in the same way as they do in their native habitats (of. Shafland, 1979). The short-term effects a new species has on an ecosystem are almost never the same as its long-term impact. The many aspects relating to the introductions of foreign organisms should be carefully evaluated since, if an exotic fish or crayfish species becomes self- perpetuating through natural reproduction, it may be almost impossible to eliminate it. On the basis of recommendations and suggestions made by ICES, EIFAC, IUCN and a number of experts (Lachner et al., 1970; Hubbs, 1977 and Shafland, 1979) the authors have prepared the following recommendations for procedures to be followed in Finland prior to making a decision regarding new fish and crayfish introductions. (i) The objectives of an introduction should be clearly stated and defined. (ii) The introduction must be shown to meet a clear ecological, economical or recreational need; and the exotic species must have the potential to fulfil this need. (iii) The new species should fill a vacant or a little used ecological niche. (iv) The biology and ecology of the exotic species, its role in its native ecosystem, its relationships with other organisms (especially with fish and crayfish in this ecosystem) and the role of parasites and diseases should be thoroughly investigated by the authorities of the importing country. (v) All proposed introductions should include an evaluation of at least several candidate species to determine which would be the best suited. (vi) The results and effects of previous introduction of the exotic or similar species in other areas should be examined. (vii) A preliminary assessment should be made of the probable and especially potentially detrimental effects of the exotic on the aquatic environment in general and, in particular, on the native fish and crayfish species and on aquatic vegetation. Evaluations should also be made on the spreading of the exotic, its possible range, its capability of forming self-perpetuating populations, its catchability and its general adaptability to the new environment. (viii) All relevant authorities, Institutes, organizations and associations should take part in the preliminary assessment of the introduction, not just the organization proposing the introduction. (ix) Preferably only that progeny of the introduced species produced from a broodstock in a closed or otherwise strictly controlled system should be used as material for stocking in natural environments. (x) Restricted trial stockings with cultivated juveniles should be made under controlled conditions in small, isolated waters to prevent escapes. (xi) The results of the trial stocking and other, related investigations should be evaluated by a panel of representatives from all agencies involved. (xii) Adequate control methods should be available to prevent over-population or undesired spreading of the introduced species. Fish and crayfish diseases are combatted in Finland by preventing their spread and by control and remedial activities at fish hatcheries and farms. There are no serious fish diseases in Finland. To prevent the spread of disease it is forbidden to import live fish and eggs without the permission of veterinary authorities. Since 1968 the State Veterinary Medical Institute has been adminstering fish disease control, covering almost all major fish farms. In addition, veterinary authorities have far-reaching powers to institute investigations and carry out preventive measures when serious fish diseases are encountered in either cultured or wild fishes. According to the fifth rule of the International Fisheries Regulations for the Baltic, issued by the International Baltic Sea Fishery Commission (IBSFC), approval of the Commission is needed for the introduction of an exotic species in the Baltic. Fish and crayfish species considered for introduction into Finland should preferably have the following characteristics: (i) Adapt either to natural waters or to altered waters (artificial, regulated, eutrophicated and polluted lakes, constructed rivers) so well that economically feasible populations would develop. (ii) No harmful impact on the aquatic nature and other fields of economic interests, e.g., fisheries. (iii) No harmful competititon with native species. (iv) No hybridization with native species in natural conditions. (v) Fast-growing. (vi) Good food source, either because of large individual size or because it swims in large, easily-caught schools. (vii) Primarily vegetarian in food habits. (viii) High reproductive potential. (ix) Resistant to parasites and diseases occurring in Finland. (x) Non-migratory tendency. (xi) Capable of withstanding predation. (xii) Resembles native species. (xiii) Easy to catch even with traditional methods and gear. (xiv) Good taste when traditionally prepared. (xv) Ability to sustain fishing pressure. (xvi) Disease and parasite free; not a host or intermediate host for diseases and parasites. (xvii) Does not accumulate heavy metals or pesticides. (xviii) Suitable for cultivation for production of juveniles for stocking purposes. Certain special characteristics relating to the enviornment may also be required for each candidate species. For example, imported crayfish species should be genetically resistant to crayfish plague (Aphanomyces astaci). It is unlikely that any one species will fulfil all of these conditions but the more characteristics it possesses, the more likely it is to survive and flourish in our climate; and the more effective it will be in enriching our fauna in a beneficial way and in finding an ecological niche in its new environment which is in balance with the rest of the ecosystem. 5. REFERENCES Ahlfors, P., P. Kummu and K. Westman, 1984 Introduction of carp (Cyprinus carpio (L.)) 1984 in Finland. EIFAC Tech.Pap., (42) Vol.2:299–312 Brofeldt, P., Evon kalastuskoeasema. 25-vuotinen toiminta ja tulokset 1892–1917. 1920 Suomen Kalatalous, 6:1–141 Hakkari, L. et al., The food of the native whitefish (Coregonus muksun) and the 1984 introduced whitefish (C. peled) stocked in the same small forest lakes in southern Finland. EIFAC Tech.Pap., (42) Vol.1:109–22 Hubbs, C., Possible rationale and protocol for faunal supplementations. Fisheries, 1977 2(2):12–4 Lachner, E.A. et al., Exotic fishes and other aquatic organisms introduced into North 1970 America. Smithson.Contrib.Zool., 59:1–28 Momot, W., introductions as possible tool in fish management crayfish. 1978 Paper presented at the NABS Workshop, May 10, 1978, Winnipeg, Manitoba. 10 p. (mimeo) Mutenia, A., O. Simola and P. Tuunainen, Results of lake trout (Salvelinus namaycush) 1984 stockings in Finland in 1957–81. EIFAC Tech.Pap., (42)Vol.2:381–91 Nilsson, N.-A. and H. Dahlstrom, Harmaanieria (Lake trout). In Kalat, kalavesien hoito ja 1968 kalanviljely (Fish, management of fishing waters and fish cultivation), edited by G. Svardson et al. Helsinki, 302 p. Pruuki, V., M. Pursiainen and K. Westman, A study of the growth and production of the 1984 native whitefish (Coregonus muksun (Pallas)) and the introduced whitefish (C. peled) (Gmelin) stocked in two small forest lakes in southern Finland. EIFAC Tech.Pap., (42) Vol.1:91–108 Rosenthal, H., Implications of transplantations to aquaculture and ecosystems. Paper 1976 presented at the FAO Technical Conference on Aquaculture, Kyoto, Japan, 26 May–2 June 1976. Rome, FAO, FIR:AQ/Conf./76/E.67 Shafland, P.L., Non-native fish introductions with special reference to Florida. Fisheries, 1979 4(3):18–24 Seligo, A., Die finnlandische Fischerei-Versuchsstation in Evois. Allg.Fischerei-Ztg., 1897 22:214–5 Sormunen, T. et al., Yhdistelma Kalataloussaation suorittamien kalamerkintojen 1976 merkkipalautusten yhteydessa saaduista saalistiedoista. Kalataloussaation monist.julk., 55:1–468 Svardson, G. and P. Tuunainen, Kirjolohi (Rainbow trout). In Kalat, Kalavesien hoito ja 1968 kalanviljely (Fish, management of fishing waters, and fish cultivation), edited by G. Svardson et al. Helsinki, 302 p. Tuunainen, P., Siika (Whitefish). In Kalat, kalavesien hoito ja kalanviljely (Fish, 1968 management of fishing waters, and fish cultivation), edited by G. Svardson et al., Helsinki, 302 p. Tuunainen, P., Relations between the benthic fauna and two species of trout in some 1970 small Finnish lakes treated with rotenone. Ann.Zool.Fenn., 7:67–120 Unestam, T., Significance of diseases on freshwater crayfish. In Freshwater crayfish, 1970 edited by S. Abrahamsson. Lund, Studentlitteratur, vol. 1:135–50 Welcomme, R.L., (comp.), Register of international transfers of inland fish species. FAO 1981 Fish.Tech.Pap., (213):120 p. Westman, K., The population of the crayfish, Astacus astacus L. in Finland and the 1973 introduction of the American crayfish Pacifastacus leniusculus Dana. In Freshwater crayfish, edited by S. Abrahamsson. Lund, Studentlitteratur, vol. 1:41–55 Westman, K. and M. Pursiainen, Development of the European crayfish, Astacus 1979 astacus (L.), and the American crayfish, Pacifastacus leniusculus (Dana), populations in a small Finnish lake. In Freshwater crayfish, edited by P.J. Laurent. Thonon-les-Bains, Institut National de la Recherche Agronomique, vol. 4:243–50 , Introduction of the American crayfish (Pacifastacus leniusculus) in 1984 Finland; impact on the native crayfish (Astacus astacus). EIFAC Tech.Pap., (42)Suppl.2:422–6 Westman, K. and P. Tuunainen, Uusia lajeja vierailta vesilta. Suomen Luonto, 1981 40(Vuosikirja 1981):97–102 Westman, K. et al., A review of fish stockings in Finland. EIFAC Tech.Pap., (42) 1984 Vol.1:252–68 Table 1 Fish and crayfish introductions made in Finland

Species introduced Latin name Local name Date From Purpose Established Distribution Remarks Coregonus Peledsiika 1965 USSR Stocking in No Over the Very useful in peled eutrophic selfbreeding whole eutrophic and (Gmelin) waters populations country regulated (plankton but widely waters feeder) stocked throughout country Salmo Kirjolohi 1897 Germany Aquaculture No In fish farms Very gairdneri 1960s Denmark and angling selfbreeding over the important Richardson USA populations whole economically country in aquaculture presently 4000 t/year production Oncorhynchus tsawytscha Kuningaslohi 1933 USA Stocking No Disappeared O. nerka Intiaanilohi 1967 USA Aquaculture No Disappeared Only small- and stocking scale experimental stockings were made O. corbusha Kyttyrälohi Stockings by Occasionally (Walbaum) USSR in Baltic and some rivers O. Keta Koiralohi Stockings by Occasionally (Walbaum) USSR in Baltic and some rivers Salvelinus Puronieriä 1895 Germany Aquaculture No In some Interest fontinalis 1965 USA and stocking selfbreeding aquaculture diminished, (Mitchill) populations stations still in in wild experimental stage S. namaycush Harmaanieriä 1955 USA Stocking in No In some Useful in (Walbaum) 1966 Canada regulated selfbreeding large and regulated waters populations deep lakes lakes in wild and Gulf of Bothnia S. namaycush Spleiknieriä 1968 Stocking and No Some test Interest × S. fontinalis aquaculture selfbreeding stockings diminished, populations still in in wild experimental stage Cyprinus Karppi 1861 Germany Aquaculture No Mainly in Useful in carpio L. 1951, Sweden and stocking selfbreeding Southern eutrophicated 1955 USSR in eutrophic populations Finland lakes, still in 1959 waters in wild experimental stage Tinca tinca Galitsian 1936 Sweden Stocking in Self- Mainly in Potentially ssp. L. suutari eutrophic breeding Southern useful in waters populations Finland eutrophicated in many lakes, little waters appreciated Micropterus Isobassi 1893 Germany Stockings No Disappeared salmoides Lacepède M. dolomieu Pikkubassi 1893 Germany Stockings No Disappeared Lacepède 1958 Canada 1966 Sweden Ictalurus Piikkimonni 1922 Germany Stockings Self- In Southern Not useful, nebulosus breeding Finland grows slowly, Rafinesque populations possible in many ecological waters competition Acipenser Sterletti 1958 USSR For No The Gulf of Also ruthenus management Finland, the migration of inland Bothnian from the waters Sea USSR Pacifastacus Täplärapu 1967– USA Aquaculture Self- In c. a Useful leniusculus 1969 Sweden and stocking breeding dozen lakes because it is Dana 1971– in crayfish populations mainly in resistant to 1974 plague in some Southern crayfish waters lakes Finland plague. Still in experimental stage. Possible competition with the native crayfish (Astacus astacus) THE EFFECTS OF FRESHWATER FISH INTRODUCTIONS INTO IRELAND

P. Fitzmaurice Central Fisheries Board Dublin, Ireland

CONTENTS

1. INTRODUCTION 2. SPECIES KNOWN TO HAVE BEEN INTRODUCED 2.1 Arctic char (Salvelinus alpinus) 2.2 Rainbow trout (Salmo gairdneri) 2.3 Pike (Esox lucius) 2.4 Catfish (Ictalurus melas) 2.5 Dace (Leuciscus leuciscus) and roach (Rutilus rutilus) 2.6 Tench (Tinca tinca) and carp (Cyprinus carpio) 3. EFFECTS AND STATUS OF THE INTRODUCTIONS 4. REFERENCES

ABSTRACT Currently Ireland's freshwater fish fauna consists of 20 species. After the last glaciation period (about 15 000 B.C.) the fish fauna probably consisted of about eight species. Documentary evidence exists to show that at least seven species were introduced into Ireland within the last 400 years or so. Species which have established thriving populations include pike (Esox lucius), bream (Abramis brama), tench (Tinca tinca), roach (Rutilus rutilus) and dace (Leuciscus leuciscus). For climatic reasons carp (Cyprinus carpio), which has been introduced intermittently over the last 350 years, has not become fully established but remains a permanent member of our fish fauna through management policies. Rainbow trout (Salmo gairdneri) was introduced to Ireland in the early 1900s and although they have been stocked into approximatley 100 waters, only three isolated lakes have breeding stocks. Of the introduced fish only three species could be said to have any adverse effects on existing fish populations. These species, namely, pike, roach and dace generate a considerable proportion of the £8 million, which Ireland earns as tourist revenue from coarse angling. RESUME La faune piscicole des eaux douces irlandaises se compose aujourd'hui de 20 espèces. Après la dernière période de glaciation (vers 17 000 avant J.C.), elle comprenait vraisemblablement huit espèces environ. On a la preuve que sept espèces au moins ont été introduites en Irlande au cours des 400 dernières années. Parmi les espèces qui ont prospéré, on citera le brochet (Esox lucius) la brème (Abramis brama), la tanche (Tinca tinca), le gardon (Rutilus rutilus) et la vandoise (Leuciscus leuciscus). Pour des raisons climatiques, la carpe (Cyprinus carpio), introduite par intermittence au cours des 350 dernières années n'est pas parvenue à s'établir complètement mais, grâce aux politiques d'aménagement, elle reste un membre permanent de la faune irlandaise. La truite arc-en-ciel (Salmo gairdneri) a été introduite en Irlande au début du siècle; environ 80 pièces d'eau ont été repeuplées mais seuls deux lacs isolés ont des stocks reproducteurs. Trois seulement des espèces introduites ont eu des effets néfastes sur les populations existantes. Ces espèces, à savoir le brochet, le gardon et la vandoise, sont la source principale des huit millions de livres sterling que le tourisme rapporte à l'Irlande grâce à la pêche au poisson commun. 1. INTRODUCTION The freshwater fish fauna of Ireland consists of 20 species. It is probable that at the end of the last Ice Age, which occured about 15 000 B.C. (Varley, 1967), the indigenous fish species were anadromous. As the ice melted and retreated the fish moved northward after the ice. At that time, Ireland, Britain and the European land mass were joined. A number of fish species reached Britain whilst it was still connected to the continental land mass but before they had an opportunity to colonize Ireland, the land bridge with Britain had disappeared. Consequently, Ireland's freshwater fish fauna has less fish species diversity than that of Britain which, in turn, has less fish species than mainland Europe. After the last Ice Age, Ireland's indigenous freshwater fish stocks probably consisted of: salmon (Salmo salar), sea trout and brown trout (Salmo trutta), char (Salvelinus alpinus), (), eel (Anguilla anguilla), Killarney shad (Alosa fallax killarnensis) and possibly two species of stickleback (Gasterosteus aculeatus and Pungitius pungitius). It is more than likely that the other species which today complete the Irish freshwater fish list were introduced into Ireland after the twelfth century. In his translations of Giraldus Cambrensis' “Topography of Ireland”, O'Meara (1951) states that, “pike, perch, roach, gardon, gudgeon, minnow, loach, bullheads and verones” are absent. The distribution of some of these species and the inconsistency in which different combinations occur suggests that they were introduced. Documentation exists which shows that not only were certain species in the past restricted in their distribution but definite documentary evidence shows that Arctic char (Salvelinus alpinus), rainbow trout (Salmo gairdneri), tench (Tinca tinca), carp (Cyprinus carpio), roach (Rutilus rutilus) and dace (Leuciscus leuciscus) were introduced by named sources (Frost, 1940; Went, 1949, 1950; Piggins and Went, 1971 and MacCrimmon, 1971). Historical documents indicate that pike (Esox lucius) must have been introduced (Went, 1957) and there has been only one recorded capture of a single specimen of catfish (Ictalurus melas) by Fitzmaurice (1977). It is also more than likely that bream (Abramis brama) and rudd (Scardinius erythrophthalmus) are introduced species but there is no documentation to indicate their dates of introduction. The results of known introductions and their current status are shown in Fig. 1. 2. SPECIES KNOWN TO HAVE BEEN INTRODUCED 2.1 Arctic char (Salvelinus alpinus) In 1969 the Salmon Research Trust of Ireland received what was purported to be a consignment of Icelandic sea trout (Salmo trutta). 135 000 eyed ova were retained for rearing. By September 1970 it was noticed that the fish were, in fact, Arctic char (Piggins and Went, 1971). Over half the original consignment was stocked out as fed fry in mid 1970 (Salmon Research Trust of Ireland Inc., 1971). Subsequently only three recaptures were made from this stocking. No recaptures were recorded from the 4 277 Arctic char stocked in 1971 as 1+. The species did not become established. 30 000 ova were sent to the Crumlin River in west Galway for hatching and subsequent release. None of the resultant planting were ever encountered. 2.2 Rainbow trout (Salmo gairdneri) Rainbow trout were originally introduced into Ireland from the United States in 1899, 1900 and 1901 (Ravenal, 1899, 1900 and 1901). It was intended to use them for stocking sport fisheries locally (Worthington, 1941) but this venture was a failure. About 1905–7, Lough Shure on Aran Island off the coast of Donegal was successfully stocked (Frost, 1940) and a breeding population persists to the present. No further introduction appeared to have taken place into Ireland until 1955, when rainbow trout eggs were imported from a trout farm situated in Surrey, England. This importation formed the core of a breeding stock. The resultant offspring were used for commercial rearing and for stocking sport fisheries. Approximately 100 different lakes and ponds have already been stocked with rainbow trout. Self-maintaining populations have been established in only three known locations, the above mentioned Lough Shure, Lough na Leibe in County Sligo and White Lake in County Westmeath. 2.3 Pike (Esox lucius) Went (1957) states that after fairly complete searches of records and historical literature, he was unable to discover any references to pike in Ireland before the sixteenth century. Indeed literature referring to the twelfth century was notable in that it specially mentions the absence of pike. There is no name for pike in the Old Irish language. The modern name (Farran, 1946) is “Gaill iasc” which literally translated means “foreign fish”. This indicates that the species is an introduced one. By the late sixteenth century pike appear to have been well established in the southern part of Ireland (Longfield, 1929; Johnson, 1810). In the late seventeenth century pike were absent from the west of Ireland (O'Flaherty, 1846) but by the early nineteenth century they were well established and had colonized the Corrib and Moy catchments (Maxwell, 1832). The species has appeared in the upper reaches of the River Bandon within the past decade. At present, pike are found in all the river systems draining the central plain of Ireland. It is absent from water systems of the extreme northwest, west and southwest. It is also absent from coastal mountain areas of the southeast. Pike is the only piscivorous fish to be found in Irish fresh waters. 2.4 Catfish (Ictalurus melas) Catfish is an introduced species on the European mainland (Blanc et al., 1971). The species is commonly used by aquarium hobbyists. During a stock assessment operation, a single specimen was captured in a gillnet in the Lough, Cork. There is no evidence to suggest that the species became established. It was almost certainly dumped in the lake by a local aquarium enthusiast. 2.5 Dace (Leuciscus leuciscus) and roach (Rutilus rutilus) Details of the introductions of dace and roach into the Cork Blackwater River are well known and documented. Evidence of the introductions, by one of the people directly involved, was given at an inquiry held in 1940. A visiting pike angler from England brought two tins of mixed dace and roach for use as bait in 1889. Whilst angling he left the two tins of bait tied by cords to a wall. The cans were swept away by a flood and about two years later the fish were noticed in the river. Since that date they have spread throughout the whole of the Blackwater system. At present dace are found only in the Cork Blackwater River system. In about 1905 roach were transferred to an artificial lake in County Tyrone in Northern Ireland and following a severe flood they escaped into the River Mourne, a tributary of the River Foyle. In the sixties roach were identified for the first time in the River Erne and within a decade had completely colonized the catchment area. By 1980, the species had been recorded from Shannon, Boyne, Dee, Liffey and Corrib catchments (Fitzmaurice, 1981). The rapid spread of roach may be attributed to sport anglers using the fish as live bait for pike. Anglers also deliberately transferred roach in the hope of improving sport angling. 2.6 Tench (Tinca tinca) and carp (Cyprinus carpio) Introductions of tench and carp into Ireland date back as far as the seventeenth century. Rutty (1772) noted that tench and carp were said to have been brought into Ireland for the first time during the reign of James I, 1603–25. Went (1950) refers to proposed introductions by Richard Boyle, the Great Earl of Cork, in 1634 and 1640. Tighe (1802), Daniel (1807), Windele (1849) and Thompson (1856) all mention locations in which tench and carp were supposed to have been stocked or were present at the time of writing. Although tench are to be found in some of the waters mentioned by these authors, carp have long since disappeared. The present distribution of tench and carp in Ireland can be considered in two parts - pre-1950 stocks and stockings since 1950. In the early fifties, tench were distributed in the River Shannon catchment and its associated lakes. It was also found to be sporadically distributed in the midlands and south of the country, mainly in small lakes and ponds. Since 1956, tench have been introduced into approximately 80 different waters, mostly small lakes, in the hope of creating sport fisheries. Wherever tench were stocked, breeding populations were established. Prior to 1950, information on carp stocks is rather vague and fragmentary. However, around 1950 it appears that the distribution of carp in Ireland was limited to a few small shallow ponds in the southern part of the country. Known locations included a private quarry pond in a Dublin suburb. The quarry was subsequently filled in with rubble. Small ponds at Blackwater, County Wexford, and at Kilsheelan, County Tipperary, contained breeding stocks of stunted fish. In 1976 both ponds dried up with consequent loss of breeding stock. A small lake near Clonmel, County Tipperary, contains a breeding stock and the largest carp captured in Ireland (14.64 kg) was recorded from this lake in 1977. Since 1950, 32 ponds or lakes have been stocked with carp for sport angling purposes. All these stockings, with the exception of two plantings, were made from endemic stocks. In 1951 some two-year olds were imported from Germany and stocked into Ballinderry Lake, County Westmeath. In 1974, despite a governmental ban on the importation of live fish into Ireland, approximately 40 small carp were brought in from England and stocked into a small pond near Ballina, County Mayo. 3. EFFECTS AND STATUS OF THE INTRODUCTIONS Legislation in Ireland prohibits the importation of live fish for sport or commercial purposes. Introductions are only allowed if the Ministry for Fisheries is fully satisfied that no damage can be done to endemic fish stocks. Rainbow trout were stocked alone or in combination with brown trout into lakes which previously held no fish or held coarse fish, which were removed with rotenone. Some lakes which held small populations of brown trout were also stocked. All rainbow trout used in stockings were an autumn spawning shasta strain. However, in the three naturalized populations the fish have reverted to spring spawning. Under Irish conditions it has been found that rainbow trout are unlikely to establish breeding populations in competition with brown trout. In sport fisheries, overwintering survival of plantings is poor. Plantings of 1+ and 2+ fish now take place. Present management policy for rainbow trout sport fisheries is to limit plantings to a few small selected lakes where the species is unable to spawn. The sport fisheries are maintained by annual stockings. In Lough na Leibe and White Lake, because of limited spawning, stocks are maintained by supplemental plantings. There are some isolated reports of rainbow trout escapees from freshwater fish farms spawning in rivers but there is no evidence to suggest that self-maintaining populations have become established. There have also been a few reports of escapees from marine cage culture locations running into rivers but no known spawnings have taken place. The introduction of pike and its subsequent spread to a large proportion of the country has had an adverse effect on the indigenous salmonid populations. In waters where brown trout, cyprinids and perch are abundant, pike prey on brown trout in preference to other fish species (Healy, 1956). Toner (1959) showed that in Lake Corrib, a large trout angling water, pike stomachs contained brown trout in percentages ranging from 51–66.6 percent, the calculated weight ratio of trout to other fish eaten was 4.2:1. Salmon smolts are particularly vulnerable to predation during migrations. In waters being developed as trout fisheries, pike stocks have to be controlled by gill-netting, trapping and local application of rotenone on known spawning areas. Pike fisheries are a valuable asset as a tourist income earner. At present, Ireland earns £4–6 million in external revenue from pike angling. The presence of dace and roach in the Cork Blackwater River have affected salmonid production and have caused the decline of brown trout fishing. Both species have colonized areas where brown trout and salmon parr were abundant in the past. They compete for food and the ecology of the dace being such that it can penetrate further up the faster-flowing regimes successfully displacing brown trout and salmon parr. Despite legislation banning fish transfers within Ireland, roach have been introduced to numerous lakes and rivers over the last 20 years. Although roach are a valuable asset as a sport fish, they have an adverse effect on the endemic species present. Within a few years of introduction into a river system, they become the dominant species. They displace brown trout and rudd stocks disappear almost to the point of extinction (Fitzmaurice, 1981). Fertile hybrids between roach, bream and rudd are produced and with back crossing, roach become the dominant species. Tench and carp are solely used for sport angling purposes. Both species are at the northern limit of their geographical distribution. In waters where tench are stocked, the naturalized populations reproduce readily although Ireland's climate is such that in summers which are much cooler than average, spawnings may fail to take place. This happens in rivers and large lakes where spawning temperatures are not realized. However, in small shallow sheltered ponds spawnings are likely to occur every year. Growth rates are good, spawnings are adequate and it is only in small ponds that overpopulation and stunting is likely to occur. Tench stockings have proved beneficial for sport angling. They do not appear to have any adverse effect on existing fish populations. They are usually stocked in waters which have either bream or an absence of bottom feeding fish. Self-maintaining populations of carp are difficult to obtain in Ireland due to climatic reasons. In only two known locations have annual spawnings taken place. The ponds concerned have a temperature of 4–5°C above national average but in 1976 under drought conditions these ponds dried out with consequent loss of spawning stock. In exceptionally warm summers there is likely to be spawnings in most areas where the species are stocked. Ecological problems, with carp such as those experienced in the U.S.A. (Stroud, 1975) are never likely to happen in Ireland. Indeed the main problem with carp in Ireland is trying to prevent the species from dying out. The introduction of Arctic char and catfish have both been failures. Overall, the introductions of fish species into Ireland have been beneficial as far as sport angling is concerned. The external tourism earning for freshwater angling in 1981 was approximately £18 million. The introduction of rainbow trout, tench and carp have been particularly beneficial in adding to species diversity, enhancing commercial production and sport fishing. With pike, dace and roach, their introduction has been beneficial to a certain extent but their impact on the ecology and their interactions with endemic species has made them undesirable. Roach are probably the least desirable of the introductions. 4. REFERENCES Blanc, M. et al., European inland water fish: a multilingual catalogue. Poissons des eaux 1971 continentales d'Europe: catalogue multilingue. Peces de aguas continentales de Europa: catalogo multilingue. Binnengewasser Fische Europas: Mehroprachiger Katalog. London, Fishing News Books for FAO, pag.var. Daniel, W.B., Rural sports. London, ii. p. 215 1807 Farran, G.P., Local Irish names of fishes. Ir.Nat.J., 8:344–7, 370–6, 420–8, 430–3 1946 Fitzmaurice, P., The Cladocera of Ireland and their importance in the diet of fishes. 1977 Ph.D. Thesis, National University of Ireland , The spread of roach (Rutilus rutilus L.) in Irish waters. In Proceedings of 1981 the Second British Freshwater Fisheries Conference. Liverpool, University of Liverpool, pp. 154–61 Frost, W.E., Rainbows in acid water: a note on the trout of a peat lough on Arranmore. 1940 Salm.Trout Mag., (100): 234–40 Healy, A., Roach and dace in the Cork Blackwater. J.Dep.Agric.Fish.Dubl., 53:67–73 1956 Johnson, S., The works of the English poets. London, iii, 413 1810 Longfield, A.K., Anglo-Irish trade in the sixteenth century. London, p. 49 1929 MacCrimmon, H.R., World distribution of rainbow trout Salmo gairdneri. 1971 J.Fish.Res.Board Can., 28(5): 663–704 Maxwell, W.H., Wild sports of the west. London, vol. 1:114–6 1832 O'Flaherty, R., A chronographical description of West or hIar Connaught. Dublin, edited 1846 by J. Hardiman, p. II O'Meara, J., The first version of the topography of islands, by Giraldus Cambrensis; a 1951 translation. Dundalk, p. 18 Piggins, D.J. and A.E.J. Went, Introduction of Arctic char (Salvelinus alpinus) into Irish 1971 waters. Ir.Nat.J., 17:144 Ravenel, W. de C., Report on the propagation and distribution of food-fishes, 1897–99 – 1899 steelhead and rainbow trout. Rep.U.S.Comm.Fish Fish., 25: , Report on the propagation and distribution of food-fishes, 1899–1900 1900 - steelhead and rainbow trout. Rep.U.S.Comm.Fish Fish., 26:100–5 , Report on the propagation and distribution of food-fishes, 1900–1901 1901 - steelhead and rainbow trout. Rep.U.S.Comm.Fish Fish., 27:88–92 Rutty, J., The natural history of County Dublin. Dublin, vol. 1:364–5 1772 Salmon Research Trust of Ireland Inc., Annual report of the Salmon Research Trust of 1971 Ireland Incorporated. Annu.Rep.Salm.Res.Trust Irel.Inc., (16):11–2 Stroud, R.H., The introduction of exotic fish species into waters of the United States. In 1975 Proceedings of the Seventh British Coarse Fish Conference. Liverpool, University of Liverpool, pp. 3–13 Thompson, W., Natural history of Ireland. London, vol. 4:135 p. 1856 Tighe, W., Statistical observations relative to the County of Kilkenny. Dublin, p. 156 1802 Toner, E.D., Pike in trout waters. Salm.Trout Mag., 5:104–10 1957 Varley, M.E., British freshwater fishes. Factors affecting their distribution. London, 1967 Fishing News (Books) Ltd., 148 p. Went, A.E.J., Giraldus Cambrensis notes on Irish fish. Ir.Nat.J., 9:221–4 1949 Went, A.E.J., Notes on the introduction of some freshwater fish into Ireland. 1950 J.Dep.Agricult.Fish., Dubl., 47:3–8 , The pike in Ireland. Ir.Nat.J., 12:177–82 1957 Windele, J., Historical and descriptive notices of the city of Cork and its vicinity. Cork, p. 1849 370 Worthington, C.B., Rainbows. A report on attempts to acclimatize rainbow trout in 1941 Britain. Salm.Trout Mag., (100):241–60; (101):62–99

Fig. 1 Current distribution and status of fish species known to have been introduced into Ireland INTRODUCTION DE NOUVELLES ESPECES DE POISSONS DANS LES PECHERIES D'EAU DOUCE DE LA ROUMANIE (INTRODUCTION OF NEW SPECIES OF FISHES IN ROMANIAN INLAND FISHERIES)

N. Bacalbasa-Dobrovici Université de Galati 6200 Galati, Romanie

SOMMAIRE

1. INTRODUCTION

2. ESPECES ACCLIMATEES NATURELLEMENT

2.1 Lepomis gibbosus (Linnaeus) 1758 2.2 Ictalurus nebolusus nebulosus (Le Sueur) 1819

3. INTRODUCTION DES ANGUILLIDAE

4. INTRODUCTION DES SALMONIDAE

4.1 Salmo gairdneri irideus Gibbons 1855 4.2 Salvelinus fontinalis fontinalis (Mitchill) 1815 4.3 maraenoides Poljakow 1874 4.4 Coregonus albula ladogensis Pravdin 1948

5. INTRODUCTION DES CYPRINIDAE

5.1 Ctenopharyngodon idella (Val.) 5.1.1 Développement et rythme de croissance 5.1.2 Nutrition et lutte contre la végétation 5.1.3 La mise en valeur des ressources végétales du Delta du Danube 5.1.4 Reproduction et extension 5.1.5 Maladies et parasites 5.1.6 La pêche 5.2 Hypophthalmichthys molitrix (Val.) 5.3 Aristichthys nobilis (Rich.) 5.4 Mylopharyngodon piceus (Rich.) 5.5 Pseudorasbora parva (Schlegel)

6. INTRODUCTION DES POECILIDAE

7. INTRODUCTION DES CATASTOMIDAE

8. BIBLIOGRAPHIE

RESUME On présente l'introduction de 15 espèces de poissons d'Amérique du Nord, d'Asie et d'Europe dans les eaux continentales roumaines, pendant le dernier siècle. Sont presentées aussi les espèces qui, sans avoir une importance économique positive, se sont acclimatées naturellement. Pour les espèces qui se sont acclimatées on donne la distribution. On insiste sur les espèces d'importance économique, pour lesquelles sont présentées des données sur la survie, le rythme de croissance, la nutrition, la pêche.

ABSTRACT This is a presentation of the introduction during the last century of 15 species of fishes from North America, Asia and Europe into Romanian inland waters. Some fish species have acclimatized naturally, including those of no economic value, and their distribution is given. However, this paper concentrates on the commercially important species, for which data is given on their survival, growth, feeding and catch. 1. INTRODUCTION Pendant le dernier siècle, on a introduit dans les eaux de la Roumanie des espèces appartenant aux familles: Anguillidae, Catastomidae, Cyprinidae, Ictaluridae, Poeciliidae et Salmonidae. Quelques espèces ont pénétré par le Danube et ses affluents et - comme on suppose pour l'anguille -aussi par la mer Noire; la plupart des espèces ont été introduites spécialement. Quelques espèces se sont acclimatées, pour les autres on pratique la reproduction artificielle et ensuite des peuplements avec des alevins ou de jeunes poissons. L'introduction d'autres espèces n'a pas réussi. Au total - sauf les espèces d'aquarium - on peut parler de 15 espèces appartenant aux familles citées plus haut. Dans le “Registre des transferts internationaux d'espèces de poissons des eaux continentales” (Welcomme, 1981) la situation dans la Roumanie n'est pas présentée clairement. 2. ESPECES ACCLIMATEES NATURELLEMENT 2.1 Lepomis gibbosus (Linnaeus) 1758 La perche-soleil, originaire de l'est de l'Amérique du Nord, a été signalée la première fois en Roumanie en 1918, dans la région inondable du Danube au sud de Bucarest (Carausi, 1952). Venue par le Danube, la perche-soleil est devenue un poisson commun dans toute sa région innondable; il a pénétré dans les zones inférieures du Siret et du Prut et on le trouve dans plusieures rivières du Banat et de la Crichane. C'est un poissons nuisible parce qu'il dévore le frai et les alevins; quant à la nourriture, il est un concurrent pour quelques espèces appreciées de poissons. On le pêche, avec d'autres espèces, à la seine, aux verveux et aux madragues de lac. La chair insipide et l'abondance des arêtes contribuent à ce que la perche-soleil ne soit pas appréciée. On ne tient pas une statistique pour cette espèce. 2.2 Ictalurus nebolusus nebulosus (Le Sueur) 1819 Le poisson-chat, originaire de l'est des Etats-Unis d'Amérique et introduit dans l'ouest de l'Europe en 1880, a été signalé en Roumanie en 1934; plus tard il pénétra dans l'ouest du pays; on l'a signalé isolément dans le Danube (Banarescu, 1964). Quoique savoureux et apprécié pour cette raison, il est considéré comme concurrent pour la nourriture des autres poissons et pendant la pêche des étangs il blesse de ses épines d'autres poissons entassés. On le pêche à la ligne et avec d'autres poissons avec différents filets. On a peuplé de poisson-chat quelques petits lacs, dans lesquels il n'y avait pas d'espèces de poissons de valeur. Les quantités pêchées sont réduites. 3. INTRODUCTION DES ANGUILLIDAE L'anguille, Anguilla anguilla (Linnaeus) 1758, est connue par une partie des pêcheurs du Danube et de la mer Noire qui capturent, de temps en temps, des exemplaires isolés. Dans les rivières de l'intérieur du pays, l'anguille est capturée encore plus rarement. On ne peut pas affirmer quelle est la voie principale de pénétration des exemplaires de l'anguille dans les eaux roumaines: par la mer Noire ou par le Danube amont; en tout cas, on ne pêche pas de jeunes anguilles. Les exemplaires pêchés sont pris aux madragues et, rarement, à la ligne. La première tentative de peuplement d'un lac par de jeunes anguilles a été réalisée dans la Dobroudja, en 1980. 4. INTRODUCTION DES SALMONIDAE On a introduit trois genres de Salmonidae: Salmo, Salvelinus et Coregonus. 4.1 Salmo gairdneri irideus Gibbons 1855 La truite arc-en-ciel, originaire de Californie et introduite dans l'ouest de l'Europe en 1880, s'est répandue plus tard en Roumanie. Ce poisson est élevé en étangs Les nombreuses tentatives de peupler des rivières de montagne avec des truites arc-en-ciel sont restées sans résultat: elle descend dans la zone inférieure des rivières, où à cause des mauvaises conditions on ne la trouve plus. Dans la plupart des étangs à truites on la maintient par reproduction artificielle. 4.2 Salvelinus fontinalis fontinalis (Mitchill) 1815 L'omble de fontaine, originaire du nord-est de l'Amérique du Nord, a été introduit en Europe en 1884. On a peuplé d'omble de fontaine plusieurs ruisseaux de la Roumanie, mais il ne s'est acclimatée que dans un nombre réduit d'entre eux, en amont de la zone de la truite de rivière de la Moldavie et de la Transylvanie. Son importance économique est insignifiante. On le pêche à la ligne. 4.3 Coregonus lavaretus maraenoides Poljakow 1874 Le corégone lavaret, autochtone du lac Peipus, a été importé sous forme d'oeufs embryonnés, en 1956. On a introduit 5 000 oeufs de Pologne et 1 000 000 de la station piscicole Volhow, près de Leningrad (Busnita, 1957). Il a été introduit en 1957 dans le lac Lacul Rosu et le lac artificiel Bicaz-Izvorul Muntelui dans les Carpates orientales, dans les étangs de Nucet et dans quelques étangs du nord de la Moldavie (Banarescu, 1964); l'action n'a pas réussi. 4.4 Coregonus albula ladogensis Pravdin 1948 Le corégone blanc, autochtone dans le lac Ladoga, a été introduit sous forme d'oeufs embryonnés (5 000) de Pologne, en même temps que le corégone lavaret. Introduit dans les mêmes étangs et lacs que l'espèce précédente (Banarescu, 1964), il ne s'est pas maintenu. 5. INTRODUCTION DES CYPRINIDAE L'introduction des Cyprinidae de l'Asie de l'est a une importance exceptionnelle pour l'exploitation rationnelle des eaux continentales roumaines, dont la plupart sont peu profondes et chaudes pendant l'été. Les eaux stagnantes ou à courant très faible sont envahies par la végétation submergée, émergée et flottante; parmi les espèces des poissons du pays, seulement le rotengle (Scardinius erythrophthalmus erythrophthalmus (Linnaeus)) pature partiellement la végétation submergée et les algues filamenteuses; jusqu'à l'introduction des Cyprinidae de la R.P. de Chine, la faune ichtyologique roumaine a été privée d'espèces macrophytophages. L'introduction réalisée a contribué à un assainissement partiel de nombreux lacs situés dans la région inondable du Danube et surtout du Delta et même de quelques portions des rivières lentes (Mostistea). Les Cyprinidae chinois sont maintenant une des composantes essentielles de la pisciculture et spécialement de la polyculture. Quantitativement ils suivent la carpe produite en étang et pendant ces dernières années on en pêche aussi dans les eaux ouvertes. Qualitativement, les consommateurs sont satisfaits de ces poissons et il n'y a aucun problème lié à leur commercialisation. 5.1 Ctenopharyngodon idella (Val.) La carpe chinoise est la plus importante espèce introduite de Cyprinidae. Le matériel provenait de 100 poissons d'une année, d'un poid moyen de 9 g, offerts par l'Académie des Sciences de Moscou, importés à l'automne de 1959; un lot de 22 555 alevins (parmi lesquels d'autres espèces), d'un poid moyen de 0,2 g, importé de la R.P. de Chine (bassin du Yang-Tseu-Kiang) en été 1960 (Nicolau et al., 1961) et un autre lot de 500 000 alevins importé de la même zone en 1962 (Mirica, 1965). 5.1.1 Développement et rythme de croissance Pendant la première année de croissance (1960) les poissons ont atteint le poids moyen de 591 g et les alevins de 46.1 g. Ultérieurement la croissance moyenne a été de 1 000–1 200 g (Nicolau, 1966). Le rythme de croissance de la carpe chinoise dépasse celui de la carpe (Cure, 1970) d'environ trois fois (Nicolau, 1962). 5.1.2 Nutrition et lutte contre la végétation A Nucet la carpe chinoise consomme plus intensivement Najas et Ceratophyllum; en général elle préfère les plantes molles submergées et flottantes; elle se nourrit aussi des plantes jeunes de la végétation dure, mais après qu'elles dépassent la surface de l'eau, ces plantes ne sont plus consommées sur tige par la carpe chinoise. Dans le Delta et les étangs elle mange surtout les Potamogetonaceae, Ceratophyllum demersum, Elodeea canadensis, Vallisneria, Lemna, etc. Les résultats les plus évidents de la capacité de consommation des plantes et du défrichement ont été obtenus à la base expérimentale Caraorman du Delta du Danube, où une population de 160–240 ex/ha de carpe chinoise de deux ou trois années, consomme presque toute la végétation caractéristique pour les zones marécageuses du Delta maritime du Danube; en même temps on obtient un surplus de production de 250 kg/ha de poisson et l'amélioration des conditions biotiques des étangs (Nicolau, 1966; Mirica et al., 1966; Cure et al., 1970; Cure, 1970). 5.1.3 La mise en valeur des ressources végétales du Delta du Danube La carpe chinoise a donnée d'excellents résultats dans le Delta du Danube; ces résultats ont été étudiés par Gh.Mirica (1962, 1964). Il a évalué l'augmentation de la production du poisson à 250 kg/ha et une réduction du coût de la production du poisson obtenu par la pisciculture mixte à plus de 30 pour cent. Pour assurer le matériel de peuplement du Delta du Danube avec des Cyprinidae importés on a construit l'alevinière de Caraorman, qui est entrée en fonction depuis 1966 (Mirica, 1965) et celle de Perisor- Lijai entrée en production en 1970. Les expériences de prédéveloppement et de croissance pendant la première année de la carpe chinoise et de la carpe argentée (Hypophthalmichthys molitrix) dans deux grands étangs: nr. 4 Calica (123 ha) et Caraorman (56 ha) du Delta du Danube, ont montré que, dans des conditions d'aménagement et d'exploitation rationnelles, les grands étangs sont adéquats pour cette fonction, car le pourcentage de survie de l'état d'alevin à l'âge d'un été est maintenu élevé (25–35 pour cent). C'est ainsi qu'on peut éliminer le prédéveloppement dans des étangs spécialement construits et les frais supplémentaires de production (Mirica, 1968). Les premières expériences de Caraorman ont indiqué l'opportunité du peuplement avec de la carpe chinoise de deux ans parce que, à cet âge, la présence de jeunes poissons ictyophages n'est plus nocive et assure la croissance de la production des autres poissons paisibles à plus de 250 kg/ha, par effet de l'amélioration des conditions du milieu (Popescu, 1962). Dès 1968, la carpe chinoise et la carpe argentée ont été élevées avec la carpe autochtone. On a établi dès la première année d'expériences que les carpes chinoises ne doivent pas participer au peuplement à plus de 30 pour cent et qu'approximativement 40 pour cent de l'augmentation de la production, dans le cadre de la pisciculture combinée, a été réalisée par les poissons phytophages (Nicolau, Popescu et Dragasanu, 1969). En régime sans fourrage, la participation des poissons phytophages à l'augmentation finale a été de 62–72 pour cent, c'est-à-dire 171–174 kg/ha. 5.1.4 Reproduction et extension Dans les eaux roumaines on n'a jamais observé la reproduction libre des Cyprinidae importées de Chine; on a accordé beaucoup d'attention à sa reproduction artificielle (Nicolau et Popescu, 1968; Popescu, Dragasanu et Giurca, 1970). On considère que 100 kg/ha de reproducteurs est un maximum pour la période végétative et on a établi que la consommation de végétaux pour assurer une augmentation annuelle de 1.0–1.5 kg/ex. est de 50–100 fois plus grande que l'augmentation du poids vif, en fonction de la qualité du fourrage. Dans le Delta du Danube, la période de maturation est d'habitude pendant la troisième décade du juin, quand la température de l'eau est de 23–25°C. Les reproducteurs sont injectés avec 2.5–2.6 mg d'hypophyse/kg corps. Maintenant la règle est de pratiquer une pisciculture mixte de la carpe avec des espèces phytophages chinoises parmi lesquelles, dans les étangs envahis par la végétation, la carpe chinoise est dominante. Sauf dans les étangs où l'on pratique la pisciculture, on rencontre la carpe chinoise, surtout à la suite de grandes innondations, dans les eaux libres. Elle est rare et on en trouve dans de dimensions différentes. Il est possible que parmi ces poissons on en trouve certains provenant de la zone de l'embouchure de la Tisza, où on a observé la reproduction des espèces phytophages originaires de Chine1/. 5.1.5 Maladies et parasites Les Cyprinidae importés de Chine ont été soumis à un examen ichtyopathologique initial puis périodique (Radulescu et Georgescu, 1962; Radulescu, 1962; Radulescu et Georgescu, 1964a; 1966). On n'a pas observé chez la carpe chinoise des affections infecto-contagieuses; elle n'a pas contracté l'hydropisie infectieuse même quand elle cohabitait avec des carpes malades, présentant la forme aigue de cette affection. Sur 38 parasites connus dans la R.P. de Chine, on a introduit dans les étangs de la Roumanie 4 espèces nouvelles pour la faune du pays, dont aucune n'est dangereuse pour homme: Trichophyra sinensis (protozoaire parasite sur les branchies), le monogène Dactylogyrus lamellatus et les parasites intestinaux Botriocephalus gowkongensis et Khawia. Sauf le D. lamellatus qui est spécifique de la carpe chinoise, les autres parasites sont aussi passés à la carpe. Comme règle, les parasitoses dues à ces immigrants ne sont pas périclitantes. La carpe chinoise a contracté quelques parasites autochtones, spécifiques des étangs du pays; les plus nocifs se sont avérés être Ligula intestinalis et Tetractyle sp. 5.1.6 La pêche La pêche d'essai pendant l'été ne donne pas de bons résultats pour la carpe chinoise; poisson fort et rapide, elle évite fréquemment les seines et on obtient un pourcentage moindre comparativement aux autres espèces. Pendant la pêche d'automne des étangs, quand le niveau de l'eau est abaissé, on la prend sans difficulté. Dans les eaux libres, la carpe chinoise est aussi pêchée au tramail et au filet maillant. 5.2 Hypophthalmichthys molitrix (Val.) La carpe argentée a été introduite en même temps que la carpe chinoise. C'est un phytoplanctonophage qui a en Roumanie un rythme de croissance supérieur à celui de la carpe chinoise. Son poids moyen atteint à la fin de l'année: 1 – 105 g, 2 – 1 800 g, 3 – 2 800 g, 4 – 5 500 g, 5 – 6 300 g, 6 – 7 500 g. Pendant le deuxième été, élevée avec la carpe, elle augmente de 200–400 g/ex. et a un indice de survie de 90. La reproduction et le développement ont été observés pour la carpe argentée parallèlement à ceux de la carpe chinoise. Les alevins de la première sont plus sensibles au transport comparativement à la seconde; ils se développent bien dans des étangs à grande surface. On rencontre la carpe argentée dans les mêmes zones que la carpe chinoise; peu nombreuse, on la trouve dans les eaux libres. Les premières années après son introduction, la carpe argentée a été très peu parasitée et ne le fut pas dans les étangs dans lesquels on l'a introduite (Radulescu, 1962). On a signalé la lipomatose diffuse (Radulescu, Georgescu, 1964b). Chez les jeunes carpes argentées on a signalé 12 espèces de parasites; le seul spécifique est Dactylogyrus hypophthalmichthys (Radulescu, Georgescu et Angelescu, 1971). Pêchées à la seine, les carpes argentées sautent de l'eau et beaucoup s'échappent; pour cette raison il est nécessaire de lever la ralingue à flotteurs. Dans les lacs artificiels on pêche un certain nombre de carpes argentées au filet maillant et au tramail. 5.3 Aristichthys nobilis (Rich.) La carpe argentée tachetée, zooplanctonophage, a été importée en même temps que les autres alevins de Cyprinidae chinois. Son rythme de croissance s'est avéré exceptionnel; il dépasse même celui connu dans son pays d'origine. Le poids moyen atteint à la fin de l'année: 1 – 164 g; 2 – 2 500 g; 3 – 5 500 g; 4 – 7 200 g; 5 – 10 700 g; 6 – 13 000 g. On enregistra le poids maximum de 17 kg à 6 ans. Sa première reproduction artificielle en Roumanie a eu lieu en 1966. Dans les étangs elle est rare et dans les eaux libres on la rencontre encore plus rarement que les autres Cyprinidae chinois. Normalement elle a des parasites des mêmes espèces que la carpe argentée. On a trouvé seulement Diplozoon paradoxum sur A. nobilis; quoiqu'on ait tenu ensemble les jeunes de jeunes de carpes argentées tachetées et de carpes argentées, on a trouvé les parasites Ichtyophthirius multifiliis, Tylodephus clavata, Botriocephalus gowkongensis et Ligula intestinalis seulement sur les derniers (Radulescu, Georgescu et Angelescu, 1971). 5.4 Mylopharyngodon piceus (Rich.) La carpe noire, molluscophage, importée avec les autres alevins de la R.P. de Chine, en représentait 2.4 pour cent du premier lot. A la station piscicole de Nucet le poids moyen de la carpe noire a été à la fin des premières années: 1 – 80 g; 2 – 800 g; 3 – 2 500 g. Le poids maximum atteint à la fin de la troisième année a été de 4.4 kg. La carpe noire n'est pas répandue. 5.5 Pseudorasbora parva (Schlegel) On a introduit cette espèce par mégarde, en 1960, comme alevins de provenance d'U-han, bassin inférieur du fleuve Yang-Tseu-Kiang, mélangés aux autres alevins de Cyprinidae de valeur. Dès le printemps 1961 il commença à se reproduire dans les étangs de Nucet, d'où il pénétra dans le ruisseau voisin Ilfov, le long duquel il s'est répandu (Banarescu, 1964). Cette espèce indésirable est maintenant répandue dans plusieurs eaux du sud de la Roumanie. 6. INTRODUCTION DES POECILIDAE La gambusie, Gambusia affinis holbrooki (Agassiz), répandue le long du littoral de l'est des Etata-Unis, a été introduite afin de combattre le paludisme (Nicolau, 1946). On l'a introduite dans plusieurs endroits, le lac Mangalia (Dobroudja), des bassins prèe de Bucarest, dans des marécages près d'Orades. Dans les étangs la gambusie est nuisible, étant une concurrente pour la nourriture des jeunes carpes (Banarescu, 1964). 7. INTRODUCTION DES CATASTOMIDAE Les trois dernières espèces introduites en Roumanie sont originaires de l'Amérique du Nord. Toutes les trois sont du genre Ictiobus: le buffalo à petite bouche (I. bubalus), le buffalo noir (I. niger) et le buffalo à grande bouche (I. cyprinellus). Premièrement on les a introduites à Nucet, où elles se sont bien développées (Giurca, Popovici et Angelescu, 1978). Maintenant elles sont testées dans plusieurs étangs. 8. BIBLIOGRAPHIE Banarescu, P., Pisces-. Fauna Republicii Populare Romine. XIII:962 p. 1964 Bucuresti, Editura Academiei Republicii Populare Romine Busnita, Th. et al., Primele incercari de aclimatizare a coregonilor in apele R.P.R. 1957 Bulet.Instit.de Cercet.si Piscic., XVI(2):5–19 Carausu, S., Tratar de ichtiologie. Bucuresti, Editura Republicii Populare Romane. 802 1952 p. Cure, V., Dezvoltarea speciei Ctenopharyngodon idella (Val.) in iazul Frasinet. 1970 Bulet.Instit.de Cercet.si Piscic., XXIX(4):31–51 Cure, V., A. Snaider et I. Chiosila, Macrofitele din iazul Frasinet (jud. Ilfov) si rolul lor in 1970 viata ecosistemului - doi ani dupa introducerea speciei Ctenopharyngodon idella. Bulet.Instit.de Cercet.si Proiect.Piscic., XXIX(1–2):5–27 Giurca, R., V. Popovici et N. Angelescu, Reproduceres si cresterea speciilor Ictiobus 1978 cyprinellus, Ictiobus bubalus si Ictiobus niger (Fam. Catastomidae). Rev.de Crest.Anim., XXVIII(10):45–50 Mirica, Gh., Importanta pentru economia noastra piscicola a problemei de aclimatizare a 1962 speciilor Ctenopharyngodon idella si Hypophthalmichthys molitrix. Bulet.Instit.de Cercet.si Project.Piscic., XXI(4):7–11 , Perspectivele pisciculturii in Republica Populara Romana. Bulet.Instit.de 1964 Cercet.si Project.Piscic., XXII(4):5–18 , Prima pepiniera pentru reproduceres in Delta Dunarii a unor pesti fitofagi 1965 originari din China. Bulet.Instit.de Cercet.si Piscic., XXIV(3–4):15–18 , Rezultatele unei experimentari cu pesti fitofagi, recent aclimatizati in Delta 1968 Dunarii. Bulet.Instit.de Cercet.si Proiect.Piscic., XXVII(1):45–49 Mirica, Gh. et al., Rezultatele cercetarilor pentru introducerea in apele Romaniei a unor 1966 specii de pesti originari din China. Bulet.Instit.de Cercet.si Proiect.Piscic., XXV(4):5–30 Nicolau, A., Sur la presence de Gambusia affinis dans le lac Mangalia. Notationes 1946 Biologicae, vol. 4:189–196 Nicolau, A., Reszultatele cercetarilor experimentale efectuate la Nucet in anii 1960, 1961 1962 si 1962 cu privire la cresterea in elestee si iernarea speciilor Ctenopharyngodon idella si Hypophthalmychthys molitrix. Bulet.Instit.de Cercet.si Piscic., XXI(4): 25–37 , Rezultatele cercetarilor cu privire la aclimatizarea in apele tarii noastre a 1966 unor specii de pesti fitofagi si fitoplanctofagi originari din China. Bulet.Instit.de Cercet.si Proiect.Piscic., XXV(1):30–37 Nicolau, A. et al., Primele rezultate obtinute in actiunea de aclimatizare a special 1961 Ctenopharyngodon idella (Val.) in apele R.P.R. Bulet.Instit.de Cercet.si Piscic., XX(2): 25–38 Nicolau, A. si E. Popescu, Reproducerea artificiala a pestilor fitofagi in conditiile 1968 climatice ale Romaniei. Bulet.Instit.de Cercet.si Proiect.Piscic., XXI(4): 38– 50 Nicolau, A., E. Popescu et St. Dragasanu, Citeva aspecte privind cresterea speciilor de 1969 pesti fito-si plantonofage, impreuna cu crapul. Bulet.Instit.de Cercet.si Proiect.Piscic., XXVIII(3):23–28 Popescu, E., Rezultatele cercetarilor experimentale intreprinse in iazul Caraorman – 1962 zona maritima a Deltei Dunarii - cu privire la comportarea si cresterea speciei Ctenopharyngodon idella in a treia vara. Bulet.Instit.de Cercet.si Piscic., XXI(4):38–50 Popescu, E., St. Dragasanu et R. Giurca, Contributii la imbunatatirea procesulul 1970 tehnologic al reproducerii artificiale a speciei Ctenopharyngodon idella (Val.). Bulet.de Cercet.Piscic., XXIX(4): 20–30 Radulescu, I., Parazitii specifici pestilor adusi din R.P. Chineza pentru aclimatizare, 1962 efectele infestarii cu acesti paraziti a pestilor autohtoni si masurile de combatere a parazitilor. Bulet.Instit.de Cercet.si Piscic., XXI(4):51–61 Radulescu, I. et R. Georgescu, Contributiuni la cunoasterea parazitofaunei speciel 1962 Ctenopharyngodon idella in primul an de aclimatizare in R.P. Romina. Bulet.Instit.de Cercet.si Piscic., XXI(3): 85–91 , Noi cercetari asupra infestarii pestilor cu Bothriocephalus 1964a gowkongensis Veh. Bulet.Instit.de Cercet.si Piscic., XXIII(1):78–84 , Un caz de lipomatoza la Hypophthalmichthys molitrix. 1964b Bulet.Instit.de Cercet.si Piscic., XXIII(1): 85–86 , Noi contributii la cunoasterea parazitofaunei speciei 1966 Ctenopharyngodon idella pe cale de aclimatizare in Romania. Bulet.Instit.de Cercet.si Proiect.Piscic., XXV(2):48–51 Radulescu, I., R. Georgescu et N. Angelescu, Contributii la conoasterea parazitofaunei 1971 speciilor de Hypophthalmichths aclimatizate in crescatoriile piscicole romanesti. Bulet.de Cercet.Piscic., XXX(2): 87–91 Welcomme, R.L. (comp.), Register of international transfers of inland fish species. FAO 1981 Fish.Tech.Pap., (213):120 p. 1 D'après les informations de collègues yougoslaves, on n'a pas observé une telle reproduction en 1980 THE EXOTIC ICHTHYOFAUNA OF THE CONTIGUOUS UNITED STATES WITH PRELIMINARY OBSERVATIONS ON INTRANATIONAL TRANSPLANTS

W.R. Courtenay, Jr. and J.N. Taylor Department of Biological Sciences, Florida Atlantic University Boca Raton, Florida, U.S.A.

CONTENTS

1. INTRODUCTION 2. SPORT AND FOOD FISHES 3. BIOLOGICAL CONTROL 4. AQUACULTURE 5. FORAGE 6. AQUARIUM FISHES 7. EXOTIC FISHES FROM OTHER SOURCES 8. PRELIMINARY OBSERVATIONS CONCERNING INTRANATIONAL TRANSPLANTS 9. ACKNOWLEDGEMENTS 10. REFERENCES

ABSTRACT We record 39 species of exotic (foreign) fishes as established in open waters of the 48 contiguous United States. Included are 13 , 7 cyprinids, 7 poeciliids, 3 loricariids, 2 gobiids, 2 sciaenids and one species each for the families Anabantidae, Clariidae, Cobitidae, Osmeridae and Salmonidae. All but nine of these fishes were introduced since the second world war. Only the brown trout (Salmo trutta), bairdiella (Bairdiella icistia) and orangemouth corvina (Cynoscion xanthulus) are generally accepted as having been beneficial introductions. In addition, we record another 57 non-established exotic fishes known from U.S. waters. Fourteen of these were formerly established and another nine were intentionally released but failed to establish. A preliminary review indicates that at least 168 native fishes have been transplanted beyond their natural ranges, with the majority appearing to have been moved as game and released bait.

RESUME Trente-neuf espèces de poissons exotiques sont établies dans les eaux libres des 48 Etats contigus des Etats-Unis: treize cichlidés, sept cyprinidés et sept poécillidés, trois loricariidés, deux gobies et deux sciénidés et une espèce de chacune des familles suivantes: Anabantidae, Clariidae, Cobitidae, Osmeridae et Salmonidae. Tous ces poissons, à l'exception de neuf, ont été introduits après la deuxième guerre mondiale. Seule la truite (Salmo trutta) est en général considérée comme une introduction bénéfique par les aménagistes et les biologistes des pêches. Quinze espèces de poissons exotiques ont des populations en expansion aux Etats-Unis, quatorze ont une répartition localisée, trois ont une aire de répartition assez vaste et plus ou moins stabilisée, quatre semblent avoir des populations en diminution; enfin, on dispose de trop peu d'informations pour pouvoir juger de la situation de trois autres espèces. 1. INTRODUCTION The continental United States now hosts 39 species of exotic fishes, established as reproducing populations. Most are aquarium fishes. Courtenay and Robins (1973, 1975) and Courtenay et al. (1974) discussed several reasons for and sources of introductions of foreign (exotic) fishes in the U.S. These included releases for establishing sport or food fishes; biological control of one or more pest aquatic plants (many of which are also exotic); and releases from culture facilities and by hobbyists. Other introductions have been made for purposes of aquaculture, insect control and forage. Two species have become established as a result of accidental transoceanic movement. In this paper, we summarize the status of established exotic fishes in the continental U.S., in phylogenetic order after Robins et al. (1980), in the categories listed above. A list of non-established exotic fishes known from U.S. waters is included (Table 1) and preliminary observations on intranational transplants of native fishes are presented. 2. SPORT AND FOOD FISHES Only three exotic fishes are regarded as beneficial introductions in this category: the brown trout (Salmo trutta, section 2.1), bairdiella (Bairdiella icistia, section 2.6) and orangemouth corvina (Cynoscion xanthulus, section 2.7). 2.1 Salmo trutta Linnaeus (brown trout) First introduced in the lower peninsula of Michigan in 1883 (Mather, 1889; Goode, 1903; Laycock, 1966), this salmonid is now established in suitable waters in Arizona, Arkansas, California, Colorado, Connecticut, Delaware, , Idaho, Illinois, Indiana, Iowa, Maine, Maryland, Massachusetts, Michigan, Minnesota, Missouri, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, North Dakota, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Utah, Vermont, Virginia, Wisconsin and Wyoming (MacCrimmon and Marshall, 1968). Kentucky recently (1980) reactivated stocking toward establishing self-sustaining populations. As of 1980, only Arkansas, Illinois and Montana had discontinued annual stocking of this fish. Illinois indicated that stocking would be reactivated upon completion of a new hatchery. The brown trout is one of three exotic fishes in the U.S. that is generally regarded as beneficial. Although reported as more difficult to catch than native trouts, it remains popular with anglers. Brown trout often occupy waters not inhabited by native trouts that prefer cooler waters. Nevertheless, there have been problems. Brown trout have adversely impacted, by predation, the golden trout (Salmo aguabonita), the “state fish” of California, in the Little Kern River, and a brown trout eradication programme is underway there (E.P. Pister, pers. comm.). The National Park Service is also eradicating brown trout from the Great Smoky Mountain National Park because of its negative impact, again by predation, on native brook trout (Salvelinus fontinalis; Reiger, 1981). 2.2 Cyprinus carpio Linnaeus () DeKay (1842) records the first release of this cyprinid fish in the Hudson River, New York, in 1831. Self-sustaining populations now exist in the 48 contiguous states, mostly from federal stockings conducted in the late 1800s (Baird, 1879; Laycock, 1966). Population densities are greatest in the midwestern states but only scattered populations exist in portions of the extreme southeastern U.S. In Florida, for example, this species occurs in only two rivers in the panhandle. Introduced as a popular food fish from Europe for aquaculture and releases in natural waters, this species has never become a popular food or sport fish in the U.S. It is considered a pest fish in nearly every state where it is established. It is claimed to have disturbed waters by its feeding habits, thereby displacing preferred native fishes and damaging waterfowl habitat (Laycock, 1966). As of 1980, 15 states conducted annual common carp control or eradication programmes at a total cost of over U.S. $ 300 000. 2.3 Leuciscus idus (Linnaeus) (ide) Now restricted to one pond in Connecticut (Whitworth et al., 1968), the ide was formerly established in New York (Bean, 1901), Pennsylvania (E.C. Cooper, pers. comm.) and the Potomac River adjoining the District of Columbia, Maryland, and Virginia (Smith and Bean, 1899; Bean and Fowler, 1929; Schwartz, 1963; Musick, 1972). Reasons for its decline and apparent absence in New York and the upper Potomac drainage are unknown; its disappearance from two sites in Pennsylvania was due to eradication and the filling of a pond for highway construction. The ide was imported for distribution (apparently with common carp) in 1877, probably as a food and ornamental (aquarium) species. 2.4 Scardinius erythrophthalmus (Linnaeus) (rudd) Due to its popularity as a sport and food species in Europe, the rudd was also imported in the late 1870s. It was introduced and became established in New Jersey (Myers, 1925), New York (Myers, 1925; Greeley, 1937) and Wisconsin (Cahn, 1927; Greene, 1935). C.R. Robins (pers. comm.) reported this fish as present in Cascadilla Creek near the Cornell University hatchery ponds in New York in the fifties. To our knowledge, the rudd is no longer extant in New Jersey or Wisconsin; reasons for its decline and disappearance there are unknown. In New York, it now appears restricted to two tributaries of the Hudson River. 2.5 Tinca tinca (Linnaeus) (tench) Imported in 1877 with common carp, the tench was distributed to many states. It became established in California (Moyle, 1976), Colorado (Beckman, 1974), Connecticut (Whitworth et al., 1968), Delaware (Schwartz, 1963), Idaho (Simpson and Wallace, 1978), Maryland (Schwartz, 1963), New Mexico (Koster, 1957) and Washington (Wydoski and Whitman, 1979). It was introduced to Arizona (Minckley, 1973) and Missouri (Baughman, 1947) but failed to establish. The tench apparently is now absent from New York (although it was collected from Cascadilla Creek, Ithaca, in the fifties; C.R. Robins, pers. comm.), New Mexico and Oregon (E.C. Raney, J.E. Johnson and C.L. Bond, pers. comm.) and probably also from Delaware and Maryland. There are no recent records from those states. 2.6 Bairdiella icistia (Jordan and Gilbert) (bairdiella) The Salton Sea in southern California was created in 1905–7 by a closure of the lower Colorado River through sediment deposition, blocking its natural flow path to the Gulf of California. The river took the path of least resistance and invaded an almost- completed irrigation canal into the Salton Sink, about 82 m below sea level (Sykes, 1937). Freshwater fishes of the lower Colorado River were washed into the sink with the creation of this inland sea. In subsequent years, the salinity rose in the Salton Sea, extirpating the freshwater species. By 1956, the salinity had risen to 33‰ (Carpelan, 1961). Many introductions were made as the salinity rose and most failed (Walker et al., 1961). In the early fifties it was decided to introduce marine species from the , collected from the Mexican coast. Among these were two sciaenids, the bairdiella, orangemouth corvina (Cynoscion xanthulus) and a haemulid, the sargo (Anisotremus davidsoni). The sargo is native in marine waters of extreme southern California (Walker et al., 1961). These fishes adapted and became established, along with some marine plants and invertebrates that became established before these fishes were introduced. The bairdiella has been a popular angling species since its introduction. It has also provided forage for the orangemouth corvina. There is some evidence, however, that bairdiella populations have been adversely impacted by introductions of tilapia in the late seventies (G.L. Black and F.G. Hoover, pers. comm., see section 3.4). 2.7 Cynoscion xanthulus Jordan and Gilbert (orangemouth corvina) The orangemouth corvina is the most popular angling species in the Salton Sea. It is common for large adults to weigh 11 kg or more (Walker et al., 1961). See account 2.6 for its history in the Salton Sea. 3. BIOLOGICAL CONTROL Fishes in this category were released for control of pest aquatic plants or insects. Evaluation studies prior to releases were conducted primarily by agronomists, entomologists or plant pathologists and plant physiologists; these evaluations, however, were target-oriented (= pest control). Only within the latter part of the past decade have studies of impact on native fishes or their habitats been included in several studies. Many introductions were conducted without prior testing; we consider that fisheries “mis- management”. In the U.S. the use of exotic fishes as biological control agents remains controversial. Fishes are not monophagous as are several insects (Courtenay, 1979a). Preferential herbivores in test situations have performed differently after release. Thus, non-target aquatic plants that shelter the young of native fishes or provide for their food organisms often become food of the exotic used for biological control. If a fish introduced as a biological control reproduces in great numbers, it becomes a biological control “out of control”. 3.1 Ctenopharyngodon idella (Valenciennes) (grass carp) Imported in 1963 from stocks in Malaysia and Taiwan (Guillory and Gasaway, 1978), grass carp were subsequently distributed to research agencies and companies with research capabilities in 11 states (Provine, 1975). The first release of this cyprinid was in Arkansas, probably in the late sixties. The first specimens caught in the wild were from the White River, Arkansas, in 1970 (Bailey, 1972); others were subsequently caught in the Mississippi River in Illinois in 1971 (Greenfield, 1973). Those specimens were of the 1966 age-class, suggesting the source of release to have been either the U.S. Fish and Wildlife Service Experimental Station at Stuttgart, Arkansas, or the Arkansas Game and Fish Commission hatchery at Lonoke (Guillory and Gasaway, 1978) or both. This species has been reported as established in the Mississippi River drainage since 1975 (Connor et al., 1980). 3.2 Tilapia aurea (Steindachner) (blue tilapia) We include this in the biological control category because two, possibly three, states (Arizona, Florida and Georgia) introduced it, primarily for algal or other aquatic plant control. There is some question as to the identity of the tilapia in Georgia. The blue tilapia was imported from Israel via the late Dr. Earl Herald of the Steinhart Aquarium by Auburn University in 1954 for studies on its potential in aquaculture and as a sport fish (Swingle, 1960). In 1961, Florida introduced stocks from Auburn in an experimental area to examine the ability of blue tilapia to control aquatic vegetation; this introduction apparently was made under the guise of testing its sport potential, already disproven at Auburn (Swingle, 1960). Fishermen subsequently moved specimens out of the experimental area, and this species is now established in 18 counties of peninsular Florida. There is some evidence that since 1972, introductions of this fish have been made by commercial fishermen for the purpose of creating new fisheries (Harris, 1978). Commercial fishing for blue tilapia was legalized in 1972 in Florida. Arizona has introduced this fish since 1975 for algal control (W.L. Minckley, pers. comm.). It is now dominant in the lower Colorado River near Yuma, where it appears to have replaced most of an earlier-introduced, largely Mississippian fish fauna. We assume its reported introduction into golf course ponds at Sea Island, Georgia, was for aquatic vegetation control. The blue tilapia is also established in thermal springs in Colorado (for aquaculture), in one heated lake in North Carolina (introduced as a sport fish), a heated effluent in an Oklahoma river (for aquaculture; Pigg, 1978) and in heated reservoirs and the Rio Grande in Texas (for aquaculture, “inadvertently released”, and bait releases; Noble et al., 1976; C. Hubbs, pers. comm.). 3.3 Tilapia hornorum Trewavas (Wami tilapia) The Wami tilapia was introduced into three counties in southern California for aquatic plant, mosquito and chironomid midge control in the mid seventies (Hauser et al., 1976; Legner and Pelsue, 1977; Legner, 1979; Legner et al., 1980). It was reported as introduced with Mozambique tilapia (Tilapia mossambica) and hybrids of the Mozambique and Wami tilapias. 3.4 Tilapia mossambica (Peters) (Mozambique tilapia) This fish is established in Arizona, California, Florida and Texas. It was purposely introduced in Arizona for aquatic plant control (Mickley, 1973) and in southern California for the same reason (see section 3.3). It is also reported as established in heated effluents in raceways on a commercial catfish farm in Colorado where it is used for algal control (R.J. Behnke, pers. comm.). It became established at two localities in Florida after escaping from aquarium fish farms and by releases by hobbyists, and in a river in Texas after escaping from a public aquarium (Brown, 1961). This species, or a hybrid between the Mozambique tilapia and an unknown congener, invaded the Salton Sea in southern California after 1976 and has become the dominant fish in this saline lake (G.L. Black, F.G. Hoover and J.A. St. Amant, pers. comm.). It, or a hybrid with the Wami tilapia, is now dominant in the San Gabriel River near Los Angeles, California (Knaggs, 1977). 3.5 Tilapia zilli (Gervais) (redbelly tilapia) The redbelly tilapia is established near Phoenix, Arizona, and perhaps elsewhere in that state (Minckley, 1973), in irrigation canals in three southern California valleys (Pelzman, 1973; Moyle, 1976) and in a golf course pond in a valley west of Las Vegas, Nevada (Courtenay and Deacon, 1982). C. Hubbs (pers. comm.) has reported its possible establishment in southern Texas. To our knowledge, all of these populations were introduced for aquatic vegetation control. Those released into some California waters were also stocked for mosquito and chironomid midge control (Legner and Pelsue, 1977). This species is reported as being cultured in warm springs in Idaho (for aquaculture) and in heated effluents in North and South Carolina (for vegetation control and aquaculture). 4. AQUACULTURE The utilization of exotic fishes for aquaculture in the U.S. was, in part, a reason for importing common carp in the 1870s (see section 2.2). Since the second world war, several tilapias have become aquaculture subjects in warm climates or where thermal springs or warm effluents are available (see sections 3.2 and 3.5). Except in sub-tropical or extreme warm temperate zones, survival of tilapias escaping from culture facilities is unlikely. In several localities where climate has permitted establishment, population explosions of tilapia have been accompanied with moderate to dramatic declines of native fishes, in some cases including transplanted native species. When blue tilapia populations reached 1 842 kg/ha in one Texas reservoir, most native fishes ceased reproducing (Noble et al., 1976; Noble, 1977). 5. FORAGE To our knowledge, only one exotic fish has been introduced in the U.S. as a forage species. 5.1 Hypomesus nipponensis McAllister (wakasagi) This osmerid was released into six reservoirs in California as forage for trout (Wales, 1962; Moyle, 1976). Moyle (1976) mentioned subsequent releases into other California reservoirs for the same purpose. The current status of this fish in California is unknown. 6. AQUARIUM FISHES Aquarium fishes are very popular in the U.S. Most of our established exotic aquarium fishes were unintentionally (rarely intentionally) released from culture facilities (Courtenay and Robins, 1973, 1975; Courtenay et al., 1974; Courtenay and Hensley, 1980). Most such introductions have occurred in states within a sub-tropical climatic zone (e.g., southern Florida where 80 percent of the U.S. aquarium fishes are cultured and southern California). Areas where most of these releases resulted in establishment contain disturbed habitats and ecosystems, a combination that has facilitated establishment. Hobbyists have been responsible for releasing aquarium fishes. Several of these releases have become established in canal systems in and around metropolitan areas. Other hobbyist releases have occurred in thermal springs in northwestern states and in remote desert ecosystems in the American southwest where they have shown adverse impacts on endemic, often endangered native fishes (Miller and Alcorn, 1946; Miller, 1961; La Rivers, 1962; Hubbs and Brodrick, 1963; Deacon et al., 1964; Hubbs and Deacon, 1965; Minckley, 1973; Courtenay et al., 1974; Deacon, 1979; Hardy, 1980; Courtenay and Deacon, 1982, 1983). No introduced aquarium fish has been demonstrated to have beneficial value in open waters of the U.S. We regard their introduction as needless but to be expected. These stock additions cannot be considered stock enhancement. 6.1 Carassius auratus (Linnaeus) () Introduced in the late 1600s (DeKay, 1842), this cyprinid was the first exotic fish to be released and become established in North America (Courtenay and Hensley, 1980). Self-sustaining populations exist at scattered locations in a majority of states. 6.2 Leuciscus idus (Linnaeus) (ide) See section 2.3. 6.3 sericeus (Pallas) (bitterling) First noted in the Sawmill River north of New York City in the twenties (Dence, 1925; Myers, 1925; Bade, 1926), the bitterling also became established in the adjacent Bronx River (Greeley, 1937). It is now apparently confined to one small area of the Bronx River where it is expected to be extirpated by industrial pollution through demise of its molluscan host (Schmidt et al., 1981). 6.4 Misgurnus anguillicaudatus (Cantor) (oriental weatherfish) This cobitid is established in one river in the southeastern portion of the southern peninsula of Michigan (Schultz, 1960; M.L. Smith, pers. comm.) and some flood control channels in southern California (St. Amant and Hoover, 1969; M.H. Horn, pers. comm.). 6.5 Clarias batrachus (Linnaeus) (walking catfish) After escaping from a fish farm in southeastern Florida in the mid sixties, followed by subsequent intentional releases in the Tampa Bay area in 1968 (Courtenay and Miley, 1975; Courtenay, 1978, 1979), this fish has spread over 20 counties in peninsular Florida (Courtenay, 1979). There have been large build-ups of local populations periodically. This fish has become a problem to aquarium fish farmers in recent years through predation on their culture fishes (Courtenay and Miley, 1975). 6.6 Hypostomus spp. ( catfishes) At least three morphologically distinct but as yet unidentified species of this loricariid genus are established in the U.S.: one in western peninsular Florida and perhaps a second near Miami (Courtenay et al., 1974); another in a thermal spring in southern Nevada (Minckley, 1973; Courtenay and Deacon, 1982); and the third in southern Texas (Barron, 1964; Hubbs et al., 1978). 6.7 Belonesox belizanus Kner (pike killifish) This poeciliid has been established south of Miami, Florida, since November 1957 (Belshe, 1961; Rivas, 1965; Lachner et al., 1970; Miley, 1978). 6.8 Poecilia mexicana Steindachner (shortfin molly) The shortfin molly is established in southern California (St. Amant, 1966; St. Amant and Sharp, 1971; Mearns, 1975; Hubbs et al., 1978), in thermal springs in Montana (Brown, 1971) and Nevada (Deacon et al., 1964; Hubbs and Deacon, 1965; Courtenay and Deacon, 1982, 1983). 6.9 Poecilia reticulata Peters (guppy) Populations of the guppy are established in thermal springs in Arizona (Minckley, 1973), Florida (F.W. King, pers. comm.), Idaho (Simpson and Wallace, 1978), Nevada (Deacon et al., 1964; Williams et al., 1980; Courtenay and Deacon, 1982, 1983; Texas (Hubbs et al., 1977; Hubbs, 1982) and Wyoming (Baxter and Simon, 1970). It may be established locally in sewage treatment ponds in California (Moyle, 1976). 6.10 Poeciliopsis gracilis (Heckel) (porthole livebearer) This fish has been established since at least 1965 in a canal north of the Salton Sea, southern California (Mearns, 1975; Moyle, 1976; Hubbs et al., 1979; Shapovalov et al., 1981). 6.11 helleri Heckel (green swordtail) The green swordtail is established near Tampa Bay (Courtenay et al., 1974) and at Satellite Beach, south of Cape Canaveral, Florida (Dial and Wainright, 1983) and in a thermal spring in Montane (Brown, 1971). A hybrid of this species and the southern platyfish (Xiphophorus maculatus) is established in a thermal spring in southern Nevada (Courtenay and Deacon, 1982). 6.12 Xiphophorus maculatus (Günther) (southern platyfish) The southern platyfish is established in two localities (Gulf and Atlantic drainages) in peninsular Florida (Courtenay et al., 1974; Dial and Wainright, 1983). A hybrid with the green swordtail is established in a southern Nevada spring. 6.13 Xiphophorus variatus (Meek) () The variable platyfish is established in Gainesville (Burgess et al., 1977) and perhaps elsewhere in Florida. 6.14 Astronotus ocellatus (Agassiz) (oscar) Introduced in the late fifties, this cichlid is established in four counties of southern Florida (Rivas, 1965; Lachner et al., 1970; Courtenay and Hensley, 1980). This species has been accepted locally in Florida by some anglers, although it is never abundant. 6.15 Cichlasoma bimaculatum (Linnaeus) (black acara) Established since the late fifties or early sixties, this fish is now dominant in many south Florida canals (Courtenay et al., 1974) and recently has entered Lake Okeechobee. 6.16 Cichlasoma citrinellum (Günther) (Midas cichlid) The presence of the Midas cichlid in southeastern Florida was discovered by personnel of the Florida Game and Freshwater Fish Commission in 1981 (Shafland, in press). It is restricted to some canals south of Miami and apparently is expanding its range. 6.17 Cichlasoma meeki (Brind) (firemouth) The firemouth is established in the Tamiami Canal system, Miami, Florida (Courtenay and Hensley, 1980; Robins et al., 1980). 6.18 Cichlasoma nigrofasciatum (Günther) (convict cichlid) This exotic has been established in two thermal spring systems in southern Nevada since the early sixties (Deacon et al., 1964; Hubbs and Deacon, 1965; Courtenay and Deacon, 1982, 1983). 6.19 Cichlasoma octofasciatum (Regan) (Jack Dempsey) The Jack Dempsey is established in three localities in east-central and southern peninsular Florida (Courtenay et al., 1974; Hogg, 1976, 1976a; Dial and Wainright, 1983). 6.20 Hemichromis bimaculatus Gill (jewelfish) The jewelfish has been established since the early sixties in canals around Miami International Airport, Florida (Rivas, 1965; Courtenay et al., 1974). 6.21 Tilapia mariae (Boulenger) (spotted tilapia) The spotted tilapia is established in three counties in extreme southern Florida (Courtenay and Hensley, 1979) and in a thermal spring in southern Nevada (Courtenay and Deacon, 1982, 1983). 6.22 Tilapia melanotheron (Ruppell) (blackchin tilapia) The blackchin tilapia has been established since the late fifties or early sixties along the eastern shore of Tampa Bay, Florida (Springer and Finucane, 1963; Finucane and Rinckey, 1965). It has become established recently south of Cape Canaveral on Florida's east coast (Dial and Wainright, 1983). This species appears in the marine commercial catch where it has become established. 6.23 Tilapia mossambica (Peters) (Mozambique tilapia) See section 3.4. 6.24 Trichopsis vittata (Kuhl and Van Hasselt) (croaking gourami) This anabantid is established in a weedy area on one side of a Palm Beach County drainage canal, southeastern Florida (Hensley and Courtenay, 1980). 7. EXOTIC FISHES FROM OTHER SOURCES 7.1 Acanthogobius flavimanus (Temminck and Schlegel) (yellowfin goby) Ballast pumped from transoceanic ships entering the San Francisco Bay area, California, appears to have been the source of this introduction (Brittan et al., 1963; Brittan et al., 1970). The U.S. range of the yellowfin goby has expanded greatly since its introduction (Kukowski, 1972; Miller and Lea, 1972; Haacker, 1979; Usui, 1981); it has been reported as far south as San Diego in 1980 (C. Usui, pers. comm.). 7.2 Tridentiger trigonocephalus (Gill) (chameleon goby) The occurrence of this fish in California waters was first noted by Hubbs and Miller (1965). They theorized that its initial introduction into San Francisco Bay may have been as fertilized eggs on the introduced Japanese oyster (Crassostrea gigas). It is now established in San Francisco Bay and a fresh to brackish water lake in Oakland (Shapovalov et al., 1980) and in Los Angeles harbour (Moyle, 1976; Haacker, 1979). 8. PRELIMINARY OBSERVATIONS CONCERNING INTRANATIONAL TRANSPLANTS 8.1 Primary source of information Lee et al. (1980) included distributional maps and information for 743 native fishes known to inhabit the fresh waters of Canada and the U.S. Our review of the data contained therein shows that at least 151 species have been transplanted via human activities beyond their natural ranges of distribution in the continental U.S. New information supplied by W.L. Minckley and J.R. Stauffer, Jr. indicates yet another 17 transplanted fishes. The exact natural ranges for several of these fishes will never be known. Early surveys were incomplete and, in some instances, drainage basins were altered or disturbed prior to thorough surveys. Moreover, what appear to be transplants may be relict populations or transfers via stream capture for a few species. The species accounts in Lee et al. (1980) typically and unfortunately did not indicate the reasons for these transplants. It is known that many fishes of the families Salmonidae, Esocidae, Catostomidae, Ictaluridae, Percichthyidae and Centrarchidae were transplanted for game (sport and food) purposes. Small fishes, particularly cyprinids and percids, were released from bait buckets. 8.2 Summary regarding transplants Table 2 is a list of U.S. fishes transplanted beyond their natural ranges through introductions, a few via manmade canals or river diversions or as results of other human activities. For many species, we used a “best guess” as to the reason for the transplant and those are indicated by a question mark (?) in Table 2. A major complicating factor in attempting to ascertain reasons and distributions for certain transplants has been the movement of mixed fishes. This has occurred when portions of lakes, rivers or streams were seined for the purpose of transplanting one or more target game fishes or salvaging fishes from drying habitats. Non-target fishes (we call these incidentals) were often transplanted in this manner in unknown and unreported numbers. Some (perhaps many) species we listed as releases may be incidentals. Locally-seined bait fishes often are of several species; these, like commercially- sold bait, are often transported to other drainages for use. Even commercially-sold bait fishes, shipped from many hundreds of kilometres away to bait sellers, may contain mixed species. Some of these lots occasionally contain fishes belonging to several families, sometimes including juvenile centrarchids. Therefore, some of the smaller centrarchids listed in Table 2 as introduced as game fishes (the expected reason), in some instances, may represent bait releases. Many perhaps were also introduced as forage species. Our preliminary analysis indicates the overwhelming importance of sport fishing as a major factor in transplants in the U.S. with 53 species probably moved as game fishes and another 58 probably released from bait buckets. We identify 11 that were moved beyond their native ranges for species survival purposes (i.e., to prevent their immediate or expected future extirpation). For some, their native habitat subsequently was destroyed or so altered that extirpation without transplantation would have been certain; for others, survival continues within the native habitat or range and these fishes remain listed as threatened or endangered. Native fishes have never matched the popularity of exotic fishes as aquarium or ornamental species, but as many as three transplants may have been from releases of native ornamental fishes. Two species were introduced to expand their ranges. Other transplants were made for the probable reasons listed in Table 2. We again emphasize that these data are preliminary and will require extensive analysis of published state fish guides, as well as federal and state introduction records, and consultation with specialists for further refinement. It would be impossible to compile a truly accurate documentation because many agency records are incomplete, sometimes inaccurate, or perhaps no longer extant in some cases. Nevertheless, our summary shows that transplants of native fishes, planned or otherwise, far outnumber introductions of exotic fishes in the U.S. Perhaps this reflects an opinion of past and present fishery managers that moving native fishes is somehow safer than introducing exotics. 8.3 Environmental concerns For intentional transplants, we know of no studies prior to release on potential impacts on non-game fishes or habitat. With the exception of a few studies beginning in the mid-to-late seventies on potential impacts of grass carp, we find few earlier studies on potential impacts of exotic fishes. Nearly all previous intentional introductions seem goal-oriented only. Those persons emptying bait buckets have no environmental concerns for that activity beyond perhaps a perception that the released bait may provide forage for game species. An introduction is an introduction, whether the source is intranational, intracontinental or foreign. Impacts, ranging from negligible to major, are to be expected. Fishery managers and biologists are under continual public or political pressure to improve or enhance sport fisheries and to correct problems in aquatic ecosystems. Since introductions long have served as a management tool, they have all too often been accepted, mostly without question, as potential “cure-alls”. For example, fishes are often introduced to fill a so-called “vacant niche”; in fishery management terms, this means a perceived vacant trophic level. Thus, fishes may be released to control phytoplankton blooms; their high reproductive success may then lead to the introduction of one or more predatory fishes, usually game species, to control the phytoplankton consumers, now regarded as forage. The predators may control this forage or eliminate it. There has been at least one instance where two predatory game fishes were introduced to reduce such a forage base; the forage was destroyed, one predator eliminated the other (Allen and Roden, 1978), and then its population collapsed. Manipulating open waters in such a manner is unwarranted, needless and costly. Open waters, including reservoirs, are not farm or culture ponds and deserve better management policies. Proper fishery management has to (and must) be based on the best available knowledge of aquatic ecology, fishery biology and , and not on the “introduce anything” psychology that has developed over the past century. Introductions remain an extremely viable tool in fishery management of open waters when conducted properly. 9. ACKNOWLEDGEMENTS We thank J.A. McCann and J.R. Stauffer, Jr. for their helpful comments on the manuscript and C.R. Gilbert, W.L. Minckley, C.R. Robins and J.R. Stauffer, Jr. for their input on our preliminary review of transplants. 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Gasaway, Zoogeography of the grass carp in the United States. 1978 Trans.Am.Fish.Soc., 107(1):105–12 Haacker, P.L., Two Asiatic gobiid fishes, Tridentiger trigonocephalus and Acanthogobius 1979 flavimanus, in southern California. Bull.South.Calif.Acad.Sci., 78(1):56–61 Hardy, T., The Inter-Basin area report. Proc.Desert Fish.Counc., 11:5–21 1980 Harris, C., Tilapia: Florida's alarming foreign menace. Fla.Sportsman, 9(11):12, 15, 17–9 1978 Hauser, W.J. et al., Tilapia - a management tool for biological control of aquatic weeds 1976 and insects. Fisheries, 1(2):15–6 Hensley, D.A. and W.R. Courtenay, Jr., Trichopsis vittata Kuhl and Van Hasselt. In Atlas 1980 of North American freshwater fishes, edited by D.S. Lee et al. Raleigh, North Carolina, North Carolina State Museum, p. 799 Hogg, R.G., Established exotic cichlid fishes in Dade County, Florida. Fla.Sci., 39(2):97– 1976 103 , Ecology of fishes of the family Cichlidae introduced into the freshwaters of 1976a Dade Country, Florida. Ph.D. Dissert., Coral Gables, Florida, University of Mami, 142 p. (Unpubl.) Hubbs, C., Occurrence of exotic fishes in Texas waters. Pearce-Sellards Series, Texas 1982 Memorial Museum, 36, 19 p. Hubbs, C. and H.J. Brodrick, Current abundance of Gambusia gaigei, an endangered 1963 fish species. Southwest. Nat., 8(1): 46–8 Hubbs, C. and J.E. Deacon, Additional introductions of tropical fishes into southern 1965 Nevada. Southwest.Nat., 9(4): 249–251 Hubbs, C.L. and R.R. Miller, Studies of cyprinodont fishes. 22. Variation in Lucania 1965 parva, its establishment in western United States, and description of a new species from an interior basin in Coahuila, Mexico. Misc.Publ.Mus.Zool.Univ.Mich., (127):111 p. Hubbs, C.L., W.I. Follett and L.J. Dempster. List of the fishes of California. 1979 Occas.Pap.Calif.Acad.Sci., (133):51 p. 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Calif.Fish Game, 61(4):251–3 Miley, W.W., II, Ecological impact of the pike killifish, Belonesox belizanus Kner 1978 (Poeciliidae), in southern Florida. M.S. Thesis, Boca Raton, Florida, Florida Atlantic University, 55 p. (Unpubl.) Miller, D.L. and R.N. Lea, Guide to the coastal marine fishes of California. 1972 Fish.Bull.Calif.Dep.Fish Game, (157):235 p. Miller, R.R., Man and the changing fish fauna of the American southwest. 1961 Pap.Mich.Acad.Sci.Arts Lett., 46:365–404 Miller, R.R. and J.R. Alcorn, The introduced fishes of Nevada, with a history of their 1946 introduction. Trans.Am.Fish.Soc., 73:173–93 Minckley, W.L., Fishes of Arizona. Arizona Game and Fish Department, 293 p. 1973 Moyle, P.B., Inland fishes of California. Berkeley, California, University of California 1976 Press, 405 p. Musick, J.A., Fishes of Chesapeake Bay and the adjacent coastal plain. 1972 Spec.Sci.Rep.Va.Inst.Mar.Sci., (65): 175–212 Myers, G.S., Introduction of the European bitterling (Rhodeus) in New York and of the 1925 rudd (Scardinius) in New Jersey. Copeia, (140):20–1 Noble, R.L., Response of reservoir fish populations to tilapia reduction. Unpublished 1977 Progress Report to Sport Fishery Research Foundation, 4 p. Noble, R.L., R.D. Germany and C.R. Hall, Interactions of blue tilapia and largemouth 1976 bass in a power plant cooling reservoir. Proc.Annu.Conf.Southeast.Game Fish Comm., 29:247–51 Pelzmen, R.J., A review of the life history of Tilapia zillii with a reassessment of its 1973 desirability in California. Inland Fish.Admin.Rep.Calif.Dep.Fish Game, (74– 1):1–9 Pigg, J., The tilapia Sarotherodon aurea (Steindachner) in the North Canadian River in 1978 central Oklahoma. Proc.Okla.Acad.Sci., 58:111–2 Provine, W.C., The grass carp. Spec.Rep., Texas Parks Wildl.Dep. (Inland Fish.Res.), 1975 1975:51 p. Reiger, G., The wildness factor. Field Stream, 86(6):40, 137–8 1981 Rivas, L.R., Florida freshwater fishes and conservation. Q.J.Fla.Acad.Sci., 28(3):255–8 1965 Robins, C.R. et al., A list of common and scientific names of fishes from the United 1980 States and Canada. Spec.Publ.Am.Fish.Soc., (12):174 p. St. Amant, J.A., Addition of Tilapia mossambica Peters to the California fauna. Calif.Fish 1966 Game, 52(1):54–5 St. Amant, J.A. and F.G. Hoover, Addition of Misgurnus anguillicaudatus (Cantor) to the 1969 California fauna. Calif.Fish Game, 55(4):330–1 St. Amant, J.A. and I. Sharp, Addition of Xiphophorus variatus (Meek) to the California 1971 fauna. Calif.Fish Game, 57(2): 128–9 Schmidt, R.E., J.M. Samaritan and A. Pappantoniou, Status of the bitterling, Rhodeus 1981 sericeus, in southeastern New York. Copeia, 1981(2):481–2 Schultz, E.E., Establishment and early dispersal of a loach, Misgurnus anguillicaudatus 1960 (Cantor) in Michigan. Trans.Am.Fish.Soc., 89(4):376–7 Schwartz, F.J., The freshwater minnows of Maryland. Md.Conserv., 40(2):19–29 1963 Shafland, P.L., Cichlasoma citrinellum (Günther). In Atlas of North American freshwater fishes, edited by D.S. Lee et al. Raleigh, North Carolina, North Carolina State Museum (in press) Shapovalov, L., A.J. Cordone and W.A. Dill, A list of the freshwater and anadromous 1981 fishes of California. Calif.Fish Game, 61(1):4–38 Simpson, J.C. and R.L. Wallace, Fishes of Idaho. Moscow, Idaho, University of Idaho 1978 Press, 237 p. Smith, H.M. and B.A. Bean, Fishes known to inhabit the waters of the District of 1899 Columbia and vicinity. Bull.U.S.Fish Comm., 1898:179–87 Springer, V.G. and J.H. Finucane, The African cichlid, Tilapia heudeloti Dumeril, in the 1963 commercial fish catch of Florida. Trans.Am.Fish.Soc., 92(3):317–8 Swingle, H.S., Comparative evaluation of two tilapias as pondfishes in . 1960 Trans.Am.Fish.Soc., 89(2):142–8 Sykes, G., The Colorado delta. Spec.Publ.Am.Geogr.Soc., (191):1–193 1937 Usui, C., Behavioral, metabolic, and seasonal size comparisons of an introduced gobiid 1981 fish, Acanthogobius flavimanus, and a native cottid, Leptocottus armatus, from upper Newport Bay, California. M.A. Thesis, Fullerton, California, California State University. 52 p. (Unpubl.) Wales, J.B., Introduction of pond smelt from Japan into California. Calif.Fish Game, 1962 48(2):141–2 Walker, B.W., R.R. Whitney and G.W. Barlow, The fishes of the Salton Sea. 1961 Fish.Bull.Calif.Dep.Fish Game, (113):77–91 Whitworth, W.R., P.L. Berrien and W.T. Keller, Freshwater fishes of Connecticut. 1968 Bull.Conn.State Geol.Nat.Hist.Surv., (101):134 p. Williams, J.E., C.D. Williams and C.E. Bond, Survey of fishes, amphibians and reptiles of 1980 the Sheldon National Wildlife Refuge, Nevada, 1. Fishes of the Sheldon National Wildlife Refuge. Contract 14-16-0001-78025, U.S. Fish and Wildlife Service. Corvallis, Oregon, Oregon State University, 58 p. (Unpubl.rep.) Wydoski, R.S. and R.R. Whitney, Inland fishes of Washington. Seattle, Washington, 1979 University of Washington Press, 220 p. Table 1 A list of exotic fishes known from waters of the United States Formerly established species

Serrasalmus humeralis Valenciennes Florida Hoplias malabaricus (Bloch) trahira Florida Oryzias latipes (Temminck & Schlegel) medaka California, New York Cynolebias bellottii Steindachner Argentine pearlfish California Rivulus harti (Boulenger) Trinidad rivulus California Aequidens pulcher (Gill) blue acara Florida Cichlasoma beani (Jordan) green guapote California C. salvini (Gunther) yellowbelly cichlid Florida C. severum (Heckel) banded cichlid Nevada C. trimaculatum (Gunther) threespot cichlid Florida Anabas testudineus (Bloch) climbing perch Florida Betta splendens Regan Siamese fighting fish Florida Ctenopoma nigropannosum (Reichenow) twospot ctenopoma Florida Macropodus opercularis (Linnaeus) paradisefish Florida

Intentionally released with no evidence of establishment

Coregonus maraena Bloch German whitefish Michigan Salmo letnica (Karaman) Ohrid trout Colorado, Montana, Tennessee, Wyoming Esox reicherti Dybowski Amur pike Pennsylvania Chanos chanos (Forskal) milkfish California Cynolebias nigripinnis Regan California C. whitei Myers California Chirostoma jordani Woolman Texas Lates nilotica (Linnaeus) Nile perch Texas Cichla ocellaris Schneider tucanari Florida

Collected but not known to be established

Anguilla anguilla (Linnaeus) European eel California Osteoglossum bicirrhosum Vandelli aruana California, Nevada Plecoglossus altivelis Temminck & Schlegel ayu California Colossoma spp. pacu California, Florida, Ohio Colossoma brachypomum (Cuvier) Florida C. macropomum (Cuvier) Florida blackfin pacu Gymnocorymbus ternetzi (Boulenger) black tetra Florida Metynnis sp. Florida Metynnis roosevelti Eigenmann Kentucky Serrasalmus sp. piranha Illinois, Kentucky, Pennsylvania Serrasalmus nattereri (Kner) red piranha Florida, Massachusetts, Michigan, Pennsylvania Barbus sp. tinfoil barb Florida Barbus conchonius (Hamilton-Buchanan) rosy barb Florida B. gelius (Hamilton-Buchanan) golden barb Florida B. tetrazona (Bleeker) tiger barb California, Florida Brachydanio rerio (Hamilton-Buchanan) zebra danio California, Florida Danio malabaricus (Jerdon) giant danio Florida Hypophthalmichthys molitrix Valenciennes silver carp Arkansas Pterodoras granulosus (Valenciennes) Florida Callichthys sp. callichthys Florida Corydoras sp. corydoras Florida Pterygoplichthys sp. Florida Poecilia hybrids Florida, Nevada micropeltes Kuhl & Van Hasselt giant snakehead Maine, Rhode Island Stizostedion lucioperca (Linnaeus)* European pike-perch New York Ameca splendens Miller & Fitzsimons butterfly goodeid Nevada Cichlasoma labiatum (Gunther) red devil Florida Geophagus brasiliensis (Quoy & Gaimard) pearl eartheater Florida G. surinamensis (Bloch) redstriped eartheater Florida Pseudotropheus zebra (Boulenger) zebra mbuna Nevada Pterophyllum sp. Florida Tilapia sparmanni Smith banded tilapia Florida Colisa fasciata (Bloch) giant gourami Pennsylvania C. labiosa (Day) thicklipped gourami Florida C. lalia (Hamilton-Buchanan) dwarf gourami Florida Helostoma temmincki Cuvier kissing gourami Florida Trichogaster leeri (Bleeker) pearl gourami Florida T. trichopterus (Pallas) blue gourami Florida

* Unconfirmed report Table 2 A preliminary list of transplanted native fishes within the contiguous United States, including the likely reason for release

FAMILY SPECIES COMMON NAME REASON Petromyzontidae Petromyzon marinus Linnaeus sea lamprey accidental via canal Acipenseridae Acipenser transmontanus white sturgeon game Richardson Amiidae Amia calva Linnaeus bowfin incidental? Anguillidae Anguilla rostrata (Lesueur) American eel commercial? Clupeidae Alosa aestivalis (Mitchill) blueback forage? A. pseudoharengus (Wilson) alewife forage A. sapidissima (Wilson) American shad commercial Dorosoma cepedianum gizzard shad forage (Lesueur) D. petenense (Günther) threadfin shad forage Salmonidae Coregonus artedii Lesueur cisco/lake herring game? C. clupeaformis (Mitchill) lake white fish commercial Oncorhynchus gorbuscha game (Walbaum) O. keta (Walbaum) chum salmon game O. kisutch (Walbaum) coho salmon game O. nerka (Walbaum) sockeye salmon game; forage O. tshawytscha (Walbaum) chinook salmon game Salmo aguabonita Jordan golden trout game S. apache Miller Apache trout species survival? S. clarki Richardson cutthroat trout game S. gairdneri Richardson rainbow trout game S. gilae Miller Gila trout species survival S. salar Linnaeus Atlantic salmon game Salvelinus alpinus (Linnaeus) Arctic char game S. fontinalis (Mitchill) brook trout game S. namaycush (Walbaum) lake trout game Thymallus arcticus (Pallas) game Osmeridae Osmerus mordax (Mitchill) rainbow smelt commercial; forage? Umbridae Dallia pectoralis Bean Alaska blackfish bait release? Esocidae Esox americanus americanus redfin pickerel game? accidental? Gmelin E. americanus vermiculatus grass pickerel game? accidental? Lesueur E. lucius Linnaeus game; forage control E. masquinongy Mitchill muskellunge game E. niger Lesueur chain pickerel game Characidae Astyanax mexicanus (Filippi) Mexican tetra bait; ornamental fish release Cyprinidae Agosia chrysogaster Girard longfin dace bait release; accidental anomalum bait release? (Rafinesque) Clinostomus funduloides Girard rosyside dace bait release? Gila atraria (Girard) Utah chub bait release G. bicolor (Girard) tui chub unknown G. coerulea (Girard) blue chub unknown G. copei (Jordan & Gilbert) leatherside chub bait release G. orcutti (Eigenmann & arroyo chub bait release? Eigenmann) G. pandora (Cope) Rio Grande chub bait release? G. purpurea (Girard) Yaqui chub species survival Lepidomeda mollispinis Miller Virgin spinedace accidental Nocomis micropogon (Cope) river chub bait release? Notemigonus crysoleucas (Mitchill) golden shiner bait; ornamental fish release albeolus Jordan* white shiner bait release N.baileyi Suttkus & Raney bait release? N. buccula Cross smalleye shiner bait release? N. cerasinus (Cope)* crescent shiner bait release? N. chiliticus (Cope)* bait release? N. chrosomus (Jordan) rainbow shiner bait release? N. coccogenis (Cope) warpaint shiner bait release? N. cornutus (Mitchill)* common shiner bait release? N. galacturus (Cope)* whitetail shiner bait release? N. gibbsi Howell & Williams Tallapoosa shiner bait release? N. girardi (Hubbs & Ortenburger) Arkansas River bait release? shiner N. heterodon (Cope) blackchin shiner bait release? N. heterolepis (Eigenmann & blacknose shiner bait release? Eigenmann) N. leuciodus (Cope)* Tennessee shiner bait release? N. lutrensis (Baird & Girard) red shiner bait release N. oxyrhynchus Hubbs & Bonham sharpnose shiner bait release? N. procne (Cope) swallowtail shiner bait release N. rubricroceus (Cope) saffron shiner bait release? N. spilopterus (Cope) spotfin shiner bait release? N. stramineus (Cope) sand shiner bait release? N. telescopus (Cope)* shiner bait release? N. volucellus (Cope) mimic shiner bait release? N. xaenacephalus (Jordan) Coosa shiner bait release? N. zonistius (Jordan) bandfin shiner bait release? Orthodon microlepidotus (Ayres) Sacramento forage; food fish blackfish Phoxinus oreas (Cope) mountain redbelly bait release? dace Pimephales notatus (Rafinesque) bluntnose minnow bait release P. promelas Rafinesque bait release P. vigilax (Baird and Girard) bullhead minnow bait release Relictus solitarius Hubbs & Miller relict dace unknown Rhinichthys cataractae longnose dace bait release? (Valenciennes) Richardsonius balteatus redside shiner bait release (Richardson) Catostomidae Carpiodes carpio (Rafinesque) highfin carpsucker accidental via canal Catostomus catostomus (Forster) longnose sucker bait release? C. fumeiventris Miller Owens sucker accidental via aqueduct C. plebeius Baird & Girard Rio Grande sucker bait release C. santaanae (Snyder) Santa Ana sucker incidental? Ictiobus bubalus (Rafinesque) smallmouth buffalo food fish I. cyprinellus (Valenciennes) bigmouth buffalo food fish; accidental I. niger (Rafinesque) black buffalo food fish Moxostoma cervinum (Cope) black jumprock incidental? M. erythrurum (Rafinesque) golden redhorse bait release? M. rhothoecum (Thoburn) torrent sucker incidental? M. rupiscartes Jordan & Jenkins striped jumprock incidental? Ictaluridae Ictalurus catus (Linnaeus) white catfish game I. furcatus (Lesueur) blue catfish game I. melas (Refinesque) black bullhead game I. natalis (Lesueur) yellow bullhead game I. nebulosus (Lesueur) brown bullhead game I. pricei (Rutter) Yaqui catfish game I. punctatus (Rafinesque) channel catfish game Noturus gilberti Jordan & orangefin madtom incidental? Evermann N. gyrinus (Mitchill) tadpole madtom incidental? N. insignis (Richardson) margined madtom incidental? N. phaeus Taylor brown madtom accidental via river diversion Pylodictus olivaris (Rafinesque) flathead catfish game Amblyopsidae Chologaster agassizi Putnam spring experimental? Cyprinodontidae Crenichthys nevadae Hubbs Railroad Valley species survival springfish Cyprinodon diabolis Wales Devils Hole pupfish species survival C. macularius Baird & Girard desert pupfish range expansion; species survival C. rediosus Miller Owens pupfish species survival C. rubrofluviatilis Fowler Red River pupfish bait release? C. salinus Miller Salt Creek pupfish range expansion? Empetrichthys latos Miller Pahrump killifish species survival heteroclitus (Linnaeus) mummichog bait release? F. parvipinnis Girard California killifish bait release? F. stellifer (Jordan) southern studfish bait release? F. zebrinus Jordan & Gilbert plains killifish bait release? Jordanella floridae Goode & Bean flagfish ornamental/bait release? Lucania goodei Jordan bluefin killifish bait release? L. parva (Baird) rainwater killifish from ship ballast; incidental Poeciliidae Gambusia affinis(Baird & Girard) mosquitofish mosquito control G. amistadensis Peden Amistad gambusia species survival G. gaigei Hubbs Big Bend gambusia species survival G. geiseri Hubbs & Hubbs largespring unknown gambusia Poecilia formosa (Girard) Amazon molly unknown P. latipinna (Lesueur) sailfin molly ornamental fish release Poeciliopsis occidentalis (Baird & Gila topminnow mosquito control; Girard) species survival Atherinidae Labidesthes sicculus (Cope) brook silverside forage Menidia beryllina (Cope) inland silverside bait release; pest control; incidental Gasterosteidae Apeltes quadracus (Mitchill) fourspine bait release stickleback Culaea inconstans(Kirtland) brook stickleback incidental? Percichthyidae Morone chrysops (Rafinesque) white bass game M. mississippiensis Jordan & yellow bass game Eigenmann M. saxatilis (Walbaum) striped bass game Centrarchidae Ambloplites cavifrons Cope Roanoke bass game A. constellatus Cashner & Suttkus Ozark bass game A. rupestris (Rafinesque) rock bass game; forage Archoplites interruptus (Girard) Sacramento perch game Centrarchus macropterus flier game; forage (Lacépède) Enneacanthus gloriosus (Holbrook) bluespotted sunfish game; forage Lepomis auritus (Linnaeus) redbreast sunfish game; forage L. cyanellus Rafinesque green sunfish game; incidental L. gibbosus (Linnaeus) pumpkinseed game; forage L. gulosús (Cuvier) warmouth game; forage L. humilis (Girard) orangespotted bait release; sunfish incidental? L. macrochirus Rafinesque bluegill game; forage L. megalotis (Rafinesque) longear sunfish game; forage L. microlophus (Günther) redear sunfish game; forage L. punctatus (Valenciennes) spotted sunfish game; forage Micropterus coosae Hubbs & redeye bass game Bailey M. dolomieui Lacépède game M. punctulatus (Rafinesque) spotted bass game M. salmoides (Lacépède) largemouth bass game M. treculi (Vaillant & Bocourt) Guadalupe bass game Pomoxis annularis Rafinesque white crappie game P.nigromaculatus (Lesueur) black crappie game Percidae Ammocrypta bifascia Williams Florida sand darter bait release? chlorosomum (Hay) bluntnose darter accidental via canal E. edwini (Hubbs & Cannon) brown darter bait release? E. exile (Girard) Iowa darter bait release? E. fusiforme (Girard) swamp darter bait release? E. zonale (Cope) backwater darter bait release Perca flavescens (Mitchill) yellow perch game; perhaps forage Percina macrolepida Stevenson bigscale logperch bait release? incidental P. roanoka (Jordan & Jenkins) Roanoke darter bait release? P. tanasi Etnier snail darter species survival Stizostedion canadense (Smith) sauger game S.vitreum (Mitchill) walleye game Haemulidae Anisotremus davidsoni sargo game (Steindachner) Cichlidae Cichlasoma cyanoguttatum (Baird Rio Grande cichlid ornamental fish & Girard) release Cottidae Cottus rhotheus (Smith) torrent sculpin incidental

* Upper New River drainage, North Carolina, Virginia and West Virginia; transplanted status questionable Questionable transplant SOME CONSIDERATIONS ON THE ROLE OF INTRODUCED SPECIES OF FISH IN THE MANAGEMENT OF INLAND FISHERIES

J.Holcik Laboratory of Fishery Research and Hydrobiology Bratislava, Czechoslovakia

CONTENTS

1. INTRODUCTION 2. THE POOR SUCCESS OF INTRODUCTIONS 3. ADVERSE EFFECTS 4. CAN INTRODUCTIONS THE YIELD OF FISH? 5. NATURE OF FISH COMMUNITY 6. CONCLUSIONS 7. ACKNOWLEDGEMENTS 8. REFERENCES

ABSTRACT Introduction of fish species into waters that are foreign to them is a highly controversial practice. Poor success has been registered in many cases, particularly those carried out before 1945. Introductions, even if successful, cannot produce biomass in excess of that determined by the natural productivity of the water body concerned. However, by filling vacant trophic niches, appropriate introductions can ensure that more of the productivity accrues to the fish community. Theoretical considerations of the influence of the nature of fish communities on fishery potential and yields of lakes and reservoirs are presented and the consequences of the concept that there are fundamental differences in the adaptive capacity of lacustrine and riverine species for the management of fisheries are suggested. RESUME L'introduction d'espèces ichtyques dans des eaux qui leur sont étrangères est une pratique très controversée. Dans de nombreux cas, les résultats enregistrés ont été médiocres, notamment dans le cas des expériences effectuées avant 1945. Cette introduction, même si elle est réussie, ne peut pas produire plus de biomasse que celle provenant de la productivité naturelle dans les plans d'eau intéressés. Cependant en comblant les niches trophiques vacantes, des introductions d'espèces appropriées peuvent assurer une meilleur productivité et accroître les communautés de poisson. On présente des considérations théoriques sur les effets que les communautés de poisson peuvent avoir sur le potentiel et les rendements ichtyques des lacs et des réservoirs et on décrit les conséquences que les différences fondamentales dans la capacité adaptative des espèces lacustres et fluviales peuvent avoir pour l'aménagement des pêches. 1. INTRODUCTION The practice of introducing fish species into waters from which they were previously absent has spread widely during the last 40 years. According to the list of international transfers of inland fish species compiled recently by Welcomme (1981), which is by no means definitive and remains to be completed, no less than 170 inland fish species belonging to 33 families have been transferred worldwide for various purposes to-date. Along with the increasing intensity of introductions, there is a growing tendency to question the wisdom and benefits of such transplants, particularly in the case of natural ecosystems. Generally speaking, there are two opposing attitudes to introductions by fishery management. The first, based on the adverse affects of introduced exotics on the native fish fauna, results in partial or total prohibition of introduction of some or all species of fish, as is the case of some western countries (Zenny, 1969). The opposite attitude, which emphasizes the positive effects of introductions, encourages transfers on a massive scale, often supported by government, as in the case of some other countries, for example the U.S.S.R., where there are many research institutions and executive organizations directly engaged in the practice. While there is no doubt as to the positive influence of introduced fishes in fish culture, where the processes leading to production can be easily managed, the situation is much more complicated in natural water bodies. Although there is a vast amount of literature dealing with the results of introduction of exotics into natural waters, there is a lack of any serious, impartial and generally valid evaluation of transplantation experiments based on thorough ecological analysis. No wonder, therefore, that controversy arises even in the same ecosystem as may be seen from the recent discussion between Markevich (1978) and Malyutin (1980) on the results of introductions into Lakes Aral, Balkhash and Sevan and, indeed, on the appropriateness of introductions at all. Without doubt, the huge amount of data and information dealing with transplantations which are frequently of different value (e.g., one is unable to allow a judgement to be made as to whether any increased catch of fish in some water body is due to introduction or due to changes in fishing effort). Thus, the evaluation of a particular introduction or even the role of exotic species is beyond the ability of one person. I will, therefore, only discuss some problems associated with introductions into the natural ecosystems. In agreement with Regier (1968) by the term “exotic” I mean any species not native to the community under consideration. 2. THE POOR SUCCESS OF INTRODUCTIONS Almost all introductions into natural ecosystems made prior to the second world war may be considered to a certain degree as a gamble. The situation improved after the war as a greater awareness of ecology developed and introduction experiments started to be planned on a scientific basis. Here the leading role has been played by the Soviet Union, where the theory and practice of introductions and the acclimatization of fish is based on ecological principles (Zenkevich, 1940; Driagin, 1954; Burmakin, 1961, 1963; Karpevich, 1960, 1965, 1975; Rass, 1965). At the present time almost all intentional introductions are more or less carefully planned. Despite this, there are a surprisingly large number of failures. According to Burmakin (1963) up to 1957 1 398 water bodies in the U.S.S.R. of which 87 percent were lakes, 7 percent reservoirs and 6 percent rivers, were planted with 51 species of freshwater fish belonging to 12 families. However, the naturalization of the transplanted species was attained only in 12 percent of these water bodies. Furthermore, the catch of the acclimated fish lagged considerably behind the effort exerted for their introductions. From the data presented by Karpevich et al. (1975) and Burmakin and Shimanovskaya (1975) one can calculate that between the periods 1957–63 and 1964–71 the mean annual number of plantings of exotic fishes rose by 123 percent, the number of water bodies stocked with exotics by 164 percent and the quantity of fishes (eggs, fry, young and adults) introduced by 229 percent. However, the catch of introduced fishes in the same period increased only by 56 percent and its percentage share to the total inland fish catch only by 61 percent. According to the last evaluation of Lifshic and Belousov (1979) only 3 percent of all introductions realized in the U.S.S.R. up to 1978 gave a commercial benefit. The discrepancy between the effort and result of introductions is explained mostly by the lack of biological justification for the introductions, insufficient analysis and generalization of experience obtained, by the slow development of the theory of acclimatization as well as by shortcomings in the actual practice and organization of introductions (Karpevich et al., 1975; Lifshic and Belousov, 1979). 3. ADVERSE EFFECTS As well as the lack of apparent benefit, there are also cases of adverse or unexpected effects arising from introductions. For instance it was observed that in both natural and manmade lakes stocked with the grass carp (Ctenopharyngodon idella) the species composition of both higher plants as well as phytoplankton changed dramatically and the total catch of fish declined significantly (Vinogradov and Zolotova, 1974; Kogan, 1974; Radziej and Krzywosz, 1979; Krzywosz et al., 1980). Decrease of population density of Salmo ischchan, an endemic trout of the Sevan Lake is attributable to the introduction of the whitefish Coregonus lavaretus s. lato (Reshetnikov, 1980). Even in such a large lake as the Aral Sea (area 66 458 km2) the adverse effects of acclimated foreign species of fish on the native ones have been observed. Due to this and also other reasons, the total annual catch of fish in this lake has decreased by some 10 000 t (Karpevich, 1975; Markevich, 1978). The negative effects of common carp (Cyprinus carpio), walking catfish (Clarias batrachus) and many other exotic species upon the ichthyofauna of the U.S.A. (Lachner et al., 1970) is also well known. 4. CAN INTRODUCTIONS INCREASE THE YIELD OF FISH? It is often thought that the introduction of both invertebrates and fish may increase productivity in a water body and thus also the yield of fish, a point of view often expressed in scientific papers (e.g., Karpevich, 1975; Karpevich et al., 1975; Ioffe, 1963, 1972). However, it is clear that as the productivity of an ecosystem depends wholly on an amount of energy entering it, any introduction of a new organism into a developed ecosystem may only increase the number of consumers of energy but not the productivity. This implies that the introduction of new fish species and/or invertebrates into an already developed ecosystem cannot lead to the increase of the total yield of fish and will only result in a change in catch composition. This postulate may be demonstrated in the case of the Balkhash Lake in Kazakhstan. This large (area 17–19 thousand km2), shallow (maximum depth 26.5 m) and endorheic lake in central Asia contained originally only five species of endemic fish (three cyprinids - Schizothorax argentatus, S. pseudaksaiensis and Phoxinus poljakowi, one percid - Perca schrenki and one cobitid - Noemacheilus strauchi). Since 1905 when the wild form of the common carp (Cyprinus carpio) escaped from a pond near Alma-Ata and entered the lake, a further 27 species of fish have been introduced, of which only eight were planned while several other species were injected illegally or by chance. Twenty species naturalized fully and six of them became so abundant that they are now the subject of commercial fishing (C. carpio, Abramis brama orientalis, Rutilus rutilus, Stizostedion lucioperca, Silurus glanis and Aspius aspius). Moreover, 17 species of invertebrates, mostly of Caspian origin, were also transplanted in the period 1957–62, ten of them quickly reproduced and spread over the whole lake (Serov, 1968, 1975; Cyba, 1974, Strelnikov and Dikanskij, 1975; Karpevich, 1975; Bashunov, 1977; Bashunova, 1976). Evaluating the results of these introductions it is claimed that the fish yield increased by 25–30 percent (Karpevich, 1975) or that it even doubled (Kozlov, 1978). However, in fact the catch of fish remained on the same level for according to the data of Karpevich (1975), Bashunova and Bashunov (1976) and Cyba (1975) one can calculate that in the period 1932–49, i.e., prior to the massive introduction of fish and invertebrates, the mean annual catch of fish in Lake Balkhash was 12.5 thousand t, while in the period 1950–73 it was 11.3 thousand t. Only the composition of the fish catch changed substantially, as the average share of native fish, which was about 32 percent in the first period, decreased to only 8 percent and even more recently these fishes are no longer found in the commercial catches (Bashunova and Bashunov, 1976). One suspects, however, that in the former period the potential fish yield in the Balkhash Lake was probably higher. At that time fishing effort was certainly lower than later on, when a trawl fishery started both here and in other large lakes (Kuderskij, 1974). Obviously, the introduction of exotics does not increase either the productivity of the lake or its fish yield and the estimates of biomass of fish are the same. Such changes in catch structure should be considered significant in economic terms in industrially developed countries but are less significant in countries of the third world, where the need of cheap fish protein is much greater (Fernando, 1977, 1980, 1980a). 5. NATURE OF FISH COMMUNITY Although the productivity of a lake cannot be increased by the introduction of exotics it does not mean that in some cases the yield of fish cannot be enlarged in this manner. To explain this apparent paradox let me discuss the results of investigations performed by Professor Fernando and myself during the last two years. It is known that in many South Asian lakes and reservoirs the generally low yields based on native fish species have increased significantly following the introduction of some African cichlids, mainly Sarotherodon mossambicus. This fact has led Fernando (1965, 1980, 1980a) to propose the hypothesis that the high positive correlation between the increased yield and introduction of this cichlid is due to its better adaptation to lacustrine conditions than the original fish species which are of riverine origin. Further analysis (Fernando and Holcik, 1982), based on lakes and reservoirs, has shown that a distinction should be made between the fishes of riverine and lacustrine origin, as their characteristics and ability to inhabit the lacustrine environment are quite different. In geologically young lakes (mainly the lakes of glacial origin) and in reservoirs one deals with fishes of purely riverine origin which are not adapted to spread over the whole area. In most of these water bodies the fish are concentrated mostly along the littoral and the density of fish decreases markedly with increasing distance offshore and increasing depth. In large, deep lakes the pelagial and profundal parts are practically without fish. The nearshore range of fish in these lakes is limited by the isobath of about 25 m. Quite another situation exists in geologically old lakes with relatively longestablished lacustrine environments (e.g., Tanganyika, Titicaca, Lanao, Baikal). Here, in spite of their extensive area and depth fishes occur not only over their whole area but also within the whole water column. In these lakes, however, beside the fishes occurring in the neighbouring river basins, there are also endemic, purely lacustrine species which are unable to inhabit the lotic environment and can be regarded to be stenoecious in this respect. Specific adaptations have evolved in these species which enable them to utilize all existing niches. The pelagial zone of such lakes is inhabited by few species only which are short-lived and have short food chains (phyto and zooplankton eaters). These species are notable for their relatively high population stability due to which high yields are reached despite high fishing pressure. It is remarkable that from about 120 families of fish inhabiting fresh waters, there are only about 12 which can be considered preadapted for lacustrine conditions and of these only five families are purely lacustrine. The majority of endemic lacustrine species comes from the families Cichlidae and Clupeidae, lower numbers are contributed by the Salmonidae, Cyprinidae and Cyprinodontidae. Exclusively lacustrine families are monotypical Indostomidae, Adrianichthyidae and Chauduriidae and also Comephoridae and Cottocomephoridae, the last two with only two species each. However, the majority of fish inhabiting recent standing water bodies is of riverine origin. Due to the constantly changing environmental conditions and the great diversity of habitats riverine fishes are able to inhabit both lentic and lotic environments, i.e., they are euryecious in comparison with true lacustrine species, although lakes and reservoirs can be considered to be only a temporary habitat for them (in the evolutionary sense of the word) to which they are not fully adapted. As there are no specialized pelagial and profundal species among them (due to the relatively small size and depth of rivers) riverine species which enter natural or manmade lakes are able to inhabit only a relatively narrow littoral belt and the pelagial zone is entered only when shallow. In support of this hypothesis I refer to the echosounding observations by Marchal and Laurent (1977) as a means of estimating ichthyomass in Lac Leman, a lake of glacial origin (area 510 km2, maximum depth 310 m). Their echo-integration survey has shown that 84 percent of the total ichthyomass is concentrated along the shores at less than 100 m depth. Fish biomass is reduced as distance from the shores increases; at depths between 100 and 200 m only 11 percent and at 200 m only 5 percent of the total biomass was present. The echosounding observations made by Balon (1974) in the Lake Kariba, the large reservoir (area 5 364 km2, maximum depth 93 m) built on the Zambezi River in Central Africa demonstrated that only about 38 percent of its total area was inhabited by fishes which were confined to shallow coves and a relatively narrow belt along the shores limited by the 25-m isobath. The pelagial zone of this reservoir became inhabited by fishes only after the introduction of the clupeid fish Limnothrissa miodon which was transferred there from the Lake Tanganyika. These observations indicate that in geologically young lakes and in reservoirs populated by riverine species of fish there are some free ecological niches and these water bodies may be classified as undeveloped ecosystems when compared with geologically old lakes occupied by true lacustrine fishes. It is therefore suggested that when deciding to introduce new fish species into a lake or reservoir its riverine or lacustrine origin should be considered along with the characteristics of the water body under consideration. In such cases, one has to consider not only as vacant trophic niche but also the spatial distribution of fishes and their ability to occupy particular spatial strata and other specific properties leading to the delimitation of a niche. The example of Lake Balkhash, where few native species were able to produce the same fish yield as a number of exotics, indicates that the native fish community occupied both trophic and spatial ecological niches of this lake. This example also indicates that the term “niche” embodies many variables, i.e., it is “the N- dimensional hypervolume” as defined by Hutchinson (1965). In the practice of introductions, however, the term “niche” is usually narrowed to the trophic niche (e.g., Karpevich, 1975). 6. CONCLUSIONS Experience accumulated during experiments with introduction of exotic species of fish indicates that, in spite of increasing knowledge in the ecology both of a particular species and of the whole fish communities, we are still unable to formulate generally valid conclusions and to construct a definitive, practical model of introductions. We are still “somewhere between total ignorance (no model at all) and a knowledge that permits decisions to be derived by simple deduction (well specified, broadly accepted model)” as emphasized 14 years ago by Regier (1968). In my opinion, there are still considerable gaps in our knowledge on the main interspecific relationships among fishes in any water body on the ecological variability and the ability of fishes to adapt themselves to new environments, on their possible impact upon the other species and on the responses of native fish to the introduced species as well as in our knowledge on the functional mechanisms of the whole ecosystem. Although the concept of Fernando and Holcik (1982) described in section 5 on the nature of fish community and on the consequences for the management of fisheries may be considered a step toward the improvement of the practice of introductions, it should be used only with strict precautions and the utmost care as each lake may be considered a unique entity. I agree with Magnuson (1976) that natural lakes are like islands; they are isolated, small and relatively young, and that the number of species in a lake is the result of an equilibrium between rates of immigration of new species and extinction of existing species. Human activity may accelerate the rate of immigration and extinction and may induce greater instability in species structure thereby adding greater uncertainty into the results of fisheries management. By choosing exotics for introduction man still takes into consideration his own advantage or immediate benefits but pays little attention to what these species can do to the whole ecosystem (Regier, 1968; Lifshic and Belousov, 1978). For the foregoing reasons one can only outline some very rough concepts applicable in the management of fisheries. In my opinion natural lakes where the native fish community is still flourishing should be stocked with exotics only after very careful considerations and every introduction must be based on the results of detailed limnological analysis. Only stenoecious species from families preadapted for lacustrine habitat should be chosen. One has to remember of course that here the species introduced need not reproduce and establish the self-sustaining population and therefore its population density has to be maintained only by regular stocking. Geologically old lakes inhabited by endemics should not be subject to introductions in order to preserve their invaluable intrinsic value. Much more simple seems to be the situation in lakes where the fish community is periodically destroyed by the winter fish-kills (as is the case of many shallow Siberian lakes) or when the fish fauna of a lake is composed of a few widely distributed riverine species. The many, mostly small lakes in northern and eastern Europe are regularly stocked with coregonines (mainly Coregonus peled) in combination with the common carp often after the total or partial eradication of the original fish community by poisoning or by complete fishing out. Their fish yields frequently exceed those obtained prior to this practice (Kuderskij, 1974), which could be recommended, although in these cases stocking has to be regularly repeated and may be financially burdensome. Reservoirs, which usually depend on stocking, should be stocked both by euryecious riverine species of the local origin and by stenoecious lacustrine exotics. Rivers also may require a regular stocking with native species in order to reach their potential. 7. ACKNOWLEDGEMENTS I wish to thank Mr. S. Ziegler, Laboratory of Fishery Research and Hydrobiology, Bratislava, for his assistance and help with the bibliography. 8. REFERENCES Balon, E.K. (ed.), Fish production of a tropical ecosystem. In Lake Kariba: a man-made 1974 tropical ecosystem in Central Africa, edited by E.K. Balon and A.G. Coche. Monogr.Biol., 24:249–676 Bashunov, V.S., Itogi akklimatizacii vobly v ozere Balkhash. 1977 Rybokhoz.Izuch.Vnutr.Vodoemov., 1977(2):28–30 Bashunova, N.N., Rezultaty akklimatizacii zherekha v ozere Balkhash. 1976 Rybn.Khoz.Mosk., 1976(1):20–1 Bashunova, N.N. and V.S. Bashunov, Istoria i rezultaty akklimatizacionnykh rabot na 1976 ozere Balkhash. Rybokhoz.Izuch.Vnutr.Vodoemov., 1976(17):7–11 Burmakin, E.V., Nekotorye voprosy teorii akklimatizacii presnovodnykh ryb. Zool.zh., 1961 40(9) 1385–95 , Akklimatizacia presnovodnykh ryb v SSSR. Izv.Gos.Nauchno- 1963 Issled.Inst.Ozern.Rechn.Rybn.Khoz., 53:1–317 Burmakin, E.V. and L.V. Shimanovskaya, Rezultaty akklimatizacii ryb v presnovodnykh 1975 vodoyemakh SSSR. Izv.Gos.Nauchno- Issled.Inst.Ozern.Rechn.Rybn.Khoz., 103:116–9 Cyba, K.P., K voprosu o dinamike chislennosti vostochnogo leshcha v ozere Balkhash. 1974 Sb.Rab.Kaz.Fil.VGBO, 2:121–4 Cyba, K.P., Akklimatizacia vostochnogo leshcha v ozere Balkhash. Izv.Gos.Nauchno- 1975 Issled.Inst.Ozern.Rechn.Rybn.Khoz., 103:175–9 Driagin, P.A., Teoreticheskie osnovy i plan akklimatizacii ryb vo vnutrennikh 1954 vodoyemskh SSSR. Tr.Soveshch.Ikhtiol.Kom.AN SSSR, 3:9–20 Fernando, C.H., The role of inland waters in relation to the development of Ceylon's 1965 fisheries, and a note on the pearl oyster fishery. Bull.Fish.Res.Stn.Ceylon, (17):291–7 , Reservoir fisheries in South Asia: past, present and future. Proc.IPFC, 1977 17(3):465–9 , Tropical reservoir fisheries: a preliminary synthesis. In Proceedings of 1980 the Fifth International Symposium on Tropical Ecology. Ecology and development. Kuala Lumpur, University of Malaysia, 16–21 April 1979, pp. 883–92 , Tropical man-made lakes, African fish and cheap protein. ICLARM 1980a Newsl., 3(1):15–7 Fernando, C.H. and J. Holcik, The nature of fish community: a factor influencing the 1982 fishery potential and yields of tropical lakes and reservoir. Hydrobiologia, 97:127–40 Hutchinson, G.E., The niche: an abstractly inhabited hypervolume. In The ecological 1965 theatre and the evolutionary play. New Haven, Connecticut, Yale University Press, pp. 26–78 Ioffe, C.I., Sostoyanie i perspektivy rabot po akklimatizacii bespozvonochnykh vo 1963 vnutrennikh vodoyemakh SSSR. Rybokhoz.Izuch.Vnutr.Vodoemov, 1963:23–43 , Teoreticheskie prodposylki, rezultaty i perspektivy akklimatizacii 1972 kormovykh obyektov dlya ryb v presnovodnykh vodoyemakh. Izv.Gos.Nauchno-Issled.Inst.Ozern.Rechn.Rybn.Khoz., 71:21–33 Karpevich, A.F., Teoreticheskie predposylki k akklimatizacii vodnykh organizmov. 1960 Tr.Vses, Nauchno-Issled.Inst.Morak.Rybn.Khoz.Okeanogr., 43(1):9–30 , Itogi symposiuma po teoreticheskim voprosom akklimatizacii ryb I 1965 drugikh gidrobiontov. Vopr.Ikhtiol., 5(1):219–23 , Teoria i praktika akklimatizacii vodnykh organizmov. Moskva, 1975 Pishchevaya promyshlennost, 432 p. Karpevich, A.F. et al., Rezultaty akklimatizacii ryb i kormovykh organizmov v 1975 vodoyemakh SSSR. Izv.Gos.Nauchno- Issled.Inst.Ozern.Rechn.Rybn.Khoz., 103:5–19 Kogan, Sh.I., O zarastanii Karakumskogo kanala i nekotorykh posledstviakh vselenia v 1974 vodoyemy belogo amura. Gidribiol.Zh., 10(2):110–5 Kozlov, A., Itogi za tridcat let. Rybovod.Rybolov., 1978(2):4–5 1978 Krzywosz, T. et al., The effect of grass carp, Ctenopharyngodon idella (Val.) on aquatic 1980 vegetation and ichthyofauna of lake Igal ielki. Ekol.Pol., 28(3):433–50 Kuderskij, L.A., O putyakh razvitia rybnogo khozyaystva na vn u trennikh vodoyemakh 1974 (ozera, vodokhranilishcha, reki). Izv.Gos.Nauchno- Issled.Inst.Ozern.Rechn.Rybn.Khoz., 87:94–119 Lachner, E.A. et al., Exotic fishes and other aquatic organisms introduced into North 1970 America. Smithsonian Contrib.Zool., (59):1–29 Lifshic, S.M. and A.M. Belousov, Itogi i effektivnost akklimatizacionnykh rabot v SSSR. 1979 Obz.Inf.TsNIITEhIRKh.(I Rybokhoz.Ispol z Resur.Mirovogo Okeana), 1979(3):1–68 Magnuson, J.J., Managing with exotics - a game of chance. Trans.Am.Fish.Soc., 1976 105(1):1–9 Malyutin, V., Ostorozhneye s prizyvom “beregis akklimatizacii”. Rybov.Rybolov., 1980 1980(1):8–9 Marchal, E. and P.J. Laurent, Premiere estimation de la population piscicole du Lac 1977 Leman par echointegration. Cah.ORSTOM(Hydrobiol.), 11:3–16 Markevich, N., Uchityvat ne tolko polozhitelnye rezultaty. Rybovod.Rybolov., 1978(2):8– 1978 9 Radziej, J. and T. Kryzywosz, Wplyw amura bialego na biomase i sklad gatunkowy roslin 1979 oraz na ichtiofaune jeziora Dgal Wielki. Gospod.Rybna, 31(9:6–8 Rass, T.S., Rybnye resursy evropeyskikh morey SSSR i vozmozhnosti populnenia 1965 akklimatizaciey. Moskva, Nauka, 107 p. Regier, H.A., The potential misuse of exotic fish as introductions. In A symposium on 1968 introductions of exotic species. Ottawa, January 1968. Res.Rep.Dep.Lands For.Ottawa, (82):92–111 Reshetnikov, Yu.S., Ekologia i sistematika i sgovykh ryb. Moskva, Nauka, 301 p. 1980 Serov, N.P., Itogi akklimatizacii ryb v ozere Balkhash. In Akklimatizacia ryb I 1968 bespozvonochnykh v vodoyemakh SSSR. Moskva, Nauka, pp. 237–42 , Akklimazizacia ryb v basseyne ozera Balkhash. Izv.Gos.Nauchno- 1975 Issled.Inst.Ozern.Rechn.Rybn.Khoz., 103:172–4 Strelnikov, A.S. and V.Ya. Dikanskij, Morifekologicheskie osobennosti sudaka, 1975 akklimatizirovannogo v vodoyemakh Kazakhstana. Izv.Gos.Nauchno- Issled.Inst.Ozern.Rechn.Rybn.Khoz., 103:195–201 Vinogradov, V.K. and Z.K. Zolotova, Vliyanie belogo amura na ekosistemy vodoyemov. 1974 Gidribiol.Zh., 10(2):90–8 Welcomme, R.L. (comp.), Register of international transfers of inland fish species. FAO 1981 Fish.Tech.Pap., (213):120 p. Zenkevich, L.A., Ob akklimatizacci v Kaspiyskom more novykh kormovykh 1940 bespozvonochnykh i teoreticheskie k ney predposylki. Byull.Mosk.O- Va.Ispyt.Prir., 49(1):1–30 Zenny, F.B., Comparative study of laws and regulations governing the international 1969 traffic of live fish and fish eggs. EIFAC Tech.Pap., (10):57 p. THE NICHE CONCEPT AND THE INTRODUCTION OF EXOTICS

N.-A. Nilsson Institute of Freshwater Research Drottningholm, Sweden

CONTENTS

1. THE NICHE CONCEPT 2. SOME PALEARCTIC EXAMPLES 2.1 Rejection 2.2 Hybridization 2.3 Elimination 2.4 Vacant niches 3. SUMMARY 4. REFERENCES

ABSTRACT The niche concept has given rise to some confusion ever since it was first introduced by Gause (1934) and Elton (1946) among others. Elton's simple definition still stands well as a working paradigm: “The status of an organism in its environment”. It soon became clear that the seemingly inflexible concept that “two or more species cannot live in the same niche” is obscure, mainly because of the obvious fact that niches very often do overlap, or may even temporarily seem identical. Hutchinson's definitions of “fundamental” versus “realized” niches, as well as his definition of “the N- dimensional hypervolume” as a handy conceptual tool to study niches, mathematically, has given rise to an ever-growing literature on “niche overlap”, “niche breadth”, etc. Related concepts of “interactive segregation” and “species dominance” have also attracted attention and many parallels have been drawn between insects (Brian, 1956), birds (Svardson, 1949) and fish (e.g., Nilsson, 1978; Svardson, 1976). Recent findings that exploitative competition by selective feeding forces fish species to segregate into their “realized niches” (literature compiled by Nilsson, 1978) has given us a possible means for monitoring introductions of “exotic species”, including subspecies, stocks, etc. It is suggested that the introduction of “exotics” leads to any of the following results. The introduced stock: (i) is rejected, because there is no “vacant niche” or predators graze down the population at early stages; (ii) hybridizes with very closely related stocks, formerly adapted to the ecosystem; (iii) eradicates a stock that is either an “ecological homologue” or a very available prey; (iv) finds a “vacant niche” within the community, which means that it adapts to resources that are not fully exploited by other species and finally makes it able to survive as a member of the community. The four alternatives suggested are demonstrated in the paper by European and North American experiences. RESUME Le concept de niche a entraîné une certain confusion et a provoqué bien des querelles sémantiques depuis qu'il a été introduit, par exemple par Gause (1934) et Elton (1946). Par sa simplicité, la définition d'Elton peut toujours servir de modèle: “le statut d'un organisme dans son environnement”. Beaucoup de spécialistes ont dénoncé l'idée selon laquelle “deux ou plusieurs espèces ne peuvent pas vivre dans la même niche”; il est en effet évident, entre autres choses, que les niches se recouvrent très souvent ou qu'elles semblent même identiques temporairement. Hutchinson et Ivlev nous ont beaucoup aidé à éclaircir nos idées. La définition par Hutchinson des niches “fondamentales” par opposition aux niches “effectives” ainsi que sa définition de “l'hypervolume de dimension N”, qui est un instrument commode pour étudier les niches du point de vue mathématique, ont donné naissance à une documentation de plus en plus abondante sur le “chevauchement des niches”, la “largeur des niches”, etc. Par ailleurs, le concept de “ségrégation interactive” a acquis une certaine importance; on a établi beaucoup de parallèles entre les insectes (Brian, 1956), les oiseaux (Svardson, 1949) et les poissons (par exemple, Nilsson, 1978; Svardson, 1976). En ce qui concerne le poisson, on a découvert récemment que la concurrence entraînée par l'alimentation sélective oblige les espèces à se séparer dans leurs niches “effectives” (documentation réunie par Nilsson en 1978), ce qui nous a donné un fil directeur pour suivre les introductions d'espèces exotiques, y compris les sous-espèces, les stocks, etc. On estime que l'introduction de poissons exotiques aboutit à l'un des résultats suivants. Le stock introduit: (i) est rejeté parce qu'il n'y a pas de niche vacante ou que les prédateurs détruisent rapidement les populations; (ii) s'hybride avec des stocks apparentés, déjà adaptés à l'écosystème; (iii) détruit un stock qui est, soit un “homologue écologique” soit une proie facile; (iv) trouve une niche vacante dans la communauté, ce qui signifie qu'il s'adapte à des ressources qui ne sont pas totalement exploitées par d'autres espèces et parvient finalement à survivre en tout que membre de la communauté. Les expériences réalisées en Europe et en Amérique du Nord servent à illustrer ces quatre possibilités. 1. THE NICHE CONCEPT The niche concept has caused some confusion and ever since early trials to define the concept (Grinnel, 1904; Lotka, 1932; Gause, 1934; Hutchinson, 1957) Elton (1946) termed the phenomenon “the status of an organism in its community”, which still stands as a simple as well as a good definition. Fisheries biologists, forced as they are to use theory as a foundation for those practical actions called fishery management, soon objected to the seemingly inflexible concept that “two or more species cannot exist in the same niche”, as they frequently observed that different fish species very often consume similar food or share other essential resources (e.g., Forbes, 1914; Hartley, 1948; Starrett, 1955; Nilsson, 1955; Larkin, 1956). Because of this, further concepts of “niche overlap”, “niche breadth”, etc., were stressed by scientists interested in species interaction (e.g., Hurlbert, 1971). The Swedish zoologist Lonnberg perhaps was the first to formulate this by pointing out what he names “det dukade bordets princip” translated by Johnson (1980) to “the smorgasbord principle”, which states that "in Nature - miscellaneous animals make use of one kind of food when it is available in plenty, even those to which that particular item is not the natural or common food (Lonnberg, op.cit., quoted by Nilsson, 1960). Hutchinson (1957, 1967) defined the term “niche” as an “N-dimensional hypervolume” designating “the requirements of an organism abstracted from the specially extended habitat. The habitat of two species may overlap completely; it is empirically probable that at equilibrium, their niches never do”. Thus he distinguished between “the fundamental niche”, which means the virtue of a species to make use of available resources through its physiological capabilities and the “realized niche”, which is that portion of the hyper-space that is actually occupied, the difference being due to exclusion from certain parts of the niche by other species in the community. This philosophy agrees well with the theories of “dominance-subordinance” and “interactive segregation”, which posits that interactions between species of subpopulations of species are fundamentally variable creating “realized niches” sensu Hutchinson (Nilsson, 1978). In other words, species occupying the same water body are forced by interaction to make the most of their assets, when resources are at a minimum. Svardson's (1976) theory of “dominance-subordinance” points to the fact that the standing crops of fish in lakes are hierarchical in nature, which means that the reduction or elimination of the dominant species tends to lead to drastic changes in the lower ranked species. On the whole species with pelagic capabilities generally are dominant over littoral species (Svardson, 1976; Skud, 1982; Ryder and Kerr, 1983). Entomologists (Brian, 1956; Park, 1954; Ross, 1957), ornithologists (Svardson, 1949; Cody, 1968; MacArthur, 1958) and fish ecologists (Nilsson, 1967, 1978; Svardson, 1976) have arrived at very similar conclusions which eventually could be of help in judging whether or not “exotics” should be introduced in a stablized ecosystem. The study of competition is, of course, closely related to these problems. Park (1948), Brian (1956) and others distinguished between two components in interspecific competition: interference and exploitation. Interference means direct damage to one or both species, for instance, by aggressive behaviour such as fighting for territories, etc. Exploitation on the other hand means an interaction that develops whenever one species is more efficient in using available resources more easily and quickly than their competitors. To turn from theory to practice, it seems to me that when introducing a new species (population, subspecies, etc.) into a new community, it may face any of the following fates: An exotic species: (1) is rejected because there is no “vacant niche” or predators graze down the population at early stages, or gets harmfully infected by native diseases, or abiotic factors like temperature, pH, etc., do not fulfil the needs of the species at crucial circumstances; (2) hybridizes with very closely related stocks, formerly adapted to the ecosystem; (3) eliminates (completely or partly) a species that is either an “ecological homologue” or a very available prey, or is sensitive to foreign diseases and parasites, carried by the exotic species; (4) or finds a “vacant niche” in the community, which means that it adapts to food, space, spawning sites, etc., that are not fully exploited by other species or stocks. It also means that because of competition, niche overlap, etc., the species within the community have to make the most of their individual advantages which results in their restriction to “realized niches” through interactive segregation. Fig. 1 is an attempt at modelling a Scandinavian example as regards the “fundamental” and “realized” niches of three salmonine species. The hypervolumes of the niches are hypothetical in the graph, whereas the indications of the zooplankton communities based on quantitative information (Nilsson and Pejler, 1973). 2. SOME PALEARCTIC EXAMPLES Immigrants into Europe and America enthusiastically tried to introduce species from one continent to another over a very long time, in attempts to “improve” the native fauna. European starling, house sparrow, European carp and brown trout are well known examples of introductions into North America. Many of these attempts either failed or proved disastrous. As Ryder and Kerr (1983) have reviewed these in some detail, I will comment on some European experiences, using the introductory scheme (1–4) above. 2.1 Rejection Theoretically, this should be the most likely outcome, as the indigenous fauna should a priori be best adapted to the ecosystem in question, and thereby should not incorporate an “intruder”. However, several experiences, such as those in Australia as well as plant introductions worldwide, have provided terrifying lessons (cf. Harlan, 1981). Fish species on the whole, however, are less likely to be disastrous as they are introduced into aquatic systems that are more closed than terrestrial ones. Rainbow trout (Salmo gairdneri), a western North-American salmonid was originally native to lakes and streams from Alaska to Mexico. It has many migratory and resident stocks and subspecies, which have been spread all over North America and later over most continents: New Zealand, Australia, Tasmania, South America, Africa, Japan, southern Asia, Hawaii and many parts of Europe (McCrimmon, 1971). In Europe S. gairdneri has, on the whole, been used in “put and take” fisheries. or cultured for direct consumption. However, as far as natural reproduction is concerned, it has not been suited to European habitats. For instance, Wheeler and Maitland (1973) stated that in the British Isles “in spite of such widespread introductions the species has appeared in relatively few places”. In fact, Worthington (1941) listed only about 14 waters in the south of England and one in Ireland where the species is found regularly. The same is true in Scandinavia, where many thousands of introductions have been made since the turn of the century but only two or possibly three reproducing stocks have been recorded. There has been much speculation on why these introductions have failed, the presence of strong competitors or predators, and the genetic inappropriateness of the strains of the species adapted as they are to the American west coast, rich in lime as a buffering substance, and with very few competing or predatory species present (cf. Nilsson and Northcote, 1979) have both been advanced as reasons. In Scandinavia the very disastrous acidification problem should imply a severe threat to the limited stocks of species that have become established. The kokanee (Oncorhyncus nerka) native to the American west coast as a landlocked variety was introduced in Sweden in 1959 in some ten lakes and also in the Baltic with discouraging results. Although the stocking of kokanee fry in some lakes reclaimed with rotenone proved to be successful. For instance, in one case, one third of the introduced fish were recaptured in excellent condition. This experience has led to the idea that kokanee might be a possibility for fish farming. There is some evidence for natural reproduction which has hitherto had very little significance. Of course, the increasing acidification problem in Scandinavia forces a lessened interest in trying to introduce species from less acid environments, for instance, the American west coast into the acid-tressed environments. The brook trout (Salvelinus fontinalis) was introduced into Scandinavia at about the same time as the rainbow trout. Although it appeared to be more successful than rainbow in establishing breeding populations it is now confined to cold head waters of small streams, where apparently it can compete with the native brown trout. This is consistent with the introductions of brown trout at the American east coast which forced the brook trout to inhabit head water refugia of small streams (e.g., Brynildson et al., 1964). The Danube salmon (Hucho hucho) was imported in 1963 to Sweden from Yugoslavia, with the general idea that, in addition to being an excellent game fish it could possibly use habitats which differed from those of its ecological homologue, the Northern pike. The introductions, however, completely failed. No Danube salmon has been recaptured in Sweden, although the places of release were chosen very carefully. Similar results have characterized many other introductions of “game fish” in Europe - mainly centrarchids such as rock bass (Ambloplites rupestris), pumpkinseed (Lepomis gibbosus) - and above all largemouth bass (Micropterus salmoides) and smallmouth bass (Micropterus dolomieu). As to the two last mentioned species, Wheeler and Maitland (1973) have stated that no population has been established in the British Isles. The same is true for Scandinavla (Svardson, pers.comm.), in spite of early attempts at introduction. Failure to acclimatize has been attributed to temperatures which are not correct for hatching (Svardson, pers.comm.). Similarly, introductions of grass carps (mainly Ctenopharyngodon idellus) into temperate European areas have mainly failed to produce self-reproducing populations where the water temperature at stages crucial for breeding is unsatisfactory. 2.2 Hybridization Mayr (1942) stressed the significance of hybridization between closely related species or subspecies, often leading to a “regressive subspeciation” (Svardson, 1970). Sibling species such as coregonids and Salvelinus may hybridize (Svardson, 1970; Nyman et al., 1981). Rainbow and cut-throat trout, cyprinids and many other taxa also hybridize, as well as “stocks” introduced to improve fisheries. Hybridization (cross breeding) can be divided into two categories: intraspecific crosses between strains “stocks”, “races”, etc., or interspecific crosses between species. Hybridization can achieve either of two favourable outcomes: viz (i) heterosis or hybrid vigour or (ii) non- heterotic effects, “the improved performance of the progeny as the result of simple combination of parental genotypes” (FAO/UNEP, 1981). 2.3 Elimination The elimination of organisms by introduced species has been one of the most delicate areas of debate since the early and uninformed attempts of the pioneers to improve natural resources. Experience from introductions of terrestrial animals or plants have taught us that from an anthropocentic view an exotic can eliminate either (a) a less esteemed species, (b) a more esteemed species, by direct competition or predation or less directly by introducing diseases or parasites to which the native species are more succeptible. Few cases of really favourable introductions where exotics have eliminated native species can be found. Nevertheless some successful introductions of carp, grass carp or brown trout have been recorded from areas where these species are looked upon as more favourable than native species. Brown trout, however, has been shown to interact to the detriment of the equally esteemed brook trout where it has been introduced on the American east coast. More severe have been deliberate or accidental introductions of species directly harmful to the native fish community, which have been documented by North American workers (Courtenay and Taylor, 1983; Ryder and Kerr, 1983). Two cases of disastrous introductions of exotics illustrate this effect in European waters. Every fisherman knows that the northern pike (Esox lucius) has threatened the existence of many species sensitive to predation. For instance, Toner (1959) estimated that 2 594 pike in two lakes consumed approximately 112.5 t of brown trout in one year, and Went (1957) gave strong evidence that pike not native to Ireland, eliminated the salmonids in some very important salmonid waters after its introduction. In Scandinavia the presence of pike has made the introduction particularly of salmonids impossible. Further evidence for this has been provided by the success of such introductions where pike have been eliminated from “coarse fish waters” by rotenone. The introduction of pike into salmonid waters, therefore, is a good example of unfortunate predation on esteemed species. The elimination of ecological homologues, however, seems to pose a still more important problem and involves competition that is not only a matter of interactive segregation but a real elimination of a species that is unable to defend the niche it was once adapted to. One example of this has been very carefully studied - the introduction of whitefish (Coregonus sp.) into brown trout-Arctic charr lakes in the north of Sweden. This is a very complicated story which has been elucidated through Gunnar Svardson's (1961, 1979) important studies of the ecology of Arctic charr-whitefish interactions. Ekman (1910) had already observed that introductions of coregonids tended to eliminate charr populations and Svardson's (1979) studies have shown that one species of whitefish especially, the “alvsik” (Coregonus lavaretus has been especially adverse to charr populations. Fig. 2 shows the elimination of Arctic charr after the introduction of “alvsik” in a northern Swedish lake. Recent investigations 'Nilsson and Pejler, 1973) have indicated “size biased” predation to be the fundamental factor leading to this effect. 2.4 Vacant niches The main philosophy regulating introductions of exotics should be to find organisms that fit into an ecosystem where there possibly is a niche that is not fully exploited by native species. A Swedish example involving three aspects of the introduction of exotics is Lake Storsjon where two food organisms (Mysis relicts and smelt) and one predator (Salvelinus namaycush were introduced. Lake Storsjon is one of the large lakes of Sweden (surface area 456 km2, with a complex fish community, dammed for hydro- electric purposes), which is important both for professional and sport fishery, but in which the Arctic charr and brown trout fishery, mainly because of the environmental modifications arising from hydro-electric constructions. To improve the declining community American lake trout (Salvelinus namaycush) were introduced in order to convert the flesh of four species of whitefish into more valuable food as well as to provide sport fishing opportunities (cf. Gonczi and Nilsson, 1983). Simultaneously the glacial relict Mysis relicta was introduced into the head waters of the lake and soon appeared as a very important member of the plankton community of the lake where it formed an important fish food organism. The initial growth rate of the lake trout was very good (Fig. 3) but after some years the growth declined, possibly because of overgrazing of pelagic food organisms, such as stickleback (Pungitius pungitius) and stunted coregonids (Fig. 4). After considerable discussion, another food organism was introduced in 1974 - the smelt (Osmerus eperlanus). The population of smelt very rapidly increased and after only three or four years it became the most important food of both lake trout and Arctic charr (close to 100 per cent in 1980). The history of Lake Storsjon provides a very interesting example of how three exotics (lake trout = predator; Mysis = invertebrate prey, and smelt = fish prey) interacted in a way that now looks satisfactory, although all interrelationships have to be carefully studied before we are ready to design an ultimate model how to manage a fish community similar to this one. 3. SUMMARY The above case histories, which are limited to the temperate/Alpine/sub-Arctic region serve to illustrate the potential dangers as well as the advantages of introductions of non-native species. The risks of hybridization with an elimination of native species makes it imperative that proper scientific evaluation of the risks and advantages of introductions be made prior to such action. Governments which do not now have mechanisms to ensure that an objective analysis of risks precedes the introduction of an aquatic organism into national waters should take immediate steps to establish such mechanisms of should seek help should they lack the necessary means themselves to do so. Genetic, behavioural and ecological data, as well as potential for introduction of disease, should be included in the analysis of risks. Governments should be aware that the probability of escape of cultivated aquatic species (even those kept only for research purposes) is so high that intent to confine imported aquatic animals does not obviate the need for such risk assessment. On the other hand, the introductions of exotics seem to have been favourable in some of the cases referred to above and on the whole the benefits of introduction are usually perceived to exceed the risks. Thus, there is still an imperative to introduce exotics. 4. REFERENCES Brian, M.V., Segregation of species of the ant genus Myrmica. J.Anim.Ecol., 25:319–37 1956 Brynildson, O.M., V.A. Hacker and T.A. Klick, Brown trout: its life history, ecology and 1964 management. Publ.Wisc.Cons.Dep., (234):1–14 Cody, M.L., On the methods of resource division in grassland bird communities. 1963 Am.Nat., 102:107–47 Courtenay, W.R., Jr. and J.N. Taylor, The exotic ichthyofauna of the contiguous United 1984 States with preliminary observations on intranational transplants. EIFAC Tech.Pap., (42) Vol. 2:466–87 Ekman, S., Om manniskans andel i fiskfaunans spridning till det inre Norrlands vatten. 1910 Ymer 30:133–40 Elton, C., Competition and structure of ecological communities. J.Anim.Ecol., 15(1):54– 1946 68 FAO/UNEP, Conservation of the genetic resources of fish: Problems and 1981 recommendations. Report of the Expert Consultation on the genetic resources of fish. Rome, 9–13 June 1980. Fish.Tech.Pap., (217):43 p. Forbes, S.A., Freshwater fishes and their ecology. Urbana, Illinois, Illinois State 1914 Laboratory of Natural History Furst, M., Introduction of the North American crayfish Pacifastacus leniusculus Dana to 1984 Sweden. EIFAC Tech.Pap., (42)Vol.2:400–4 Gause, G.F., The struggle for existence. Baltimore 1934 Gonczi, A. and N.-A. Nilsson, Results of the introduction of lake trout (lake charr, 1984 Salvelinus namaycush) into Swedish lakes. EIFAC Tech.Pap., (42) Vol.2:392–9 Grinnel, J., The origin and distribution of the chestnut-backed chickadee. Auk, 21:364– 1904 82 Harlan, J.R., Who's in charge here? Can.J.Fish.Aquat.Sci., 38(12):1459–63 1981 Hartley, P.H.T., Food and feeding relationships in a community of freshwater fishes. 1948 J.Anim.Ecol., 17(1):1–13 Holcik, J., Review on experiments with introduction and acclimatization of the huchen – 1984 Hucho hucho (Linnaeus, 1758) (Salmonidae). EIFAC Tech.Pap., (42) Vol.2:289–98 Hurlbert, S.H., The nonconcept of species diversity: A critique and alternative 1971 parameters. Ecology, 52(4):577–86 Hutchinson, G.E., Concluding remarks. Symp.Quant.Biol., 22:415–27 1957 , A treatise on limnology. Vol. 2. Introduction to lake biology and the 1967 limnoplankton. New York, John Wiley and Sons, 1115 p. Johnson, L., The Arctic charr, Salvelinus alpinus. In Charrs, edited by E. Balon. The 1980 Hague, Junk, pp. 15–98 Kerr, S.R. and E.E. Werner, Niche theory in fisheries ecology. Trans.Am.Fish.Soc., 1980 109:254–60 Larkin, P.A., Interspecific competition and population control in freshwater fish. 1956 J.Fish.Res.Board Can., 13(3):327–42 Lotka, A.J., The growth of mixed populations, two species competing for a common food 1932 supply. J.Wash.Acad.Sci., 22:461–509 MacCrimmon, H.R., World distribution of rainbow trout (Salmo gairdneri). 1971 .Fish.Res.Board Can., 28(5):663–704 Mayr, E., Animal species and evolution. 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Oxford, Blackwell Scientific Publications, pp. 303–25 Nilsson, N.-A. and B. Pejler, On the relation between fish fauna and zooplankton 1973 composition in North Swedish lakes. Rep.Inst.Freshwat.Res., Drottningholm, (53):51–77 Nilsson, N.-A. and T.G. Northcote, Rainbow trout (Salmo gairdneri) and cutthroat trout 1981 (S. clarki) interactions in coastal British Columbia lakes. Can.J.Fish.Aquat.Sci., 38(10):1228–46 Nyman, L., J. Hammar and R. Gydemo, The systematics and biology of landlocked 1981 populations of Arctic char from northern Europe. Rep.Inst.Freshwat.Res., Drottningholm, (59):128–41 Park, T., Experimental studies of interspecies competition. 2. Temperature, humidity and 1954 competition in two species of Tribolium. Physiol.Zool., 27(3):177–238 Ross, H.H., Principle of natural coexistence indicated by leafhopper populations. 1957 Evolution, 11(2):113–29 Ryder, R.A. and S.R. 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Fig. 1 Model of the “dimensions” of the niches of brown trout (Salmo trutta), Arctic charr (Salvelinus alpinus) and whitefish (Coregonus sp.) in allopatry and sympatry, and the dominant species of zooplankton (after Nilsson and Pejler, 1973)

Fig. 2 Decrease in the gillnet catch of charr (Salvelinus alpinus) as the catch of the introduced whitefish (Coregonus sp.) increased, Lake Västansjö, north Sweden (from Nilsson, 1967)

Fig. 3 The food of lake trout in Lake Storsjön

Fig. 4 The growth of tagged lake trout in the Lakes Storsjön and Kallsjön REDUCING THE RISK OF FISH INTRODUCTIONS: A RATIONAL APPROACH TO THE MANAGEMENT OF INTEGRATED COLDWATER COMMUNITIES

R.A. Ryder Ontario Ministry of Natural Resources Thunder Bay, Ontario, Canada and S.R. Kerr Marine Ecology Laboratory, Bedford Institute of Oceanography Dartmouth, Nova Scotia, Canada

CONTENTS

1. INTRODUCTION 2. ECOLOGICAL CONSIDERATIONS 3. GLACIAL LAKES 4. CASE HISTORIES 4.1 The sea lamprey 4.2 The smallmouth bass 4.3 The cisco 5. DISCUSSION 6. CONCLUSION 7. ACKNOWLEDGEMENTS 8. REFERENCES

ABSTRACT The introduction of exotic species into native fish communities has been a popular approach toward the management of oligotrophic lakes during the past century. The high level of risk involved, however, all but precludes the usual approach to fish introductions which are as likely to be damaging as successful. Three case histories of introductions, looked at retrospectively, suggest that in some cases, at least, the outcome of an introduction may be predictable. For north-temperate, oligotrophic waters, the greatest likelihood of success lies in the use of species that have co-evolved in glacial refugia but may have become allopatric through the vagaries of redistribution following glacial recession. The risks involved in planting new species may be greatly reduced through a priori consideration of several ecological principles such as niche theory, interactive segregation, dominance-subordinance, and resource partitioning. Each of these hierarchically-associated principles is consistent with the concept of niche, both fundamental and realized, and major niche dimensions of candidate species for introduction should be quantified whenever possible and compared with those species comprising the indigenous fish community. A resulting high level of niche complementarity of the candidate species with the various components of the native community will increase the likelihood of success of an introduction. RESUME La faune piscicole des régions du Canada envahies par les glaces au Pléistocène a réintégré son habitat original il y a 8 000–12 000 ans, lorsque les glaciers ont reculé. Elle a survécu à la glaciation dans des refuges situés dans le sud, l'est et le nord-ouest, où il y a en stabilisation des communautés sous l'effet de très longues interactions spécifiques et, par la suite, de la complémentarité des niches. Elle a réintégré son habitat original en passant par les grands lacs du Pléistocène et les cours d'eau qui les relient, et une nouvelle différenciation des niches a eu lieu sous l'effet des interactions collectives et des contraintes écologiques. L'introduction d'espéces exotiques dans ces réseaux assez fragiles n'a parfois eu aucun effet notable; en revanche, dans d'autres cas, elle a eu des conséquences très importantes, rendant la pêche imprévisible et non rentable, notamment lorsque les dimensions des niches des espèces indigènes se sont trouvées réduites par la concurrence ou le prédatisme des espèces exotiques. Souvent, un seul facteur écologique abiotique a déterminé les résultats de l'interaction. Les auteurs présentent plusieurs études de cas, avec leurs consêquences pour l'aménagement. 1. INTRODUCTION The introduction of an exotic fish species into an indigenous fish community is often viewed as a constructive effort to increase fishery yield by the manager who has experienced repeated frustration in restoring an ailing fishery to a semblance of its pre- stressed condition. Undoubtedly, this approach may have measurable effects on the autochthonous species comprising the natural fish community, but the outcome may be viewed as either beneficial, mitigative or destructive, depending on the anthropocentric values that might be perceived to derive from such introductions. The greatest initial impact, however, will be expressed politically, in the sense that the fishery manager may appear, to both his superiors and the public alike, to have judiciously applied modern management technology in order to enhance a fishery. Essentially, the fishery manager is then perceived to have acted with alacrity and intelligence, and thereby improved his image amongst his clientele public, if not his peers. On the other hand, public or peer retribution for a damaging fish introduction is rarely exacted, as a sufficiently lengthy time span will usually lapse before the effects of an ill-advised planting is conclusively documented. This typical sequence of events precludes the immediate assignment of blame in such actions, the results of which may be amenable to various subjective interpretations. Because of the somewhat cavalier approach to fish introductions during the past century, when most stocking took place without informed regard for the biological consequences, the practice of management by species manipulations has, perhaps, been overly maligned. In North America, the almost immediate penetration of carp (Cyprinus carpio) into indigenous fish communities, following deliberate introductions, and their subsequent widespread invasion of contiguous waters has been amply documented (e.g., McCrimmon, 1968). Evidence of the effects of the successful establishment of carp on native fish species, particularly in shallow, more fertile bays of oligotrophic lake basins, has been less well documented but is generally felt to be negative (Scott and Crossman, 1973). Other notable, well publicized, and apparently unfortunate introductions into North American waters emanating from the Old World fauna include those of the grass carp (Ctenopharyngodon idella) and the walking catfish (Clarias batrachus). The former species was first introduced into an open system in Arkansas in 1971 (Guillory and Gasaway, 1978). Special consideration was directed to the ecological role the grass carp might play in the elimination of macrophytes from lakes (Mitzner, 1978) but relatively less scientific rigour was directed toward the potential side effects it might have had on an indigenous fauna, either locally or within the major accessible watershed. The walking catfish, apparently introduced inadvertently as an escapee from the ornamental aquarium trade in Florida, is usually considered to be an ecological disaster (Courtenay and Robins, 1975). Numerous other examples of potentially disruptive introductions of non-indigenes occur in North America, but some exotic species that have become established in the form of substantial, reproducing populations, are often viewed favourably by certain user groups. Of the latter, two introduced trout species, the brown trout (Salmo trutta), a transplant from European waters, and the rainbow trout (Salmo gairdneri), introduced widely throughout eastern North America following its importation from the west, have often been considered as not only successful emigres (e.g., Courtenay and Robins, 1975) but also as highly desirable ones by the sport fishing faction. In both instances these species probably have displaced the native brook trout (Salvelinus fontinalis) or the Atlantic salmon (Salmo salar) to varying degrees. This displacement has likely been effected through food or reproductive competition in the case of the former species, or because of the proclivity of the two emigres to be better adapted physiologically to partially degraded (on the trophic scale), oligotrophic environments than was the Atlantic salmon. Despite the fact that the rainbow trout was introduced into the Great Lakes region in 1895 (Scott and Crossman, 1973), it is still in the process of adaptive radiation, extending its range as cultural influences on environmental factors swing the balance of survival in its favour, to the detriment of the native brook trout. Cutting of the forests and the attendant increase of mean stream temperatures, appear to be critical factors in this process, which, in many instances, has created marginal or sub-marginal environmental conditions for brook trout, particularly in the lowest reaches of some streams tributary to the Great Lakes (Fig. 1). The rainbow trout/brook trout relationship therefore would seem to be dependent on the ambient range of one or more physical factors such as temperature, calcium carbonate concentration or pH, and the determination of which species might be dominant, in the sense of Svardson (1976) may depend on the values of these variables, or other physical factors at critical periods, such as those of spawning or larval emergence. The ready adaptability and further expansion of the rainbow trout in eastern North America has been viewed by many as an enhancement of the sport fishery resource. Its impact has penetrated the socio-economic system because of the need for specialized gear for the capture of rainbow trout, and the associated requirements for special seasons and creel limits. On the other hand, inveterate brook trout anglers often view the continuing encroachment of the rainbow trout into brook trout territory with dismay, particularly when an obvious decline in the availability of their favoured species ensues. The need for reconciliation of these differences of opinion between two user groups is one that should be anticipated by the fisheries manager before an introduction is effected, in order that appropriate measures may be taken to mitigate the perceived negative effects by one or more client groups. Following an introduction, experience shows that solutions to user group conflicts may become tedious and, often, totally intractable. To this day, the dilemma persists of judging the relative benefits of exotic fish introductions versus their potentially disruptive influence on native fish communities. We shall approach this problem with the perspective that native fish communities, comprised of both indigenous species and stocks (e.g., Loftus, 1976) in environments relatively unperturbed by man, best optimize the resources of their environment, all factors considered, and consequently produce the highest sustainable yields (Ryder et al., 1981). But in the face of ubiquitous and enduring environmental degradation, a realistic attitude may opt for maximizing yield through introductions of stenoecious fishes (narrow range of environmental tolerance) rather than wait for the implementation of environmental rehabilitation (Regier, 1968), which itself may not be economically practical in the foreseeable future, if ever. In essence, then, our approach condones exotic species introductions in cases of extreme and persistent environmental degradation, or in other exceptional instances, with the rigorous proviso that adequate background studies afford supportive evidence for the likelihood of success, not only of the introduction, but also of the successful integration of the remnants of native fauna that exist in the degraded environment, insofar as practicable. The decision of when to forego rehabilitation in favour of a fish introduction that may prove ultimately to be irreversible, is not only a philosophically tenuous problem, but is pragmatically intractable as well. For example, a technological solution to a persistent environmental stress may be just around the corner, but its subsequent utilization may be preempted through the introduction of a new species. Consequently, the vital decision between environmental rehabilitation as opposed to species introductions, is one of the most difficult ones that a fisheries manager will ever have to make and should be given careful consideration from ecological and socio-economic points of view, and should be measured against various time-scales in order to determine optimum long- term benefits. In offering these recommendations, we issue the caveat that a substantial to moderate probability of failure can be expected in successfully integrating the emigre with the native fish community. In other words, the introduction of exotics must always, in some measure, be a game of chance (Magnuson, 1976) although the seemingly insurmountable odds against the fisheries manager may be perceptibly reduced through careful, prior application of fundamental ecological principles to any contemplated introduction. 2. ECOLOGICAL CONSIDERATIONS When environments have been altered or degraded to a point of unacceptable return, an introduction of an exotic species may make possible the development of a viable fishery where none previously existed. In such instances, careful assessment of various ecological factors should be made, insofar as relevant, both in the abiotic and biotic sectors of the ecosystem. A most convenient framework within which to pursue this assessment is the niche concept, which embodies all the environmental limitations and interactions of an organism, together with those affecting other organisms in the community and its abiotic environment. The niche concept we espouse, combines the synecological approach of Hutchinson (1957) with the autecological approach of Fry (1947), as set out by Kerr and Ryder (1977). Suffice it to say here that we consider the ecological niche to be an organismal attribute which is genetically determined and which constrains the role of an organism within the aquatic community and within the abiotic environment, mediated, in turn, by the feedback restrictions imposed on the organism by both the community and its abiotic environment. Accordingly, niche comprises multiple dimensions that describe the feeding, or reproductive or other capabilities of an organism and its ability to survive within the environmental boundaries established by various physical and chemical factors such as temperature, oxygen, light and depth as an important subset of limiting or controlling factors. Each fish species, therefore, might be conceived as enclosed and constrained by a multi-dimensional envelope of survival capability, the contours of the envelope determined by innate genetic characteristics phenotypically expressed as genetic potential or potential niche. In nature, this genetic potential along each dimensional axis is compressed through the organism's interactions with other organisms as well as with the abiotic environment, to create an operational or realized niche. Within fish communities which have co-evolved, the realized niches of component species have achieved levels of mutual compatability or complementarity, such that a measure of undirected mutual altruism has been attained. We have labelled these aquatic communities as “harmonic” (Ryder and Kerr, 1978; Ryder et al., 1981) because the consequent longterm co- evolution has engendered a perceivable degree of mutually beneficial integration among component species. Harmonic communities lacking severe anthropogenic stresses tend to persist as predictable entities, and exhibit robust levels of resilience under substantial removal rates (e.g., Grossman, 1982). Astatic communities (Ryder and Kerr, 1978) on the other hand, are consequences of stochastic assemblages of fishes and other organisms, resulting from unplanned introductions or inadvertent invasions of highly opportunistic species, often r- selected, and following environmental degradation (e.g., MacArthur and Wilson, 1967). Astatic fish assemblages are characterized by a boom or bust syndrome, providing highly variable yields and highly variable species composition within composite yields from year to year. The fishing history of Lake Erie over the last 150 years reveals a change from a harmonic community to, more recently, an astatic assemblage of fishes responding opportunistically to various cultural stresses including harvest, eutrophication, toxic wastes and exotic species introductions or invasions (Regier and Hartman, 1973). We suggest that had not the first three stresses been present on a long-term basis, the fortuitous presence of exotic species, from whatever source, would have made much less of an impact on the native harmonic community, whose community niche attributes would have kept them closely in tune with their relatively unstressed environment. This condition, in turn, would contribute to the capacity of the indigenous community to successfully resist the intrusions of new opportunistic species (e.g., Christie et al., 1972). To propose a “thought experiment”, if all the fish stocks of Lake Erie were catastrophically eliminated, and all other cultural stresses were suddenly removed from the lake, what would be the most appropriate measures a fisheries manager could take to restore a viable and predictable fishery once the environment returned to its previously natural state? One possible approach might be to select a cross-section of the world's best food or game fish species for introduction into Lake Erie, without regard for any ecological considerations, but with the knowledge that natural environmental rigours would eliminate many species from the mix in short order, usually during the passage of only one winter in the case of tropical species. In time, the effects of interaction among the survivors might run its course, and co-evolution might again be perceived as the sculptor of a new harmonic fish community. Unfortunately, the time scale of evolution hardly coincides with that of a fisheries manager's expectations, and an equilibrium ecological solution to this scenario would not likely ensure before additional perturbations to the system occurred because of realized impatience on the human time-scale. A more realistic approach to the problem, given Lake Erie in a limnological condition similar to that of 150 to 200 years ago, would be to attempt a reconstruction of the indigenous fish community as it was perceived to be at that time (e.g., Ryder, 1972). For the most part this would involve reintroduction of originally native fish species done in the most appropriate manner, that is, by planting sub-species and individual genetic stocks to fill the community hiatuses that would exist, in numbers and kinds proportional to the energetic processes of each trophic level. While the fishes utilized would be drawn from lakes other than Lake Erie and therefore would not be perfectly nichetuned to that particular lake, on an evolutionary basis they should be sufficiently close to the archtypal genetic strains that one might expect the development of a new harmonic community over several generations, rather than aeons. The greatest chance for success in harmonic community establishment, therefore, would be close emulation of the available, co-evolved natural scheme insofar as possible. In accomplishing this goal, the niche concept is implicit at each taxonomic and hierarchic level of ecological organization, up to that of the integral biotic community. 3. GLACIAL LAKES North-temperate lakes comprise a major portion of the world's natural lakes (Fernando, 1980) and of these, most have formed following the recession of continental glaciers (Flint, 1957). In Canada, because of the immense areas of refractory substrate of the Precambrian Shield, most glacial lakes are relatively low in nutrient content, that is, they are oligotrophic or at the low end of the mesotrophic range. Oligotrophic lakes are apparently more vulnerable to the destabilizing effects of exotic fish introductions than lakes which are eutrophic (Li and Moyle, 1981). Accordingly, we confine the remainder of this discussion to glacially derived lakes of North America, including exemplary case histories, assuming that the principles underlying the debilitating effects of exotic introductions may be clearer, but similar in principle, to introductions in eutrophic lakes and other complex systems. Hence the fundamental concepts revealed by oligotrophic introductions should be applicable to eutrophic and similar systems, but differently scaled. The biological buffering effect of large assemblages of many fish species in complex eutrophic systems may mask the ecological effects of exotic fish introductions that we wish to demonstrate. Many glacially-derived lakes of North America were repopulated with aquatic fauna from three or more major refugia (McPhail and Lindsey, 1970), while the Laurentian Great Lakes received their fauna principally from two refugia, located within the Mississippi and Atlantic drainages. The native fish fauna of the large to medium- sized inland glacial lakes is moderately depauperate, relative to the Laurentian Great Lakes, and species often number less than thirty (e.g., Keleher, 1972) but more commonly less than twenty in total, depending on post-glacial accessibility (Ryder et al., 1964). Very small, glacially-derived lakes might have ten fish species or less (e.g., Fraser, 1972) and occasionally such lakes are barren of any fish because of the lack of access to Pleistocene refugia or because of episodic events of ecological significance such as periodic winterkill. Because the numbers of fish species in glacial lakes are low, the numbers of potential interactions amongst them are relatively few, and consequently, alternate pathways for energy transfer are restricted. Egress to glacial lakes by fishes may have occurred on the order of 8 000–12 000 years ago (McPhail and Lindsey, 1970) a short time-scale in evolutionary terms, but it may be assumed that co-evolution of coldwater fish communities occurred not only within glacial refugia, but during several to many interglacial periods as well. Recent (8 000–12 000 BP) natural immigrants to glacial lakes, therefore, may be essentially co-adapted in terms of niche compatability or complementarity, but because of the restricted numbers of alternate pathways for interaction in a moderately depauperate fauna (e.g., Welch, 1967) may be presumed to be in a relatively delicate balance of dynamic equilibrium. Invasion or introduction of exotic species into these relatively uncomplicated, harmonic, species complexes would generate reverberations throughout the community that would decay monotonically over long time periods until new dynamic steady states are reached (Ryder et al., 1981). One current ecological theory suggests that removal of a perturbation from a system does not necessarily guarantee the system's return to the original dynamic equilibrium, if the perturbation took it beyond its response capabilities. In such an instance the system tends toward the equilibrium of a new resident attractor region (Peterman et al., 1979) or new median of central tendency (Ryder and Kerr, 1978). In terms of species introductions into glacial lakes, an essentially salmonid community in a marginally mesotrophic lake could convert readily to a predominantly percid community with the introduction of only one or two of the latter species (Ryder and Kerr, 1978). Events such as this should be predictable by fisheries managers, given the appropriate application of unifying concepts such as niche theory (Kerr and Ryder, 1977), dominance principles (Svardson, 1976) or the concepts of interactive segregation (Nilsson, 1967) and resource partitioning (Schoener, 1974). These hierarchically related concepts, together with their governing principles and modes of utilization, converge in their application, as we shall demonstrate subsequently. 4. CASE HISTORIES 4.1 The Sea Lamprey The best documentation showing the effects of the invasion of an exotic species into oligotrophic aquatic systems in North America, is probably that of the sea lamprey (Petromyzon marinus) penetration into the three upper Great Lakes following the construction of the Welland Canal in 1829, which allowed passage around the natural physiographic barrier created by Niagara Falls (Applegate, 1950; Lawrie, 1970). Because of the massive literature on this inadvertently caused invasion, only a brief account will be provided here, specifically related to some of the major ecological considerations. Prior to the ingress of the sea lamprey into Lakes Huron, Michigan and Superior, the indigenous fish communities of the limnetic waters were essentially of the salmonid (salmonine-coregonine) type (e.g., Koelz, 1979; Lawrie and Rahrer, 1972) which were regulated by two large, piscivorous top predators, the lake trout (Salvelinus namaycush) and the burbot (Lota lota). The former species was an active predator in all zones of the lake where ambient temperatures were favourable, which effectively excluded them only from shallow bays and some epilimnetic waters during summer months. In Lake Superior, numerous phenotypic varieties of lake trout co-existed, which were variously adapted to feed or spawn or otherwise occupy characteristic habitats at particular times. For example, some lake trout were river spawners (Loftus, 1958) while others spawned on shallow or deep lake shoals. Spawning times for lake trout varied from June through November (Eschmeyer, 1957) although most spawning probably occurred in October. Some stocks fed pelagically while others fed on the benthos. The lake trout then, appeared to be exploiting its environment efficiently through a remarkable phenotypic radiation of specialized morphotypes, which themselves developed in response to multiple environmental opportunities. The second major piscivore of the three upper Great Lakes, the burbot, fed benthically for the most part, and competed with the lake trout for a limited supply of forage fishes (Bailey, 1972) mainly in the form of a species flock of coregonines, Coregonus (Leucichthys sp.), although other prey species were important at times, particularly one of the cottids, Myoxocephalus quadricornus, and the nine-spined stickleback, Pungitius pungitius (Smith, 1968). There is substantial evidence that the lake trout and burbot effectively partitioned their food resources on the basis of temporal and spatial separations, and that the combined effects of these two top predators were the primary force in the maintenance of the steady-state condition within the remainder of the coldwater community (e.g., Smith, 1968). The effects of the penetration of the sea lamprey into these coldwater communities were difficult to assess initially because of the complications afforded by other exogenous stresses already in place and probably incrementing on an annual basis (Loftus and Regier, 1972). These stresses included exploitation, cultural eutrophication and related effects, and the introduction of, or invasion by, other exotic species such as the rainbow smelt (Osmerus mordax) and the alewife (Alosa pseudoharengus) to mention but two that inhabited the same limnetic areas of the lakes and comprised a large proportion of what had become an astatic assemblage of fishes, particularly relative to the pelagic factions of the coregonine and lake trout complex. The initial impact associated with the advent of the sea lamprey was the rapid and virtual elimination of the two large predators, the burbot and lake trout, and the subsequent release from predation of various prey species, some of which had apparently been held in predation refugia by these two efficient predators (Ryder et al., 1981). Indeed, it is conceivable that the phenotypic variability expressed in the cisco complex (Koelz, 1929) was created by isolation in predation refugia caused by the highly effective predation of lake trout and burbot. The subsequent release of the small-sized stocks of this genus from efficient and persistent predation was followed by radiation from the predation refugia, which, in turn, may have created conditions favourable for introgression (Smith, 1964). The evidence for introgression between stocks in the cisco complex was expressed in terms of morphological blending, to the point where many of the original systematic characteristics no longer had utility for the identification of the phenotypic stocks originally described by Koelz (1929). In summary, the total coldwater community reverted to an astatic assemblage, a condition of flux and of unpredictability. The loss of the two top predators not only released prey species from predation pressures, but perhaps paved the way for additional exotic prey species to invade or expand (e.g., Christie et al., 1972). The alewife invaded Lake Superior about 1954 (Miller, 1957). The rainbow smelt, which had already become established prior to the advent of the sea lamprey, continued to expand, presumably at the expense of the native limnetic coregonines, through increased competitive and predation pressures (Anderson and Smith, 1971; Selgeby et al., 1978). By 1954 the lake trout was already well on the way to its precipitous decline in Lake Superior (Lawrie, 1970) a factor which, no doubt, contributed to the opportunistic expansion of the alewife in that lake (Miller, 1957). Both the alewife and the smelt, which benefited from the sea lamprey-induced losses of the lake trout and burbot, in turn, affected other native species through their ability to opportunistically capitalize on abundant food resources. Reverberations within the total fish community persisted (e.g., Smith, 1970: Anderson and Smith, 1971) from the secondary effects of rainbow smelt and alewife invasions, and new steady-state conditions have not yet been attained in Lake Superior, more than thirty-five years following the initial recorded invasion of the sea lamprey. The case history of the sea lamprey in the Great Lakes has interesting implications within the context of exotic fish introductions because of its clear exemplification of the utility of the niche concept (Kerr and Ryder, 1977). Prior to the advent of the sea lamprey in the upper Great Lakes, there were no terminal predators apart from man, that consistently and effectively preyed upon adult lake trout and burbot. The strategy of the sea lamprey was, in effect, opposite to most predators in which maximum food size may be related to mouth gape or body dimensions (Keast and Webb, 1966). In the case of the sea lamprey, the larger the prey, the greater the likelihood it would survive the attack (Hall and Elliot, 1954) and thereby lessen the need for repetitious food search by the individual lamprey. The niche role of the sea lamprey, therefore, was more akin to that of a parasite than a predator, and its particular niche function was unrealized in the Upper Great Lakes Basin prior to its invasion. It may be hypothesized that given sufficient evolutionary time, a steady-state between the sea lamprey and the lake trout and burbot may be achieved through mean size reduction of the lamprey with a proportional reduction in the mortality rates caused by the lamprey on lake trout and burbot. This relationship has apparently been realized in the finger Lakes of New York where the sea lamprey is believed to be endemic and the primary host (lake trout) maintained itself at high levels (Webster et al., 1959). In that instance, the sea lamprey mean size is substantially less than those found in the Upper Great Lakes. The management implications of the foregoing considerations are intriguing. If the sea lamprey were economically valuable as it is in Europe, an enterprising fisheries manager may have recommended its introduction into the Upper Great Lakes as an alternative food source, or perhaps even as a valuable gourmet item. Further thought along ecological lines would have revealed that the sea lamprey's niche role was not being realized in the Great Lakes, and that once introduced it would likely have a substantial chance of survival as its potential food resources were abundant, in the form of large standing stocks of lake trout and burbot. Further consideration along ecological lines might have revealed that many of the potential sea lamprey spawning streams had temperature regimes below optimum for sea lamprey reproduction and rearing (e.g., Applegate and Smith, 1950) but that the prevalent cutting of the forests in the Great Lakes watershed promised to provide streams with amenable spawning and nursery conditions (Lawrie and Rahrer, 1972). In short, the example of the sea lamprey as a successful invader of the Upper Great Lakes could perhaps have been predicted on the basis of analyses of relatively few niche dimensions, that is, preferred food, optimum spawning, and early life history temperature regimes. Similar analyses for other species considered for introduction could be undertaken and while the outcome may not necessarily be as predictable as was the retrospective case with the sea lamprey, reasonable assessments with acceptable measures of confidence could probably be made. 4.2 The Smallmouth Bass The smallmouth bass (Micropterus dolomieui) has been successfully introduced into many oligotrophic lakes of the Precambrian Shield in southern Canada and the northern United States (Scott and Crossman, 1973). Of particular concern has been its potential effect on the lake trout, another top predator native to these lakes, as well as to the salmonid communities in which the lake trout normally occurs (Ryder and Kerr, 1978). Although the smallmouth bass on the Precambrian Shield originated from at least one of the same glacial refugia as the lake trout (Mississippi), the total extent of their subsequent northward radiation in Canada following glacial recession, was rather restricted relative to the lake trout. The temperature limitations seem to be the most obvious factor to be considered in this regard (e.g., Hubbs and Bailey, 1938; Rawson, 1945; Coble, 1967; Christie and Regier 1973; Shuter et al., 1981). Generally, temperatures less than 10°C inhibit feeding, reproduction, growth and certain other metabolic processes in the smallmouth bass while the lake trout functions well within these thermal bounds. The temperature limitation, however, has not been nearly as rigorous nor as general as the 18°C summer isotherm limitation proposed by Radforth (1944). Many robust populations of smallmouth bass exist well north of that line in the Hudson Bay watershed of Ontario as a result of introductions and they appear to be extending their range to this day. The northward radiation of the smallmouth bass from its , or refugia, may well have been restricted by physiographic barriers such as watershed divides, rather than thermal ones, of which the latter might have delayed its radiation to a substantially later date relative to another sympatric predator in the Mississippi refugium, the lake trout. Effectively, lake trout reinvasion may have been better assisted by the cold, ephemeral, post-glacial drainage patterns than was the smallmouth bass with its higher temperature optimum. These physiographic barriers then, may have prevented the smallmouth bass from access to both the Hudson Bay watershed and the Algonquin highlands in Ontario, although they demonstrably can thrive there today in both locations following introduction (Scott and Crossman, 1973). Whether these successful smallmouth bass introductions north of their natural range are enduring or not, depends on several facets of climate and demography interacting to determine survival rates (Shuter et al., 1980). Most of these introductions have a history of less than fifty years. The significance of glacial history, glacial refugia and subsequent radiation patterns of the smallmouth bass, relates to its successful integration into, or at least, coexistence with salmonid communities. Essentially, the smallmouth bass has evolved in at least one glacial refugium in which substantial components of salmonid communities occurred. In a strict sense then, the smallmouth bass should not be considered as an exotic species when reintroduced into salmonid communities, especially in the boreal forest zone of the Precambrian Shield (Ryder and Kerr, 1978). The smallmouth bass was introduced into the Algonquin Park Region of Ontario in the early 1900s and subsequently spread or was reintroduced into many lakes within the area (Christie, 1957). The native fauna of most of these lakes consisted of salmonid communities, integrated through the predation of lake trout and burbot (Martin and Olver, 1980). There is no indication that the introduction (or reintroduction in evolutionary terms) of the smallmouth bass into the salmonid communities of Algonquin Park had any appreciable effect on any of the community elements with the possible exception of the brook trout (Martin and Fry, 1973). The brook trout was vulnerable to the invasion of the smallmouth bass for evolutionary, zoogeographic and ecological reasons. It most likely survived Pleistocene glaciation in an eastern refugium (Radforth, 1944) and sympatry in such a refugium with the smallmouth bass is unlikely. Much of the original ranges of the brook trout and smallmouth bass (e.g., Scott and Crossman, 1973) were mutually exclusive and overlapped mainly in the Great Lakes region where fauna originated from both Atlantic and Mississippian refugia (Bailey and Smith, 1981). Even there, the brook trout and smallmouth bass tended not to co-occur and were spatially segregated according to their physiological tendencies to seek different abiotic optima. In lakes, both species are essentially littoral-bound, and the potential antagonistic behaviour may be another possible mechanism for interactive segregation of the species (Nilsson, 1967). In total, too many forces of an evolutionary, zoogeographic and ecological nature are in place to ever allow mutual compatibility in a single habitat between the brook trout and smallmouth bass. The decline of brook trout in lakes in which smallmouth bass were introduced was, therefore, predictable. The apparent compatibility of smallmouth bass with lake trout was equally predictable on the basis of present knowledge. Sympatric co-evolution has favoured adequate phenotypic or genotypic response to allow each to adequately explore the limitations of its realized niche within the total niche-space of the salmonid community (Ryder et al., 1981). Zoogeographic considerations, such as patterns of glacial recession, have prevented a large overlap in the natural range of these two species in Canada. In certain lakes, both the ecological attributes of the environment, together with speciesspecific and interspecific behaviour, have separated the lake trout from the smallmouth bass in both time and space. The preferred temperature of the lake trout is 8–10°C (Rawson, 1961) about the same as that at which the smallmouth bass commences feeding on an ascending scale in springtime or reduces feeding activity on a descending temperature cline in the autumn (Coble, 1975). Lake trout, on the other hand, feed actively although at differing rates, in both shallow and deep lake waters from fall overturn, throughout the winter, until the onset of early summer thermal stratification. During most of this period smallmouth bass feed only occasionally if at all, and spend most of their time inactivated in substrate hibernacula (Webster, 1954). During the period of summer stratification, high temperatures prevent lake trout from feeding in the littoral waters of the epilimnion where smallmouth bass actively feed, principally on crayfish, insects and forage fishes (Fedoruk, 1966). In the boreal forest lakes of Ontario, lake trout spawning normally occurs in October, at temperatures at which the smallmouth bass is inactive, while the latter species spawn in June in shallow littoral areas at temperatures not normally tolerated by lake trout. Interactive predation between lake trout and smallmouth bass occurs but rarely. Martin and Olver (1980) report only occasional predation of smallmouth bass by lake trout for lakes in which the two species are sympatric. Adult lake trout, by virtue of their size alone, would not be vulnerable to smallmouth bass predation, and young lake trout normally inhabit deep waters (Martin and Olver, 1980) and, therefore, do not co-occur with adult smallmouth bass. In certain isolated instances, young lake trout are found in the stomachs of adult smallmouth bass in the fall of the year (J.A. MacLean, pers. comm.). Adult lake trout predation on young smallmouth bass may be more common in specific circumstances. In four small lakes (200–1 000 ha) of eastern Ontario, adult lake trout predation on young of the year and a few yearling smallmouth bass ranged from 3 to 9 percent frequency of occurrence during a winter creel census (D. Loftus, pers.comm.). It may be conjectured that the small lake sizes, relative environmental homogeneity, large littoral to pelagic zone ratios have forced the lake trout to feed benthically more often than would be normal in a large lake. Smallmouth bass are typically closely associated with substrate materials during the winter months (Webster, 1954). In summary, available evidence points to the fact that smallmouth bass and lake trout are spatially segregated for much of the year (Carlander, 1975) and any interaction between the two species may occur only through the sequential use of an overlapping food resource. Even here, little evidence can be found to support sequential feeding competition as the primary food requirements of the two predators are substantially different (e.g., Fedoruk, 1966; Martin and Olver, 1980). Any potential interaction between these two species, therefore, must occur indirectly through food web inter-relations or at an even lower hierarchic level through energetic or nutrient “competition”. In the latter instances, if smallmouth bass were introduced into a lake trout lake, the annual production of the latter species may be reduced accordingly, although perhaps not appreciably, through the various interactive and integrative pathways of the energy and nutrient transfer systems which form the basis for a common resource for both species. From a purely anthropocentric point of view, however, the addition of smallmouth bass to a lake trout lake and its associated salmonid community (brook trout are assumed to be absent), would probably increase both the total yield and variety to the angling fishery without a noticeable decrease in lake trout yield per se. This has already been demonstrated in Lake Opeongo where the smallmouth bass was introduced in 1928 (Martin and Fry, 1973). Again, a careful evaluation of niche dimensions, resource partitioning, dominance traits and potential or interactive segregation would have provided a fisheries manager with all the scientific information required to have predicted this compatible outcome of the co-existence of lake trout and smallmouth bass. 4.3 The cisco The cisco (Coregonus artedii) is a co-habitant with the lake trout in many boreal forest lakes of the Precambrian Shield in North America and co-evolved with the latter species in the Mississippi refugium (McPhail and Lindsey, 1970). Because of long-term co-evolution, a natural dynamic equilibrium has been attained by these species through their predator-prey relationship, with adult lake trout preying upon both adult and young cisco. The other major relationship between these species involves “competitive” feeding interactions at the zooplankton level, where young lake trout and all ages of ciscos feed on zooplankton of various size-classes. The lake trout-cisco relationship is a closely coherent coupling in that it is mutually beneficial, although not necessarily mutually obligatory as alternate food pathways may often be available to the lake trout. Hence, the lake trout, through predation, retains ciscos at a predictable steady-state, that is, a level that allows ciscos to operate well within the carrying capacity of its zooplankton food base. Where lake trout predation stress has been removed from cisco stocks, the latter have been observed to expand opportunistically, even beyond the long-term limitations imposed by the environment. Such loss of predation control has been described for the Great Lakes cisco complex following the sea lamprey incursion and the subsequent virtual extinction of the lake trout (Smith, 1968). Lake trout, in the absence of ciscos in the depauperate faunas of the boreal forest lakes of the Precambrian Shield, and in the absence of alternative forage fishes, must remain with a planktonfeeding habit as they grow. In such instances, the lake trout grow more slowly and do not reach as great an ultimate size as piscivorous lake trout, and mature at a smaller size (Martin, 1966). The other alternative for lake trout in the absence of ciscos is usually to feed on other species of fishes such as the yellow perch (Perca flavescens) or white suckers (Catostomus commersoni) neither of which are as readily available to the lake trout, nor as nutritious as the cisco (Martin, 1970). In some extreme instances, the energy expenditures required to enter a thermally hostile environment in order to capture alternate prey species may not be justified by the caloric augmentation following digestion (e.g, Kerr, 1971). In a comparative study of lake trout lakes of Algonquin Park, Fry (1939) noted the presence of rapidly growing, large-sized piscivorous lake trout in some lakes and relatively small, slower-growing lake trout in other lakes. Martin (1966) showed conclusively that both the growth rates of lake trout and the upper asymptotic sizes they attained were related to food habits and were essentially dichotomous in nature, depending upon the availability of zooplankton forage or prey species of fish. Most directly, this relationship depends on the particle-size density of the prey relative to predator body size (Martin, 1970) which affects the availability of prey and the ensuing growth efficiency of the predator (Kerr, 1979). Other considerations are equally important to predator efficiency. In instances where ciscos and lake trout were sympatric, it was noted that their migratory and feeding habits were somewhat parallel (Fry, 1939), a condition that augments a continual availability of the prey to the predator and precludes the necessity of the lake trout temporarily invading a hostile thermal environment to capture alternative prey in the epilimnion, such as perch. In addition, Fry (1949) noted a depression in the growth curve of Lake Opeongo lake trout at a size when they had become inefficient in feeding on small particle sizes, but were not yet large enough to prey effectively upon the available forage fishes. The importance of such discontinuities in the prey sizespectrum has since been formalized, and recognized as a useful tool in the management of predator stocks (e.g., Kerr, 1974a, 1979; Ware, 1977). The obvious management solution contrived to help the lake trout accelerate their growth past this observed growth depression was to introduce a prey species that would provide an optimum size range for adult and sub-adult lake trout to feed upon, and be readily available at all times of the year without the necessity for the predator to enter unfavourable environmental conditions. Accordingly, ciscos were introduced into Lake Opeongo in 1948 (following a failed introduction in 1940) and subsequently became one of the most numerous fish species in the lake (Martin, 1970). The effects of the cisco introduction into Lake Opeongo have been summarized by Martin (1970) from the data taken from the first three decades of a creel census established in 1936 at the suggestion of the late W.J.K. Harkness (Fry, 1939). The first effect of cisco introduction has been the elimination of the growth depression in lake trout during the feeding transition from zoo-plankton and other small invertebrates to forage fishes. The fishery perhaps became less attractive to the angler seeking more lake trout rather than fewer, large individuals (Martin, 1970). The lake trout matured one year later, a factor that resulted in more trout being caught before an opportunity to reproduce occurred. Egg production, however, became greater and the eggs were larger and perhaps more likely to survive. Our assessment of the effects of the cisco introduction into Lake Opeongo in 1948, with the added benefit of another ten years' data, supports the observations of Martin (1970). We have smoothed the available annual data by approximate decades (i.e., 10–12 year units) to eliminate some of the detailed variability in the fishery caused by sampling, socio-economic and climatic changes affecting the fishery, and to accentuate major trends. Of obvious significance is the fact that about 10 000 trout were captured in each of the first three decades (Table 1) but in the fourth decade nearly 19 000 lake trout were taken by the angling fishery. This was undoubtedly a response to a mean fishing effort smoothed by decades, that increased by almost an order of magnitude from the first to the fourth decades. Concurrently, catch-per-unit of effort (CPUE) in both weight and numbers has shown a steady decline from 0.67 trout per hour (0.51 kg/h) in the first decade to only 0.18 trout per hour (0.14 kg/h) in the fourth decade. Mean weight of lake trout (Table 2) increased markedly in the decade following introduction of the cisco but thereafter declined almost to its original value of the 1936 decade. Mean length has shown a similar, but less obvious pattern. Mean age has generally decreased over the four decades (Table 2). Apparently, the observations indicate that the lake trout responded to the cisco introduction first with a growth response and, secondly, with a recruitment response, however, contrary to current ecological theory the conservatism of yield was not demonstrated (Kerr and Martin, 1970) and yield increased by about a factor of two between the first and fourth decades of the fishery (Table 1). This response, however, is correlated with the marked increase in mean annual effort (1 653– 11 858 h) and the equally marked decrease in CPUE previously noted. This fishery, therefore, bears many of the signs of a heavily exploited stock as indicated by accelerated fishing effort, increasing catches in the face of decreased CPUEs, decreased mean lengths and weights, and declining mean ages (Tables 1 and 2). In the light of the dynamic consequences of an increasingly exploitive fishery, which tends to mask the effects of the cisco introduction, we further investigated the changing status of the lake trout stock by preparing estimates of ponderal indices by decade, made from large samples of lake trout both before and following cisco introduction, with the understanding that density-dependent response to exploitation may have about maximized by the third decade (1958–68) and that further incursions into the standing stocks of lake trout would not likely elicit any further demographic response. The ponderal index is a descriptor of the relative wellbeing of a fish and may vary with season, sex, maturity, age and other factors (Carlander, 1969). It is essentially a length- weight relationship where

and K is the ponderal index or condition factor. Other analytical procedures may sometimes be more satisfactory for certain purposes, but ponderal indices have the convenient advantage for large and long-term samples of mitigating many of the confounding factors that may contribute to variability when calculated over shorter time spans. Examination of the ponderal indices by decade (Table 2) shows that the mean index for the second decade was already substantially higher than that of the first decade, despite the fact that the ciscos were not introduced into Lake Opeongo until 1948, the first year of the second decade. Martin (1970) provides evidence that this was, in fact, a transitional decade for the feeding habits of the lake trout. Ponderal indices increased markedly in the third and fourth decades to higher values (Table 2). It is interesting to note that the index in the fourth decade, although similar to that of the third decade, represents appreciably younger trout, with noticeably lower mean weights and lengths. Authors cited by Carlander (1969) noted a general increase in ponderal index with increase in length for lake trout. It would seem, therefore, that the effect of the intensive fishery on Lake Opeongo has been effectively masked by the conservative behaviour of the ponderal index over several age-classes of lake trout. The two indices for the third and fourth decades (ten and twenty years after cisco introduction) were significantly different from that of the decade before introduction as determined by a student's t test (P 0.01). It must be concluded that this case history exemplifies one of the most successfully planned introductions in the short history of fisheries management in Ontario. Our account has touched only the highlights and does not set out much of the solid background of detailed information that went into the initial management decision and the subsequent evaluations of the results. Essentially, a growth depression in the age-length relationship of the lake trout was observed and attributed to a single factor, namely, a lack of appropriately sized forage fishes accessible to the lake trout at all times of the year, with relatively little energy expenditure required for its utilization by the trout. A natural safety factor was built into the introduction, that is, the cisco is a co- evolutionary cohabitant of the lake trout in many lakes, and their interrelationships and interactions with other community components of oligotrophic lakes were known. Predictably, it was unlikely that the cisco introduction would harm this cold-water community. Rather, the introduction might have stabilized the indigenous community under the intensive fishing pressure it is increasingly receiving, by providing it with an alternate food resource which would reduce its vulnerability to angling. In a parallel instance, Forney and Eipper (1961) have described angling success in a walleye fishery to be best when natural food supplies are low. 5. DISCUSSION We have provided brief synopses of the histories of invasion or introduction of the sea lamprey, smallmouth bass and cisco, as examples of new species entering natural communities, together with observations of the ensuing consequences. Some lessons may be learned from these examples. As fish communities in oligotrophic lakes can be expected to be more sensitive to change than those in eutrophic systems (Li and Moyle, 1981), the results in our examples have, perhaps, been more decisive and readily discernible than would be likely for similar introductions in eutrophic systems. For that specific reason, study of comparatively simple, oligotrophic systems may sometimes provide useful principles that could be revealed only with great difficulty, if at all, in more complex eutrophic systems in freshwater or marine communities of any kind. At the present stage of our knowledge, and with a restrospective advantage, we can say with some confidence that the eventual major effects of each of these three introductions on the indigenous communities could have been predicted beforehand with reasonable precision by competent managers utilizing available scientific information tempered with simple ecological logic. Lest this statement be misinterpreted as a “carte blanche” for exotic species introductions, we issue the caveat of the “emergent surprise” (Kerr, 1974), which no mathematical theory can be expected to predict. Accordingly, despite our retrospective confidence in predicting the consequences of the foregoing three examples, we caution as a general rule, against introductions. However, as with all general rules, exceptions do occur, and we will attempt to deal with these now. Natural lakes devoid of any fishes can be advantageously stocked with fish species intended to enhance human cultural values. In these situations an opportunity exists for the fisheries manager to attempt to reconstruct a “natural” fish community by introducing several species in proportions appropriate to their various trophic interactions. The suitable choice of species would involve those indigenous to adjacent lakes with similar habitat characteristics, subject to approximately similar edaphic and climatic regimes. First, however, an assessment of the habitat status quo should be made, particularly with respect to the reason for the depauperate condition of the lake. If barrenness is attributable to the vagaries of zoogeography, then the likelihood of establishing something reminiscent of a natural community would seem to be good. Otherwise a habitat constraint such as low oxygen concentration or pH level might be suspect and should be investigated. Intermittent and widely-spaced oxygen deficits on the other hand, in lakes with a winterkill or summerkill history, may prove to be ideally suited for management purposes, depending on the length of interval between the episodic events, because of the presumptive ease of managerial control. Political expediency or public pressure often forces the fisheries manager's hand, and introductions may have to be made in some instances regardless of best personal judgement. On such occasions, the manager has many ecological tools at ready availability and should utilize these to best advantage in order to reduce the relatively high likelihood of failure of the introduction (e.g., Magnuson, 1976) or worse, experiencing a damaging introduction as in the case of the carp in North America. Many concepts exist in the ecological literature that may be extremely constructive in the process of considering an appropriate species or community for introduction. A hierarchy of four closely related, ecological concepts that are concerned with species interactions within the context of ambient environmental conditions are listed in Table 3. These concepts differ mainly in emphasis, level of generality, or the degree to which they are quantifiable as well as in other, more subtle considerations. Two of these concepts of particular application at the community level of organization, chosen in this instance, from the Swedish literature, are the related ones of interactive segregation (Nilsson, 1967) and dominance-subordinance (Svardson, 1976) together with their further elaboration in marine systems by Skud (1982). Nilsson (1967) observed that most fish species of north-temperate zones exhibit “interactive segregation”, an expression to be preferred over “competition” which is itself difficult to demonstrate conclusively (Reynoldson and Bellamy, 1971; Fraser, 1978). Interactive segregation depends on the degree of overlap of critical niche dimensions (our terminology) between two allopatric species which might otherwise demonstrate niche complementarity in sympatric situations. That is, the niche dimensions that overlap when the two species are geographically separated become mutually exclusive when the two species co-occur. Each species, therefore, functions in sympatry in a reduced niche space as measured along one or more critical dimensions. In extreme instances, if a formerly allopatric species cannot operate effectively within its newly restricted niche space, continued survival of the species becomes uncertain. Nilsson (1967) has listed some of the possible mechanisms underlying the interactive segregation process, including exploitation, territoriality, agonistic behaviour during feeding, predation and other possible interactions between species. When considering the introduction of a fish species which is a normal constituent of the fish communities of other nearby and closely similar lakes, careful attention should be directed to the potential of the candidate species for interaction with the native stocks, particularly with respect to the ecological mechanisms indicated by Nilsson (1967). Rigorous perusal of the literature for documentation of observed interactions of the candidate species may lead to the prevention of a potentially catastrophic introduction. Nilsson (1978) suggests that when a new species of fish is introduced into a lake, several alternative events may potentially occur. The three principal alternatives include rejection, in which the new species cannot compete successfully along one or more of the major niche dimensions; displacement of an indigenous community component by the introduced species, an indication perhaps of successful functioning along major niche dimensions to the point of competitive exclusion of another species (Hardin, 1960) or segregation of the introduced and native species to portions of one or more of their major niche dimensions that are utilized only in the face of major competitive interactions, that is, interactive segregation to avoid undue niche overlap. A fourth consideration that should be included is the potential for total community disruption as it decays into an astatic condition (Ryder and Kerr, 1978). Ideally, a new introduction into an existing fish community should utilize food and other resources not previously used by the native fish community and it should differ functionally from other community members. For one example, introductions should seek to augment or capitalize upon a particle-size hiatus in the prey resource, as exemplified by the cisco lake trout examples offered earlier. In other words, in looking for a candidate species with a high probability of survival, without creating unacceptable disruption of the native community, the degree of niche complementarity (Werner, 1977) will likely determine the relative level of success. In some respects, the sea lamprey in the Great Lakes fits this definition precisely, except for its apparent inability, as yet to establish a dynamic equilibrium with its principal prey species, the lake trout and burbot. A second concept of use to the fisheries manager in the context of a fish species introduction, is the level of interspecific population dominance in fish communities, or more simply, the dominance-subordinance principle (Svardson, 1976). In freshwater fish communities relatively few species dominate numerically, and the standing stocks of fishes in a lake are heirarchically organized. Reduction or elimination of a dominant species will tend to elicit drastic changes in the trophic hierarchy. In general, the terminal predators, unless heavily exploited, are governed by abiotic variables generated by climatic and edaphic conditions or lake morphometry, while the lower ranked species are primarily regulated through one or more interactive pathways determined by the dominant species. Svardson (1976) has elaborated on this pattern by the careful empirical observation of dominant and subordinate sympatric species pairs and has shown that species with pelagic capabilities are dominant over those species that live, generally, in the littoral zones of lakes. The survival advantage conferred on pelagic species apparently reflects their ability to be efficient plankton-feeders. In a recent elaboration of Svardson's (1976) dominance-subordinance principle, Skud (1982) demonstrated that changes in relative abundance of dominant and subordinate marine fishes, caused by changes in the physical environment, improved the survival of the dominant species while reducing the survival of subordinate species. The subordinate species, therefore, were controlled by the dominant species although environmental changes could reverse the dominance relations between species. In a recent review of replacement of depleted marine stocks by other species, Daan (1980) was unable to demonstrate, with any degree of confidence, direct replacement of one species by another in exploited stocks. Only for the closely coupled - fisheries of California, was approximate compensatory replacement of one species by another a reasonable explanation for changes in stocks. However, Daan used only species pairs in his analysis of complex marine communities. In such instances, compensatory mechanisms may not be particularly obvious without a careful examination of all community components. Even where a total yield conservatism is demonstrated in a fishery of many species, individual species yields may fluctuate widely (e.g., Sutcliffe et al., 1977). Perhaps most marine systems are too complex with too many alternative energetic pathways available for such changes to be easily detected by dominance-subordinace analysis alone, reinforcing the notion that such mechanisms are best studies first in relatively simple freshwater systems before extension to the marine environment. Application of the dominance principle is especially relevant to culturally disturbed lakes, those subjected to acidification or eutrophication to cite just two examples. Changed pH or nutrient levels may be sufficient to confer advantage on a species other than the one currently dominant (e.g., Fig. 1) and corresponding reverberations may occur throughout the fish community. In environments that have been severely degraded beyond any realistic prospect for rehabilitation, non-indigenous species more tolerant of the agents of degradation may be the only option for a successful introduction. Again, careful evaluation of the dominance-subordinance relationships of many species pairs may provide additional clues of the likelihood of niche complementarity of an introduced species. Another important consideration for the fisheries manager is the partitioning of resources by fishes in ecological communities in general and, more specifically, how different fish species divide the available resources of the environment (Schoener, 1974). In some respects this concept is complementary to the concept of interactive segregation (Nilsson, 1967), particularly with respect to food resources, but in another sense it adds a different perspective to the competition concept. Of particular interest in what follows, is the effect of habitat partitioning by related species. In a study of three centrarchid species in a small Michigan lake, Werner et al., (1977) observed that two species occupied the same habitat in the littoral zone where they utilized broad but disjunct ranges of food size. The two species were segregated in terms of food-size rather than habitat, with the larger species being functionally better adapted to capture and ingest large food particles than the smaller species, which selected correspondingly the small food particles. A third centrarchid species that was functionally intermediate between the first two species did not overlap with them on a spatial basis. By feeding in isolation in extremely shallow waters, the third species effectively avoided direct food competition with the two functionally efficient occupants of deeper waters. The third species thus demonstrated a niche complementarity that allowed it to co-exist with the other two centrarchids which, in turn, were effectively segregated along the food-size dimension of nichespace. Whether or not a candidate species for introduction will be spatially segregated from other components in a natural community is an important consideration. Supporting evidence required empirical observation from a broad range of habitat types. The lake trout, for example, generally occupies approximately similar habitat to the cisco on a time-space basis (Fry, 1939), such that a steady-state predator-prey relationship is typical. Such a steady-state relationship was observed by the late D.S. Rawson (in Cuerrier, 1954) in Lake Minnewanka, Alberta, prior to the construction of a major impoundment in 1941. However, by 1952 Cuerrier observed that the lake trout no longer preyed upon ciscos and attributed this fact to a spatial separation of the species, induced by a marked increase in water levels following impoundment. Accordingly, spatial separation of species may be caused by environmental heterogeneity, including changes in depth, temperature, light or other abiotic factors, as well as by interactive segregation per se. Earlier, we demonstrated that effective time-space separation of the walleye (Stizostedion vitreum) and sauger (S. canadense), when sympatric, is attributable to both the particle-size of the food ingested and the respective capabilities of these predators to feed most efficiently at differing light intensities (Kerr and Ryder, 1977). In this instance, at least two niche dimensions come into play in the time-space separation of these two closely similar ecological homologues. Besides the concepts of interactive segregation, dominance-subordinance and resource partitioning, many other approaches to assessing the consequences for fish introduction exist in the literature, such as the application of graph theory (Saila and Parrish, 1972) or loop analysis (Li and Moyle, 1981). Regardless of the approach taken, quantification of both fundamental and realized niche properties as defined by Ryder et al. (1981) are critical to the evaluation process for a species introduction. Realized niche boundaries may be determined empirically from direct observation of natural communities comprising various species. Unfortunately, as many candidate species for introduction evolved in communities other than those in which they are destined to be planted, this type of evidence can be difficult to interpret. Because community interactions, together with a multitude of abiotic variables, shape the realized niche, only broad generalizations will likely be obtained in this manner for truly exotic species. For example, a fish species functioning as a piscivore in one community will likely retain this characteristic if introduced in a community in which it did not evolve, assuming that appropriate forage fish species are available. Similarly, a filter-feeder will likely retain the planktivore habit, again assuming that the right kinds and sizes of food particles are readily available. Even these assumptions may be tentative, however, as shown by the occurrence of numerous stocks of plankton-feeding lake trout in highland lakes lacking suitable forage species. In most fish communities in which lake trout occur, however, they function typically as piscivores (Martin, 1966). Despite the existence of these and similar emergent surprises, attempts to evaluate the realized niches of candidate species will increase the safety factor of introductions, by emphasizing incompatibilities that may not otherwise be obvious. For situations where quantification is impossible, or tenuous at best, some indication of interactive processes may be determined qualitatively through the establishment of a multi-dimensional interaction matrix between the candidate species and the indigenous community. Each of the major interactive niche dimensions of the candidate species should be examined in this fashion (e.g., food, reproduction, behaviour, etc.), with respect to environmental and prey resource factors of primary ecological importance (type, size, time, space, density, nutrients, energy). The latter set of factors may be further subdivided as necessary to any level of resolution for which relevant information exists such as day-night, pelagic-littoral, eutrophic-oligotrophic, predation type or other appropriate division. Each niche dimension, in turn, should be considered in relation to the broader concepts of interactive segregation, dominance-subordinance and resource partitioning. Careful evaluation of the fundamental (potential) niche (Ryder et al., 1981) is also desirable in order to determine the likelihood for adaptation of a candidate species into a particular environment. Usually, evaluation of major limiting or controlling environmental factors such as temperature, oxygen, light or pH (e.g., Fry, 1947; Scherer, 1971; 1976) will provide suitable information to determine the potential of a species to utilize the resources of its environment, or to survive at times of the year when some of these dimensions may be suppressed. Multi-variate or matrix analysis may be used where appropriate, depending on the quality and resolution of available data. 6. CONCLUSION Fish community manipulation, through the introduction of new species, is generally risky and therefore the degree of success likely to be experienced is relatively unpredictable. Conservatively, the probability for damaging an indigenous community is at least as great as that of enhancing it. These odds against the fishery manager can be reduced by taking a stand toward the protection of, or rehabilitation of, indigenous co- evolved fish communities. Where introductions are unavoidable, because of public or political pressure, efforts may be most effectively directed toward barren lakes, lakes degraded to the point of irreversibility, or lakes with an obvious functional community component missing. In each instance native community stocks should be utilized for introduction where possible, although in irreversibly degraded systems of the boreal forest zone, it is unlikely that a useful candidate species will exist. In such cases it may be necessary to search for a likely species elsewhere, where fish communities or individual species have perhaps developed some evolutionary tolerance to culturally degraded systems. An alternative option which should always be considered as a means for reducing the risk of introductions, is to choose stenoecious species or sterile hybrids as candidates for planting, thereby increasing the possibility that catastrophic errors can be rectified. In summary, intelligent approaches to community manipulation involve careful evaluation of appropriate niche dimensions of both the candidate species for introduction and the indigenous community. 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J.Fish.Res.Board 1977 Can., 34(3):360–70 Table 1 Mean values for numbers, yield, effort and catch per unit of effort of lake trout entering the sport fishery at Lake Opeongo, Algonquin Park, for the four decades (approximate) between 1936 and 1979

Total no. No. Yield Effort C.U.E. C.U.E. Years1 lake trout .yr-1 .yr-1(kg) .yr-1(hr) .yr-1(no.hr-1) .yr-1(kg.hr-1) 1936–47 9 227 839 1 047 1 653 0.51 0.67 1948–57 10 298 1 030 1 449 4 103 0.28 0.40 1958–68 10 044 913 1 645 6 857 0.13 0.24 1969–79 18 604 1 691 2 114 11 858 0.14 0.18

1 Data for each variable was not available for every year

Table 2 Age, fork length, weight and ponderal indices (K = W105/L3) for lake trout from the angling fishery of Lake Opeongo, Algonquin Park, for the years 1936 to 1979 by approximate decade. The indices for the last two decades (1958–79) were significantly different from that prior to the cisco introduction according to a Students t-distribution (p 0.01)

Mean fork Mean length weight Years1 Mean age (cm) (kg) Ponderal index value (K) Comments Mean Range 1936–47 8.0 47.9 1 271 1.14 0.99–1.27 Ciscos not present 1948–57 8.1 47.8 1 446 1.32 1.22–1.50 After cisco introduction (1948) 1958–68 7.8 48.8 1 906 1.58 1.42–1.76 -"- 1969–79 7.5 43.5 1 273 1.55 1.44–1.64 -"-

1 Data for each variable was not available for every year Table 3 A hierarchy of four ecological concepts relating to the interrelationships of two or more species within the background of ambient environmental conditions. Despite the difference in nuance or emphasis among the four concepts, they all describe a conceptually similar phenomenon.

Ecological Objective Mode Level of Level of Concept Generality Quantification Authors Realized niche Most passive Most general Low Fry (1947) Hutchison (1957) Interactive Nilsson (1967) segregation Dominance- Svardson (1976) subordinance Resource Most active Most specific Relatively high Schoener (1974) partitioning Werner et al. (1977)

Fig. 1 Changes in relative proportions of rainbow trout and brook trout in response to changing environmental factors

1 Contribution No. 82-06 of the Ontario Ministry of Natural Resources Research Section, Fisheries Branch, Box 50, Maple, Ontario, Canada 2 Bedford Institute of Oceanography Contribution PARASITE RANGE EXTENSION BY INTRODUCTION OF FISH TO HUNGARY

K. Molnar Veterinary Medical Research Institute Budapest, Hungary

CONTENTS

1. INTRODUCTION 2. MATERIALS AND METHODS 3. RESULTS 4. DISCUSSION 5. REFERENCES

ABSTRACT Several new species of parasite have spread into Hungary as a consequence of fish transfers. Some of these parasites were brought in with introduced fish, while others, originating from fish introduced to the neighbouring countries, have actively invaded the country through natural waters. The parasite fauna has increased especially after the introduction of the E. Asian herbivorous fishes, which - apart from specific parasites also carried - several parasites which have also affected the native fishes. The probability of introducing pathogens during the transfer of older fish is very high, while in yolk-sac fry it is minimal. The spread of parasites cannot be stopped by country borders and so fish naturalizations bear international implications. After the several pathogenic parasite species have passed across natural barriers forming the borders of fauna systems, their spread, on appropriate host fishes, is only a matter of time. RESUME Plusieurs nouvelles espèces de parasites se sont propagées en Hongrie à la suite d'introductions de poissons, soit dans le pays même, soit dans des pays voisins dont les eaux communiquent avec celles de la Hongrie. La faune parasitaire s'est accrue, notamment après l'introduction de poissons herbivores, quand, outre les parasites spécifiques plusieurs parasites affectant également les poissons indigènes se sont établis dans les eaux hongroises. Le risque d'introduire des agents pathogènes lorsque l'on transplante des poissons assez âgés est très élevé; en revanche, il est minime avec des alevins dont le sac vitellin ne s'est pas encore résorbé. La propagation des parasites ne connaît pas de frontières et l'introduction de nouvelles espèces de poissons est donc une affaire internationale. Une fois que des parasites pathogènes ont franchi les barrières naturelles, leur propagation - en présence de poissons hôtes - n'est plus qu'une question de temps. 1. INTRODUCTION During the last hundred years several new fish species have become permanent members of the Hungarian fauna (Pinter, 1980). Some of these have reached the country by invasion, while others have been introduced. In most cases the fish also brought with them elements of their parasite fauna from their original biotope. The first detailed survey of introduced parasites took place after the translocation of herbivorous fishes to Hungary, when Szakolczai and Molnar (1966) reported the appearance of several parasites previously unknown in the country. The simultaneous spread of fish parasites and their hosts is well documented in the literature and in the case of Asiatic herbivorous fishes and the Amur wild carp diffusion throughout the water system of Central Asia and Europe can be followed in the works of the Soviet researchers (Babayev, 1965; Bauer and Strelkov, 1972; Malevitskaya, 1958; Kulakovskaya and Krotas, 1961; Musselius, 1967 and Vismanis, 1962). Data concerning the transcontinental spread of the parasites of herbivorous fishes and salmonids in America (Hoffman, 1970; Bauer and Hoffman, 1976) and in New Zealand (Edwards and Hine, 1974) can also be found. Some of the introduced parasites proved to be highly pathogenic on their new biotope, causing animal health problems (Musselius, 1976; Molnar, 1970, 1972; Korting, 1974; Hoffman, 1980). The present paper deals with the mode of introduction or spread of non- indigenous parasites into the country, and with the pathological significance of these parasites. 2. MATERIALS AND METHODS In our Institute we have continuously studied the parasite fauna of fish living in the natural waters and fish farms of Hungary since 1960. Apart from these zoological investigations, we have also carried out animal health inspections in some fish farms regularly, in others occasionally. The fish studied were transferred alive to the laboratory where the parasites were collected by dissection under a microscope. Squash preparations and histological sections were examined for the presence of tissue parasites. 3. RESULTS The first undoubtedly introduced parasites were found by Molnar (1963) on the brown bullhead (Ictalurus nebulosus), a fish transferred from the United States in 1902 and on the pumpkinseed (Lepomis gibbosus) which was transferred to Hungary in 1895. Monogeneans including Cleidodiscus pricei (Mueller, 1934), Haplocleidus dispar (Mueller, 1936) and Urocleidus similis (Mueller, 1936) collected from the gills of the above fishes proved to be typical North American species, and due to their relatively high host specificity these have not spread to the native fishes. Cleidodiscus pricei has only been found on the brown bullhead and on Ictalurus melas originating also from America, which were transferred to Hungary through Italy. In this case, it was not possible to decide whether the I. melas brought its parasites from Italy or acquired them from the previously acclimatized brown bullhead. The first alternative seems to be more likely, because there were no brown bullhead near the quarantined I. melas stock. In 1963, large numbers of herbivorous fishes were transferred to Hungary from Chinese natural waters. Four species, grass carp (Ctenopharyngodon idella), silver carp (Hypophthalmichthys molitrix), bighead (Aristichthys nobilis) and Mylopharyngodon piceus were introduced to the country. A fifth species, Pseudorasbora parva was also introduced accidentally. Several parasite species were detected on the quarantined fish (Szakolczai and Molnar, 1966) some of which proved to be undoubtedly Far Eastern parasites. The majority of the species then detected are regular parasites of cultured herbivorous fishes even at the present time. Two species, the grass carp and the bighead were infected with monogeneans, among which Dactylogyrus lamellatus (Achmerov, 1952) and D. nobilis (Long and Yu, 1958) are specific parasites of the grass carp and the bighead, respectively. At the same time, Myxobolus pavlovskii (Achmerov, 1954) and the intestinal parasite Eimeria sinensis (Chen, 1956) proved to be common parasites of both the silver carp and the bighead. Cryptobia branchialis (Nie, 1956) found on the gill rakers and Spironucleus sp. living in the intestine, have established themselves on carp in addition to the herbivorous fishes. During the investigations carried out in 1963–64 two additional parasites of Far Eastern origin, a Myxidium sp. (Protozoa) and Amurotrema dombrovskajae (Achmerov, 1959) (Trematoda) were detected in herbivorous fishes, though these parasites have not been observed since then. On the other hand, no introduced parasites were found on Mylopharyngodon piceus and Pseudorasbora parva, which had been introduced only in small numbers. During our investigations, particular attention was paid to Bothriocephalus acheilognathi (Yamaguti, 1934) syn. B. gowkongensis (Yeh, 1955) infection of the quarantined fish because according to Soviet authors this parasite was a dangerous pathogenic species. In spite of regular inspection, the parasite was detected five years after lifting the quarantine in both the naturalized fish and in carp kept with them. At the same time, Bothriocephalus infection was detected in an isolated minnow stock five months after naturalization. That this infection spread to this biotope by ten grass carp fry removed from quarantine was found out only several years later. After its detection B. acheilognathi became widely distributed in the country and is now one of the most significant pathogens in carp stocks. Two more parasites, Trichophrya sinensis (Chen, 1955) a gill parasite of the silver carp and bighead, and Myxobolus drjagini (Achmerov, 1954) a head skin parasite of the former two species were also introduced as a latent infection with the first transport from China. The occurrence of these parasites was detected only a year after the naturalization of the fish. Between 1964 and 1969 we imported artfically-hatched fry of herbivorous species from the U.S.S.R. These fishes did not bring any parasites with them and the parasite species originating from China caused infection only in those fish farms in which previously imported fish were kept. The parasite fauna of the native herbivorous fishes was further enriched in 1966 when broodstocks were imported from the U.S.S.R. Numerous Balantidium ctenopharyngodonis were present (Chen, 1955) in the intestine of grass carp transported at that time, while numerous Datylogyrus ctenopharyngodonis (Achmerov, 1952) parasites were detected on their gills. Entamoeba ctenopharyngodoni had probably also spread into Hungary on this occasion (Chen, 1955) and was detected only later. Only a few of the parasites introduced with herbivorous fishes have established themselves on native species (including Cryptobia branchialis, Spironucleus sp., Bothriocephalus acheilognathi). Of these, only the Bothriocephalus species has pathological significance. Infection with Lernaea has been known in Hungary for a long time, but has gained pathological significance only since the naturalization of the herbivorous fishes. At the present time, we cannot decide merely on morphological grounds whether the increased pathogenic importance of lernaeosis can be attributed to the introduction of Lernaea elegans Leigh-Sharpe, 1925, or to the changed conditions arising from the expansion of polycultural fish rearing promoting the reproduction of the native Lernaes cyprinacea Linne, 1758. An intensive introduction of fish from China and from the region of the River Amur to Romania and the European part of the U.S.S.R. had been carried out prior to the transport of herbivorous fishes to Hungary. These fish brought with them a considerably larger parasite fauna than to Hungary (Radulescu and Georgescu, 1962; Musselius, 1965). Owing to the common water system, these parasites will probably spread into Hungary sooner or later. Indeed, the appearance of Dactylogyrus suchengtaii (Gussev, 1962) a gill parasite of the silver carp in fish farms situated in the vicinity of the Romanian border already shows the capacity of such species to spread. An eel parasite, Myxidium giardi Cepede, 1906, is brought into the country in large numbers with the annual import of elvers. This parasite cannot be demonstrated in older fish but considerable infections with Bothriocephalus claviceps (Goeze, 1782) can be detected in the intestine of the more developed eels. Fortunately, eel bothriocephalosis does not affect other fish species. The fact that up to now we have failed to detect specific parasites on Carassius auratus gibelio introduced to Hungary in 1954 and on Neogobius fluviatilis, which appeared in Lake Balaton in 1970, is remarkable since these fishes possessed a rich parasite fauna in their original biotopes. Fish transfers have also had indirect negative effects which increasingly make themselves felt in Hungary. There have been no wild carp introductions into Hungary from the Far East or Eastern Europe. In spite of this, several common carp parasites are supposed to have spread into Hungary through rivers from the U.S.S.R. and Romania. Ivasik and Karpenko (1967) reported that Amur wild carp (Cyprinus carpio haematopterus) had been first transferred in 1955 from the River Amur to the Ukraine, which borders on Hungary. These fish brought with them three parasites previously unknown in Europe, Myxobolus amurensis Achmerov, 1960; Khawia sinensis Hsu, 1935, and Philometroides lusiana (Vismanis, 1962), which established themselves on the local mirror carp. In addition to the parasites listed above, presumably other species have also appeared in the Ukraine, from whence some carp parasites of Far Eastern origin have already spread into Hungary either directly or through Romania. Among these, the gill parasite Dactylogyrus achmerowi Gussev, 1955 and the tapeworm Khawia sinensis have been detected in Hungary for the first time, but recently Thelohanellus nikolskii Achmerov, 1955 and T. hovorkai Achmerov, 1960, have also been demonstrated. The above-mentioned parasite species have obviously spread to our fish farms on carp living in natural waters, and among them Khawia sinensis and Thelohanellus nikolskii already have an undoubted economic significance. A regular treatment is applied for the control and eradication of the introduced tapeworms in the Hungarian pond farms which is included in the standard technology. Many regulations concerning the disinfection of fish ponds have also been necessary to destroy the introduced spore-bearing parasites. 4. DISCUSSION Parasitological analysis of the European and, primarily Hungarian, fish transfers has proved that during fish transfers which lack the advice of expert pathologists, the transported fish may carry numerous parasites from remote biotopes to the site of naturalization. Among these parasites many keep on causing damage to their hosts in their new biotope. This is a relatively negligible factor since calculations concerning the probable usefulness of a fish species include not only its favourable properties but also its proneness to the diseases affecting it. Those parasites cause problems in the first place which affect not only the introduced fish species but also the native fishes. These include the species of loose host specificity which can live on several fish species, e.g., Bothriocephalus acheilognathi, which affects all carp species. There are significant differences among the parasite groups as to the probability of naturalization of the introduced parasites. Those parasites which spread directly from fish to fish and can multiply even on the same individual, e.g., flagellates, or those which develop on the bottom for a short time, e.g., gill parasites, have great advantages because a single introduced parasite can eventually infect a whole fish stock. Rapid spread may be expected also in spore-bearing protozoans, where, if the introduced fish species is bred on the same water area in the following year, fish of the new generation will be able to ingest the spores of the parasite which generally has a seasonal developmental cycle, (e.g., Myxobolus drjagini). Experience proves that parasites requiring an intermediate host can also multiply if the proper organism is common in the new environment for example, cyclops in the case of Bothriocephalus or tubifex in the case of Khawia. On the other hand, the naturalization of the trematode species Amurotrema dombrovskajae has obviously been prevented by the lack of the proper molluscan intermediate host. During introduction of broodstock even the most careful quarantine fails to assure freedom from parasites and most parasites characteristic of the fish species have the possibility of spreading to the new biotope. The situation is similar in the case of the already feeding fry though these sometimes bring along with them only those diseases characteristic of the young fish. The introduction of the grass carp to Hungary may serve as an appropriate example. Knowing the biology of the parasite Dactylogyrus ctenopharyngodonis and Balantidium ctenopharyngodonis, which are characteristic parasites of older fish, have spread into Hungary with the broodstock and not with the fry. Thus herbivorous fishes brought into the country as yolk-sac fry remained free from specific parasites as long as they had not come into contact with the generations introduced earlier. If we consider the cost of medication and labour necessary for the control of bothriocephalosis and gill parasitosis of grass carp, the saving of one year by the introduction of the first infected stock from China was not worthwhile. If this introduction had not taken place, we could have hoped to maintain a relatively parasite- free herbivorous fish stock in Hungary for some considerable time. This supposition is, of course, not wholly true because fish naturalizations exert their influence not according to country borders but through water systems and the spread of both host and parasite along water courses cannot be stopped by even the most radical animal health regulations. thus, fish introductions have international significance, and if these occur across geographical barriers they will always be followed by unfavourable consequences. During his studies on monogeneans Gussev (1969) distinguished five faunal complexes in Eurasia. It is known that the parasite fauna of the amuro-sino faunal complex is much richer than that of the palaearctic region in which Europe is located (Achmerov, 1952; Gussev, 1955; Ivasik, 1968 and Ergens, 1969), and Europe has already become partly acquainted with this rich fauna, which has been a severe lesson. We can only hope that ichthyologists and fish culturists will profit from these unfavourable experiences and will organize fish naturalizations more circumspectly, choosing to introduce eggs and fish larvae rather than adult fish. In this case, there is hope that fihs species of high productivity will maintain their freedom from parasites in their new biotope and will not endanger the health of the native fish fauna. 5. REFERENCES Achmerov, A.Ch., New species of monogenetic trematodes of fishes from the River 1952 Amur. Parasitol.Sb.Zool.Inst.Akad.Nauk SSSR, 14:181–212 (in Russian) Babayev, B., Distribution of Bothriocephalus gowkongensis Yeh, 1955 (Cestoda, 1965 Pseudophyllidae) in the water bodies of Cara-Kum Canal. Zool.Zh., 44:1407–8 (in Russian) Bauer, O.N. and G.L. Hoffman, Helminth range extention by translocation of fish. 1976 Wildl.Dis., pp. 163–72 Bauer, O.N. and Yu.A. Strelkov, The effect of acclimatization and transportation on fish 1972 parasite fauna. Izv.Vses.Nauchno-Issled.Inst.Ozern.Rechn.Rybn.Khoz., 80:123–31 (in Russian) Edwards, D.J. and P.M. Hine, Introduction, preliminary handling, and diseases of grass 1974 carp in New Zealand. N.Z.J.Mar.Freshwat.Res., 8:441–54 Ergens, R., Monogenoidea from Cyprinus carpio haematopterus and Carassius auratus 1969 gibelio (Cyprinidae) from Mongolia. Folia Parasitol., 16:201–6 Gussev, A.V., Monogenetic trematodes of fishes of the Amur basin. 1955 Tr.Zool.Inst.An.SSSR, 19:171–398 (in Russian) , The history of the fauna and adhesive adaptation of freshwater 1969 monogeneans from Eurasia and North America. Parasitol.Sb.Zool.Inst.AN.SSSR, 24:106–27 (in Russian) Hoffman, G.L., International and transcontinental dissemination and transportation of fish 1970 parasites with emphasis on whirling disease. Spec.Publ.Am.Fish.Soc., (5):69–81 , Asian tapeworm, Bothriocephalus acheilognathi Yamaguti, 1934, in North 1980 America. In Beitrage zur Fischpathologie und toxicologie. Stuttgart, Gustav Fischer Verlag, pp. 69– 75 Ivasik, V.M., On the centre of incidence of the common carp and its parasite fauna. 1968 Vopr.Ikhtiol., 8:342–9 (in Russian) Ivasik, V.M. and I.M. Karpenko, The change of parasite fauna of Cyprinus carpio 1967 haematopterus Tem et Schl. at its introduction into fish ponds of the Ukraine. Parazitologiya Lening., 1:149–50 (in Russian) Korting, W., Die Bothriocephalose der Karpfen. Vet.Med.Nachr., 1974, 2:152–8 1974 Kulakovskaya, O.P. and B.A. Krotas, On Khawia sinensis (Caryophyllaeidae, Cestoda) a 1961 parasite transferred from Far East to the fish farms of the USSR western districts. Dokl.Akad.Nauk.SSSR, 127:1253–5 (in Russian) Molnar, K., Mono- and digenetic trematodes from fishes. Allattani kozlemenyek, 50:103– 1963 6 (in Hungarian) Molnar, K., An attempt to treat fish bothriocephalosis with devermin. Acta 1970 Vet.Acad.Sci.Hung., 20:325–31 , Studies on gill parasitosis of the grass carp (Ctenopharyngodon idella) 1972 caused by Dactylogyrus lamellatus Achmerov, 1952. 4. Histopathological changes. Acta Vet.Acad.Sci.Hung., 22:9–24 Malevitskaya, M.A., On the transfer of a parasite of complex development, 1958 Bothriocephalus gowkongensis Yeh, during acclimatization of Amur fishes. Dokl.Akad.Nauk.SSSR, 123:572–75 (in Russian) Musselius, V.A., Parasites and diseases of plant-eating fishes and methods to control 1967 them. Moskva, Kolos Publishing House 81 p. (in Russian) , Parasites of phytophagous fishes from the Far East, cultivated in ponds 1969 of the European part of the U.S.S.R. Parazitologiya Lening., 3:236–43 (in Russian) Pinter, K., Exotic fishes in Hungarian waters their importance in fishery utilization of 1980 natural water bodies. Fish.Manage., 11:163–7 Radulescu, I. and R. Georgescu, A contribution to the knowledge of parasite fauna of the 1962 grasscarp, Ctenopharyngodon idella in the first year of its acclimatization in Roumania. Bull.Inst.Cerc.Proj.Piscicole, 21:85–91 (in Roumanian) Szakolczai, J. and K. Molnar, Veterinarmedizinische Untersuchungen an den in Ungarn 1966 eingeburgerten pflanzenfressenden Fischarten, Z.Fisch., 14(1/2):65–77 Vismanis, K.O., Carp philometrosis in fish farms of Latvia. Izv.Akad.Nauk.Latv.SSR, 1962 4:93–6 (in Russian) IMPLEMENTATION OF A REVIEW AND DECISION MODEL FOR EVALUATING PROPOSED INTRODUCTIONS OF AQUATIC ORGANISMS IN EUROPE AND NORTH AMERICA

C.C.Kohler Fisheries Research Laboratory and Department of Zoology Southern Illinois University Carbondale, Illinois, U.S.A.

and

J.G. Stanley Maine Cooperative Fishery Research Unit University of Maine Orono, Maine, U.S.A.

CONTENTS

1. INTRODUCTION 2. MODEL REFINEMENT 3. THE PROTOCOL 3.1 Protocol Committee 3.2 Opinionnaire 3.3 Review and decision model 3.4 Research facility requirements 3.5 Permit, transportation and disease-free certification requirements 3.6 Report requirements 4. PLANNING 5. CONCLUSION 6. REFERENCES ABSTRACT A protocol is presented that we recommend be used to evaluate proposed introductions of aquatic organisms in Europe and North America. The protocol requires establishment of an evaluation board or committee, promulgation of a formal proposal for each proposed introduction and evaluation of the proposed introduction employing a Review and Decision Model. The model is presented as a flow chart in the form of a decision tree. Recommendations would be arrived at by computer analysis of an opinionnaire completed by committee members and experts. The opinionnaire would generate, reject or accept statements and identify where significant gaps of knowledge occur. The presented protocol is a refined version of a previous protocol we suggested be adopted in the United States. RESUME Vu les transplantations massives de poissons exotiques qui ont actuellement lieu à l'échelle mondiale et les graves conséquences écologiques qu'ont souvent entraînées ces introductions faute d'être suffisamment bien conçues, il est manifeste qu'il faudrait mettre au point un mécanisme permettant d'évaluer systématiquement les projets d'introduction de poissons exotiques. L'idéal serait de parvenir à un accord mondial. Toutefois, son application serait difficile, sinon impossible, compte tenu des exigences particulières des pays en développement qui, pour la plupart, se trouvent dans l'hémisphère sud. Nous avons proposé dans la presse un protocole pour l'évaluation des projets d'introduction de poissons exotiques aux Etats-Unis; nous pensons qu'il pourrait s'appliquer à tout l'hémisphère nord. Ses points principaux sont les suivants: création d'un organisme d'évaluation, formulation d'une proposition officielle pour chaque projet d'introduction (exception faite des espèces exotiques déjà largement établies et de la plupart des poissons ornamentaux) et analyse du projet. Le modèle utilisé pour l'analyse et très souple et prévoit cinq degrés d'examen et cinq types de décision. Chaque degré d'examen suppose une étude de plus en plus poussée du projet même si des décisions peuvent souvent être prises aux premiers stades de l'évaluation. 1. INTRODUCTION The current massive transfer of aquatic organisms occurring on a global scale, and the severe ecological consequences that have often manifested when such introductions were not well conceived, clearly indicates the need for developing a mechanism for systematically evaluating proposed exotic introductions. Ideally, a single protocol could be developed for worldwide adoption. However, because of the disparate priorities of lesser developed countries, most of which are in the Southern Hemisphere, such a protocol would be difficult, if not impossible to apply. We have previously suggested a protocol (Kohler and Stanley, in press) for evaluating proposed exotic fish introductions in the United States that we feel would have general utility throughout much of the Northern Hemisphere, as well as for the more developed countries of the Southern Hemisphere. The protocol requires establishment of an evaluation board or committee, promulgation of a formal proposal for each proposed introduction (excluding those exotics already widely established and most ornamental fishes) and analysis of the proposed introduction employing a “Review and Decision Model” (Fig.1). Four categories are considered in the evaluation: (i) Feasibility, which deals with the validity of the proposed use, the status of the organism in the native range, the location and type of system into which it would be introduced, disease control measures, and various legal restrictions; (ii) Acclimation potential of an organism, which is based on habitat requirements, reproductive viability and migratory behaviour; (iii) Control potential, which deals with methods that could be used to eliminate organisms introduced but later deemed undesirable or to prevent (limit) reproduction, and (iv) Prediction of impact, which is defined as the balance between perceived benefits and risks. The model is highly flexible and is comprised of five levels of review and five “decision boxes”. Although each level of review mandates progressively greater scrutiny of the proposed introduction, decisions can often be rendered during early stages of the evaluation because the more basic criteria for analysing introductions are considered at the outset. Here, we present refinements to the proposed protocol and suggest how it could be implemented in Europe and North America. We refer our readers to our initial paper for a discussion of the factors that were used as a basis for synthesizing the protocol. The protocol presented here is a reflection of the author's views and does not necessarily coincide with that of their employers or professional affiliations. 2. MODEL REFINEMENT We have reworded some of the questions generated in the initial review and decision model so that answers more sharply focus on approval and rejection decisions. Questions asked in the initial model simply required answers of “yes” or “no”. However, we recognize that such definitive answers would rarely be possible and that a degree of subjectivity would often exist. Consequently, we have incorporated a decision scale in the model for the purpose of replacing verbal answers with numerical values that illustrate the level of uncertainty of each answer. The scale ranges from 1 (an absolute answer of “no”) to 5 (an absolute answer of “yes”). Values would be obtained by an opinnionaire (Table 1) completed by the evaluating entity and by outside experts. The opinionnaire is based on the premise that opinions of experts are justified as inputs to decision-making when absolute answers are unavailable, and that a concensus of experts will provide a more accurate response to a question than a single expert (Fusfeld and Foster, 1971). The revised review and decision model (Fig.1) contains five decision points for approval and seven for rejection of an introduction. A computer analysis we developed assists in tabulating answers on the opinionnaire. We recommend that scale values of 3 and 2 be used for approval and rejection decisions, respectively. 3. THE PROTOCOL The proposed protocol requires establishment of a Protocol Committee to evaluate proposals for introductions of aquatic organisms. The Protocol Committee would employ a review and decision model (Fig. 2) that is a decision tree in which a hierarchy of factors are considered in successive levels of review. In the subsequent sections we describe components of the protocol and suggest how it could be implemented in Europe and North America. 3.1 Protocol Committee A separate committee would need to be established for Europe and North America. The committees should be composed of qualified individuals representing government agencies, academia and the private sector. Ideally, the European and North American committees would operate under the auspices of the European Inland Fisheries Advisory Commission (EIFAC) and the American Fisheries Society (AFS), respectively. The Protocol Committee would: (i) receive proposals for introductions; (ii) select experts for the review of proposals on case-by-case basis; (iii) exercise the review and decision model to generate decisions; (iv) make necessary recommendations, and (v) evaluate reports and records regarding impacts of realized introductions. We recommend that the opinionnaire be completed by committee members prior to sending to experts. Approval could be reached if all criteria are met in the first level of review. 3.2 Opinionnaire An opinionnaire (Table 1) was developed that would be used in generating a data base for the review and decision model. The opinionnaire consists of ten questions designed to evaluate any proposal to introduce an aquatic organism. The committee itself could complete the opinionnaire to possiby arrive at an early decision. Subsequently, the proposal and opinionnaire would be submitted to experts. We developed a computer programme (available upon request) to analyse the opinionnaire. The programme has outputs of “Reject”, “Approve” and “More information needed”, each with an explanation of why that particular decision was reached. 3.3 Review and decision model The model is composed of five levels of review and five corresponding “decision boxes”. Components of the model are listed below and described essentially as they appear in Kohler and Stanley (in press) but with the addition of scale values. (1) Proposal for exotic fish introduction An entity desiring to introduce an aquatic organism would prepare a proposal that includes the answers to the following questions: (i) What exotic species do you propose to introduce (common and scientific name)? (ii) What is its native range? What is the present range? (iii) What is the purpose of the introduction? (iv) Where and into what type of system would this organism be introduced, and how many would be introduced? (v) What precautions have been or will be taken to ensure that the organisms are not harbouring communicable pathogenic organisms and parasites? (vi) If the organisms are to be maintained in a closed system, what measures would be taken to guard against accidental escape to open waters? (vii) What is the current state of knowledge concerning the acclimatization potential of the organism? (a) Thermal requirements: tropical, temperate, Arctic; (b) Habitat requirements: freshwater (stream, river, lake, pond, etc.) or marine (tide-pool, coral reef, demersal, etc.); (c) Reproduction: describe the spawning habitat and reproductive strategy of the species. A bibliography of pertinent literature should be appended to the proposal. (2) Level of Review I (a) Purpose of introduction: Does the proposing entity have valid reasons for introducing the aquatic organism? Could no native species serve the same function? (b) Abundance in native range: Knowledge of the population abundance of the organism in its native range is an important aspect of the evaluation. Is it endangered, threatened or rare? Is it exploited from the wild or under culture? (c) Communicable pathogenic organisms and parasites: The evaluation would include assessing the safeguards for avoiding transmission of communicable pathogenic organisms and parasites to the proposed receiving system(s). (d) Site of introduction: It is important to discern from the outset whether the organism would be stocked in an open or closed system. Would it be stocked in or have potential access to a major drainage? If it is to be maintained in a closed system, the proposing entity must identify steps it would take to guard against accidental escape. (3) Decision Box I A proposal for an introduction would be rejected if: (i) reasons for introductions were not deemed valid; (ii) the species is endangered, threatened or rare in its native range, or (iii) the proposing entity has not established that adequate safeguards would be taken to avoid introduction of communicable pathogenic organisms and parasites. The proposal would be approved at this stage when the above criteria are met, and provided that the introduction is perceived as being limited to a closed system. When this last condition is not fully met, the evaluation process would proceed to the next level of review. (4) Level of Review II This and subsequent levels of review are directed to experts selected by the committee. In Level II, the acclimation potential is assessed (question 5 of the opinionnaire; Table 1). Should pertinent information be insufficient, as evidenced by more than 50 percent marking “don't know” on the opinionnaire, the Protocol Committee might grant the proposing entity permission to conduct research with a limited number of specimens under confined conditions for the purpose of obtaining the required data. If the proposing entity is not qualified to conduct the research, it would be its responsibility to subcontract to a qualified laboratory. For some species, the Protocol Committee may require that all research be conducted within the organisms' native range. (5) Decision Box II The proposal for the introduction would be approved when there is a strong chance that the organism will not establish a self-sustaining population (average value ≥ 3 for question 5 in Table 1). Alternatively, further evaluation would be mandated for those organisms that would likely produce self-sustaining populations, or when evidence is insufficient for making a reasonable prediction. (6) Level of Review III This level of review is based on predicting the potential impact of the organism on the ecological integrity of the system(s) where it is proposed for introduction. In addition, the analysis of benefit and risk would include assessing the array of potential impacts on man. Review at this level requires detailed knowledge on the ecological relations of the organism in its native habitat, as well as considerable information on the community structure of the proposed receiving system(s). (7) Decision Box III The introduction would be rejected if the available information suggests (average opinionnaire values ≥ 2) that the organism would exert a major adverse impact on the receiving system(s) or to man. The proposal would be approved when indications are for the opposite outcomes. If the available information is not considered conclusive, the evaluation should proceed to Level of Review IV. (8) Level of Review IV Level of Review IV requires development of a detailed literature review based on the format for a Food and Agriculture Organization (United Nations) Species Synopsis. However, additional sections concerning impacts of transplantation (documented or potential) would also be required. Once the synopsis is obtained, this information will be sent again to the panel of experts so they can attempt to arrive at a recommendation. (9) Decision Box IV On the basis of an analysis of the second round of opinionnaire data, the Protocol Committee would either approve or reject the proposed introduction. Additional review (Level V) would be necessary whenever the current data base is not considered sufficient, or if it is unclear whether the introduction is desirable. (10) Level of Review V This level of review requires that research be conducted to complete the species synopsis or to assess the potential impact of the introduction to the indigenous species and habitats. Research might be conducted under controlled conditions near the site where the introduction is contemplated or the Protocol Committee may require that all studies be carried out within the organisms' native range. In either case, the qualifications of the staff and research facilities would be evaluated by the Protocol Committee before the studies were conducted. Topics would be investigated as specified by the Protocol Committee. (11) Decision Box V Using all information collected to this stage, the Protocol Committee should be able to make an informed recommendation regarding the proposed introduction. However, the Committee may find it necessary to specify additional research if important questions remain to be resolved. In such a situation, the fifth and final evaluation stage would become a loop of the “Review” and “Decision” modes until a ruling could be made. 3.4 Research facility requirements Research mandated by the model will be conducted by qualified individuals at approved sites. Administrators of proposed research facilities will be required to submit to the Protocol Committee a description of their staff, capabilities and the security procedures they would take during the course of the research. The Protocol Committee will have to approve the proposed research staff and facilities prior to any experimental studies with exotic organisms. As previously noted, the Committee may at their discretion specify that all, or part, of the research be conducted within the species' native range. 3.5 Permit, transportation and disease-free certification requirements Prior to an approval of an introduction, the proposed importer will be required to submit to the Protocol Committee copies of exportation/importation permits. The Protocol Committee will also check that the importer is following all local regulations. The importer will be required to have the organisms certified by an approved fish pathologist as being disease-free before they are introduced to the wild. We suggest that every practical measure be taken to prevent translocation of diseases. The exact methods depend on the species, life stage, point of origin, and use of the organism at the receiving site. 3.6 Report requirements Although the Protocol Committee will lack the authority to require that an environmental impact analysis be conducted following an introduction, it could strongly recommend to the local governmental agency(s) having such authority that this be done, and that the reports generated be made available to the Committee for their review. Thus, the Committee would be able to evaluate whether the protocol is effective in ensuring that exotic organisms are being wisely used. 4. PLANNING The Review and Decision model may facilitate planning. Private importers or public officials could use the model to identify the kinds of information that will be needed to evaluate a proposed introduction. Anticipation of the informational needs could also lead to more efficient literature searches and better-designed scientific research. In many cases, an entity desiring to make an introduction should be able to estimate the chances for approval prior to expending a great deal of time, effort and money. 5. CONCLUSION The proposed protocol is an effective mechanism for considering progressively more complex, and uncertain information to arrive at decisions to approve or reject proposals for introductions of aquatic organisms. The goal for adopting such a protocol should not be to eliminate or overly restrict such introductions, but rather to reduce the risk of an exotic becoming a pest. 6. REFERENCES Fusfeld, A.R. and R.N. Foster, The Delphi technique; survey and comment. 1971 Bus.Horizons, 14(6):63–74 Kohler, C.C. and J.G. Stanley, A suggested protocol for evaluating proposed exotic fish introductions in the United States. In Distribution, biology and management of exotic fishes, edited by W.R. Courtenay and J.R. Stauffer (in press) Table 1 Opinionnaire for appraisal of introductions of aquatic organisms. Each member of an evaluation board or panel of experts circles the number most nearly matching his/her opinion about the proability for the occurrence of the event. If information is unavallable or too uncertain: “don't know” is marked.

Response Variable Question No Unlikely Possibly Probably Yes Don't know VALID 1. Is the need valid and are no native species available that could serve the stated need? 1 2 3 4 5 X STATUS 2. Is the organism safe from over-exploitation in its native range? 1 2 3 4 5 X DISEASE 3. Are safeguards adequate to guard against importation of disease/parasites? 1 2 3 4 5 X ESCAPE 4. Would the introduction be limited to closed system? 1 2 3 4 5 X SUSTAIN 5. Would the organism be unable to establish a self-sustaining population in the range of habitats that would be available? 1 2 3 4 5 X IMPACT 6. Would the organism have only positive ecological impacts? 1 2 3 4 5 X HAZARD 7. Would all consequences of the introduction be beneficial to humans? 1 2 3 4 5 X SYNOPSIS 8. Is there a species synopsis and is it complete? 1 2 3 4 5 X DESIRED 9. Does data base indicate desirability for introduction? 1 2 3 4 5 X BENEFIT 10. Would benefits exceed risks? 1 2 3 4 5 X

Fig. 1 Revised Review and Decision Model for evaluating proposed introductions of aquatic organisms. Mean opinionnaire values (see text for discussion) are used at decision-making points EXOTIC FISH SPECIES ACCLIMATIZED IN HUNGARIAN NATURAL WATERS

J. Tóth Hungarian Danube Research Station Alsógöd, Hungary and P. Biró Biological Research Institute, Hungarian Academy of Sciences Tihany, Hungary

CONTENTS

1. INTRODUCTIONS INTO HUNGARIAN WATERS 2. REFERENCES ABSTRACT Since the turn of this century, altogether 18 fish species have improved the Hungarian Native fish fauna. Nine of them were systematically introduced into Hungarian waters, five species were introduced by chance, and only four elements immigrated from Ponto-Caspian or northern territories. Now about seven exotic species have more or less economic significance. None of them is endemic to the Carpathian Basin.

RESUME Depuis le début du siècle, 18 espèces de poisson ont amélioré la faune indigèene hongroise. Neuf d'entre elles ont été systématiquement introduites dans les eaux du pays, cinq l'ont été par hasard et quatre seulement ont immigré de la zone du Pont caspien ou des territoires septentrionaux. Environ sept espèces exotiques ont à présent une importance économique plus ou moins grande. Aucune des ces espèces n'est endémique du bassin des Carpathes. 1. INTRODUCTIONS INTO HUNGARIAN WATERS Hungary (93 000 km2) lies entirely within the River Danube catchment area of 817 000 km2. The first more or less planned introductions of exotic fish species into the catchment area took place early this century when bullhead (Ictalurus nebulosus Le Sueur), pumpkinseed (Lepomis gibbosus L.), largemouth bass (Micropterus salmoides Lacepede) and rainbow trout (Salmo gairdneri L.) appeared in rivers and lakes of the Hungarian reaches of the River water regime (Arvai, 1907; Vutskits, 1914; Unger, 1927; Hankó, 1932). After more than half of a century the status of these species is as follows: The pumpkinseed commonly occurs in all suitable waters in Hungary, although it is never excessively abundant. The bullhead has multiplied enormously and occurs in great quantities especially in the side arms of the Körös and Tisza Rivers, as well as in the side arms of the lower reaches of the Danube itself. In these waters it had altered the composition of native fish stocks but since 1970 the stock density of bullhead has decreased significantly. The occurrence of largemouth bass was relatively sporadic. Its occurrence was regularly reported from the River Drava and the side arm of the Danube, forming the border between Hungary and Yugoslavia. Some specimens also occur at the confluence of the Drava and Danube rivers or in adjacent reaches of the Danube. At the beginning of this century, this species was also recorded from Lake Balaton. Nowadays it can be found especially in those permanent ponds between Lake Balaton and the River Drava, which cannot be drained entirely. Here, the stocks maintain themselves with natural propagation. Presently, it is not actively introduced although in the late fifties some 10 000 specimens were bred in fish farms and stocked into the side arm of the Danube at Soroksár, which is utilized exclusively for sport fisheries as well as into the Baja-Beszdan canal close to Yugoslavia. No recaptures have been reported from these latest stockings. The rainbow trout was introduced into Hungarian waters because its temperature and oxygen requirement appeared more suitable for the stocking of Hungarian lowland and mountain waters than the native brook trout Salmo trutta m. fario L. The occurrence of rainbow trout has since always been connected basically with trout-breeding stations. Specimens inhabiting cold rivulets originate from these stations as due to its very rare natural spawning, there is no self-maintaining stock. In 1975, kamloops trout (S. gairdneri L.) and speckled trout (Salvelinus fontinalis Mitchill) were introduced into the Viszló stream flowing into Lake Balaton (Biró, 1981). Because the stockings made at the turn of this century resulted in bad experiences, new non-native fish species were not imported into Hungary for a long period. New stockings were only undertaken later when the Soviet and the Romanian scientific literature reviewed the results of introductions of phytophagous fish species from the Far East. These species were then imported into Hungary from China and from the bigger fish-breeding stations of the southern part of the U.S.S.R. during the early sixties. The grass carp (Ctenopharyngodon idella Valenciennes), silver carp (Hypophthalmichthys molitrix Valenciennes) and bighead (Aristichthys nobilis Richardson) became acclimatized exclusively in fish ponds in Hungary (Antalfi and Tölg, 1972). Their introduction into Hungary was preceded by the Romanian one and thus the first appearance of these phytophagous fish species in the Danube catchment area was in Romanian territory where most of the stockings were from fry imported directly from China. The first Hungarian stocking was also made with fry imported from China. Although later stockings were with homogenous fry of one species originating from Soviet fish-breeding stations, the original stocking material imported from China contained large numbers of fry of other species which were introduced by accident. These were, for example, Pseudorasbora parva Schlegel, Megalobrama terminalis Richardson, Parabramis pekinensis Basilewsky as well as Milopharyngodon piceus Richardson, the interested species. Of these species only Pseudorasbora parva now occurs abundantly in ecologically suitable Hungarian waters (Biró, 1972a; Toth, 1970). The other species have apparently not become established. In addition to the above species which were introduced either accidently or intentionally, other species have appeared in Hungarian waters by natural immigration. These are the three-spine stickleback (Gasterosteus aculeatus L.), the goby (Neogobius fluviatilis Pallas) and above all the crucian carp (Carassius auratus gibelio Block) (Holčik and Zitnan, 1978; Holčik, 1980). The threespine stickleback was recorded in relatively small numbers in the Hungarian Danube above Budapest (Botta et al., 1980). Neogobius fluviatilis was found exclusively in Lake Balaton (Biró, 1972) having arrived there by way of the River Danube and Sio Canal. Not one specimen, however, has been caught in the Hungarian section of the Danube River. In both these species a gradual invasion from the lower regions of the Danube is expected. The crucian carp appears in relatively great quantities in all suitable water bodies (Antalfi and Tölg, 1972; Holčik, 1980) but such mass appearances as have been observed in the Danube delta and in the Yugoslavian tributaries have not yet been recorded in Hungary. Because it achieves high densities of stunted fish through uncontrolled reproduction in certain fish ponds and thence gets into various natural waters of the country by migration or by transportation mixed with carp fry, it is highly probable that its stock density will eventually increase significantly in Hungary. In the mid fifties this species was experimentally introduced into a Hungarian lowland fishpond for commercial culture, thus its appearance in Hungarian waters is not only due to natural immigration from the lower Danube basin. Whitefish species have appeared temporarily in Hungarian waters through the natural connexions of water systesm. Coregonus lavaretus L. and C. peled Gmelin probably escaped from Czechoslovakian fishponds and only few specimens have appeared in Hungary during a short period of time. Vendace (Coregonus albula L.) was stocked in Lake Balaton to exploit offshore zooplankton during the fifties (Ribiánszky and Woynárovich, 1962) but was unable to withstand high summer temperature. The eel (Anguilla anguilla L.) is a native member of the Hungarian fish fauna albeit very rare, when its occurrence depended on natural migration. During the last two decades, however, since the mass introductions of juveniles caught in West European river estuaries into Lakes Neusiedl and Balaton and other waters with commercial aim, eels often escape and appear in various other Hungarian waters. Some of the species described can maintain themselves by natural spawning. These are bullhead (Ictalurus nebulosus), pumpkinseed (Lepomis gibbosus), largemouth bass (Micropterus salmoides), rainbow trout (Salmo gairdneri), crucian carp (Carassius auratus gibelio), Pseudorasbora parva, Neogobius fluviatilis and the three-spined stickleback (Gasterosteus aculeatus). There are no confirmed observations concerning the natural spawning of white amur (Ctenopharyngodon idella), silver carp (Hypophthalmichthys molitrix) and bighead (Aristichthys nobilis). The present frequency of occurrence of these species is primarily due to offspring which are produced in great numbers and stocked into fish farms in various parts of the country. Specimens escape from these ponds and establish themselves in most natural waters excepting Balaton, Velence and some other lakes. Their natural propagation under favourable weather conditions cannot be excluded however and it has been observed already on the River Tisza floodplain. Consequently, these species may be present continuously in the waters of the middle Danube basin. REFERENCES Antalfi, A. and I. Tölg, Növényevö halak. Budapest, Mezögázdasagi Kiadó, 202 p. 1972 Árvai, S., Amerikai jövevények. Halászat, 9:139–40 1907 Biró, P., Neogobius fluviatilis in Lake Balaton - a Ponto-Caspian goby new to the fauna 1972 of central Europe. J.Fish Biol., 4:249–55 , Pseudorasbora parva a Balatonban. Halászat, 18(65):37 1972a , A Balaton halallomanyanak strukturalis valtozasai. A Balaton kutatas ujabb 1981 eredmenyei. 2. VEAB Monogr., 16:239–76 Botta, I., K. Keresztessy and I. Neményi, Faunisztikai es akvarisztikai tapasztalatok az 1980 édesvizi akvárium üzembehelyézésevel kapcsolatban. Állattani Közlem, 67(1–4):33–42 Hankó, B., Ursprung und Verbreitung der Fischfauna Ungarns. Arch.f.Hydrobiol., 1932 23:520–56 Holčik, J. and R., Zitnan, On the expansion and origin of Carassius auratus in 1980 Czechoslovakia. Folia Zool., 27(3):279–88 Ribiánszky, M. and E. Woynárovich, Hal, halászat, halgazdaság. Budapest, 1962 Mezögazdasági Kiadó, 310 p. Tóth, J., Fish fauna list from the Hungarian section of the River Danube. 1970 Ann.Univ.Sci.Budap.Rolando Eötvös Nominatse, (Biol.), 12:277–80 Unger, E., Amerikai pisztrángsügér a Lunában. Halászat, 29:11–2 1927 Vutskits, Gy., Elszaporodott a naphal sé a pisztrángsügér a Drávában. Halászat, 1914 15:230–1 EIFAC TECHNICAL PAPERS ISSUED DOCUMENTS TECHNIQUES DE LA CECPI PUBLIES

EIFAC/T1 Water quality criteria for European freshwater fish. Report on finely divided solids and inland fisheries (1964) Critères de qualité des eaux pour les poissons d'eau douce européens. Rapport sur les solides finement divisés et les pêches intérieures (1964) EIFAC/T2 Fish diseases. Technical Notes submitted to EIFAC Third Session by Messrs. J. Heyl, H. Mann, C.J. Rasmussen and A. van der Struik (1965) Maladies des poissons. Notes présentées à la troisième session de la CECPI par J. Heyl, H. Mann, C.J. Rasmussen et A. van der Struik (1965) EIFAC/T3 Feeding in trout and salmon culture. Papers submitted to a Symposium, EIFAC Fourth Session (1967) Alimentation dans l'élevage de la truite et du saumon. Communications présentées à un symposium, quatrième session de la CECPI (1967) EIFAC/T4 Water quality criteria for European freshwater fish. Report on extreme pH values and inland fisheries (1968) Critères de qualité des eaux pour les poissons d'eau douce européens. Rapport sur les valeurs extrêmes du pH et les pêches intérieures (1968) EIFAC/T5 Organization of inland fisheries administration in Europe, by Jean-Louis Gaudet (1968) Organisation de l'administration des pêches intérieures en Europe, par Jean-Louis Gaudet (1968) EIFAC/T5(Rev.1) Organization of inland fisheries administration in Europe. Revised edition (1974) Organisation de l'administration des pêches en Europe (édition révisée) (1974) EIFAC/T6 Water quality criteria for European freshwater fish. Report on water temperature and inland fisheries based mainly on Slavonic literature (1968) Critères de qualité des eaux pour les poissons d'eau douce européens. Rapport sur la température de l'eau et les pêches intérieures basé essentiellement sur la documentation salve (1968) EIFAC/T7 Economic evaluation of inland sport fishing, by Ingemar Norling (1968) Evaluation économique de la pêche sportive dans les eaux continentales, par Ingemar Norling (1968) EIFAC/T8 Water quality criteria for European freshwater fish. List of literature on the effect of water temperature on fish (1969) Critères de qualité des eaux pour les poissons d'eau douce européens. Références bibliographiques sur les effets de la température de l'eau sur le poisson (1969) EIFAC/T9 New developments in carp and trout nutrition. Papers submitted to a Symposium, EIFAC Fifth Session (1969) Récents développements dans la nutrition de la carpe et de la truite. Communications présentées à un symposium, cinquième session de la CECPI (1969) EIFAC/T10 Comparative study of laws and regulations governing the international traffic in live fish and fish eggs, by F.B. Zenny, FAO Legislation Branch (1969) Etude comparée des mesures législatives et administratives régissant les échanges internationaux de poissons vivants et d'oeufs de poisson, par F.B. Zenny, Service de législation de la FAO (1969) EIFAC/T11 Water quality criteria for European freshwater fish. Report on ammonia and inland fisheries (1970) Critères de qualité des eaux pour les poissons d'eau douce européens. Rapport sur l'ammoniac et les pêches intérieures (1971) EIFAC/T12 Salmon and trout feeds and feeding (1971) CECPI/T12 Aliments du saumon et de la truite et leur distribution (1973) EIFAC/T13 Some considerations on the theory of age determination of fish from their scales - Finding proofs of reliability, by R. Sych (1971) EIFAC/T14 EIFAC consultation on eel fishing gear and techniques (1971) Consultation de la CECPI sur les engins et techniques de pêche à l'anguille (1971) EIFAC/T15 Water quality criteria for European freshwater fish. Report on monohydric phenols and inland fisheries (1972) CECPI/T15 Critères de qualité des eaux pour les poissons d'eau douce européens: rapport sur les phénols monohydratés et les pêches intérieures (1973) EIFAC/T16 Symposium on the nature and extent of water pollution problems affecting inland fisheries in Europe. Synthesis of national reports (1972) Symposium sur la nature et l'étendue des problèmes de pollution des eaux affectant les pêches continentales en Europe. Synthèse des rapports nationaux (1972) EIFAC/T17 Symposium on the major communicable fish diseases in Europe and their control. Report (1972) CECPI/T17 Rapport du symposium sur les principales maladies transmissibles des poissons en Europe et la lutte contre celles-ci (1973) EIFAC/T17 Suppl.1 The major communicable fish diseases of Europe and North America. A review of national and international measures for their control, by P.E. Thompson, W.A. Dill and G. Moore (1973) CECPI/T17 Suppl.1 Les principales maladies transmissibles des poissons en Europe et en Amérique du Nord: examen de mesures nationales et internationales sur la lutte contre ces maladies, par P.E. Thompson, W.A. Dill et G. Moore (1973) EIFAC/T17 Suppl.2 Symposium on the major communicable fish diseases in Europe and their control. Panel reviews and relevant papers (1973) CECPI/T17 Suppl.2 Symposium sur les principales maladies transmissibles des poissons en Europe et la lutte contre celles-ci: exposés des groupes et communications apparentées (1973) EIFAC/T18 The role of administrative action as a tool in water pollution control, by G.K. Moore (1973) CECPI/T18 Le rôle instrumental de l'administration dans la lutte contre la pollution des eaux, par G.K. Moore (1973) EIFAC/T19 Water quality criteria for European freshwater fish. Report on dissolved oxygen and inland fisheries (1973) CECPI/T19 Critères de qualité des eaux pour les poissons d'eau douce européens. Rapport sur l'oxygène dissous et les pêches intérieures (1973) EIFAC/T20 Water quality criteria for European freshwater fish. Report on chlorine and freshwater fish (1973) CECPI/T20 Critères de qualité des eaux pour les poissons d'eau douce européens. Rapport sur le chlore et les poissons d'eau douce (1973) EIFAC/T21 Water quality criteria for European freshwater fish. Report on zinc and freshwater fish (1973) CECPI/T21 Critères de qualité des eaux pour les poissons d'eau douce européens. Rapport sur le zinc et les poissons d'eau douce (1973) EIFAC/T22 Ecological diagnosis in salmonid streams - Method and Example, by R. Cuinat et al. (1973) CECPI/T22 Diagnose écologique en cours d'eau à salmonidés. Méthode et exemple, par R. Cuinat et al. (1975) EIFAC/T23 Report on the Symposium on methodology for the survey, monitoring and appraisal of fishery resources in lakes and large rivers (1974) Rapport du Symposium sur les méthodes de prospection, de surveillance et d'évaluation des ressources ichtyologiques dans les lacs et grands cours d'eau (1974) EIFAC/T23 Suppl.1 Symposium on the methodology for the survey, monitoring and appraisal of fishery resources in lakes and large rivers - Panel reviews and relevant papers. Vol. I and II (1975) CECPI/T23 Suppl.1 Symposium sur les méthodes de prospection, de surveillance et d'évaluation des ressources ichtyologiques dans les lacs et grands cours d'eau - Exposés des groupes et communications apparentées, Vol. I et II (1975) EIFAC/T24 Report on fish toxicity testing procedures (1975) Rapport sur les tests de toxicité sur les poissons (1976) EIFAC/T24 (Rev.1) Revised report on fish toxicity testing procedures (1982) CECPI/T24 (Rév.1) Rapport révisé sur les tests de toxicité sur les poissons (1983) EIFAC/T25 Workshop on controlled reproduction of cultivated fishes - Report and relevant papers (1975) Réunion sur la production contrôlée des poissons d'élevage. Rapport et communications apparentées (1975) EIFAC/T26 Economic evaluation of sport and commercial fisheries. Report and technical papers (1977) Deuxième consultation européenne sur l'évaluation économique de la pêche sportive et commerciale. Rapport et communications apparentées (1977) EIFAC/T27 Water quality criteria for European freshwater fish. Report on copper and freshwater fish (1976) Critères de qualité des eaux pour les poissons d'eau douce européens. Rapport sur le cuivre et les poissons d'eau douce (1976) EIFAC/T28 Joint ICES/EIFAC Symposium on eel research and management (Anguilla spp.). Report (1976) Symposium conjoint CIEM/CECPI sur la recherche et l'exploitation des anguilles (Anguilla spp.). Rapport (1976) EIFAC/T29 Water quality criteria for European freshwater fish. Report on the effect of zinc and copper pollution on the salmonid fisheries in a river and lake system in central Norway (1977) Critères de qualité des eaux pour les poissons d'eau douce européens. Rapport sur l'effet de la pollution par le zinc et le cuivre sur les pêcheries de salmonidés dans un système fluvio-lacustre du centre de la Norvège (1977) EIFAC/T30 Water quality criteria for European freshwater fish. Report on cadmium and freshwater fish (1977) Critères de qualité des eaux pour les poissons d'eau douce européens. Rapport sur le cadmium et les poissons d'eau douce (1977) EIFAC/T31 Report of the Symposium on Finfish Nutrition and Feed Technology (1978) CECPI/T31 Rapport du symposium sur la nutrition des poissons et la technologie de leurs aliments artificiels (1978) EIFAC/T32 The value and limitations of various approaches to the monitoring of water quality for freshwater fish (1978) CECPI/T32 La valeur et les limites des diverses méthodes de surveillance biologique de la qualité des eaux pour les poissons d'eau douce (1978) EIFAC/T33 Guidelines for sampling fish in freshwater (1980) EIFAC/T34 EIFAC fishing gear intercalibration experiments (1979) CECPI/T34 Essais CECPI d'interétalonnage des engins de pêche (1979) EIFAC/T35 Report of the EIFAC workshop on mass rearing of fry and fingerlings of freshwater fishes (1979) CECPI/T35 Rapport du stage CECPI sur la production massive du frai et des alevins en eau douce (1979) EIFAC/T35 Suppl.1 EIFAC Workshop on mass rearing of fry and fingerlings of freshwater fishes Papers (1979) EIFAC/T36 Report of the EIFAC/IUNS and ICES working group on standardization of methodology in fish nutrition research (1980) CECPI/T36 Rapport du groupe de travail de la CECPI, de l'UISN et du CIEM sur la normalisation de la méthodologie dans la recherche sur la nutrition des poissons (1980) EIFAC/T37 Report on combined effects on freshwater fish and other aquatic life of mixtures of toxicants in water (1980) CECPI/T37 Rapport sur les effects produits par la combinaison de toxiques dans l'eau sur les poissons d'eau douce et sur d'autres formes de vie aquatique (1981) EIFAC/T38 Report of the technical consultation on the allocation of fishery resources (1981) CECPI/T38 Rapport de la Consultation technique sur la répartition des resources ichtyologiques (1981) EIFAC/T39 Utilization of heated effluents and recirculation systems for intensive aquaculture (1981) CECPI/T39 Rapport du Symposium sur les récents développements de l'utilisation des eaux réchauffées et des eaux recyclées en aquaculture intensive (1981) EIFAC/T40 Problems of fish culture economics with special reference to carp culture in eastern Europe, by M. Leopold (1981) EIFAC/T41 Report of the EIFAC Workshop on fish-farm effluents, by John S. Alabaster (1982) EIFAC/T42 Report of the Symposium on stock enhancement in the management of freshwater fisheries (1982) CECPI/T42 Rapport du Symposium sur l'amélioration des stocks dans le cadre de l'aménagement des pêcheries d'eau douce (1983) EIFAC/CECPI/T42(Suppl.) Documents presented at the Symposium on stock enhancement in the management of freshwater fisheries, Volume 1: Stocking, Volume 2: Introductions and Transplantations (1984) Documents presentés au Symposium sur l'amélioration des stocks dans de cadre de l'aménagement des pêcheries d'eau douce, Volume 1: Repeuplement, Volume 2: Introductions et transplantations (1984) EIFAC/T43 Water quality criteria for European freshwater fish. Report on chromium and freshwater fish (1983) CECPI/T43 Critères de qualité des eaux pour les poissons d'eau douce européens. Rapport sur le chrome et les poissons d'eau douce (1983)