__________,Ea- Numerical ecology Coastal nursery Fish community Spatio-temporal variability Ecological guild Pleuronectiform juveniles Écologie numérique in the structure Nourricerie littorale Peuplement ichthyologique Guilde écologique of a nectobenthic fish nursery: Pleuronectiformes juvéniles a descriptive study Patrick SAFRAN Laboratoire d'Écologie Numérique-SN3, Station Marine de Wimereux, Université des Sciences et Techniques de Lille, 59655 Villeneuve-d' Ascq Cedex, France. Present address: Department of Fishery Sciences, Laboratory of Marine Living Resources, faculty of Agriculture, Tohoku University, 1-1 Tsutsumidori Amamiyama­ chi, Sendai 981, Japan. Received 16/ll/88, in revised form 717/89, accepted 13/7/89. ABSTRACT When the by-catch of an artisanal brown shrimp fishery in the Canche bay demonstra­ ted the existence of an important nursery of nectobenthic fishes, a survey of this nursery was performed along the coast of the Pas-de-Calais (Eastern Manche) from January 1985 to June 1986. Tne samples were obtained from 210 hauls of an exper­ imental trawl containing 51 600 juveniles of 37 species. Complementary multivariate statistical methods, such as diversity index, rank-frequency diagrams, particularly the smoothed rank-frequency diagram have proved to be powerful tools in assessing the relationships between the populations of different species in a community. Qualitative and quantitative similarity, cluster analysis and correspondence analysis showed that: 1) the fish community in this area was qualitatively, quantitatively and structurally homogeneous; 2) this single, "multispecific" community was maintained by a seasonal flow of individuals, which passed through the nursery and induced a turnover of the dominant populations, without any modification to the general faunistical pattern (structure); 3) Dab (Limanda limanda, L.) constituted the predominant population. Three other important populations were: dragonet (Callionymus lyra, L.); sand goby (Pomatoschistus minutus, P.); and plaice (Pleuronectes platessa, L.). Less important species were: sole (Solea so/ea, L.); sprat (Sprattus sprattus, L.); hook-nose (Agonus cataphractus, L.); and poor cod (Trisopterus minutus, L.); 4) The three pleuronecti­ forms, dab, plaice and sole, formed a structural guild, referred to here as the LPS guild (L. for Limanda, P. for Pleuronectes and S. for Solea), within the nursery community. Within the LPS guild, seasonal succession showed autumnal and hibernal dab populations (1986); hibernal (1985) and coastal estival plaice; and a printempal sole. Their various growth-rates (plaice > dab >sole) induced the cohabitation of indivi­ duals of various size classes, without obvious exclusion of any of the three species. The distribution of individuals in space and time, which was the result of a regular flow of individuals passing into the nursery, replenishing the stocks with juveniles, maintained this cohabitation. Oceano/ogica Acta, 1990. 13, 1, 97-106. RÉSUMÉ Variabilité spatio-temporelle de la structure d'une nourncene de pois­ sons nectobenthiques: une étude descriptive Suite à une étude de la pêche accessoire dans la pêcherie artisanale de crevettes grises en baie de Canche, qui a démontré l'existence d'une importante nourricerie littorale de poissons nectobenthiques, une étude écologique de cette dernière a été effectuée de janvier 1985 à juin 1986 à partir d'un échantillonnage comprenant 210 traits de chalut, répartis le long du Pas-de-Calais, et qui ont permis la collecte d'environ 51600 individus juvéniles de 37 espèces. Des méthodes statistiques multivariables complémen­ taires les unes des autres : diversité spécifique, diagramme rang-fréquence, similarités qualitatives et quantitatives, analyses factorielles des correspondances et classifications 0399-1784/90/01 97 10/$ 3.00/© Gauthier-Villars 97 .., P. SAFRAN numériques représentées par des dendrogrammes, ont permis de démontrer: 1) une homogénéité faunistique qualitative, quantitative et structurelle dans cette zone; 2) l'existence d'un peuplement unique, réparti sur l'ensemble de la zone d'étude, soumis à un flux saisonnier d'individus qui traversait en permanence la nourricerie et entraînait le renouvellement des populations structurantes, sans modification de la composition faunistique générale; 3) La limande (Limanda limanda, L.) représentait la population dominante de ce peuplement. Trois autres populations étaient structurantes : celles du callionyme (Callionymus lyra, L.), du gobie (Pomatoschistus minutus, P.) et de la plie (Pleuronectes platessa, L.), accompagnées par celles de la sole (Solea solea, L.), du sprat (Sprattus sprattus, L.), de la souris (Agonus cataphractus, L.) et du petit tacaud (Trisopterus minutus, L.). Parmi ces espèces, les Pleuronectiformes (Limande, Plie et Sole) representaient une guilde structurante (nommée ci-après guilde LPS) du peuplement; 4) Au sein de la guilde LPS, les remplacements saisonniers successifs faisaient apparaître une limande automnale et hivernale (1986), une plie hivernale (1985) et estivale collectée principalement à la côte, et une sole printanière, Leurs vitesses de croissance différentes (plie>limande>sole) semblaient induire une cohabit­ ation d'individus de toutes tailles sans entraîner d'exclusion. Oceanologica Acta, 1990. 13, 1, 97-106. INTRODUCTION It has been established that the coastal zone is an area of reproduction, egg-laying, growth and nursery target-species without taking into consideration the grounds for many species of offshore fishes (Peres, fluctuations of the others. This reflects the realization 1972; 1976; Bou gis et al., 1976; Margalef, 1977; 1983; that single-species population dynamic models are Ross, 1977; Lackey and Nielsen, 1980). The factors insufficient for fisheries management purposes; multi­ influencing migrations and temporary concentrations species interactions must be considered, as well as the have yet to be determined. Fishery studies tend to focus impact on stocks caused by fishing, etc. on commercial adult populations, and knowledge of The work presented here documents, for the first time factors that influence the juvenile populations is often in this area, the fish community of a coastal multispe­ poor (Weinstein, 1982; Alderdice, 1985). Recruitement cific nursery, and concludes that there is one single to adult stocks depends on the events occuring during community. Multivariate statistical methods are used the juvenile growth period. Nurseries ensure the perma­ to analyse the multispecific samples obtained by this nence of populations, but at the same time, owing to survey. their geographical localization, their limited area and Moreover, special attention is given to dab, plaice and the possible competition between species, they consti­ sole. These three flatfish populations are studied here tute a limitation, and most probably a regulatory mech­ in more depth for two main reasons. Firstly, they anism for stocks. are of primary economie importance, constituting pre­ Only a few ecological studies are performed on recruitment stages of stocks exploited by an artisanal nurseries; these are generally focused on commercial fishery (off Boulogne-sur-mer and Etaples, Pas-de­ species, as for example, herring ( Clupea harengus L. : Calais, France). Secondly, they are the most numerous Hodgson, 1957; Burd, 1962; Cushing, 1968), plaice species in this survey. Dab is the main species and (Pleuronectes platessa L.: Gross, 1947; Beverton and plaice is a dominant one of the single coastal nursery Holt, 1957; Dawson and Grimm, 1980), or managed community sampled along the Pas-de-Calais. Another in restricted areas such as estuaries, hays and lagoons important species is sole, which constitutes an escort (Marchand et al., 1983; Labourg et al., 1985). The population, while dab and plaice are structural (in study area in the present work is located in the open terms of data analysis). These three species belong to sea, along the coastal zone of the Pas-de-Calais in the the same family Pleuronectidae. They may have similar Eastern Manche (English Channel), a zone harvested ecological requirements and foraging strategies (Barrett by an artisanal brown shrimp (Crangon crangon, L.) and Yonge, 1958; Wheeler, 1978; Bauchot and Pras, fishery. The study of the bycatch of this shrimp fishery 1980; Muus and Dahlstrom, 1981; Quero, 1984), and in Canche Bay demonstrates the existence of an import­ so fit the definition of an ecological guild (Lincoln et ant nursery of exploited flatfishes (Safran, 1984; 1985; al., 1982; Barbault, 1983), which is referred to here as 1987 a) including plaice, dab (Limanda limanda, L.) and the LPS guild (L. for Limanda, P. for Pleuronectes and sole (Solea solea, L.). Many other species, whether S. for Solea; in decreasing order of importance). exploited or not, occur together with these flatfishes: Concentration on the three structural species of the the area is a multispecific nursery. lt is therefore clear gui1d provides a rouch better understanding of the that spatio-temporal variations, abundance fluctu­ general results, and specifies cohort succession and ations, and cohort successions of sampled populations growth during the period the fishes spend in the "foster inside this coastal stretch cannot be analysed for one mother" zone. 98 VARIABILITY AND STRUCTURE IN A FISH NURSERY MATERIALS AND METHODS Study area The sampling area was located along the French "Opal Coast" (Pas-de-Calais, Eastern Manche), between the southern mouth of Canche Bay and the northern