SYNOPSIS of BIOLOGICAL DATA on the EEL Anguilla Anguilla (Linnaeus, 1758)

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FAO Fisheries Synopsis No. 80, Revision i FIRI/S80(Rev. 1) SAST - Anguilla anguilla - 1,43(02),002,01

SYNOPSIS OF BIOLOGICAL DATA ON THE EEL Anguilla anguilla (Linnaeus, 1758)

Prepared by C.L. Dellder

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"7- -

FOOD AND AGRICULTURE ORGAMZATION OF THE UNITED NATIONS FIRI/S80(Rev. 1) FAO Fisheries Synopsis No. BO. Revision I SAST - Anguilla anguilla - l,43(02),OO2,O1

SYNOPSIS OF BIOLOGICAL DATA ON THE EEL

Anguilla anguilla (Linnaeus, 1758)

prepared by

C.L. Deelder Netherlands Institute for Fishery Development Haringkade i NL 1970 AB Ijmuiden The Netherlands

FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS Rome 198G 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 oftheUnitedNations concerning thelegal status of any country, territory, city or area or ofts authorities, or concerning the delimitation of its frontiers or boundaries.

M-40 ISBN 92-5-102166-X

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FAO 1984 FIRI/S8O Anguilla anguiI]a (Rev.l) iii

PREPARATION OF THIS SYNOPSIS

The first edition of this document, published in 1970, was prepared at the request of the European Inland Fisheries Advisory Commission. It proved to be one of the most popular FAO species synopsis and has been out of print since many years. As the author has been able to gather sub- stantial new information on the species in the course of the last few years, it was decided this year to produce a revised version rather than a simple rerun of the original document. In order to avoid further delays in its publication, it appeared practical to simply insert the new information provided by the author in the original text, rather than retype the entire document, even if this procedure involved some shortcomings in the presentation of the publication.

ABSTRACT

This synopsis compiles and reviews the scattered information on the identity, distribution, life history, populations, exploitation, management and culture of the eel, Anguilla anguilla (Linnaeus, 1758), a species that has been harvested in Europe for many centuries.

For bibliographic purposes this document should Distributinn: be cited as follows: FAO Fisheries Department Deelder, C.L., Synopsis of biological data on FAO Regional Fisheries Officers 1984 the eel, Anguilla anguilla (Linnaeus, Regional Fisheries Councils 1758). FAO Fish.Snop.. (BO)Rev.l: and Commi.qsions Selector SM and SI 73 p. Author pmi/seo A.npuilla a illa (Rev.1) y

CONTENTS

IDENTITY

1,1 Nomenclature

1,11 Valid name i

1.12 Objeotivo synonymy i

1.2 i

1,21 Affinities i 1,22 Taxonomic status 5 1.23 Subspecies 5 1.24 Standard oommon names, vernacular names 5 1,3 5

1.31 External morphology 5 1.32 Cytomorphology 8 1.33 Protein specificity 8

2 DISTRIBUTION 9

2,1 Total area 9

2,2 Differential distribution 9

2,21 Spawn, larvae arid juveniles 9 2,22 Adults 9

2.3 Determinants of distribution ohanes 9

2,4 Hybridization 9

2.41 Hybrid.a* 2,42 Influence of natural hybridization in ecology and morphology*

3 BIONOMICS AND LIFE HISTORY lo

3.1 Reproduction lo

3,11 Sexuality 10 3.12 Maturity 10 3.13 Matins 11 3,14 Fertilization il 3,15 Gonads ii 3.16 Spawning ii 3,17 Spawn 11

3.2 Proadult phase 11

3.21 Embryonic phase 11 3.22 Larval phaco 11 3023 A'Jolec cent phase 12

3.3 Adult phase 12

3.31 Longevity 12 3.32 Hardiness 12 3.33 Competitors 12 3,34 Pvedators 13 3.33?araiter, ioeaoem, injurico and abnormalities 13 vi FIRI/S80 Anguilla anguilla (Rev.l)

Page No. 3.4Nutrition and growth 14 3.41 Feeding 14 3.42Food 15 3.43 Orowth rate 16 3.44Metabolism 20 3.5 Behaviour 21 3.51 Migrations and local movements 21 3.52Schooling 49 3.53Responses to stimuli 49 4 POPULkTION 51 4.1 Structure 51 4.11Soz ratio 51 4.12 Ae composition 51 4.13 Size composition 51 4.2Abundance and density 51 4.21 Average abundsnoo* 4.22Changec in abundance' 4.23 Aver o denoity 443Changoc in donoityt 4.3 r U1y and reoruitmnt 51 4.31 Reproduotion rateefl 4.32Bactorn affecting roproduotion' 4.33Rocruitment* 4,4Mort4and morbidity 51 4.41 Mortality rates'. 4.42Factors causing or affecting mortality' 4,43Factors affecting morbidity' 4.44Relation of morbidity to mortality rates'

4.5 Dynamics of population 51 4.6 The population in the community and the ecosystem 51

5 EXPLOITATION 52

7.1 Fishing equiploent 52 52 5.11 Gears 5.12 Boats 53

5.2 Fishing areas 53 5.21 General geographic distribution' 5.22Geographic ranges' 5.23 Depth ranges' 5.24Conditions of the grounds' 5.3Fishing seasons' FIRI/580Anguilla anguilla (Rev.1) vii

Page No. 5.4 Fishing operations and results 53 5.41Effort and intensity 53 5.42 Selectivity 5.43 Catches

6 PROTECTION AND MANAGEMENT 57

6,1 Regulatory (legislative) measures 6.11 Limitation or reduction of total catch 6.12 Protection of portions of populations 57 6.2 Control or alteration of physical features of the environzoent*

6.3 Control or alteration of chemical features of the environment* 6.4Control or alteration of the bioloical features of the environment*

6. Artificial stookin 57

6.51 Maintenance stocking* 6.52Transplantation; introduotion*

7 EEL CULTURE 58 7.1 Procurement of stocks 58 7.2 Genetic seleotion of stocks 58 58 7.3 pawni 58 7.4 Holding of stock 58 7.5 Management 58 7,6Foods and feeding 58 7.7Disease and parasite control- 58 7,8Harvest 58 7.9Transport 8 REFERENCES 60

As no information was available to the author, these items have been omitted from the text. FIRI/580An lila angiiiia(Rev.1)

i IDE'PITY Tongue preconi,.Lape thaok,Fronial bones paired, not ownogoher,Pelatorterygoida 101 Nomenclature well-developed, ftomalllzir.esot developed ec die inoelemento in adulte, Pectoral 1.11Valid name girdle with7to9(woo 11 in the young) radia]. elemente, Cauilal vertebrae dthout A i].lu anguilla (Linxiaouc,1758) transverse processes. o 1uraena anrilla Linnaeus 1758, Syct, Nat,Ed,10 s 245), Specific

1.12Objective synonymy lia La (Linnaeus,1758), (Fig, 1 There are no junior objective synonyms of the name. Swainson(1839)was the first to use the combination Anguilla antuilla. 1,2Taxonomy Linnaeuc did not deeignate any typo 1,21Affinities maerial, but a speoimem named by him le in the British Museum (Natural History), Londori - Suprenerio Linnacus Collection, No,80,

Phylum Vertebrata Typo localitysEurope,Linnaeus(1758) Subphylum Craniata (see below) referred to the abundance of tho Superclass Onathostomata epsolco in Leko Comacchio, noaFerrera, Italy, Series Pisces but this cannot be regarded as restricting the Class Teleostomi typo lOOClito Subolans Aotinopterygii Order Anguilliformos Definition of Muraena anguilla by Linnaous SuborderFily tillid Aiilloidoi (1758) s guilleo.1, mxilla inferiore longiore, Generic corpore unicolore. D, 1000,P. 19 V. O.A. 100,C, -. Genus An'uilla Shaw, 1803, Oem, Zool, Art, spec, 66, gori, 24, syn,39 Fn, Svec, 290. (Paccon),4TT)T57'Typo epcooe by monotypys uranna unicolor, manilla inferiore longioro. uU1a vulgarie Shaw, 1803 (a oubjoetivo Habitat ici Europa;sne:ima ii le.cu Comacchio oynonym of Mtaraona anguilla Lin aosxe,1758), Forrorionoa non fert Danubaum, Genders feminine, Nocturna;latet in coono duplioi forammo; corcetur trunco albo Betulas;cutis tenaois- "Head smooth. Nostrils tubular, Eyoc simio parat vivapera, sub cenacula. covered by the common slzin, Gill.membrano ten-rayed, Bod.y roundireh, amooth, mucous, Body nearly cylindrical, slightly compressed, Dorcsal, caudal, and anal f.ns united. especially near caudal end. Tail end rounded, Spiraoles behind the head or pectoral fins." Length 40 cm toiin or more, Head compressed. (Shaw, 1803). Snout rather elongated, sometimes broad, de- pressed, Jaws and vomer provided with a As the family Anguiliidae consists of series of teeth. Mouth-slit oxtonding to only one genus, the family characters may be about level of middle of eye, Front nostril considered as generic characters too, The tubular near tip of snout;hind-nostril open- following (family.-) concept is oxpressed by ing in front cf the upper half of eye, rg(1949)s Eye round with yellowish iris,diamoterY8to Y12 length of head;about half pro-orbital Body elongate, snake-like.Dorsal and distance, External branchial aperture about anal fina confluent with the rudimentary caud- as long as eye-diameter. al fin, Pectoral fine proBent, ventrale ab- Pectoral with 14 to18fin-rays,Anterior end sent, Body covered with minute scales, of dorsal fin well behind head, in front of Lateral line well.-devoloped. Vent remote anal opening, at about 2/5 total length, from the head, Mouth terminai;jaws not Dorsal and. anal fins becoming higher near tail, particularly elongate, Teeth small, peoti- where thoy fors one rounded fin. Lateral line nate or setiform, in several series on the quite clistanot, Small elliptical scales em- jaws and the vomer. Minute teeth on the bedded in skin, Colour variable;dorsal sido pharyngeal bones, forming an ovate patch on varying from greyioh-brown, olive-brown or the upper pharyngoale. Gill openings lateral yellowish, to black;ventral eide yellowish. vertical, quite well developed, well separated FinB yellcwioh, dorsal fin darker, from each other, Inner gill slits vide, The specimens approaching sexual maturity ('Anguilla Klonen' Kaup, 'ABibroni' Kaup, FIRI/S80 An uf lia an uilia Revi

Fic, 1. Eel, after C. Geener (1575). (By courtesy of Dr. SJ, de Groot).

etc.) have their eyes enormously enlarged (dia.- Subjective synonyms meter much greater than pre-orbital distance), dorsal side black, ventral side white with Muraena anguilla Linnaeus,1758; Anguilla silvery reflections (silver eels), pectorals vulgaria Shaw,1803; placed in synonymy of dull, dark, even black, Anguilla aouti,roetrje Rieso,1826,by Yarrel]. Vertebral numbers 112 to 117, of which45to (1836). Reasons not disoussed. 46are pre-anal. Leptocephalus larva (Lepto. cephalus brovrostris Kaup), attaining88mm, Anguilla cloacina Bonaparte,1846; uilla shaped like an olive leaf, Beginning of dor.. platyrhynchus Coste.!!'; placed in eynonym of sai fin always in front of anus; pectorals A illa latirostrie Risso,1826,by Yarrell well developed; caudal fin deep; last hypural 1.49 Reasons not discussed, large, with5rays; last but one slit, with4 rays; last but three with 1 ray. Muraeria ozyrhina Ekstrthn, 1835; Anguilla acutirostris Risse,1826; piaoed in synonymy (translated from d'Ancona,1938) of AL illa migratoria Kryer,1846-1849,by Kröyer 16-49), Reasons disoussed. Diagnoeis2 Vert. 111 to119,normally 114 to 116; D245to275; A176to249;C 7to12; Anguilla latiroetris Risso,1826; Anguilla P 15to21. (Berg et al,,1949). obtusirostris Risso.i./; placed in synonymy of Anguilla fluviatilis Anelijn,1828,by Heckol Prehaen, vert. 44 to 47; vert. 110 to119; and Kner(1858). Reasons not discussed. R,br.9to13; P. 15to 21, (Ege,1939).

Vert, 112 to117; D 245 to275; A205 to IJDate of publication not known. 235. (1beeler,1969). .a anuìlla(Rev.1) 3

Anguilla fluviatilis Anslijn, 1828; length without head, in percent of Anguilla mediorostris Rieso, 1826;Anguilla total length, ca. 27.0 to os, 27.2. ratoyia Kryer, 146-1849;placed in synonymy of Anguilla vulgaris Shaw, 1803, by Von * Average maximum valuo of distance Siebold (1863), Reasons not discussed. between verticals through anus and origin of dorsal fin, in percent of Muraena platyrhina Ekstrftn, 1835;placed total length ca. 9.0 ...... in synonymy of A .lila vulgaris Shaw, 1803, A. celebesenais KAUF. by Van Bemmelen 1. Reasons disoussed. *0 Average maximum value of distance Anguilla canariensis Valenciennes, 1836- between verticals through anus and 44;Anguilla Callensis Guichenot, 1850; origin of dorsal fin, in percent of Anguilla Cuvieri Kaup, 1856;Anguilla Bibronii total length ca. 13.0 ...... Kaup, 1856;Anguilla Savignyi Kaup, 1856; A. interioris WHITLEY. Anguilla morena Kup, 1856;Anguilla marginata Kaup, 1856;Anguilla microptera Kaup, 1856; B. Number of prehaemal vertebrae 40 to An«uilla altirostris Kaup, 1856;Anguilla 44, total number of vertebrae 108 to platycehala Kaup, 1856;Anguilla nhlotica 116. Average maximum value of the Kaup, 1856;Anguilla aej.tiaca Kaup, 1856; preanal length without head, in per Anguilla hibernica Couch, 1.5;placed in syn- cent of total length, ca. 25,9 onymy of Arilla vulgaris Shaw, 1803, by A. oma KAUF. Unther (ff70). Reasons not discussed. II.The average breadth of the intermaxillary-. A ills anguilla Swainson, 1839; vomerine band of teeth, measured in the An illa Linni ?lalm, 1877;plaoed in synonymy middle, greater than or equal to the great- o Anguilla vularie Shaw, 1803, by Day (1884). est breadth of the maxillary bands. Reasons not 6.isoussed. A. Skin with variegated markings. Anguilla yo.a]a de la Pylaie, .1834; placed in synonymy of Anguilla vulgaris Shaw, * The three rows of teeth forming the by Moreau (1881), Reasons not discussed. main part of the maxillary bands are regular, longitudinal groove distinct Anguilla ICieneri Kaup, 1856;placed in without interruptions;a projection synonymy of Anguilla vuigaris Shaw, 1803, on the inner side of the bands ante- by Pries, Ekstrtim and Sundevall (1895). Rea- riorly. Primary pigment of the eons not disoussed.. elvers forming a distinct mediolateral streak on the end of the tail, Le,tooephalua brevirostria Kaup, 1856; (Dentition not knom in A. ancestralis, placed in synonymy of Anguilla vulgaris Shaw, only represented by elvers). 1803, by Grassi and Caland.ruccio (1897).Rea- SOns discussed. 1.Number of prehaomal vertebrae 37 40 ...... 4. A. ancestralis EGE. Anguilla septembrina Bonaparte, 1846; According to Willi.amson (1974), this Anguilla oblongirostris Bianchard., 1880; Anguilla ourystoma Heckel and Kner, 1858; shoutd be: A. celebensis juv. placed in synonymy of Anguilla vulgaris Shaw, 2,Number of prehaemal vertebrae 39 Reasons discussed, to 43. 1803, by Schmidt (1914v Average maximum valuo of distance Anguilla rostrata (Le Sueur);placed in between verticals through anus and synonymy of Anguilla anguilla (Linnaeus, 1758) origin of dorsal fin, in percent by Tucker (1959). Reasons discussed. Not of total length, ca. 11.7 to ca. -accepted as a synonym by present author (cf.1.31), 11.9 ...... A. nebulosa McCLELLAÎD. x Total number of vertebrae 106 to 112 ...... 5a. A.nebulosa nebulosa For a comprehensive list of synonym-authors, MoCLELLPJD. see:Hureau and Monod (1973). xx Total number of vertebrae 107 to 115. .,... 5h. A.nebulosa labiata PETERS. Key to the species of Anguilla Shaw, 1803. Average maximum value of distance (after Ego, 1939). between verticals through anus I. The average breadth of the intermaxillary- and origin of dorsal fin, in per vomerine band of teeth, measured in the cent of total length, ca. 16.3 middle, amounts to little more than half 6. A. marrnorata QUOY & GAIMARD. the greatest breadth of the maxillary 0* The three rows of teeth in the maul- bands. lary bands irregular, longitudinal A. Number uf prehaemal vertebrae 38 to 42, groove less distinct with interruptions; total number of vertebrae 101 to 107. no projection of the inner side of the Average maximum value of the preanal FIRI/S80 Anguilla anguilla (Rev.i)

bands, The dyers without stre&. of CS Average maximum value of distanoe be-. pr!niaxrp i.odiole.tcval pigment on end tween verticale through anuo and ori1n of '.aib ,, ....7.A. reinbard,ti of doradfin, in percent of Cotai length, ca. 0.2 to on,3.6.

B, Skin without variegated markings, 1. Constriction of theintormaxillary- voniarino bund of tooth begino on * .Leurd value of dietance be- an average before the middle of the teen vo&!oa]Ø throu1i aiuc anct oragin band. Averde maximum values of of ciorsai. i'in in percent of ì:otal preanal length without head, in løugthOao 91O O. 14.6. peroent of total langh, ca, 27.0 to ca. 28,2. Number of prohaerncil 1. Average aaimuu vlue oflengthof vertebran 40 to 45, gapo in peroorn of he'.d length, a, Average maimure value of length of ca. 32 o oa.36. inermazillary-vomer1no band of a, ria.Ulary bando of tooh 7Lth teeth, n percent of diotanoo from longiud1na] roove, Average maz- front margin of that band Cc poe- imuni valuc of pree.nnl length, tenor end of right maxillary band., 71thou head,, in percent of total cc. 82 to cc.86, Average maximum length, ? ca. 27 8. value of distance 'OeC'tTQOnI 'iertioaio A. borneonsis POPTA. .hrough anus and origin of doron]. b. Maxillary bands of teeth without fin, in paroeni of total length, longitudinal groove, Aver.xe max- ca. 02 to on, 0,8 ....o. A. bicolor imum value of preanal length with-- MoCLELLAND, out head, in percent of total z Total number of vertebrae 103 to length,0e,29. 111 ....,. 14 a. A. bicolor nacifica x Total number of vertebrae loo to SCHMIDT. 106, Average maximum value of xx Total numberofvertobrac106to 115 distance betsen vobale through 14 b. J, bicolor bicolor floCLELLtND, anus and origin of dora]. fin, in b. Average maximum value of longth of percent of tcal loagth, cc.14.6 interr.ìoxillary-vomonino band of 9. A. sos sbica PETERS. teeth, in poncent of distance from xx Total number ci' vartebreo 109 to front margin of that band to poni-. 116, Avorege se:iium velue of tenor end ofrghtmaxillary band, distance beean vr!c'la through ca, 71. Average maximum value of anus and origic do,aal f in in distance bctveon verticale through percent of total lengtb, ca. 11.1 anus and origin of domedfin, in IO, A. dieffenbachi GRAY. percent of total length, on,3.6 2, Avere aaimum velue cf length of 15.A. obscure.GtJNT}tER. gape, in percent of head length, 2, Constriction of the intermaxillary- ca, 25 to ca, 27. voinerino band of teeth begins on Maxillary bands of teeth with longi- an average far behind the middle tudinal groove, Average maximum of the band. Average maximum value of preanal length without value of preanal length without head, in percent of total length, head, in percent of total length, ca. 26.9 11, A. japonlica ca.30.3. Number of prehaemal TE1'O4INCK & SCHLEGEL. vertebrae 44 to 48 ,... A, australia Maxillary bands of teeth without RICHARDSON. longitudinal groove. Aver e max- Avor&ae maximum valuo of distance imum value of preanal length with-. between verticals through anus and out head, in percent of total dorsal fin, in percent of total length, ca. 30.1 to ca, 30.2 length, ca.1.2 Average maximum x Total number of vertebrae 103 to value of head length (measured in 111, Aver&e maximum value of females), in percent of total length distance between verticals through ca.14 ....., 16 a. Aaustralia anus and origin cf dorsal fin, in achmidti PHILLIPS. percent of total length, ca. 9.1 Aver&e maximum value of distance 12, A. roatrata (LE SUEUR). between verticals through anus and xx Total number of vertebrae 110 to origin of dorsal fin, ìn peroerit 119. Average maximum value of of total length, ca. 2,6. Aver e distance between verticale through maximum value o± head length anus and origin of dorsal fin, in (measured in femdlei) in poncent of percent of total length, ca. 11,2 total length, cc,12 o,,,.,, 16 b. 13.A. anguilla (LINMAEIJS), A. auctralio auntrailo RI,ARDON. Uil 580 Ar iiia(Rev.

1,22Taxonomie status 1.3 Morphology

The species is well established by bio.. 1031External morphclogy (For des- logical data.(cí'. Schmidt, 1922;Bortelsen, oription of spawn, larvae, and 1967)0 adolescents, see 3.17; 3,22; 3.23). 1.23Subspecies lu additiono the descriptions and di ei No subspecies existe. nois given under 1.2, come more morphological data are shownn Table I. 1.24Standard common naines, vornaoular riamos One of the eoecontroversial subjects regarding indivicluo.l variation conoarns broad- Standard common names i nosed and sherpriosed eels, Am these names imply, in an oeleíook corno individuals mey he Algeria 111e, Selbah, Bou observed with a relavoly broad head and mokhriat others with a narrow heath (Some moristic Arabia Bou rnekhriat characteristics of theme difî'rengroups aro Austria Aal mentioned in Table II), Belgium AalPaling, Anguille Coeclioclovakia- Ubor This phenomenon has given riss to two Dniar1c hypotheses Egypt - Hannash 1re Eel,Silver eel i) the differenoss betwsen broad- and sharp- land Eel, Stiver eel nosed eels are genetically determined; Eothonia Illa..ngerjae Finland Airolws, Ankeri as 2) the differences originate from different 11anoo Anguille feeding. Galicia - Wen&ura rmany Aa].:, Blarikaal After a thorough investigation Thurow Greece Ichlie, Chéli (1958) derived the following conclusiones Iceland ill Israel - Zlof ach,Zelofab An eels hock mainly consists of animals lealy Anguilla,Capitone with intermsd].ae charaoterntics. Narrow Latvia Zutis and broadnosed eels are variants of a contin- Lebanon Un ahmed uoum population ithichehousgreat flexibility, Lithuania Ungurys and their 000urrence io determined by environ- Eire mental conditions:, especially the prevailing 1alta - Sallura food,(cf.3.11). Iaro coo -Anguila, Noune Netherlands - Aal, Paling, Schieroal No data aro available which conclusively Norvny point to distinct gsoraphical variation, Portugal Engula,Eir6 Differences in certain characteristics, e.g. Romani e Ogor, Ahele, Helios, lion colour of the skin, which have been alleged to balac, Anghila show geographical variation, invariably may be Rus s i a Tgorj ascribed to strictly local circumstances with- Spain Anguila out any geographical connection, According Swede n to the theory of Tucker(1959),Eu.ropeaa aoci Switzerland- Aal, Anguille, Anguilla American eels should be regarded as geographi- Tunisia Hanek, Sallour, Sannour cal variants of the same species, but this Thrkey Yuan balyghi view cannot be accepted, since the respective Ukraine - Oglir chromesome patterns differ f ron each other Hungarh - Köznsge ngolna Yugoslavia - Jegulja (A. rostrata: 2n=38, 2M+18A; A. anguilla: 2n=38, 32N+6A). (Ohne etal., 1973). Besides, Vernacular names i Camparini and Rodfn (1950) confirmed the pre- It is impossible to mention oil the names sence of two species of Anguilla leptocephali ascribed to the eel in any country within its in the spawning zone, disoription area (e.g. Fischer, (1961) for German names), No list of vernacular naines No suhpoeulatons can he distinguished. is given ht3re, Description of morphological changos which occur during growthr During the eel stage no external morphological changes occur, When maturing, however, the changes aro striking Some morphological data on the eel (after Ego, 1939) TABLE I 100-199 200-299 300-399 ¿j 4"st.45O 99 most.450 400-499 500-5 99 a) 41.72 (15) 42.83 (29) 41.93 (54) 42.63 (49) 41,56 (57) 42,77 (81) o)b) 10.3128.33 (15) 11.5229.44 (29) 29.85 (38) 11.34 (103) 29,71 (46) 29.76 (57) 11.31 (139) 29.99 (81) 41.56i1,io30,27 (47) (41)(47) 41.60 (49) 30.40.11,08 (49) (49) r)e)d) 25.6113.3918,02 (15) (15) 26.5113.3917,93 (29) 25.1112.27 (54) 18.31 (103) 25.7213.00 (50)(so) 23.8511.78 (58) 18,45 (139) 26.4312.79 (82) (82) 26.0911.2919.17 (7) (4v)(47) 19.32 (49) 25,6711.20 (49) (49) a) .. 100 Variation of preanal length, in % of total length b)o) .L100 Variation of preanal length without head, in Variation of distance between verticals through axais and origin of dorsal fin, of total length ci) . 100 Variationin % of total of predorsallength length without head, in % of total length r1 f)e) ì. 100. 100 Variation inin lengthlength ofof head,gape, inin %% ofof totaltotal le leath of head h fr.. fr.. FIRI/680 uillaanguille (Rev. 1)

TABLE II Some meristic characteristics conoerning the heads of broadnosed and sharpnosod eels (after Thurow, 1958)

a)Skull-measures of' broadriosed eels in relation io body length (in cm).

number letth SL OB 6H PFB POB DL ML DH DAH

4 46.7 2,60 1.05 0.54 0.50 0,21 2.05 1,29 0.29 0,09 0,39 4 49.2 2.79 1,16 0,55 0.57 0,20 2.24 1.45 0.34 0,14 0,43 3 53.6 3.24 1.41 0.70 0,62 0,20 2.63 1.66 0.33 0.14 0.52 6 60.1 3.94 1.71 0.85 0,74 0.36 3.25 2,06 0.43 0,18 0.73 5 64.9 4.15 1.74 0.86 0,75 0.33 3.36 2.15 0.48 0.19 0,74

) SL skull-length; OB ocoipital-broadth; SH skull-height; PFB postfrontal-broadth; POB postorbital-broa4ith; DL i .ntaJ..lenth; ML ¡ao.xillar-length; }'I&B praemazil].ar-broadth; DH dentl.height; DAli dental-arch-height.

b)Skull-'meaaures of sha.rpnosed. eels in relation to body length

number le7th SL OB 5H PFB P03 DL ML PMB DH

3 46,5 2,62 1,10 0.58 0,51 0,21 2,10 1.29 0,27 0,09 0,39 3 48.3 2.76 1,16 0,64 0.58 0.21 2,09 1.38 0,27 0,09 0.39 3 50.9 2,78 1.12 0,59 0,55 0,20 2,19 1Q38 0,29 0.07 0,40 3 53.8 2.83 1.19 0.62 0.55 0,20 2,18 1q40 0,27 0,08 0.39 5 57.0 3.03 1,30 0,66 0,65 0.24 2,39 1.51 0.31 0,07 0.44 4 59,7 3.29 1.44 0,69 0.61 0.23 2.59 1,62 0.35 0,09 0,45 1 64.7 3.68 1.45 0.73 0.53 0,26 2.87 1,79 0.43 0,11 0.53

o)Relation of skull-length to skull-parts SL DH PM POB ML DL OB PFB 5H*L Broadnosed loo 4.32 11.42 8.04 51.3 80.842,3 19,3 21.1 Sharpnosed 100 2,86 10.27 7,48 4904 78.3 41.7 19,7 21,8 oc) +1.46 +1,15 0,56 +108 2,5 +0,6 -0.4 -0.7

Diff.in% 33.9 10,1 7,0 3.5 3,1 1.4 2.1 3,3 ) Difference between values of broactnosodandsharpnosed eels

(cf. 1.2 and 3.514). 2.7 to3 percentof the animal's weight. When the eyes enlarge significantly during this the skin weight roaches 4,2 percent, the eel period of life. Their diameter Increases 1 to shows the first signs of becoming silvery. In 2 mm, their weight at least doubles. The ol- fully developed silver eels the skin accounts factory organs atrophy (Pankhurst and Lythgoe, for over 6 percent of the total weight, The 1983); the lateral lines bec mareconspicuous; belly becomes tougher too, so that itiQdif- ficult to press it together in a silver eel, the dorsal side turns black, as do the pectoral in complete contrast to the weak belly of an fins; the ventral side, in contrast, turns white immature yelloweel. with silvery reflections.A further obvious change is that the skin becomes tougher, harder Increase of dermis-thickness was confirmed by Pankhurst (1982), who also noticed increase of and mare shining. Havinga (1943) stated that in the great mass of eels the skin weight arrounts to scale areas from 50'!, to 1457e in sexually maturing eels. Occasionally eels are captured with a much more sexually advanced appearance, pro- 8 FIRI/580Anguilla anguilla (Rev.1)

sumably attained by eels on their spawning- quenoy distribution greodrith the distribu- route in tho sea, With regard to this "par- tion expected on asoumption of genetic oontrol tial nuptial dress", Svdrdson (1949) described of the main transferrne by three alle].io genes. the presence of very enlarged eyes with a dia- (Fine et 5.1,,1964; Drilhon et al., 1966). meter of 1 cm at least, very long and pointed black pectorals, a reddish-brown colour most It should be mentioned that a complicating prominent on the belly, and a moro blunted factor was encountered in a oomparieon of snout Atlantio samples and samples from two aro in the Mediterranean. (Drilhon et al., 1966, 132 Cytomorpho logy 1967,1967e). Differences in the transferrin frequencies were found both between the Sick et al,, (1962,1967)studied the Atlantic and the Mediterranean specimens and chromosome-number of the eel and concluded between sples from the eastern and western that Â. anguilla has38diploid chromosomes. Mediterranean, The latter in particular involved the presence of a three banded pattern According to Chiarelii et al.(1969) in the western group. (cf. also de Ligny,1969). A anguilla shows18pairs of perfectly paired and 1 pair of dissimilar chromosomes. Pantelouris and Payne (1968) concluded that esterase zymcgrams of eel sera exhibit up (Cf. also 1.31). to eight fractions. The individual differ- oiou aoservect ouggost genetic polymorphism 1.33Protein specificity and the sane appl3.es to the esterase pattern of various tissues. Preliminary olectrophoretic studies of the eel revealed only one haouoglobiri pattern Subsequently Pantelouris et al..(1970) dis- (Sick et al,, 1962), An azonoion of this study, involving 848 eels of 'ithich .302 spo- covered significant differences in the frequences omens wore collected in Greece and Spcan1 of phenotypes in the electropherograms between confirmed this result. samples freni Germany and samples from Iceland and Scotland. By moans of ele otrophori;., serum trans- forrins were also studied, ,u:' variable Bel blood contains a serum, whoh bolongs, iron-.bindj fractions were d;ectod, Five ocordingwunder (1967),to e. group which is phenotypes were observed, of iioh the fre- very poisonous for mammals, Heating to 58 70°Cdestroys the serum. FIRI BOO Anguillanguilla(Rev.l) 9

2 DISTRIBUTION

2.1 Total area 2.3 Determinants of distribution changes

The Atlantic from the spawning area (see Until recentl.y it was the general conviction 3.5.1) towards Europe and Northern Africa; that Gulf stream and North Atlantic current pri- coastal areas of Europe and its islands from manly determined the distribution of the eel Pechora to the Black Sea; Mediterranean and its coastal areas of Middle-East and Africa; larvae and in this way of the species. Due to Atlantic shore of Morocco; Canary Isles; the work of Worthington (1976), in which the Azores; Madeira. existence of these flow-systems is denied, it must be assumed that the larval distribution in Boundaries in Europe aree lower part of Pechora, Volga-system, and Black Sea, with oc- the Atlantic is determined by other, hitherto currence in Danube, Dniester, Bug, Dnieper and unknown factors (cf. 3.511), with subsequent dis- sometimes Don. persion in coastal areas after the glass eel metamorphosis assisted by possible sea-currents If not barred by obstacles (e.g. weirs), inland areas may be reached by eels swimming near and on the continental slope. upstream in rivers and water-courses; density of eel-stock decreases with increasing distance Further distribution depends on accessi- from the coast. bil.ity of interior water regions and on transplantations. Due to the easy way in which eels may be handled, transplantations over great distances Ali water-areas, be it in the interior or are carried out, and water-areas are stocked along the coast, which are fit for fish and offer with eels which otherwise would be absent. a sufficient food supply, may contain eels. This Japanese eel ponds have been stooked in applies from a warm saline lagoon on the 1969 for the first time with elvers from France Mediterranean coast to a cool. fresh trout-brook (see also 6.5), in a mountainous region, e.g., the eutrophication of the North Sea area along the Dutch coast not 2.2 Differential distribution only gave rise to an increase of shriap stock, 2.21 Spawn, larvae and juveniles: but promoted an important eel stock as weil. see 3.1.6 and 3.5.1 Local winter hardships are overcome by under- ground hibernation. 2.22 Adults: see 2.1 2.4 Hybridization

No hybridization is known. 10 FIRI/S80 Anguilla anuilla (Rev.l)

3 BIONOMICS AtD LIFE HISTORY (unpublished) noticsd a majority of females in an enclosed polder, which is inaccesible to 3.1 Reproduction eels and was deliberately kept understocked with elvers by the manement. 3.11 Sexuality In all probability the phenomenon mentioned According to d'Ancona (1943), who studied above gave rise to the opinion that salinity of the development of eel gonads from the elver ste onward, the gonads at first have a non-. the surrounding water-area determines thesex of differentiated appearanoe, with some parts eels, which is denied by Sinha and Jones (1966). tending to show male characters, some female, and some intermediate. The sexual differen- Sexual dimorphism: In the stage nearest tiation of the gonads takeB pisos with the mature (silver eel), males and femalescan be development of elements of the male gainetogenic recognized by their length, see under 3.12 serios and degeneration of the female or vice versa. Heermans and van Willigen (1981) demun- Differentiation of female gonads may some- strated that broadnosed eels turn mainly into times be recognized in eels of 20 orn or even females, which normally are characterized by a smaller. Most eels, however, romain in a con- much lower fat content than maies (cf. Wiehr, dition of sexual indetermination up till 1932; 3.42). lengths of 30 cm or moro, During their de- This is in agreement with the velopment, a stock of eels shows a gradual observation of Thurow (1958) who mentions that series from distinct females to distinct males broadnosed eels have a lower fat content and linked by moro or less intermediate specimens. reach the silver eel stage later than sharpnosed eels (cf. 1.31). The conditions 7hich d.'Anoona obsorved. point to a fairly long otage of cexual Indeter- mination with the coexistonoc of prospective 3,12Maturity fornaio colle beside potentially sacouline ele- monts in thu latter the coxun]. ohiraoterit- Havinga (1945) stated that female silver los are not evident booauso of cìalsyod matura- eels are longer than 45 cm; the smallest ho tion. noticed had lengths of 47, 48 and 51 orn, In his opinion male silver eels are smaller than All observations, moreover, lead to the 45 cm. Deelder (unpublished.) observed in the oonolusion that in the osi the gonotypio sexual IJaselmeer male silver eels as small as 25cm. dotoz'mlnation is very labile end that it may be influenced by plienotypio factors, whichot on Ehrenbaum (1929) mentioned the invastiga- the erm=.collo throuGh ho eomtio substratum tion of 5,260 eels, among which were males of of the Gonad. 45cm (i), 46 orn (3), 47cm (1) and. 49cm (i). Graphs of Meroor (unpublished) show a olear Sinha and Jones (1966) concluded that the division between male arid female silver eels "organ of Syrski", which was supposod to b' the in Lough Nejh (N, Ireland) at 47 cm, The diagnostio of maleness, is ambiguous in nature, length-frequency of malo silver eels increases therefore it is difficult to determine the mx from 31orn to a distinot mode at 37 to 42 orn; of eels which have these organs. Their conclu- lengths of female silver eels reaoh 80 orn, and sion is fully supported by Passakas and Tesch the length-frequency graph shows a distinct mode at 53 to 55 orn. The largest male silver (1980), who found not less than 16 females in 42 eel observed by Frost (1945) had a length of Elbe-eels, which previously had been determined 43.0 orn, the smallest female was 44.0 cm; as males in their organs of Svrski (see also "such fish are most unoommon", Wiberg, 1983). In this respect it should be mentioned that Fidora (see d'Ancona, 1959) concluded nowadays Italian eel farmers manage to produce after stocking-experiments that different con- male silver eels of well over 45 cm (figure 16). ditions of crowding affect sexual determination, viz, a denso stock results in a majority of From the above, the conclusion must be drawn males and decreasing population density in- that attaining maturity has a connection with creases the percentage of females. length, and should be primarily a matter of (cf. Parsons et al., 1977). This con- growth-rate in the preceding period. ception tallies with other observations, e.g. a 100 per cent female stock in an understockod For size and weight of silver eels see area like the upper reaches of long rivers, e.g. 3.43- Rhino (see also Gandolfi-Hornyold, 1929, for the Loire); a great male preponderance in the dnse eel population in the ostuarine area of the same river (e.g. about 95 por cent in IJsselmeer); and again an increase of females in a less pop- ulated coastal sea-area next to it, Deelder FIRI/S80 Anguilla anguilla (Rev.?) ii

3.13/3.14 Mating/Fertilization According to Yevseyenko (1974), by further development eel eggs will swell to diameters of Not observed in the wild. Boëtius and 2.3-2.9 mn. Boëtius (1980) described artificially induced mating behaviour, from which it may be assimed 3.2Pro-adult phase that mating is promiscuous and fertilization is external. 3.21 Embryonic phase

Boëtius and Boëtius (1967) studied cap- Unknown tive silver eels. The authors found that the 3.22 Larval phase maximum temperature at which normal sexual development would take place ws 5° to 260C. Pro-larval phases According to Sohmidt A mathematica;l approach to the data on tempe- (1923) the yolk sao of larvae contains an oil globule. rature and maturation period gave an indication of an optimum temperature of about 20°C. Post-larval phasesThe development of the The biological zero for experimental matura- eel larvae up to an average length of 75 mm tion was about 10°C. (Leptocephalus breviroetris) has been desoribed by Schmidt (1923), See 3.5.le

3.15 Gonads It must be borne in mind in this connection, that the leptocephalus increases in length dif- Kokhnenko et al. (1977) and Boëtius and ferentially, growth in length being greater post- Boëtius (1980) described the development of anally than pre-anal].y. With regard to the gonads and eggs after treatment with gonado- position of anus and dorsal fin it has been shown that neither changes its position relative to the trophic hormones, both in appearance as weil myomeres until metamorphosis (Ford, 1931). as in chemical corrposition. Kokhnenko calculated mature female eels to contain Grassi and Calandruccio (1897) described the 3 million eggs per 1 kg of body weight. The metamorhosjs of eel larvae into tzis slyer stage, which was nicely illustrated afterwards with ovary weight increases up to 90-120 g (normal photographs by Schmidt (1912)ø ;sides the con- 3-16) and colour is rose-pink due to many spicuous change from the larval olive leaf form bloodvessels, into the eel form of the elvers with a simultaneous deoreaRe in length and veight (Schmidt, Meske and Cellarius (1973) as well as 1923), a remarkable change in the relativo posi-. Bieniarz and Epler (1977) dealt with the tion of dorsal and anal fins and of the anus takes place; the first fin rays and anus move development of male gonads and described forward in a real movement and change their eel spermatozoa, the average concentration position entirely with respeot to the myomeres, of which might amount to 9 million per inn3. so that fins and anus are made to lie immediately opposite the myomeres from which they are inner- vated, instead of remote from them as in the 3.16Spawning laptocephalus stage (Ford, 1931). It may be concluded, therefore, that the dorsal and anal (See also 3.14/3.15.) fins are temporarily held in a posterior position moving forward to their true positions during Flydrographical data from the supposed metamorphosis. spawning area for eels in the Sargasso Sea Bartels (1922), who studied the anatomy of indicate a spawning depth of 100-200 m in the larvae and elvers, noticed remarkable changes in winter months,if an optimum spawning tempera- some organs, e.g. the lateral line and intestines. ture of 20°C is presupposed. He moreover mentioned the existence in the leptocephalus of a big jelly-filled cavity, occupying the greater part of the body and with the neuro- 3.17Spawn chord suspended in it. ThiB cavity disappears The jelly pre- Kokhnenko et al. (1977) and Boëtius and completely during metamorphosis. sumably keeps the animals in te upper wter Boëtius (1980) observed that the pelagic eggs layers by its low specific gravity, since have a thin chorion and a diameter of about leptooephali do not possess swim bladders. 1.2 inn. Due to their transparency the presence of one too manyoil droplets in them could be established. 12 FIRI/S80 Anguilla anguilla (Rev.1)

3,23Adolescent phase. Hivers. 3.32Hardiness The most oonepiouous featurec of elver Provided enough oxygen is available to devolopment ers the graduel pigmentation breathe and the skin is kept wet, with s.c air (starting with tho "aoho o6r6bralo') and the temperature not exceeding 15°C (Krogh,1904), reduotioninlength and weight (Gileon,1908; hardiness of eels and silver eels is great. Strubborg, 1913). Deorease in length of It is because of this characteristic that their elvers after reaching the Danish ooast may economic value is great too, since it enables amount to3.5mm (from about7mm to 6.5 mm); storing, handling and transporting over huge deorease of weight from about 0.32 g to distances. Although no exact data are avail- 0.135g, a deficit of0.185g. This roduc. able it is a well-known fact that silver eels tion isdue to not feeding while migrating and may be densely packed in live-basins for months. swimming. Mann (1960) kept starving eels for 3 and even 4 mo in aquaria filled with tapwater, Because At Den Oever (Holland) the greatest re of their physiological state in which they need oorded length of immigrating elvers is 92 mm; no food, silver eels may be kept loner in cap- the smallest,56mm. The annual average tivity than solaiBotius(1967)experimented length of immigrating elvere varies here from with silver eels during9successive months, 72to77mm. and Botius and Botius (1967) kept a silver eel which was able to maturo normally after a Diseases and parasites are unknown for period of starvation of over 3 yr. this stage. Botius and Botius(1967)showed that the Predators aro found in the whole scale of thermal amplitude of survival is roughly O to animals big enough to devour elvers many 30°C for both yellow and silver eels, The kinds of fishes (although elvers are known to vegetative temperature range of the yellow eel escape through the gill..slit) including eels is from about 10 to 30°C. This deduction has (Sinha and Jones 1967a), rats (e.g., at boon nicely confirmed by the experience of elver-passes), and gulls (Moriarty, 1984). Japanese eel'culturists, who in1969imported lvers from France. First there was great Losses due to man are very important. satisfaction;in the summer, however, with They may result from impassible barriers in water temperatures of over 30°C, a groat mortal- waterways, where olvers may try to pass u.ntil ity occurred without signs of diseases or para- they die, or by oatohing elvers forstocking, sites. The conclusion was that the European food or manure. An early descrietion of this was eel could not withstand very well these high given by Redi(1684)(of. Bertin,1956),who temperatures (Korringa, personal communication). possessed a remarkable insight into eel biology. According to him, on a day in 1677 more 3.33Competitors than3,000Tuscany pounds(1,200kg) of elvers were caught in the Pisa stretch of the Arno Rogers (1964)correctly pointed out that within5h. one may still query the suitability of the cri- tori upon which competition is jud.ed. It is The total damage inflicted to an elver obVious that what doss, in faot, constitute stock by man and animals must be tremendous. "competition" is still subject to considerable debate. To some it seems quite wrong to as- 3.3 Adult phase (Mature fish) sume that because the same food is found in the stomachs of different species, competition Although eels and silver eels are not must be taking place, for there may be an strictly mature fish, they will be discussed abundance of that food. Conversely, if an here since they represent the most important organism is found in the stomach of one species phases in the life of Anguillaguilla. of fish and not in another, both species occurring in the same area, it cannot be said that 3.31 Longevity there is no competition;it could be that one species has been markedly successful in the corn- The total life span a.o. depends on the petition for that item of diet, time taken to attain the silver eel stage. Since this, as already has been pointed out, Competitors for spawning-area are unknown. is determined partly by sex and growth-rate, If all animals which share items of diet which are strongly influenced by the environ- with eels are to be regarded as competitors for food, then most species of fish which occur ment, it is difficult to give a general state- with eels must be included in this category. ment. 0f these, no list will be presented because cf its extensiveness, due particularly to the Absolute topper is Putte, kept for 85 diverso areas in which eels may live. years in aquaria (Svärdson, 1949).Bertin (1956) mentioned an eel which lived 37 years Since silver eels do not take food, no in captivity, and eels staying for 50 years food competition exists here. in a Swiss lake. FIRI/S80 Anguilla anguilla (Rev.l) 13

With regard to competition for shelter, Bottriocephalus olavices it may be noted that eels like to hide them- Diphyllobothri.um latum (larval) selves in the bottom, in tubes, plant-masses Proteocephalus macrocephalus etc. No serious competition by other species Sphaeroetoma brarnae for these habitats is known. Plagioporus angulatus Podocotyle atomon 3,34 Predators Contracaecum squalii (larval) das cris ecos For the youngest eel stages, all fish Spino otus inermuis which are able to swallow elvers may act as Iohthyobronmmo, gnsc&ini predators, although eels are more protected Camellenus leoustris by their wuf living. Orown eels are preyoi C. truncatus on mostly by bigger eels (Sinha and Jones, Neoechinorhynohus rutili 1967)and by birds, Cormorants may take a Pseudoeohinorh nchus clavula heavy toll as they are able to dive, and herons Metechinorh, nchus salmonie are important in shallow areas. Acanthoce halus guillee A, luau Van Dobben(1952),who studied a cormornt Pomphorh-nchue laevis rookery, observed that the food composition is Corynosomna semorme (larval) ruled by the accessibility of the eel. The Ergasilus gibbus catch of eels increases with the approach of Lernaea cyprinacea the summer, until on warm days in July the sein Argulus foliaceus become the principal food, As almost ail erío caught are injured at the snout and thebiri This list may be supplemented with other exclusively catch in daytime, it must be asour observations, e.g.: Dipiostomum sp. in the eyes that the eels are caught when hidden in theboc- tom with their head protruding, No efficararu (Frost, 1949); cysts of Glugea sp. in eel's defense reactions of the eels are known, ovaries (Willemse, 1966); Apiosoma cylindriformis in skin and gills (Ghittino, 1970); The length cf the eels caught may range Helicostoma buddenbrocki in skin (Purdcm and from 18 to 60 cm, Van Dobben calculated thet a cormorant rookery with about 2,000 ner Howard, 1971); Spinitectus inermis in intestines situated near the IJsselmeer consumes in a (Moravec, 1977); Ligula intestinalis and summer about200,000kg of eels, Pomphorhynchus bosniacus in intestines (Hristovski and Riggio, 1977); Sphaerostoma Moriarty (1984) observed predation by gulls bramme in intestines (Lacey et al., 1982); on elvers released for stocking purposes. Dermcystídium anguiilae in gills (Wootten and McVicar, 1982). With regard to fish predation Hegemanìn's (1958)interesting observation must be mentioned regarding pike in the Oreifswalder Bodden Under certain conditions parasites may (Rugen) consuming eel, Eels made up4per- exert a devastating effect on eel stocks, cent of the prey of 50 pike, Ichthyophthirius is able to wipe out total stocks of free-living eels (Timmermann,1939) No data are known about predation as a fac-as well as of stored eels (Deelder, unpublished). tor controlling the eel population in any area, The great massacres of eel populations, however, are caused by just a few diseases:Pseudomonas 3.35 Parasites, diseases, injuries punctata and Vibrio anuillarum;"red disease" and abnormalities for both, and the "cauliflower disease". Parasites and diseases According to Schperclaus, (19) who described this illness extensively, the "red eel These are manifold, Reichonbach-Klinke disease" may be caused by the two bacterial (1966) mentioned, besides common parasites such species mentioned above, Pseudomonas thrives as Ichth'ophthirius multifiliis, no less than in water with a salinity less than 0.8 per cent 30parasitic speoies NaC1, Vibrio in water with a salinity of about 1.5 to 3,5per cent NaCl, Dutch eel merchants Trypanosoma granulosum aro well aware of this peculiarity and if pos- Cryptobia markewitohi sible change their stocked eels from freshwater Eimeria anguillae to seawater arid back again in caso of "red Myxidium giardi disease", to restore the condition. Trichodina anguillae Trichodinella epizootica Gyrodactylus anguilles Triaenophorus nodulosus 14 FIRI/S80 Anguilla anguilla (Rev.1)

In addition to stocks of stored eels, the As with other animals, some abnormalities "rod disease" may inflict heavy casualties in may occur occasionallye.g. stone in the body- eel populations, e.g. in the Baltic (Wolter, cavity (Tesch,1958); kidney-disease (Scheer, 1960)and Adriatio (Canestrini, cf. Wolter, 1934). 1960) as well as in interior waters, e.g. in Holland (Deelder, unpublished). 3.4Nutrition and growth An outbreak of the so-called "cauliflower- 3.41 Feeding disease" of silver eels, starting in 1944 in Denmark, was reported by Christiansen and Eels, being nocturnal animals, feed mostly Jensen(1950). These silver eels presumábly during the night, when they search for food by descended frein Baltic regione, Obviously swimming to and fr0. Aquarium experiments and assisted by eel storage and transport, the ill- direct observations (e.g. Berry, 1935), however, ness attacked eels as well, and spread over reveal that they feed. in daytime too. Europe, to the south part of Holland in the west and toward the Black Sea in the east (Raduiescu Feeding takes place all over the area where eels occur and where food. is available. In and Angelescu, 1972), in brackish, as well as in estua.rine areas daily movements may occur to and fresh water.As indicated by the name, diseased from the foodetaples in the brackish area (see eels bear tnnors, irnstl.y on the snout but other 3.51; Koendzinsj,1958). In springtime, parts of the body might be affected as well. spawning areas of coarse fish may attract large Despite serious efforts (cf. Schäperciaus, 1953; numbers of eels, wbioh feed upon the spawn. Koops et al., 1969; Peters and Peters, 1970, 1979; Here we may draw attention to the statement Bremer and Ernst, 1972; Einszporn-Orecka, 1974) no of Albertus Magnus (1545, cf. von Siebold,1863) exact cause coul.d be established, although a which runs translatedi "The eel allegedly crawls out of the water during the night and (polyhedric formed) virus might be the malefactor enters the fields, where it finds sowed. lentils, (Koops et al., 1969).As the turimrs prevent eels peas or beans". Nileson(1860)pointed out from burying in the mud and from foraging, a that such eels might go for slugs and. worms. gradual deterioration in condition is the result As Deelder (unpublished)and Beraann (1970) have provedthat eels may leave the water to (cf. LUthmann and Mann, 1956).In addition, forage, and knowing their ability to locate the deseased eels become unsalable because of their nearest stretch of water (see 3.513), it must be repelling appearance. assumed that Magnus and Niisson were right about In captivity, especially with low stocking this peculiar feeding ground. density, conditions might be favourable to pro- The eel exhibits some peculiar ways when duce a stress-situation for subordinate eels foraging, depending on circumstances, Small (Peters et al., 1980; Peters, 1982) as well as objects are taken without diffioulty. A large elvers (Willemse et al., 1984), resulting in, object, e.g. a dead fish, which is too big to e.g., degeneration of the gastro-intestinal swallow, is attacked in the following way: the eel bites into the flesh and. then tears off the tract and several other degenerative changes, mouthful by rotating around. its long axis of which are characteristic for the General high speed.. Adaptation Syndrome of Selye's stress principie. Buried eels catch food in a peculiar way, of which Berry(1935)gives a vivid descriptioni Injuries and abnormalities "They lie buried in mud. or gravel with only the tip of the snout down to the eyes projecting. As far as known, injuries with regular The spot selected, in e.g. a river, is out cf appearance are only those inflicted by turbines the main current, either in front of, or behind and pumps on eels and silver eels passing down- a large stone or other barrier. Any unwary stream through power and pumping stations. creature of suitable size which passes within a These injuries may vary from complete cutting few inches of this hidden danger, is seized. to damage visible only after some time with surprising suddenness and bolted at a gulp (Butmchek and Hofbauer,1956). or swallowed.grad.ually head first. To see an eel catapult two thirds of its length from its Abnormalities are not common. Most con- tunnel to seize its prey im only less remarkable spicuous is the yellow eel, reported. from time than to watch it suddenly draw backwards out of to time (e.g. Thumann, 1953). Another pheno- sight; the flattened. tail is evidently used as menon observed now and then is the eel with a sort of spring, but it is difficult to under- undulated spine ("plekosondylie"). No cause stand how they accomplish this feat". is known (Schrider,1930). FIRI/S80 Anguilla anguilla (Rev,i) 15

Big eels which are not buried in the sub- when occurring in great masses (Schiemenz, stratum presumably swim round for a while after 1910). Instances of fish too big to swallow having seized prey. Long-line fishermen cope being attacked and devoured gradually by eels with this habit by using special long cross- are also known, particularly when they are lines or by fishing exclusively for big eels on caught in gill nets (Louth Neagh, Deelder un- hot days, when eels are lazy, using special published;Morrison, 1929), sized baits euch as ruff or small perch. Their voracity is amazing. In an aqua- At all times eels seem more dependent on rium,aneel of 30 cm consumed an average of scent than on sight for getting food. An 12 g, or about 25 earth worms, per day. Such imitation worm of other bait dangled in front an eel can consume a dozen worms each about of them produces little if any interest, but 5 cm long in as many minutes, and after only a any object which has been placed for some time few hours will be ready for more (Berry,1935). in a tin of worms causee immediate excitement. (of. von Fritsch, 1941;?ohr, 1969), Because cf the big economic implications, the food relationships of eels and salmonids (hen baiting their longlinee, fishermen always arouse great interest. It is commonly should take care that their hands are clean; assumed that eels cease feeding each year be- such things as oil or kerosene on the hands fore the earliest salmon spawn, and that they cause a sharp decrease in the catch). do not become active again until after the ova have hatched (Menzies, 1933;Sinha and Jones, Besides the scent, the taste of food also 1967a). This does not, however, apply to seems to play a role in food-uptake. Wunder char eggs (Frost, 1952). Meazies (1933) refer- (1927) showed that meat drenched in quinine red to an eel trap which was for some time bait- is immediately spat out. ed with worms without any success;when, however, some roe cf a dead salmon was placed in it, Eels in the northern part of Europe ab- it caught 44 eels between 11.00 h and 17.00 h stain from feeding in wintertime. Sinha and the same day. Jones (1967a), who studied eels from Welsh rívers, stated that feeding is mainly restrict- About salmonid fry, Berry (1935) remarked ed, to the months of April to September. Ex- that eels account for large numbers of miller's ceptions are known, however (Frost, 1952). thumbs and bach, which are amongst the most It must be assumed that in the warmer southern serious enemies with whioh alevins and young regions of their area eels abstain from feeding salmon and trout fry have to contend, but it for a much shorter period, if anysuch to the must be doubted whether destruction of such benefit of e.g., Italian pond eel farmers. enemies will make up for the numbers of small parr which eels themselves consume. Most According to the experience of fishermen coarse fish fry rest near the surface or among using bait gear, it is obvious that in mid-sumîier weeds, whereas young parr rest on the bottom, and it is owing to this fact that eels probably an interruption in feeding is a conìnn phenomenon. consume a high proportion. 3.42Food A similar problem exists in areas with stocks of freenwater crayfish, as in Sweden, Eels are fully catholic with regard to The eels are known to attack the crayfish when animal food, provided it is alive or extremely they have shed their shell during the moult. fresh. A list of species serving as food for In this way presence of eels means a certain eel has to include virtually the whole aquatic danp-er for the stock of crayfish.(cf. Svdrdson, fauna (freshwater as well as marine) occurring (19'72). in the eel's area (see e.g. Frost, 1946; Sinha and Jones, 1967a!)aniel., 1968; Wiehr (1932) investigated the relation be- Biró, 1974).The food iisrmight tween foodfats and their biological consequences. easily be augmented with animals living out of No chemical-histological differences can be de- water, e.g. worms, while fresh meat is taken tected between broadncsed and sharpnosed eels. as well, With eels in captivity, unlike most The latter category shows the greatest deposi- other creatures, thereIBno difficulty in tion of fat, particularly in the livor.Sharp- inducing them to feed, and they will thrive on nosed eels contain up to 27 per cent cf fat, practically any diet, broadnosed eels up to 12 per cent. Eel fat consists mainly of glycerids of fatty acids, The kind of food taken is greatly in- while the fat cf sharpoosed eels contains moro fluenced by size and availability (Schiomenz, ole ins. 1910). Thus, big eels are more inclined to prey on fish they can swallow than small The difference in fat-percentages of sharp- eels, and as eels are mainly bottom dwellers nosed and broadncsed eels muet be ascribed to they prey more on the bottom fauna, Excep-. diffrei&oes in îeedingviz, the first group tions are known, and plankton may be taken takes the fattest food. The quality of the eel fat moreover depends on the quality of the food fat, 16 FIRI/S80 Anguilla anguilla (Rev,l)

3.43Growth rate Negative growth-rate has been studied by Mann (1960), who kept starving eels and silver Concerning the Leptocephalus stage, eels for 3 to 4 months in basins.His eels lost van Utrecht and Holleboom (in press) showed that up to 20 percent of their weight.The greatest at the metamorphosis the eel, larvae are not of loss observed was 28 percent.Small. eels the same age.With fair certainty their age can With regard to eels, all. decrease in weight more quickly than large ones. vary from 1 to 6 years. The same holds true for the fat content.Eels, investigators agree that eels belonging to one with their lower fat content, are therefore year group (i.e., from the giasseelstage) may much more affected by loss of fat than are show very different lengths, as has been experi- mentally confirmed by Meske (1968),Thus in a silver eels. certain area a certain length may be Toccupiedt The following examples concern the maxinurn by several year-groups simultaneously. To sizes which eels may attain.Day (1880 to 1884) judge an eel's age solely by its length, would mentioned an eel with a length of 1.12 ni, a be to risk an error of one to five years either maximum girth of 25,4 cm and a weight of 3.3 kg. way" (Bertin, 1956; p.43), He also reported an eel with a weight of Nilsson (1860) mentioned eels of 2,5 The length-frequency of an unfished year- 4.53 kg. In 1939 an eel was group of eels shows a Gaussian normal distribu- to 5.5 kg from Sweden. tion; the points fail on a straight line when caught in the Aller (Germany) with a length of 1,15 m and a weight of over 5.5 kg (Anon., plotted on Gaussian paper (Deelder, 1963; 1939), and Waiter (1916) mentioned three catches figure 2). of eels weighing 6 kg, each in Germany.Record- Eels of the same length vary in weight. holder is the 7.65 kg eel caught in Dalsland Examples of this are given in Table III, from (Sweden) in the first half of the 19th Century the results of Marcus (quoted by Ehrenbaum, 1929) (Svärdson, 1972). who investigated 5 260 eels,As the graphic Growth of eel in natural waters have been "length" - "average weight" relation is curved subjected to many investigations, either: (cf. Deelder and de Veen, 1958), Sinha and Jones (a) on the basis of stocking results in eel-free (1967), using the equation: WaLn, plotted waters; (b) by age-determination on otoliths, the relation on double logarithmicpaper and got straight lines for eel stocks from different (a)Stocking results:Benecke (1881) stated areas, the lines for female eels nearly coin- that elvers, when set free in East Prussia, ciding with those for male and undifferentiated grew in one sumner to eels of 20 cm, and eels. reached next sunnier lengths up to 35-52 cm. Bertin (1956, p.51) concluded "that the Walter (1910) studied several stocking increase in the size and weight of eels is more results and concluded that eels in (tempe- a question of food than anything else".The rate) German waters coul.d reach a length results of some investigations indeed indicate of 25 cm after the first sunnier, 52 cm that food and its composition exert a deciding after the second and 65 cm after the third influence upon the condition of eels. sunnier (cf. Deelder, 1981).Einsele (1961) There are no indications that a high density released elvers in May 1958 in the eel-free of eel stock decreases the growth, provided there Neusiedler See and caught in 1960 eels weighing 5CXJ-8CX) g, with maxima up to 1 kg. is enough food.Meyer-waarden and Koops (1968) Next sunnier the average weight was minimal reported on a pond on the Mosel bank with a den- 1 kg, maxíni amounting to 1.5 kg.In 1961, sity of no less than 1.8 eels per m2.Food was in Lake Constance, 100 kg of elvers were available in plenty.Notwithstanding the crowd- released, giving a catch rise of about 100 ing, the mean weight increased by 76 percent in percent (conpared with 1961) in 1963, 400 one surrmer from 93.3 to 164 g. percent in 1965 and 550 percent in 1966 In this respect the study of Thurow (1959) (Berg, pers. cormi.).This result cannot be most also be mentioned.He observes that the ascribed to increase of catch-intensity factor K in the equation; G = K1L is depen- alone.DahI (1967), who carried out stock- dent on age, season, environment and sex of the ing experiments in Danish ponds found mean eels, lengths at the end of the first three sum- mers of about 17.5; 28.9 and 43.9 cm, respectively. FIRI/S80 Anguilla anguilla (Rev.1) 17

(b)Age-determination on otoiiths: This approach As the hunan eye is definitely unable to is executed in order to deduce the age from distinguish between the several zones in the the zonation in the eel otoliths, according otoliths, special. methods have to be applied the formula: to obtain a correct evaluation,Io are known L = 0.04.L + 0.58 (Appelbaum and Hecht, o ee. and were controlled with eel otoliths of known 1978). age, respectively proposed by Wiedemann Smith To prate a universal otolith preparation, (1968) and Deelder (1976; 1978b; 1981).The the "Joint ICES/EIFAC Working Group on Eel" results obtained are in agreement with those in 1979 unanimously agreed upon the "proof eel stockings (see above). duction of a slide of 0.2 cm thickness obtained by cutting an otolith embedded in Moreover, Deelder showed that growth plastic along its longitudinal axis passing interruption in mid-maimer occurs frequently through its centre so as to make clearly (cf, 3,41: feeding), presumably being a main visible the circumferential outline of the cause for supernumerary zonat ion. leptocephalus otolith", Experiments with eel.s kept in basins It is to be deplored that many otoliths are (Kuhlmann, 1975; Sadler, 1979) showed that with still treated by other methods, e.g., burning, sufficient foodsuppl.y growth is regulated by although in this way great errors are likely to temperature. hen relating this datum to eel be introduced (Deelder, 1981).The major problem growth in open waters, it is to be reminded for age determination in this way líes in the that monthly tarperature averages in absolute eel's characteristic to form at times in the oto- value deviate only to a slight extent from the 1 iths more than one band per year (supernumerary normal average, inplying that usual.l.y eel zones), which phenomenon caused in the past growth will be nearly constant from year to extreme errors, and, it is sad to notice, is still year. causing them, as recognition of that phenomenon In this respect, Deelder (unpublished) is still eluded. studied growth of eels in an enclosed water From the very beginning the age-determination area on their otoliths during the respectively on otoliths has been based on the "one band = one 'normal", "warm" and 'cold" surrmer of 1975, year" foLllfula of Ehrenbaum and Marukawa (1914), 1976 and 1977 (with added average deviations who founded their opinion on results with bas infor the five-month s'aimer periods, respectively: kept eels,This caused great errors (Deelder, +1.4; +3,5 and -5.0°C) and according expectancy 1981), as Dahl (1967) proved that in eels, trans- established the feature that no obvious diffe- planted as elvers to an eel-free pond, two zone- rence with a trend of significance could be indications in one year were possible.Following detected in the annual, length-increments.In this, Moriarty (1976) discovered on otoiíths from complete contrast to fishes of other species Hungarian eels, that up to three zones per year that grow by increasing the volume of small could be laid down,These observations indicate muscle fibres alieady present, eels grow by that errors up to 200 percent have to be reckoned forming relatively large numbers of new muscle with, implying that most age-determinations fibres (white and red ones as weil.), which hitherto have lost their significance, which also originate from already existing fibres by a counts for future determinations if the super- "budding' process (Wiliemse and Lieuwna- numerary zonat ion is neglected. Noordanu1983; in press). 18 FIRI/S8OAn uflia an utila (ev.1)

70 60 ¿V 50 40 30 20

12 14 16 18 20 22 24 cm Fie. 2 Length-frequency of' an unfiahed eel-yearolass in the IJae1meer, plotted on Gauei1pape2' (Deelder, 1963). Eel-lengthe rounded off to neareSt orn downwarde. FFP,/S8Onguil1.a anguilla (Rev.1) - I TABLE III Relation between length and weight of eels (after Marcus, in Ehrenbaum1929)

Lsngth Number of inves- Average veiht Weight-limits (orn) tigated animals (g) (g)

lo 10 1,2 l-2 ii li 1.7 1-3 12 23 2,5 1- 3 13 38 3,3 1-5 14 44 3.8 2-6 15 50 4.6 2-7 16 55 5.1 3-12 17 71 6,4 3-10 18 123 7.5 4-19 19 159 8.7 4-12 20 150 12.7 6-15 21 188 12,1 7-20 22 196 13.4 5-23 23 198 15.3 9.-23 24 256 17.3 11-25 25 362 20.1 12-31 26 379 22.1 14-33 27 312 92 25,5 24.4 15- 43 17- 31 28 237 113 29.2 26,8 19- 45 15- 40 29 210 141 31.8 31.1 20-46 18-44 30 187 139 36,0 33.2 22- 49 22- 52 31 127 139 40.0 37.5 27- 56 26- 63 32 84 108 45.5 42.9 29- 63 28- 63 33 37 86 48.2 45,6 34- 66 30- 66 34 23 86 54.5 51.5 45- 75 38- 70 35 11 92 65.5 55,6 52- 93 36- 81 36 10 74 66.0 62.1 57- 75 43- 98 37 5 53 72.4 65.3 64- 95 50-lOO 38 9 37 82,7 76.5 68-i16 51-110 39 3 41 109.3 82.8 100-120 60-104 40 3 27 112.3 90.2 100-125 63-110 41 - 17 - 100.5 - 84-128 42 - 22 - 110.3 - 89-140 43 - 13 - 116,2 100-137 44 1 15 150 131.4 - 105-165 45 1 18 157 140.5 - 119-165 46 3 17 175 149.1 160-195 112-197 47 1 15 171 162.7 - 134-202 48 - 22 - 166.0 - 122-230 49 1 12 182 183.5 - 144-220 50 - 19 - 205.3 - 152-287 51 - 14 - 227,0 - 170-303 52 - 26 - 244.4 - 200-295 53 - 23 - 247.4 - 185-327 54 - 27 - 266.4 - 178-413 55 - 29 - 279.2 - 218-340 56 - 25 - 292.0 - 240-340 57 - 25 - 310.4 - 255-375 58 - 23 - 317.0 - 265-405 59 - 24 - 358.4 - 290-425 60 - 22 - 360,2 - 290-440 61 - 18 - 380.9 - 300-460 62 - 5 - 406,6 - 352-520 63 - 8 - 385.0 - 340-420 64 - 5 - 448.4 - 375-595 65 - 4 - 455.8 - .30-470 67 - 2 - 481.0 - 462-500 68 - 2 - 465,0 - 450-480 75 - i - 620,0 - - 76 - 1 - 667.0 - - 20 FIRI/S80 Anguilla anguilla (Rev,1)

3/4/: Metabolism Jaeinsi (191) ami. aloe Llir P'ano (1940) otucliod, the pituitary gland of the ool Tho Metabolic rates. mt or dic coveredcertain oham200conne otod nith the ooleo biology9i luiin, the 6.ovoloy- Several investigators have studied the oxygen mont of tho gonade, consurrton of eels, Krogh (1904) found a con- 1Tith regard to oomotio rolc.tiono9 citten= sunption of 40, 45 and 60.5 mg/kg/h at 8°, 16° and tion muet be draum to Parry90(1966) ccmpilc' 25°C,respectively. Wienbeck (1979) mentioned a tion about aonotio aptaioa in fiohoo0 Zon consumption of 29.3 and 55.1 mg 02/kg/h at 150 and it como itomo nay bo montionod. hero, 25°C, respectively. After feeding these values Moat otrilcing changeD in tioouo conpoci- increased thrice: 91.1 respectively 147.5 mg tien have boon oboorvod. riagan eol'o lifOo 02/kg/h. Nicol (1960) mentioned 63.4 mg 02/kg/h The loptocophalue contciino 94 por cent uator at 16,5°C. Precht (1961) found for two eels kept elvoro paocing into oetuorioc bave 86 to 805 after one ruelt in frech ratos' 82 por ooat the at 18°C values varying from 49 to 67 mg 02/kg/h. untar oouont of mucoloc doolinoc from 73 por Boëtius and Boëtius (1967) studied the oxygen cant in yellow celo to 63 por cant in cuver consumtpion of captive silver eels at 12.5°C it ocio, The decline n untos' content io no amounted to 20 mg 02/kg/h.At about 30°C the coolatod. uxth fat and nitrogen noowsulatiom but also with ino: in e1ootrolytes consumption was about 111 mg/kg/h after acclimation for 3 to 9 days, and about 94mgwhen the Organic(gflcgwater) acclimation period was 10 to 15 days. At a nitrogea fat temperature of 220_25°C, oxygen consumption was Elvers 25 292 about 57 mg/kg/h. The authors concluded that Yellow eels 42 40 8 respiratory rate and heart rate of the silver eel show a maxirrum at about 25°C. Lie cìtrolrtoo (ceouiv, ¡hg rater) oodiuo potaco Iwo onloiwo nagn0 ohlos'.do Attention must be drawn to th statement of Gayer and Mann (1940) that fishes kept to. Elvoro 36 670 157 162 192 gather, consume less oxygen than they would if Voilou kept separately. This opinion is in full oele 63 890 195 171 276 arsement with the experiences of eel merchants, who manage to keep very many eels in tanks, The nucoloc of oele in ooaratorprior to the provided the eels are put in gradually and not opauning migration9 hayo ohoun a 10 to 15 per simultaneously, co-it incroaao in oleotrolyton.

Byczkowska-Siuyk (1958) calculated the Integrity of theuc in is important in nain- gill surface of a 1 kg eel to be about 9900 cm2, tainin internal, stability. Impaired oonotio which is extremely low compared with the gill control reoulte if the mucous ooa io not intact surface of other fish species. Krogh (104) as is the caco nith elvers. Salinity chango had, however, previously explained this dis- is followed by a grcua1 chango in blood concon- crepancy by showing that eels respire through tration with a grown eel 50 h io neodod for their skin, which provides for about 3/5 of the blood conoontraton to oabilieo within tho the total oxygen uptake, made possible by a normal ro, special capillary system,

Endocrine systems and hormones. Experimental, maturation of both male and Corrpared with yellow eels, silver eels show female eels with simultaneous increase of gonads, a lower cortisol level, an increased redistribu- growth of the eyes, atrophy of jaws, internal tion and utilization of fat, a changed carbo- atrophy in the head and changes of coloration hydrate and protein metabolism and an enhanced have been induced by several authors (cf. content of dark muscle fibres (Lewander et al,, Boëtius and Boëtius, 1980; see also 3.17). 1974). Attempts at fertilization succeeded in few cases, but erroryonic development ceased at the gastrula stage. FIRI/S80 Anguilla anguilla (Rev.l) 21

Silver eolo hava boon ato oc 305 pooially (cf. PaZTy 1966) Migrations and local movement Xt appoorohntthoi alinontary oonaloplayox 351 otnaotio rolo . 1nen filled with important liratiom000moc bo tho moat ioportent ooai7ator they absorbedthto 14 por cont of phenomenon in tha life cycle of the 0010 In thoir volume of iater from the medium0 Thia feet all oc,00 of eolo chow a moro or loso pcaBoiVo inZlu.O1 t7atoruao oaor in alinont- pronounced miratio12 and with. the oxooption ary canals from animals c.pted o soavator than in those from ammala in freohaater0 Thoro of theyelloceol otme all otaco ero fully io also a pasnivo ooclium oí'i'lui from the ut at eneed in porfornirnZ oxtonaivo migratory aovo-- rato 16 to 35 par cent in 2h0 It iraus- manto0 Only tho yellow ocio ohow O.iotict a which aro needfor fooO.in3 geotod that there ioaninitial loss of body osdontary poriodo uaor in the -gut followed by a loco of oodiuo and °nttenim0 from thout into tholoodend. thon aboorption of water fromtho ut fluid0 Prom whet io lnoun about colo the 000ln cien msv be oafoly drawn that theyhove ci close In tho cuverooi lomorular filtration connection with doapoea raGio000 $0mo opocieo rato eod fron 110o5 ni//ilay in frooh=' have ranoforred. their foodin eroe from tho wc,torto loso than 25 mla/dayin coaator coo to froohatozandthoso opooioo all chow the dofiitivo urina volumoo coro845and migratory movements which aro o logical eom' 15 mi roopootivoly0 This adaptation to a aouenoa of the combination8O.00poea spouîmin moro calinoonriromnent thun invoivoo bot crony otronZ oceanic ourrenZlowinj nuy from in lonorular filtrationroto and i thatarea,, ooactol area touched bythio ourret tubularr000rption0 and freohueertorou up enO. maturo in.

Unter anO. oodium ehiorido follow osmotie Apparently this combination iooosomtinl anO. ioule roíliontc0 The illo of eolo adapt for all the mircitory eel o3OAOOyimplying cO. to conuator loco oodinm ohloriO.otonti000 thatthepeculiar lifehistrey of the i?uropcan factor in oocvaator thanin frocherator, uhilo eel choco comoidorablo nioiherit,Yte that of thoso of froohuator ocio loco colt inO.opondont other opeoioo. ly of theecucontration radion0 It issuggeted that thlife history Adaptation to a given external mediun seems of the Luropean col is a recuit ocontinental drift,, oreatinaap between Amarice end to be important. Isaia and Hirano (1975) conclu- It may be ded that it brings about a decrease in the osmotic EuropeLrioa (c2. v.rbicch,, 1924). notedhero that the relation of all eel speoioa permeability so that water gain in fresh water or with doepooa roiono may be ro,ardodanpri-. water loss in salt water is minimal. nary, while the relation of sorno apaoioo with a freohuator area io eacondery. In the cneo

Tho poifi leone for nalt oohono wee of continental drift, thin would imply thatbo-- roctriotod by Ioys and Ullinor(1932) to aei fore the driftinZ started a doe000 area cae clophii collo tn the ill'opitholiwa at the alreadypr000ntbotwoanthe continente, which baoso of thoanollo The 'Koyatilloor hoc remained near the American continent. ccllo haveboonoubjeotoO. to an oarrioO. out by can »yc!s (1966) . ronorobio 3.5111iratory movomonte o2 the ineocco in tho ummbor of lCoyc.Tillner eolio larval or Loptoosphalue otaZo woo oboorvod ir injured oolc heinZ iz ooavatora the noon number ofofls perlamolla baso im After tentativo ouGootione by various uroaned from3q12 to 4.790 1oroovor the eolio anthem,, anor whom certainly must be mentioned ohano in mopoct9in siso and in number durim both Rodi and $pallancnni, who deduood that diZforo.t otoeo irthe dovolopmont ofho colo epaunod in the osa (of. Lortin,, 1956, o animalo whom thoso aro exposedto cator of 4),, it tian Soha4t (1922, 1y23) whooveloped different oclinitios oitho'under natwal or a theoio on the oiçjratory movements of the lar.- oxperimental eondition valeel. Thin wanbooed on numerous data col.- looted by him in the Atlantic and the Mod.itor- manean eitor the first lnx'va.=fincl in the At-. Sargent et ai (1978) continued the study on lantlo by hm !.n 190a off the Paroe Isles. structure and function of chloride cells in the eel gills and on basis of their observations pro- fli,mat ion otario at the spauninoree posed that sait punping in the gills stems basi- which Schmidt aoouried to be situated between about 220 and 30011, andabout 48°anO. 65°U, cally from an unusually high concentration of with the contrai part cit a lattitude of about sodium pumps delivering a very high concentration 26011,, ooixteidisZ with the depths of the Sar- of NaCl into a specialized space between adjacent cganoo Son, chloride cells that is essentially open" to sea water. 22 FIRI/S8O Anguilla anguilla (Rev,1)

rtoioon (1967) defined, the spatmin.g area With regard to the Atlantic journey,however, acme coro prooioo]4r during the "Dana"-oxpedition of doubt rises, Brongerama (1967) traced a case of a logger- 1966e Iwnorouo email larvae were found. in head turtle (Carotta carotta (Lo),which stranded at the April in the South.4oatern part of the area dea- Shetlands when being just over a year old and which must oribed. by Schmidt, end. only there0 Aa no lar- have been carried over the Atlantic by the prevailing veo tero found. during the preoeding February cruise, he conoludad that spavning beine at current streams in that period. Moreover,00iabining data the of aroh in this area, vhioh is situa- on the movements of derelicts that have floated from ted near thg interasotion of the Tropic of encan Atlantic waters to European ooasts,an earlier Ccer with the 60°W meridian, worker estimated the duration of the voyage to be 13 to 17 mo. This fits in nicely with the size of the smallest On account of their recent investigations, turtles recorded in British waters. According to oceano- Schoth and Tesch (1982) obtained results that are graphical evidence,the shortest time needed by a float- essentially in accordinace with Schmidt (1924), ing object to drift from the Florida Strait to Great especially true when latitude is concerned,Assum- Britain can be estimated at ten months,and some turtles ing that occurrence of 7 em larvae is congruent may have completed the crossing in that -timö,bizt under leas favourable circumstances the trip will take a year with the spawning area, it must be concluded that or even more. its western limit lies beyond 69°W, The course of the larval migration may be This evidenoe, assembled by Brongorsma, cloocribod beet in Sohmid.t'a otin ordaa "Spaun- does not tally at first sight with Schmidt's ing commences In oariy spring, lasting to veli ideas of the Atlantic crossing of the lepto- on in summer0 The tiny larvae 7 to15 mn cephalus, and the subject therefore needs mcx's long,float in tiatoriaysra about 200 to 300 in investigation. fron the surface, n a temperature of about 20°C, The larvae grow rapidly during their Latest developments in this respect are made first months, and in their first summer nverore about 25mmin length, They nora nove up into by German scientísts (cf. Tesch, 1982); e.g., the uppermostaterlayora, the great majority Kracht (1982) found that the distribution of I being found, botusen 50 and 25 n, or at times and II-group eel larvae is neither in agreement even at the ourfaos itself0 Then they com- with hypothetical Atlantic drift charts, nor mence their journey towards the choree of urop aided by the eaatrsrd. movement of the surface'-' with the assumption of distribution by either unter itself0 Durig their first summer, they Gulf Stream or North Atlantic Current. aro to be found. in the western Atiantio (west o' 50° long, T0)0 their second summer they This conclusion is fully supported by the hayo ttainod. n average length of50 to 55 mm, c the bul1 aro now in to central Atlantic0 age determinations on larvae otoliths by 1y the hi'd summer, they have arrived, off the van Utrecht and Hol.leboom (in press), who proved coastal banks of Europe, end aro noi' full- that eel larvae do not possess a uniform life groun, averaging about 75 mm in length, but history. As the otolith zones show the same still retaining the compressed, loaf-shaped larval form" (Fig, 3), structure as the annual zones in otoliths of Ang. rostrata (cf. Liew, 1974), the authors with Consid.aring the frail appearance of fair certainty credit European glasseels with an ",,htooephalus breviroetris", it is obvious age of 1-6 years, with a majority of 3-4.As that it loads a plaxiktonic, life, so that its distribution is determined by movements of the eel larvae hAve been observed without exception ourrcn-tm in the sea of vhich the Qullbtream with empty gut only, the question rises how they and the North Atiantio Current oro the most manage to stay alive and grow without feeding. important0 A solution is offered by Marshall. (1979), quoting This assumption explains the distribution Hulet (1978),whoafter close study of leptoce- during the winter months in the Mediterranean, phali of the bandtooth conger eel, Ariosoma where eel larvae have been observed as far east balearicum suggested that before metamorphosis as the Sicilian Channel and the Straits of leptocephali derive their nutrition from Messina, transported there with Atlantic water from the Straits of Gibraltar by the Mediter- dissolved and minute particulate material in the ranean surface currents, as Scmidt alrsad,y sea, and there is preliminary evidence (from presumed in 1912 (Schmidt, 192&; Sverdrup et tracer work) of their uptake of solutes. al., 1952). 80 n.1ir. e .1

60 50 40 30 20

Fig. 3 Showing the supposed distribution of eel larvae in the Atlantic (after Schmidt, 1928), The lines on the dotted areas show limits of occurrence of larvae; i.e. larvae less than 10 mm have only been found up to the curve denoted by 10; u.l. indioates boundary of unnetamorfosed larvae. See also: Schoth arid Tesch (1982) and Kracht (1982). number of elverS x 1000 45 25 -

20 -

15 -r- 4035 -

10 5 o r ç Fig. 4. Graph showingthe night the number of glassee}.s, annually caught at DenOever (Netherlands) with a dipnet at 2-hour intervals during 30

1940 45 50 55 60 65 70 75 80 year FIRI/S80 Anguilla anguilla (Rev.1) 25

Due to the fact that several year-classes of Jccording te Benin (1956) qutitiso of eel larvae are on their way at sea, they sce- elvero reaching the coastline show a definito times are called the migratory fish "par excellence", pOriodiOitym not only yearly but aleo a oix yearly. Lll voriatonn .nhc abundance of although oceanic currents might influence their elveren bin opinion nuo avo their otigin movements.Mien full grown, they stay in the in variations in ,hooat 7aeo. vicinity of the 1 fOO-m depth line around the Hiver oatchoo at Don Ocmver (Heilend.), European continent (Schmidt, 1928). (No data however, orrriod. out from 1938 onward in about this feature in the Mediterranean are otriotlyho nane woy, do not vovoai c'ny known.)Here they manage to stay and here they periodiolty citcill,, Tho name coaolueion nay metamorphose, i.e., they turn into elvers in be derived. from oatohoo at tho rlwor £nn (N. Ireland) end. tho Bavera (Eoglcnd) (of0 autumn. After having bec elvers the migration Deelder, 1952),(Figo4) is resumed in winter. Heermans and Van Willigen (1982), who It is of interest to speculate about the studied data of nearly 70 O glasseels (average cbi1ity of lophtocophal±do to r000nLnothe length: 73 mii; minimum: 56 orn; rnaxisnwì length: OOO dopth area no that they interrupt their 92 ma) along the Dutch coast, observed a reja- miatiom and otoy tero for nomo timo to nota-. hoeo n thin roopoot attention mny be tion between water temperature, migration and d.rautoootonber,°n idea that many polaio body length. Normally the water temperature is tol0000 are able to dinoorn different dopthn about L5°C at beginning of the migration period. Ncotonborg ('952) otatedhacertain Xndononian fioon ere able to otin reral].nl to a ooaet; After a cold winter, the migration starts later they ore even able to teop a].oua certain depth and lasts shorter than normal; in this case the contour (an the lept000phali do)0 Ho explained. animals are taller (up to 77 mn average).After thin foaturo not an duo to the nurf but to the a warm winter, migration starts earlier, lasts ruffled uater ourf ace oending vibrtionn down-. 'ocird which aro rofiootod from the bottom,, Thin longer; the glasseels are shorter, down to 77 mn will set up renonanon with an average wave- average. longth of four timen the depth, no that a part- Tho ni,rcition of olvoror.e oton orouced iouler 1ow-.freqzenoy rooOnanoo will oorreopond conoidorablo intoroei end. conooquoutly han to a certain depth, which r000nanoe ehould be often been thesubject ofociontifie otu,y, discornable with the fish' auditory and/or lat- enpecially in conel ucitero,thero invootia eral line system. tiono could bo ocrriod. out mont oonveniotly. Thnoo resulto,wIth nomo oztrepolatam, load 3.512Elver migration to a tentative hypothonin. Th.iring the period between the two mete- Beoauee tho aroci of metamorphooio in the morphosea of lcirvao tuz'ning to elvere end olvern Atiantio in at tho ecigo of the oontinontal turning to eels, the olvera do not feed (Joban-. ohoif, the migraion hero 000uro mainly in sen, 1905). Comroquently they dininieh in sine.water orean under tidal influauoo, and for thin roaoon one nay be inclinedtoaenure a Granai and. Calundruocio (1897) shoved with relation between olvor migration and tidoo. aquarium experimente that the metamorphosis of Studies carried out in the cocotal watoro of the eel lorae into elvere is accompanied by a the North Sea have ohownthatalvaro hero uno diminution in breadth and iength, The oharac- tidal movements, but one ohould not forgot teriatio of early-season elvers being larger that a groat many olvorn migrate annually in than the elvero arriving later is veil known the ilediterranean obero tidal infXouoe in too. Johannon (1905) measured elvoro in the praotioaily nil. Nevertheless elvers still North Sea and off Denmark and noticed a differ- manage to roach oreme as far n.e 4t or the ence of about 6 mm between mean lengths at the North of the Adriatic in winter or early start and the end of tho season0 Similar ob- spring. It ioapity that comprohoneivo servations have been made by Pagot (1923, see data about migration in the Lloditerranoan ore also LUbbert, 1930), Heidt and Heidt (1930), who lacking, an in our opinion renults from here observed a difference of' 6 between the mean may be revealing about the naure of olver lengths of elvers, and Menzies (1936) who pub- migration in the open seo. In thin respect lished graphs on the lengths of aivors reaching the observationo of Pc-got (1923neo aleo N. Ireland which show a decrease of almost 4 mm L1bort, 1930) deserve attention. He noticed during the season. Monzies moreover noticed that in Egypt, olvormmiration occurs in that at the end of the noneon the olvero are two distinct waves, w±th oomo menthe between. obviously thinner than before. It may be This feature contraste with the oituation in sa.fely concluded thst the proceDo of dotoriora- Western Europe and in the Western 1editor- tien during bho metamorphic otage io progron-' raneen, and in our opinion nuggoste that aive until the elvero otail feedingzain, olvers reach Egypt via two different migration and Schmidt (1906) put the total loon in routes in the eastern part of tho IodItor- length at 1cro, taking about a year. ranean. 26 FIRI/580 Anguilla anguilla (Rev,1) A000rding to Schmidt (1906) elvers can Nothing conclusive iknown about the be oovered every year in the Atlantic from die start of elver migration at sea; the end pnriod, September to November onward,andwe may as- when the elvere strike land, io strongly in sume that elver migration starts in this fluenoed by the water temperature (Tabla XV). period from the area of motanorphonietorardo inland waters. Schmidt also produced n map of In mild winters first elvers arrive, e.g., in Western Europe, later completed by Uniter (1910), Holland as early in December. Assuming which showed several areas and the rmst Lmportant the some etartir time, thia could lead to months of elver irrmigration, although in the the conolucion that elvero are able to achieve a much higher migration speedin thesea than South Atlantic regions freshly arrived elvers Nsyor-Wasrdon found, It io herd to imagine might be observed the whole year round (cf. how these tiny animals oucoeed in nohievi Charlon and Blanc (1982) for the French coast riuoh a velocity by their oun strength, whioh near the Spanish border). ooneideration mtregtheno theassumptionthat The data by ScFnnidt and Walter can be elvero uny uoe the soc currents, io. the used to calculate tho ,oigratioe epood at son. tidal strenna, for their transport. The latest attempt uno o by ieyer-11carden (1965)uho mentioned n value of about7hin per As stated above, the lver migration in day, but the cannot be mors than n rough result estuarine waters has been a subject of some conoidering the unoertaintieo of the beginning studies, selder(1960)from 1946 to51, and end of the migration. To shou th.e, one and also Creutzberg(1961)from 1955 to57, ozample may be put forward heros 6..idt (1906) observed the phenomenon that, superimposed up.- mentioned immigration into the Severn (England) ors a vertical doy-rs.ght rhythm, olvere suits in in February; Crautzborg (1961) and Iloyer- the upper water layers during flood tide and Waardon (1965) put the Severn in the December keep to the bottom during ebb tide, thus caus- area, while the commonopinion in the Severn ing extensivo inward migratory movements if area in that normally elvor iemigraion begtho Ílood coincides uith darkness. Hence it le hero in LIpril. obvious that elvers discriminate between the

TABLE IV

The dates on which elvere were first noticed at Den Oever, Holland, in s serios of years and the water-temperature of the coastal sea in front of Dan Oever. (After Deeldor,1952)

Elvers Temp,°C Temp.°C Pemp.°C Elvers Temp,'C

1938 1939 1940 1941 6.111 5.1 22,11 - 4.1 29.111 - 4,4 14,111 - 309 5,5 4,1 - 4.8 - 3.9 5,6 4,5 4.6 - 4.0 5,8 4.5 5,4 - 4,2 - 6.1 4.6 - 5.8 - 4.1 - 6,2 - 4.6 - 506 - 4.2 12.111 23 6,0 28,X1 43 404 4.1V 83 5,8 20,111 5 4.7

1942 1943 1944 1946 6,1V - 4.7 27.11 - 5.1 15.111 - 4.1 5,11 - 4,2 - 5.4 - 5.2 - 4.3 4,3 5,3 - 505 - 4.6 4.5 5,5 5,7 - 4,7 - 4,b 5.1 - 5,9 408 407 5,4 - 5.0 4,7 - 403 12.1V 2 6,0 5.111 11 5.4 21,111 3 407 11.11 3 4,7 1947 1948 1949 1950 5.1V - 4,6 11,111 - 4,5 11.11 - 4.1 3,111 4,7 - 4.4 - 4.8 - 4.4 5.1 4.4 - 5.0 - 4.3 5,3 3,7 - 5.1 4,7 5.3 4,4 503 - 4,9 5,0 4.2 - 4.3 - 4,9 5,3 11.1V 5 4.8 17,111 3 5,5 17.11 6 407 90111 16 5.6 FIRI/S80 Anguina anguilla (Rev.l) 27

two tidecihoa migreting in an estuary. Wheth- Creutzberg (1961) pointed out that in suoh e; in the open sea, they let themse].ves be a transition area elvers are carried to and fr0 transported in a certain dirootion by water by the tidal streams (if these are present) so movements and stay near the bottom during any that they staY in the same water mass of about water movement in the reverse direction, as sup- 8 to 11 per mille Cl and, a temperature of about posed by Croutzberg (1961), is still an opon 4°C and higher. question. Such a phenomenon, however, would not form an isolated case, since De Veen (1967) Lower temperature seems to have to be cor- showed that soles (Solea golea) discriminate be- related with higher salinity and reverse, tween different directions of the tidal ourrents Deelder (1952) noticed the disappearance of in the North Sea. olvers in a certain region during a sudden cold spell, Creutzberg (1961) noticed not on1y that During the estuarine migrations, Deelder during a cold spell elvers stopped their mi- (1952, 1960) could not find a relation between gration much nearer the sea than normally, slyer movements and salinity or temperature viz, in an area with higher salinity, but he gradients. With regard to temperature it is also made the remarkable observation that un- sometimes supposed that temperature differences der those circumstances migration with the are responsible for the migration of olvers ebb tide might be greater than with flood tide, from the sea into estuaries. The warmer water implying that many elvere were retreating from of an estuary should "attract" the elvers away the brackish water. (This again is an indi- from the colder sea, That this opinion is not cation of the sophisticated use which elvers true in all circumstances is evidenced by make of tidal movements). figures published by Poetma and Verwey (1950) for the water temperature at a certain point in From the foregoing it is evident that the the North Sea, at the entrance to the Waddensea area of migration delay or transition area can- and in the Waddensea, They showed tnat at not be fixed from year to year at the same exactly the time when migration is most intense position. It is obvious that.under certain (March to April) the average temperature is the conditions, especially later in the season, same at all those placed, so that there can be elvers move up as high as possible, that is no attraction by warmer water in the Waddensea just in front of barriers such as weirs or estuary. sluices, Deelder (1958, 1960) had the opportunity to observe several aspects of transi- In a series of well planned experiments, tion in much an area, viz, at the sluices at Creutzberg (1961) did not get satisfactory re- Den Oever, Netherlands. Those observations sults when trying to get a relation between may be discussed here in brief since the trans- salinity changes and the activity of elvers; ition seems to be an important st e in the he even observed an apparent indifferenoe to- migratory life of elvers, It divides migra- ards salinity. On the other hand he could tion in the sea from that in freshwater, and release a flood- and ebb-swimming behaviour by apparently is necessary to convert the elvers raising and diminishing the inland-water odour into the necessary conditions for living in content of the water, thereby proving that the freshwater. odour of inland water is a factor with which elvers discriminate between the tides in an At this place during every elver season estuary, (One cannot assume, however, that from 1938 onward, a haul was made with a fino this factor exerts any, activity in the open sea, meshed i m dipnet, at intervals of two hours so if it is proved that elvers have a tide- every night in about 5 s of water, The catch- discriminating ability there too, it must be to es (Table V) not only gave a comparison of the another factor). numbers of elvers im this brackish area during consecutive years but also revealed some other After havin2 arrived in an estuary or an area which can be considered peculiarities: e.g. that under certain oondi- as tions elvers may be present with a temperature estuary-like, the olvers cone to a delay of as low as l their migration in which the second metainorpho- or 2°C, but they mostly arrive in 'this area with a temperature of about 4°C, as sis or transition takes place. The factors we already learned (Table IV). One of the which define the area of transitionare not yet fully known, but it may be presumed that salin- most important items was the activity of the elvers in the upper water layers, which also ity together with water-temperatureare the most important. here turned out to be greatest during the night (Fig.5a & 5b). De Vean (of. Deelder, 1952) Transition may even take place near the calculated the average pattern of this nightly coast in open sea, as might be deduced from activity in four successive fortnights and observations by Charion and Blanc (1982) for showed that it is mainly determined by the the Atlantic, and by Lecomte-Finiger (pers. beginning and end of dusk (Fig. 6). In the culli!.) for the Mediterranean. daytime the elvers bury themselves in the bottom or they retreat to much deeper water of 28 FIRI/S80 Anguilla anguilla (Rev.l)

70 60 50 40

20 22 24 2 4 6h

Fig. 5 The average course of the nightly giasseel activity at Den Oever (Netherlands), plotted on Gaussian paper. During the season swirmìing glasseels have been caught with a i n2 dipnet, hauled straight from bottom to surface at 2 h intervals every night at the same place, just in front of the sluices.After Deelder, 1952 (cf. Figure 4 FIRI/S80 Anguilla anguilla (Rev.l) 29

astronomical dark

Fig. 6 The average couree or tno n1gtiy slyer activity at 'n Oever, Holland, during four successive fortnights of the elver season. (see also Fig. 5a), After Deelder,1952. 30 FIRI/S80 Pngui la anguilla (Rev.?)

about 20 to 30 n if available (Deolder, 1960). Another method to investigate how far an In this respoct the experiment of Van Heusden elver has changed its oónduot is to shine a (1943) may be mentioned. He kept elvers in strong light on it when it swims at the surface. an Lquariuw for a long time, and by turning Only at the end of the transition period are out and on the light in the aquarium room elvers attracted by a strung ligat. They are every day an hour earlier, he finally got an repelled by it at the beginning of the period, artificial night, and consequently "nightly" when they are, however, attraoted by a faint elver-aotivity in daytime, and reverse0 light, which induces them to congregate at a small distance under the surface in the cirole Comparison with catches made at the other of light. Elver flehermen in estuaries know end of the Afaluitdijk showed that no relation this phenomenon perfectly well, and for this exists between behaviour andhases ci' the moon, reason use a candle or faint kerosene light to although this is often alleged. Presumably çbtin thbiggest catches. Deeldor (198), this last opinion, is influenced by local situa- 1ie (1979) and Cantrelle (1981) described this tions in which tidal movements, which are re- resp. tortheSevern, Loire andGironde. lated with phases of the moon, play a decisive In this cnnection it may be mentioned role (Deelder, 1952). that De Veen (personal communioation) made preliminary observations with a partly covered Obvious differences in conduct between new- elver aquarium at the beginning of the transi- ly arrived elvers and those which have been tion period, using a movable faint kerosene present for some time have been shown by regular lamp. He showed that elvers at that stage observations in the transition area and lead to are attracted only by a certain light intensity the conception about the temporary dèlay in the and are repelled when this (small) intensity migration, These differences relate, among is increased or diminished. other characteristics, to vertical movements, schooling behaviour, artificial light, flowing The phenomena occurring during the transi- freshwater and orientation, which phenomena tion period undoubtedly indicate that in estua- have been checked by aquarium trials (Deelder, rime areas the elvors stop their migration for 1958, 1960). some time and resume it after a presumably physiological change-over has taken place, which After arrival in the estuarine area, results in an adaptation to freshwater life. elvers gradually develop tendencies to swim at the surface and to form schools, preferably This migration-stop is very conspicuous moving along the banks, Although newly ar- in small brooklets which flow directly into

rived elvers do not have the slightest inclina-- the sea, Mr. D.o'Leary (personal communication to migrate ainet a flow of freshwater, tion) from Arthiaorusha (Eire), stated in this and even try to swim actively out of it if they respect: "We find that large numbers of elvers get in by accident, they gradually alter to a arrive at the junction of the brackish water pr'ference for freshwater, This peculiar and the fresh in February and usually do not oh e-over offered an opportunity to collect enter the fresh until May or June ....". data during the transition period in an aquarium about the percentage of olvers swimming Deelder (1960) observed that elvers, when ainst or with a flow ci' freshwater, thereby entering artificial water-outlets, need not indicating their willingness to resume or do- migrate at the surface, On the contrary, at lay their inward migration (Fig.7). these places they seem to prefer to migrate at the bottom, If this is not possible it will take some days more before elvers start their migration at the surface, viz.7°c,according repeated observations by Deelder (1954a). FIRI/S80 Anguilla anguilla (Rev.1) 31

40 *

3°

20 *

/** 29 5 Io 15 20 25 301 A PRI t. 1956 APnII.

Fig,7 Perconts of elvers staying in a small aquarium provided with a oonstant flow of fresh water. The lower graph refers to olvers frishly caught in sea with a dipnet, The upper graph refers to elvers caught while climbing against s freshtrater-tricklet. All experisent lasted15minutes, except the first three of the lower series. Then the elvers had disappeared within50,100 and 140 seconds respectively. (after 1elder,1958). 32 FIRI/S80 Anguilla an utila (Rev.l

Once the olvers have finally arrtved in a served at a distance of 120 km from the sea sbcibor ars&their mirabìon, if continuad, (Deelder, 1960), which area, however, was occurs cb the nurfcice0 The orientatiOn of still influenced by tidal movements, so the these olvero sesee to be ci complicated phenom- elvers might have benofittad from flood-tides, anon0 In ooaplobsly still and pitch-'black (vi0 completely covered)Tate. areaeflash- cause of their oonspicuousnoss, not light ob ervatioiw of Deelder (unpublished) have least at obstadas which have to be surmounted, diooloesd that alvaro ccï'o scpabe an Orientating schools of calvare always attract much atten- themselves along the banks0 If such a water tion. Some vivid descriptions have been area is enclosed (e0g0 a ship-look) this oharac- given, including that by v.Siebold (1863), teristio results in an endleos swimming round and many attempts have been made to correlate and round, at a speed of about IO to 15 orn/seo0 their appearances with external factors. An cause of the total absence of light, visual elver school is obviously the final result of orientation must beirnpossiblo and it le for a simultaneous responding of masses of olvers bhie reason presumed that the lateral-line sys- to external influences within the estuary, tem plays an mporant rBle in the orientation. whereas the guiding (river-.) bank acte no a It not only enables them to looate the idang concentrating factor, A study of a aightod bankeven at a dabance of I n or sorebut elver school ahouJ.d therefore start from the also to dodge impeding objects ahead0 This transition area onward. presumption of course does act imply that visual orientation,phonposiaable2 is not applied, Statistical data on elvar movements at the Herbrum-weir la the river Eins have been Visual orientation of olvore eigraang up- analysed by !leyer and KLii1(1952to 53). river night have beanpocoablein the following They concluded that the phase of the moon, aituation (Dsclder, nn-acbliahed)o Ia '1955 tho air temperatura, air pressure, weather condi- reconstruction of the ?ash- and olverpass of tions, salinity and water temperature aro all the ri uec-oix' at Lith, Holland, wcicompleted0 without influence on the migration, although Elvora, ahioh had. climbed the olvor'paes suo- at the beginning of the migration season the cesafully, either oIoec to follow the riverbank water temperature must be of' decisive import- aG their riGht sido or the concreto das bel-usen anoe. From what we have learned above, it the alvar-pase and the usar ab thear loft side. is obvious that elvers require a certain rain- (Obviously the riverbank at the right was too imum temperature to migrate to inland water far away(2ni) to be perceived byhe last areas;this, according to Schineidler(1957, group)0 1963), is 9°C for the first few, although 10°C seems to be necessary to start full-scale Tho recuit uns that at the end of the darn migration. all elvers turned alonG it and oonseuently then swam tousrds the weir with its heavy over- According to Meyer and Kilhl (1952 to 53) flow, which swept the dyers downstream again. full-scale slyer migration only occurs at To prevent -l-hie uninbondod devaioprnsnl- a screen night and. especially when the tido io ricing; had to be bualt, running from the dam towards Ito 2 h after haghweter the migral-iontOp. the riverbank forcidistance of about20 cm, Thay calculated that the migration speed in The dyers now followed first the dam, then the the Erna is about2.2to2.5km/h, i.e.55to coreen (still cit their left aida) and at the 70ein/sec. This high value of coursa refers end of the acreen racked up ab their right side to the earth and must be mainly the result of the rivorbankwhich wan duly t'ollowed upstream tidal water movement, whioh feature gives uc together with their fellows from which they had a clue how to explain the start of the slyer recently aeparated. immigrat ion. A roughly similar situation, viz, at the It is commonly known that immigrating underside of the Eibweir at Geesthacht, has olvers are aware of counter-currents. The been depicted by Teach (1965). stronger the oounter-ourrent, the more alvers are inclined to swim close to the bank or even In natural circumstances continued migra- atop migrating at all, hiding themselves be- tion of olvers takes placo mostly in rivers tween stones, weeds and so on, until the and may cover considerable distances, In counter-current in over, They then resume Holland surface-migrating elvers have been ob- their original direction, which they keep once they have started their inward migration.

°' It must be borna an sind that the torni "freshwater" is used here solely to distinguish the invaded water area from the sea, without referring in the least to the salinity, In this respect we must remember that slyer immigration may also occur into highly brackish or even saltwater areas, e.g. the lake of Tunis (Heidt, 1928; Heldt and Heldt, 1929). Such knowledge, combined with that of the results of Creutzberg'a experiments (1961), strongly supports the idea that it is not the salinity but the odour of the water which influences the inward migration cf the elvera. FIRI/S80 Anguilla anguil]. (Rev.1 33

Moreover we have aen that olver activity in- spicuous when olvore como out of the wat.r and. creases considerably after sunset and that climb a wet area, Meazies (1936) observed at freshwater in a river mouth (of which the temp- the Bann woirs that elvors stop mounting the erature has ttained at least the necessary slyer-passes during frosty nights or with cold minimal value) occurs during ebb-tide, it stirn- northerly winds, No matter how great tho ulates all those elverm which are ripe for in- alvar run might bo, if the wind moves to the ward migration to rise to the surface during north, they immediately disappear from the the next night and start their sitration up-' passes end the run stops. stream en the current gradually subsides,i0o. whoa the water level is rising0 In doing so At the French Mediterranean coast northerl.y the olvors seek the vicinity of a riverbank for winds on the contrary exert a stimulating mf lu- guidance, thus creating the well-known huge ence on the elver-irmiigration into the étangs choolo of dyers, which profit from the upward- ly dire oted water-surge. (Finiger, 1976; Finiger (Lecomte), 1983). Due to these winds an outward superficial waten low is In this respeot it may be noted here that created into the Mediterranean, arousing the Teoh (1965), who studied elver behaviour in elvers being present there in their transition estuaries of rivers in Germany, noticed most slyer schools after nights when high-water at area. the upper boundary of the tidal flow 000urred. 3.513 Eel migration at midnight, Ho observed moreover that elvers may start their migration oven in daylight, Eolo, i.e. the fooding and fully pigmont- when the ebb 000urs in the early morning. This od animals into which elvers turn, may also migration breaks off during the i.fternoon0 show migratory movements, of which those per- formed in the summer seem to be of groat bio- }'rom what has boon dicussod above, one logical importance as they are roeponsiblo for gets the impression that the orientation of the 000upatzon of water oreas Lar from the sea. dyers is solely based on ono of the banks of This statement does not imply of oourso that the water-area oonoarnocl0 That this impras- all eels migrate in summer. This is olearly ezon is not quite true, however, hes boon proved demonstrated by the ovororoudod stocks of alow- by Neubaur (1933) whose oboorwaions indicate growin« eels in river mouths, hieh in some that alvaro aro wall able to icoop a certain casos (e.g. in Germany and Holland.) gave rise direction for some timo, notwithstanding the to full-soalo fishing, for trazisportig and dioappc arenco of the guiding bank. This may releasing in other waters for stocking purposes. be duo to the outflow of sidewaters from the water in which the migration takes place or to The summer-migration may occur from estua- tributaries joining it. Thun the elvers have riss up to the sources of rivers and is most to cross these branches to pick up the b conspicuous at the sol-passes alongside weirs, which they previously followed0 Neubaur ob- Moving upstream2 the migrating eels show an served that even great d.rops in aalinity need. increase in average sise, and consequently in not divert the elvore from their original course age. although afterwards the elvers have to swim into water with higher salinity ein. In this respect Trybom and Schneider's (1908) remarks may be mentioned about summer According to this author elver-orientation migration in the Baltic, which can be consid- may be explained by assuming that olvers smell ered. as a big river-estuary with several trib- the odour of the water in which they swim and. utarise. According to these authors the that they follow their original course if a phenomenon of increase in average siso in an joining stream does not smell as attractively. eastward. direction is obvious, They mention Neubaur's assumptions were completely confirmed the results of Nyetrhn who mesurod. 1,000 oels by Creutzberg (1961). at the Trollhttanfall (Geta Alv) in Southern Sweden and who found lecths varying between Finally we must draw attention to the 8 and 65 cm, with a majority between 20 and phenomenon that, once the elvers are on the 30 cm. The following table is based on these move, they nevertheless stop when the tempera- data and others given by Nordqvist (1917, cf. ture becomes too low. This is especially con- Wulff, 1921).

* It must be stressed that the immigration into waters around. the ltie should not be oompared with that in the Mediterranean, although this has been done sometimos (of. d'Ancona,1958). The summer migratory movements in the Baltic area are carried out by eels, nomatter what size they are. On the contrary, immigration into Mediterranean countries is carried out by elvers, ful- filling their non-stop trip from the area of metamorphosis toward. the inland feeding region. As eels and elvers form two different groups with strongly diverging characteristics, any comparison between migration of Baltic eels and Mediterranean elvers must be rejected. 34 FIRI/S80 Anguilla anguilla (Rev.1)

when the water temperature had not reached15°C. NumberAver e Avere Avor&e Moreover he concluded from hie experiments that ago length weight larger eels show less temperature sensitivity than small eels, and activity is decreased by Trollhgttan 183 3.7 year25.7 cm25.7 g low water temperatures. (GSta L) Mann who studied eel migration Motala near 114 5,5 304 34.3 (1963), ,Norrk3ping at the Geesthachtweir in the Elbe, concluded Alvkarleby 127 63 36.2 49.7 that eels, the majority with a length between (Da].X) 17 and 22 cm, started their summer migration at a water temperature of8to9°C. He more- In Finland, immigrating eels axe also big. over noticed a considerable increase in migra- Examples aro given by Trybom and Schneider tion intensity when the water temperature was (1908)who uotod measurements of eels from the high (about 22°C), and a decrease when the Kuno-elf, carried out by Nordqvist; the lengths temperature dropped. Mann(1961)also noticed varied betueen31and54cm, with the majority a distinotly higher migration intensity during between36and43cm. the night than durindaylight, a conclusion which Srensen(1950)had earlier reached from Ii is not known what part of the local eel the outcome of hiexperiments. This latter population participates in the summer migration. author remarked, moreover, that smaller eels Observations of Mann(1963)at the Geeathacht- are less inhibited by light than larger ones, woir in the Elbe showed that the summer migra- and thus axe more inolined to continue their tion may occur from the beginning of April till upstream movement in daylight. As such a late autumn (even till November-December), so migration may occur in huge numbers at the sur- it is quite probable that a considerable propor- face of rivers, this phenomenon does not fail tion of a local sel population may be on the to attract attention (of. v.Siebold,1863). move during some period of the "summer". Lü.bbert(1923)described an eel school Certain observations give the impression migrating in the jilbo, By close examination that summer migratory movements can only take he conoludod that this school had a length of place against a flow of water, Thus, in still at least 10 to 12 km, and a width of 1 m or waters, of which there ara so many in the low more, Although the depth of the dense school countries, a distinct directed summer migration was unknown it was clear that the total number starts only if considerable amounts of water of eels must have been several billions. from another area are discharged into it, Besides the summer migration, eels also In some DutchHpoler5t (e.g.reclaimed carry out other migratory movements, some of lakes im which the water im kept artificially whichmay be briefly mentioned here. In at a constant level) water is introduced in spring and autumn eels may migrate between summertime to compensate for evaporation, while their feeding and hibernation grounds. Fisher-- the excess of water is pumped out at another men who are aware of this habit try to profit place0 The peculiar observation can then be from it. made that at the site of water discharge eels try to invade the polder against the outflow Another well directed movement may occur and "polder eels" try to get out of it by swim- in spring towards the spawning places of ming against the inflowing water, coarse fishes to prey on the spawn. Again fishermen are the main witnesses of this phon- Preliminary experiments showed that an out- omenon, and, like the migration mentioned flow without turbulence failed to attract eels above, it cannot be assessed properly from a during summer migration time, To attract eels scientific standpoint since no adequate data by turbulent water, a simple apparatus may be are known. used, which is also effective in guiding elvers (cí. Deelder,1958),but it must be stressed Thirdly, short diurnal migrations in es- that water must be used from another natural tuaries seem to take place between resting area. places and feeding grounds. Koendzinsj (1958)demonstrated such a migration by putting From what is known with elvers, it is a number of longlines with baited hooks, I to tempting to credit the water-odour with mf lu- 1.5km from the mouth of a river flowing into ence in relation to the summer migration. It the Gulf of Riga. Observations covering20

is bard, however, to explain the upstream migra-- nights showed that in the evening most eels, tion in rivers of which one has to assume that with empty intestines, were caught by the up- their consistency does not change noticeably in stream lines; at sunrise most eels were a short time. caught by the downstream lines, their stomachs filled with a crustacean specieslivingin the With regard to the influence of temperature sea. and light, more consistent data aro known. Sorensen(1950)mentioned that at the Trollhät- Although the following subject does not tan catching station, from1919to1950no ap- belong exactly to the chapter "migration", it preciable numbers of small eels were noticed may best be discussed here, since it deals FIRI/S80 jtnguilla anguilla (Rev.1) 35

with the ability of eels to proceed straight In the low countries of north-western Eur- forward towards a certain area when liberated ope the migration may start in June and already after displacement. be in full swing in July. In some small water areas, this month may even be the peak of the In this respeot the marking experiments of season (Deeldei', 1957), but this is exceptional. Mann (1965) must be mentioned. He proved, that In most regions the peak of migration falls moro non-migrating eels, living in a river, tend to towards the end of the year, e.g. October to stay in their home range of about 40 km length November, and return to it if they are transplanted. This characteristic is also dis- cernable under laboratory conditions; e.g. Tesch (1967) greatly extended these experi- Boëtius (1967) demonstrated a pronounced autumn ments by tagging eels and releasing them in the maximum of activity during a nine-month agua- Heligoland Bight, Some of his eels homed from riuin experiment, in addition to a spring max- distances of 180 km, Moreover the eels de- mum of activity (Fig, 9), This last item may monstrated a high ability to find sm8ll areas, be related to the phenomenon that silver eels such as the rocks of Heligoland (wnounting only may interrupt their autumn migration and resume to a few km2), from far away places like Cux- it afterwards, Cold spells or freezing periols haven and the island of Fahr, at distances of immediately interrupt the migration, which may over 60 and 70 km, According to Tesch's cal- start again even well into the next year, up culations, his eels demonstrated an orientation to and even including May. A delay of similar with arz accuracy within 5. Finally Teach re- duration may occur in small Danish brooks after corded in two cases a minimum speed of Iand 3 a dry summer (Jacobsen and. Johansen, 1922), km/hour respectively. The attention must be drawn here to the Deelder and Tesch (1970)transplanted about phenomenon that sexual segregation may be ob- 2 800 tagged eels and observed that even over dis- served during the autumn migration. E.g. Moyer placement distances exceeding 133 km a fair nwber (1938) and Nolte (1938) both discussed the fact, well-known among Rügen-fishermen, that relative- of eels were recaptured on routes pointing toward ly many more females migrate at the end of the the home area. Single eels returned over a range season than at the beginning. According to of mere than 200 km. Moreover Tesch (1970) was Nolte the percentage of big eels fesalos) able to demonstrate that eels need not use their caught at Ragen in 1936 increased gradually from 39 in the first 10 days of September to olfactory system for homing, nor do they need 90 in Deoember, Deelder (unpublished) observ- salinity or other factors, as, e.g., terqerature ed an analogous phenomenon in catches if and light, as orientation clues. IJsselmeer silver eels, with no big eels in August 1964 and 11,0, 25.7, 46.5 and 60.0 por In agreement with persistent rumours, eels cent during the next months. are able to discover the nearest water area when liberated on land, even at relatively great The average size of male silver eels may distances, In this respect SchLffer (1919) also change during the autumn migration. In made several experiments in the water-land area that Oase the largest ones travel first. E.g. south of Stockholm; his results were rather Deoldor (unpublished) measured male silver remarkable. Even when he transported the eels eels in 1960 and observed an average length of over relatively great distances, many hundreds 36.0, 35.5, 34.0 and 33.0 cm in July and sub- of meters, and over hills, they still were able sequent months, to locate the nearest water surface immediately after liberation, even when the wind was blow- Safe conclusions about the daily rhythm of ing from them towards the water, miring these the migration can be drawn from experiments SchÍ'fer's attention was drawn twice catches, showing that it is limited to the by his eels to another water-course nearer than night. As a result of his experiments, Br.u- that he originally thought to be the nearest. tigam (1961) was able to define the limits more closely and stated that the daily migration of Presuming the ability of eels to recognize silver eels in freshwater (unfavourable condi- water from a great distance, no data are avail- tions absent) ocours almost entirely in the able about the factors or se'nses which makes hours between sunset and midnight, With cir- this possible. However, with regard to their cumstances favourable to eel migration, Br.u- tendency to change from time to time water for tigam observed that the main daily migration even concluded in the first period of nightly meist land (Bergmann, 1970) it is obvious that darkness, frequently in less than an hour. this ability is of paramount interest for their survival. The intensity of migration of silver eels 3.514 Silvereel migration may show distinct peaks, even of considerable magnitude, We may conclude from what is known Because of its great economic importance, now that those peaks aro the result of a typ- the migration of the silver eel has always at- ical ethological occurrence, evoked by the com- tracted a big amount of interest by many scient- bination of an internal migration-urge and exists, ternal impulses. As will be discussed below, these impulses may be of different character The migration season may well extend over and strength. several months, mostly during the second half of the year (Fig, 8a & b, Deelder, 1954), but It may be noted that, with a sequence of in fact migrating silver eels can be observed external impulses at short intervals, the last somewhere the whole year round, 36 FI1I/S8O Anguilla anguilla (Rev,1)

e , o, o, o

Fig., 6a The averre catchof silver eels in Dutch canals by 5-day periods, from1947to1952,expressed as percentaes of the total catch. (After Deelder, 1954), FIRI/580 iinguilla &nguilla (Rev.1) 37

z 2a o oo s 26

± 1 AUG. ± I SEPT ± 1 OCT. ± NOV.

Fig. 8bAverage weekly catch-percentage of silver eels in the Ijseelnieer in the years1950andl951 also showing periods of new and full moon. (See aleo Fig.lo). 38 FIRI/S80 Anguilla anguilla (Rev.1)

lunations number of escapes U9ó1 1962) 10 20 30 40 50 60 70

947/

8/125/1 6/i/2 6/33/

4/4 3/5 4/l%

Fig.9 Graph showing the escape-activity of aquarium-held silver eels per lunation through the total period of experiment. (After Botius,1967). FIRI/S80 Anguilla a uilla (Rev.1) 39 impulses of course do not release the sano in- of silver eels, scientists as well as fisher- tensity of migration as the first one, because men, is that exerted by the moon, For example, most silver eels ready to migrate leave the area Meyer(1938)and Nolte (1938), who published when the first impulsos are received. extensive data about silver eel catches at Ri'igen in the Baltic, mentioned that the best Not much can be said about the behaviour of catches of silver eels are made in'periods be- migrating silver eels, largely due to the shy- twoon the last and first quarters of the moon. ness of the animal, Lowe(1952)watched sev- Frost (1950) and Lowe(1952)also mentioned eral migrating silver eels in anEnglishbrook moonlight as affecting the numbers of silver and noticed that they swain head--first down- eels migrating in the British Isles, stream along the bottom, Jens (1952 to 1953), and independently of Deelder (unpublished) observed the be- him Doolder (1953,1954),analysed statistical- haviour of silver eels in a big aquarium during ly the relation between the phase of the moon the night and was astonished by their apparent and silver eel catches, the former of the upper- clumsiness, With slow snake-like movements, Rhine and the Baltic, the latter of Dutoh in- their round pectorals wide-spread, they seemed land waters, Both authors agree that the best to "glide" lazily through the water, giving the catches are made around the last quarter of the impression that all movements aro fulfilled moon (Fig. 10 and Table VI). with as small an effort as possible. The last quarter is that part of the lunar Factors affecting the intensity of migration month during which moonlight fails early in the night. As according to Brutigaia(1961)and Depth others, the migration of silver eels occurs only between sunset and midnight, it is tempt- Study of regular catches of silver eels in ing to assume that it is the absence of moon- different regions reveals the remarkable fact light then which stimulates the migration, that shallow waters coincide with an early summer migration, while deep waters have an obvious According to Jens (1952 to1953),however, retarding effect. Deelder (1957) mentioned the moon's influence is certainly not exerted several examples, including a striking one of through its light intensity, He basedhis two shallow canals in Holland loading to and opinion on the results of hi study of the from the small lake "Axnstelmeer", which has catches of silver eels in the river Rhino on depths of 10 n and more, In1954the percent- cloudynights. They have been classified ac- age catches during the months July to October in cording to the phase of moon and averages have the first canal werei 22, , 19and3,thus been computed which cover a whole lunar period. demonstrating a distinct majority in August. In Since they still exhibit a lunar rhythm, Jens the meantime the percentage catches in the sec- inferred that moonlight has no effect. ond canal, through which the former silver eels had to pass too, were z O, 0, 6 and per cent, This deduction might not be correct, An i.e. with a migration peak fully two months eel is subject to the influence of moonlight later, throughout its life, hence wo must consider the possibility that silver eels have become Instances of this phenomenon have been fully familiar with the lunar cycle, For found for more regions and invariably lead to this reason one may not assume that the lunar the conclusion that deep-water areas tend to effect does not assert itself because of' a hold up migrating silver eels for one or two temporarily clouded sky and invisiblò moon, months, whereas shallow waters are cleared of It might be supposed that direct moonlight may silver eels at the beginning of the migration set up an inner rhythm during the eel's life season. span, which finally operates whether moonlight is present or not, as is the case with other Trapping experiments, e.g. carried out by animals (Korringa,1947). Lowe (1952) in England, lead to theconclusion that silver eels congregate near the outflow of It must be mentioned here, however, that a lake and there await a stimulus (e.g. a the moon's effect does not appear to be felt "flood") to migrate further, This tallies well everywhere at the same time, According to with Frost's observation (1950) that silver eels Jon's data (1952 to 1953) the height of the evidently congregate near the outflow from Lough migration in the Upper Rhine occurs distinctly Neh (N. Ireland) before migrating on and thus before the time of the soon's last quarter, give rise to illicit seine-fishing. while in the Baltic (Jens) and in Holland (Deelder) it takes place after the last quarter. Moon No explanation for th1phenomenon has been found, up till now, In the Upper Rhine the One of the best-known relations, mentioned eels aro all females, whereas in the other re-. by nearly everybody dealing with the migration gions mentioned sales are very abundant or even FIRI/S80 Anguilla anguilla (Rev,l)

o oao 30' V° 28

26 Lake yssel

22 20 ta

16 C) o 14

12 Io 'dutch cna1s

nwbc

I 8 so V I I8 I 40 g V 3

o V vO 30 V : IJ) upper rhine V 20 D D V

o s Io IS 20 25 30 0 ) o O

Fig, 10Avorago catch ofilvor oela per autumnal lunation in Eolland andn tho Upper-Rhine0(After Deelder, 1954, and Jome, l952/53) FIRI/380 1.nguilla anguilla (Rev.l) 41

TLE VI

Escapo activity in relation to the phases of the moon, (vter Botius,1967)

Total exp. period Period 1O.IX-8,X1 Period ,XII.'20VI eeoepe % of escape- % of oocepc- % o number total number total number total

1ou moon 21 9 7 7 14 11 I'irotquarter 31 13 12 11 19 15 Full noon 42 18 17 16 25 19 Laut quarter 144 61 71 66 73 55

prodoiainato the poosibility thorefore exists d) Turbidity that the phenomenon io caused by difference of Dcx, Froot (1950) mentioned the reporte of Bann .i'ichermon hal cliscoloratAon of the n o) Waterfiow and/or level (N. Ireland),e.g0by peat bog drainee,, makes fishing better than under the came con- A000rdin to fiohormen,amincrease in dit ions without turbidity0 waterflou may be aecociatod with en increase in numboro ofmigrating cuver oele This opinion ) Deprossion..generated rnicroseisms oocmo to differ from that of coverei ooiontists. Jonc (1952 to53)examined the relation between Nolte(1938)studied diaries of Rìgen the catoli of oil7cr eolo end the water level in fishermen and concluded that during dark the river Rhine (Pigo ii) end reu the conclusion tormy nights unusually rich catches could be that moot celo aro caught when the water level made, although favourable wind directions ob- io higheot. Ho aleo mentioned better catches viously were not necessary. with rioinwater level0 ost(1950), and Lowe (1952) also concluded for British rivero Prost(1950)and Lowe(1952) alsostated that an increase in catch usually coincides with that on stormy nights silver eels in kitish a rise in ter level. rivers may migrate in large numbers0 Deeldor (1954)studied catoh resulto i Dutch canale In our opinion, however, itiSnot80much during the period. 1947 to52and cue inguichod. the high water level ao the inoreased flow whii 28cases of distinct increase in oatch thic encour ee the eel to migrate, In this oonneo- offered an opporuthty to check the entent to tion the many Dutoh "polders" must be mentioned, which weather condituono may be hold respons- low-lyic creee enclosed by a dike, in which ible for the occurrence0 He boned that in the water io kept at e constant level by pumpe every case, without excoptionweather con&L- (formerly by windmills)0 Ao coon as these be- tiens over the Nethorlando were governed by a gin to eject water out of' the polder, the water depression in tho vicinity. ]xaninationof in the polder oenale starts to flow towards the the particular factors commonly aosooiatod pumps0 hile the flow occurs larger catches with a depression, e.g. wind strength and dir- of silver cols are made, which feature suggests ection, air temperature, baromeirio pressure, that the cuver ocio ere etimuietod. into migra- cloudiness and precipitation, auled to show tion by the flow, since the water level remaine any which might be actingana stimulus to approximately the earns or us even falling migration. In six cases the prevailing (Bicker Caarten,1946), Obviously then it is weather conditions were in no way favourable the mere ed flow, which in rivers necessarily to eel migration in the opinion of fisheries ooinoides with a rise in water level, which experts, acts a stimulating factor. In this respect it is important to know.that Frost(1950)plot- Ono factor, however, was discovered which ted silver eel catches in the river Bann (LIre- may provide the stimulus, viz, microseisme, land) against the amount of water flow over a These are vibrations cf the ground and are fre- weir and showed a good relation between in- quently caused. by depressions at oea (Scholte, crease in catch and inorease in tho amount of 1954). The vibrations are probably set up by water passing. the forces exerted on the sea-bottom by the 42 FIRI/S80 Anguilla anguilla (Rev.l)

A /\ av. daily catch per year / - av. annual water level (maxau)

500

450

400

1938 39 40 41 42 43 44 49 50

Fig. liAnnual average daily catch of silver eels per Upper-Rhine fisherman compared with average annual Rhine-level at Naxau, Germany. (tfter Jens,1952/53), FIRI/S80 Anguilla anguilla (Rev.1) 43 high waves at the centre of the depression. The available data, however, point to the From there they pass through the ground at a assumption that silver eels in freshwater aro speed of several kilometres per second. They not uninfluenced by seawater. Those silver may be resolved into three components, two hori- eels tend to swim ainst a seawater flow zontal and, one vertical, and may last for hours. (again according to fishermen) which conduct is in contrast to that towards a freshwater The period of the vertical component de- flow. (See section o, and under). Moreover, pends on the depth of the sea, the amplitude captured. silver eels, transported in a fish- depends on the intensity of the depression and well show high activity when transferred to varies in Holland between iand 20 p. Depres- seawater. sions in the Atlantio produce microseisms mainly with periods of 6 to 8 seo;/ with those over Fishermen in Holland are of opinion that the North Sea periods are noticeably shorter seawater, artificially introduced so that it

(Fig. 12). Since there is a sharp division creeps over the bottom, stimulates the migra-- between the depths of the Atlantic Ocean (1,000 tion. As in this case depression-influence to 4,000 s) and. those of the oontinental shelf does not exist, one might indeed presumo that (loo to 200 m), the corresponding mioroseisms inflowirig seawater exerts a stimulating effect. in Western Europe can be put into two fairly distinct groups. Mioroseisms with a period of Factors influencing the migration route 4 seo and amplitude of cause pressure fluo- tuations at the base of a 10 m water column of Flow direction a magnitude of 2 dynes/oin2, i.e. 0,002 millibar. Silver eels migrating in inland waters Careful study of the available data, cover- where there is no obatruotion tend. to swim ing a series of about 1,000 fishing days, re- with the current if there is one. Deolder vealed the remarkable fact that the numbers of (1953)showed that the direction of migration migrating silver eels only rise sharply, without changes with the flow when he compared. oatoh- exception, after the 000urrenoe of microseisms results on both sides of a net barring a canal with a 3- seo period, caused by depressions over with an alternating weak flow. the North Sea and the English Channel. As we already learned the silver eel's The assumed. relation between mioroseisms migration activity is greatly stimulated by and. migratory stimulation implies that oaptive an inoreaso of the water current. The bio- silver eels should be motivated by mioroseisms logioal consequence of both phenomena obvious- too. In this regard the laboratory observa- ly is that migrating silver eels which have tions of Lowe (1952) must certainly be mentioned; themselves been guided by a watorflow invari- her indoor aquarium-kept silver eels showed con- ably reach the sea and. moreover do so with the siderably increased activity only on those least effort. nights on whioh free living silver eels migrated in large numbers. In opinion micro- During their migration silver eels indeed seisma, generated in the relatively shallow try to profit from water movements, which is Irish Sea, must be held responsible for this clearly demonstrated by the "schokkerfishermen", phenomenon. at the N.W. European rivers, e.g. the Rhine, They are keen to get the mouths of their Muir Nrana (1940) stated that in Norfolk funnel-shaped nets at the line of greatest it was the custom for eel catchers to keep a few flow, and, according to them, the catch hero silver eels in a perforated trunk in the water. is greater than could be expected in relation They did. not put down their eel-setts unless to the flow-rate. As the line of greatest these eels became restless, which was a signal flow in rivers mostly coincides with the line that the other eels were running and that the of greatest depth, it follows that one certain- night was favourable for a good catch. ly has to bar this region if one intenda to catch silver eels in rivera. f) Seawater As already disoussed, the tendency to According to fishermen's allegations silver swim with the freshwater current is reversed eels staying in freshwater are greatly s'timulat- when seawater flows into the freshwater region. ed. by inflowing seawater. This phenomenon is known especially from lagoons and river mouths Wind direction around the Mediterranean. Since inflowing sea-- water occurs at those places mainly during gales, The course of the silver eel migration in the question remains if the silver eels are aot- broad water stretohes, e.g. lakes, is obvious- ivated by these storms (see section e. above), ly influenced by the direction of the wind.. by the seawater or by both phenomena. Frost (1950) mentioned that a strong southerly FIRI/S80 Anguill.a anguilla (Rev.l)

b /f\(\ /ç \ \V( ''AM ç 7j'A-w o

Fig. 12Vertical seismograms, registered on the Galitzin seismograph at De But, Holland,

Day of microsaismio calm 31 July - 1 August1953 Microseisms caused by a depression over the Atlmntio, 22 23December1953 Microseisms causad by a depression over the North Sea, 8 9August1948

The relation between actual and registered amplitudes is 1 725 for b), and 1 575for o). 1 horizontal centimetre covers 20seconds, FIRI/r30 "'1 a auilltRev.1 45

wind in Lough ITeagh (x£rcland) mckee for good upatre whoa shining vertically downward f ihing in thecan river, uhiok floue from the theyre loss offeotive In her laboratory northern oido of the laize0 At Tindormero lolco experimento, silver eels choco a dark ohannol (nglancL) on the contrarya northerly uincl, !n proforonoc to a lit one0 In field experi- bicuing to?Tardo the oulet there9in regarded ments,onthe coatrory, nilver eels migraing oc having the cano offoot on the f idling itbo dotinstroom did not choose the de.rlc oido moro balzo' o outflou0 oftenthanthe lit one uhon o bright light vas presented0 A very remarkable recuit vas Djeldez'(1953)tried to gea correlation moreover thatrtifioiol lights hod moot of- bouoon uind dirootion and catohoc in the feet on those nights uhenho oele trere soot IJ000lmoor (Holland)0 Thio laize iobarred fron active0 the ooa by a 35 icu dikeprovided. vith a cot oí' oluicoo at ea end0 Conooqueitly a big fyko BrsLtigan (1961) experimented. extonoive- not f iohory forsilvereolo hoe developed horo0 ly at the Rcgen coast vith a rango of umboom- Gatoli reoulto revealed. thaiith strong uecrtorly edunderuaberlighto and. concluded. that on wj.ado relatively nary soro clivercolo (83Y2 of come nirhts ootoheo at certain oiee could be ba total) oro caught at the oaetern sido than inoreaeod several timeo0 in periodo uih moderato eactorly uindo(5O) One say preoueio that uiad strengtho aleo Of It is cerain that oliver cols may react importance here9 oc a otrong uind makeo itobf to artificial liglit how hio phenomenon io felt rnore than a uaok one0 evoked,however, ioby no moans known0 Uith out doubt the degree and, kind. of turbidity of Uind influence io clearly denono Grated in ho ue.tor playan,Lmporent r8lo, together the 3altio tOOOogoalong the floncoaiet with the intonelty and type of light-source0 (Rnoiphorot 930 floyo 1938g Iolte 1938) Bue to these factors auon othere,thewhole 7ith Ito oevoal boyo Lacing different diroc phenomenon io otili of ouch a ooafuoing nature tieso0 Windo undoubtedly inoreaoo eel oatohe thatnoreliable procedura hao been found for on thoco aoctione of to coaot touardo tihiob diverting cliver eels with light towar*o cer- they blotî0 Ao eolo ore oaubt here vrith fyko tain oatoh sites and so inoreasi the catches0 notocosneood tothe ehoo th.o phenonenon conoequently abo domonotrates that uith coastal Apart fron very occasional and scattered ujado the oliver oelo cono guito near the choro0 trials, noregular useof lighto influence According to the oano suhorthe uind influenoe ho commercial oach of silver solo io knorr, io likely to be trae mitted to the oele by the and. this provides the beet proof that thio difforont d..=genoratod. oea ouontsbut thio oyotem doesnotyet work satiofoctorily0Tho certainly d000 not explain the oobooloco ap- only oxceptione known arehoeo at (omaoohio proach to the choreo bo erlag he huge Beitioo near Venice and in Irish rivero0At Comacchio, fishermen used fires to reduce the intensity o) Aztifioial light of large mIgrations and to calm down the cliver eolo already caught (tralor, 191O)Irioli Persistent reports from f ishormen indicate reports toll of blacthgorcbeo placed. to that artifioial light maydeflootthmigration chock the run cl' silver eels while the neto routo of silver oels, e.gstre t lights along were emptied (Went, 1944). It must be rrtres a rivorb may onuse much reduced catch s at sod, howovor, that the oases mentioned hero the lit sido Petersen (1906)mentioned scv refer to barring and not to diverting the eral of these f iohermeno allegationo, and migratior Rumphorot(1930)too citan an example at RUgen in the Baltic9 uhere catoheo at a certain nito d). El O ctricity only inoreasod uhen the bathing season uoe Over and the ligh3 on the nearby promenado lied been With the development of modern technical eztiaguiohod0 It is of importance to notice facilitiesrpeople have naturally oonoiderod that all these caceorelatoto a number of faint the poesibility of influencing the miation lighte placed ueil abovetheuater surface0 An routa of silver eels with use of electricity, experiment of Potoreen (1906) ueing kerosene aiming at an increase in catches by guiding lampo oonfirmed thi.s deduction0 Then using a them tcuarde previously selected. catch sit0 strong light.beau in shallot? olear unter, The sore fact that thic, technique is not uni- Petersen moreover could clearly chou a ohunning versally used, to a cign bhat, up till now, of the light by the silver eele Hisoxperi sufficient knowledge about tbsubject is monte, houever, uoro not continued.0 lacking Of the trials nods here and. there, i'G is of neroot to mention hora one, oarried Lowe(1952) made several experiments in out in the river Shannon (Ireland) on behalf English brooks with underwater lights of 2 to of the Electricity Supply Board 4 W and stated that the migration of silver eels may be deflected 1y thom In that oase, A gap with e. width of about 12 s wae barred hover, the lightboams had to be pointed well by three nets, side byide0 In front of the 46 FIRI/S80 Anguilla anguilla (Rev,l) middle net a positive electrode and., more up- When considering Mèr's results plotted stream, at eaoh bank a negativo electrode was on a map, one is convinced. that the uninflu- placed. Tests were made at a terminal voltage enced. migration in the Baltic proceeds like of 250 and an electrode loading of 7 amps. The that of birds, viz. over a broad front in a gap was fished for ten consecutive 2-. hour certain direction. If such a migration periods with the electrode alternately "on" and. strikes an impassable area, in this case a "off", coastline, the phenomenon of the concentrated. migration is evoked 'running in In the five 2-hour periods with the elec- that direction along tue coastline tuat cor- tricity system switched "on", the catch of both responds most nearly with the original direc- side nets amounted to 31.8 kg each and that of tion of migration. (cf. Deelder, 1970). the middle net to 108.1 kg. The five 2-hour periods, with the electricity system switched. From the recaptures in experiments "off" resulted in total catches of 54.1, 73.1 (Fig.13), ii is obvious that the uninfluenced and 117.7 kg respectively. From these results migration route runs in the direction about one gets the impression that! a) not all silver 240° or W 30°S, so that the Baltic is definite- eels let themselves be guided towards the posi- ly crossed. This explains fully the concen- tive electrode but only about the half of them; trations of silver eel fisheries in certain b) the total catches without use of electricity area's, e.g. at the western side of the Baltic, are conspicuously greater. which are fixed by the areas of departure at the other side of the Baltic, i,e, by the big Obviously many silver eels are repelled. by river mouths at the eastern side. an electrio field. and seek other possibilities to escape. Mr's resulte, moreover, give rise to some more considerations. One is that the Uninfluenced migration silver eels certainly are not migrating with the help of a "sense of geographical position", Rea]. uninfluenced migration is only pos- which sometimes is ascribed to animale by sible in huge water-stretches, such as the sea. migration experts. In that case the silver As a matter of fact it is very difficult to eels should try to reach their spawning area catch migrating silver eels at sea, and if the by the shortest way, namely the great cirole eels tend to swim at groat depths it will be- route through the start (Baltio) and finish come technically impossible to catch them, the (Sargasse Sea) of the migratory journey. more because migratory fish can be caught best They then should. migrate in the direction by stationary gear. about N BO°W (Deelder, 1949), which isnot their migration course there (Deelder, 1970). It has been shown possible to catch silver eels in the shallow waters along the oontinent, A third region where observations on the e.g. in the North Sea, with fast-moving towed. uninhibited migration of silvereel.s have been nets, but from these rare catches no consistent data about the migration route or intensity of carried out is the Bay of Biscay. Here Tesch migration can be derived. (1978) liberated silvereels with ultrasonic transmitters and found the mean direction during Great quantities of migrating silver eels off-shelf migration to be about 260°, or W10°S. oan only be daught by stationary gear, e.g. fyke nets, standing at that part of a coast where the Comparing these results: 240° in the Baltic, eels ooncsntrate. If such a situation is used. for marking experiments, distinct results may be 340° in the German Bight and 260° in the Bay of acquired. Some of these investigations, Biscay, one has to consider the possibility that will be discussed here. sil.vereel.s are able to fulfill, the 'angular migration", i.e., in different regions different Iithma.nn and Mann (1958) marked. silver eels courses, like some bird species as for example and released. them at Gorleben on the Elbe. Two of these were recaptured on the Danish North Sea the Scandinavian Chaff inch, Fringil.la coelebs coast, thus showing movement in a rather unex- (Tinbergen, 1967). In the Baltic the 240° course pected direction, which result, however, was also leads the silvereels toward the outlet; in the obtained by Tesch (1974). He liberated silvereels, North Sea heading 3400 brings them to the Atlantic tagged with ultrasonic transmttternear Heligoland (apparently the Strait of Dover is not incorpora- and observed at 6 of them a swirrming direction of ted in eel 's migratory behaviour) and from SW 340°, or N20°W. In the Baltic the silvereel migra- Europe the 260° direction nearly coincides with tion shows quite a different pattern, according to the straight course toward the Sargasso Sea (from the illustrative results of the rrmrking experiments Cape Finisterre about 2550). carried out by Mär (1947) during the period 1937 to 1939. FIRIIS8O Anguilla anguilla (Rev.l) 47

Fig. 13 Map showing the resulte of marking-experiments with Baltio silver eels, carried out by Màr (1947). The places of recaptures show clearly the existence of oonoentrated migration along certain coastlines originated by a broad-front crossing of the Baltic in a direction about N 1200 W. (Enoirclod marks show the places of release). FIRI!S80 Anujlla anui11a Rev.

When discussing the orientation of silver A possible adaptation to life at the sur- eels it must be remarked that Edeistam (1965) face is seen in the pure white belly of the ascertained by tank experiments that silver silver eel contrasting sharply with the colour- eels possess a star-compass moehanism He also ed belly of the bottom dwelling yellow eel, tried to investiato the orientation of silver Such a white ventral side will be most effec- eels in the Baitlo by tagging them with long-S tive in the top water layers, preventing the stringed balcons before releasing them off the body contours from contrasting strongly with southern coast of Sweden, The uniform be- the light water surface; contrasting colora- haviour of the animals on each occasion was tion would undoubtedly result in attacking striking, Immediately upon release, they predators coming from beneath, In that case would often rush into deep water in arbitrary the silver eel would be helpless. directions and, sometimes continue on those courses for a few minutes, but within a short A surface migration would tally well with timo they would all, or nearly all, head in the the silver eel's ability to orientato itself by same direction. On two particular days the by using star-light (Edeistam, 1965), This majority of the eels had already chosen a dir- will only be of biological value when swimming ection before touching the water. They would at the surface. twist in the hands of the operator so as to point their heads in the final vanishing direc- Probably the star-compass mechanism is tion, and, then rush off in that direction im- made possible by transformation of the silver mediately on release, It could. be ascertained eel's eye before the seaward journey,Accord- that the precision of orientation was not as- ing to Carlisle and Denton (1959) sociated with the visibility of the sun nor and Strarrke (1972) this trans- with the timo of day or night. formation, in which the eye becomes larger with Marking experiments also offer the pos- a relatively even larger pupil, makes it a more sibility of getting an idea about the migration efficient light collecting organ, whilst also speed of silver eels, Ld.hmann and. Mann (1958) the increase in the total retinal density of mentioned. a minimal speed. of 13 km a day, fit- photo-sensitive pigment makes the retina more ting well with values mentioned. by Trybom and efficient in usefully absorbing the light in- Schneider (1908), who also found in their ex- cident on it, perimonts maximum speeds up to 50 km a day, to- In this respect it may be mentioned that gether with average top speeds of about 36 km a Tesch (1978) with his telemetric observations day. These data have been nicely confirmed. by MHr (1949) who found maximum speeds of 62,8 km demonstrated that silvereels are extremely sensi- and 54,5 km a day and. average high speeds of 25 tive to light. Vertical migration in sea takes to 30 km a day, The average travelling rate place at dawn and dusk over a range of several was much less, viz. 16.4 km a day, Consider- hundred meters. ing these values, one must bear in mind that these eels had. been caught by fyke nets, which The silvereels' surface migration in the were only emptied during the morning, so that the eels may have stayed for several hours in coastal sea, mentioned before, is the last aspect the fyke nets already, As the top speed. of of the eels' activity which can be observed pro- 62,8 km a day was developed by an eel which was perly during its life span. From certain obser- only 42 h under way, covering a distance of at vations, however, one may draw deductions about least 110 km, the possibility is great that the actual speed was higher. what happens afterwards. Although Bobtius and Bobtius (1980) treated silvereels with hormones and got Telemetric observations of Tesch (1974; 1978) strip-ripe animals of which interesting photo- also revealed swirming speed data:maximum speeds graphs are shown, one cannot assume that these over a period of i h: 415 km for silvereels and fish should be able to cover a huge stretch in 6.22 km for yelioweels, while Westinand Nymann the Atlantic with their belly full of swollen (1979) found 2.2 km/has max. speed for silvereel.s in the Baltic, There are strong indications that gonads. Hence, one has to conclude that the fish the Lu1tiJ migration Of silvereels takes produced by the authors are not in concordance place in the upper water layers, Petersen with those that cross the ocean first and then (1906) stated as his convictionas wøll as that of many Baltic fishermen "that most of develop for the spawning act. the eels go over the guides of the traps", Blegvad (cf. Sch. 1929), when on board. ofa ship equipped with a search-light, observeda distinct migration of silver eels in theup- per water layers of the Great Belt. He mentioned. the slow movements of the eels, with which they kept themselves up, while being transported by the fast water-flow towards the North Sea, 49 FIRI/S80 Anguilla anguilla (Rev.l)

Svirdson(1949)described four eels caught With regard to a discussion whether eel is in swedish waters in, what he calls, "partial able to cross the Atlantic or not to reach its nuptial dress". These eels undoubtedly give spawning area (cf. Tucker, 1959; d'Ancona, 1959a, the impreosion of already possessing many of the characters which might occur during the 1960; Jones, 1959; Deelder, 1960a; Gemeroy and trip through the Atlantic0 Boyden, 1961; Sinha and Jones, 1976b) the attention might be drawn to the study of Boët tus and All four had enlarged eyes, with a dia- Boëtius (1980) who concluded that the total meter of about 1cm; the pectorals were al- sufficient to most black, some of them very pointed and energy reserves of a silvereel are lengthened; moreover, the snout of the animals cover the costs of a4 000-km active migration seemed to be moro or less blunted. Finally, and full sexual. development as well. the ventro-lateral parts of the body were not as white as usual with silver eels but had as- 3.52 Schooling sumed a rather reddish-brown colour. The eel, in all its stages, is an indiv- Some years afterwards Rasmussen(1951) idualist, Virtually no schooling, where the also mentioned the catch of two female eels, individuals react towards each other, can there- both with big eyes and black pointed pectorals fore be noticed. The 'schools" of elvers and (one pair bilobate) and one with bronze colour- young eels which are observed from time to ed sides. Wundsch(1953)mentioned some eels time in estuaries and livers are the result with strikingly big eyes and a bronze coloured of a mass response to outward conditions and. ventral side not of active assembling. As those characteristics must be considered A certain phenomenon might be regarded as premature anomalies, they give some indica- as a kind of schooling, namely the massive tiens of the way of life which may be expected "balls" of silver eels which sometimes alleg- during the Atlantic journey. Undoubtedly the edly come downstream in a river, but this can very large eyes point to a deep-sea life, in be explained by their urge to keep ici close completo contrast to the silver eel stage to contact with an object, starting with some which we earlier ascribed a life in the surface debris and ending with silver eels themselves, water layers, This conclusion is in agreement (of. Nilsson, 1860), This phenomenon, how- with the much darkened underside of the body. ever, seems to be extremely rare and no com- A white belly would be of no uso in the deep sea prehensive data are available,

An excellentindicationof a future deep- 3.53Responses to stimuli sea life has been discovered by Carlisle and Denton (1959), who studied the eyes of eels. Environmental stimuli: Influence of They observed that the retina of silver eels mechanical stimuli, as e.g. vibrations, may change to a golden colour, like that of the be observed during migration; see3.51. photosensitive chrysopsius of deep-sea fish. The With regard to chemical stimuli it has already maximum of absorption shifts by about33mu to- been pointed out that eels are highly sensi- wards the shorter wavelength end of the spectrum, tivo to olfactory impulses. Teichinann(1957) and the final total retinal density curve of the showed that eels are able to perceive B-phenyl- silver eel is almost identical with that of the ethyl-alcohol n a concentration of one part oonger eel, Conger oonger (L.), and of a deep- in2.857x 1O. He concluded that a mono- sea eel, Synaphobranchus sp. Their conclusions molecular stimulation is highly likely. In have been confirmed by Straue (1972) and this respect the attention must be drawn to Pankhorst and Lythgoe (1983). a) the impressive bulbi olfactorii of the eel The peculiar change in the pectorals offers (Lissner,1923),which demonstrate the great us a tentative explanation for the eel's way of importance of olfactory stimuli; b) the en- life in the deep sea too. Strong elongated and trance of the big nose-cavity, which is tube- pointed pectorals indicate a behaviour that re- like and starts immediately over the mouth sults in swimming long stretches at constant (see also3.41). depth, which only now and then is changed and then kept constant again0 Tunniesand horse Muir Evans(1937)studied the brain of mackerels are good examples of this way of life. the eol and observed a remarkable resemblance to the brain of the common sole (Solea solea): Round pectorals, on the contrary, indicate a highly developed olfactory system together a swimming behaviour in which the level is con- with well-marked facial and somatic sensory tinually changing, so that the fish is well lobes and small optic lobes, which features adapted to overcome obstacles and find its way are related by him to the eel's feeding habit through them, Groupers, tenohes and silver by smell, taste and touch during the night. eels are good examples of this. This view apparently is justified, as The phenomenon that after the silver eel stage the eel acquires elongated pointed pector- Pankhurst and Lythgoe (1983) found the olfactory als is in our opinion a strong indication that organs to atrophy in matured eels, thus with it changes over to swimming for long uninter- feeding stopped. rupted stretches at a certain level. 50 FIRI/S80 anguilla anguilla (Rev.1)

For the influence of temperature upon Artificial stimuli: Highly repellent are migration, see 3.51. Botius and Botiu.a such artificial stimuli as objects drawi over (1967), in their profound study, mentioned the bottom of the water, as is the case with the influence of water temperature upon res- eel trawis. Regular eel trawling, which eng. piratory rate and heart rate, with a maximum occurred at the IJsselmeer up till1970,obviously at 25 to 26°C. Investigations of Deelder causes a spreading terror among the eel stock, (unpublished) showed that eels become highly resulting in diminishing catches, of eng. fyke active at a water temperature of about 200C nets situated near the banks, and higher.

Optical stimuli: see 3.51. ke has been mentioned before, eels shun daylight, although some activity may take place then. FIRI/580 illa anguilla (Rev.1) 51

4 POPULLT ION 4.3Natality and recruitment

4.1 Struoture No reliable data are known.

4.11 Sex ratio. See under 3.11 4.4Mortality and morbidity

Sex oomposition may vary exceedingly, from Mostly because of the secret life and, the nearly loo per oent males in river estuaries to different environmental conditions occurring in loo per oent females in tributaries of big the eel's area no reliable data are available. rivers and in coastal waters. Conpare, e.g., Factors affecting mortality are discussed under Penáz and Tesch(1970): the percentage of f1es 3.2, 3.34, 3.35 and 5.4, Concerning the physical factors affecting mortality, mention must is highest in the vicinity of Heligoland. It be made of olvors and eels migrating towards in- falls to the lowest value in the inner estuary of accessible or uninhabitable areas and of winter the Elbe and rises again above Harthurg to the mortality due to freezing of the water followed high value typical for trost inland waters. by lack of oxygen (Muus, 1967).

4.12Age cceposition 4.5 Dynamics of population Because of frequent neglect of the existence Deelder and do Veen(1958)tried, with re- of supernumerary zones in eel otoliths (cf. gard to the eel fisheries of the IJsselmeer, to calculate the consequence of a possible raising Deelder,1981),reliable eel age data are few. It of the legal minimum size. For this purpose may be presumed that no correlation exists between they built a 3-dimensional model using the age composition and such items as depth, time of following simplified formula, which is based on the ideas of Beverton and Holt day, season, etc. Density of age-groups is mainly (1957): determined by the size of elver inmigration or by stocking rates, See 3.31 and 3.43. tx-t- FR ...M(te.t2)F -(F+M)) -(F+M)q 4.13 Sian composition. Vw=e l -e

F+M I q-0 In enclosed areas, size composition is de-. pendent on immigration and stocking. In un- Vw s weight of total annual catch enclosed areas, in view of the eel's habit of F and. M: coefficients of fishing and natural migrating regularly, no consistent composition mortality occurs, See 3.43 and3.513. s moment of entering the IJsselmeer anolvers 4.2Abundance and dens it t1 s moment of entering fishery moment of leaving the IJsselneer as Calculations about the density of eel tA silver eels stocke, the eel being an elusive fish, are al- R i average magnitude of elver stock most impossible,, According to Havinga (1945), entering the IJsselmeer one eel per 9 mlived in the IJsselrneer, but weight of eel at the moment t. density in river-mouths may be far higher and Wt in tributaries far lower, Muus (1967) found The ,poulat ion in the community in Danish fjorde, at places with a dense vege- 4.6 and the ecosystem tation of Zostera, a density of 0.5 to5eels per 10 in2 and in wintertime he noticed once a This subject is sufficiently dealt with in density of eels per n2. 4 the preceding part of the synopsis. Under artificial conditions very high den- sities may be attaineds Meyer-Waarden and iCoops (1968) got an excellent growth of pond- eels with a density of 1.8 eel per in2. For the Comacchio lakes about1975Rossi(1979)cal- culated265eels/ha, producing a catch of 61silvereels/ha. 52 FIRI/S80 An:uilla anguilla (Rev,1)

5 EXPLOITATION fyke nets, eelpots, baited hooks. With the well-known fyke nets the fishermen make use of 5.1 Fishing oquipment the eel's tendency to swim along walls. A leader of netting guides the eels into the 5.11 Gears trap-net provided with funnel-shaped inlets Study of the extensive reports on this sub- which block the eels' return to freedom, Fyke ject by Meyer-Waarden (1965, 1966, 1967) and nets are considerably adapted to local circumstances z small ones may be used in the small- McGrath (1971) are strongly recommended. est ditches, really huge fyke nets are used in open sea (of. also Meyer-Waarden,1965,and Elvers may be caught either in an active or '65, f66, '67;Loebell, 1965). in a passive way, adapted to their behaviour. In the active fishery scoop-nets are used toge- Eel-pots may be considered as small fyke ther with a faint light to attract and concen- nets with a rigid frame, without a guiding system, The attracting agent is formed by trate the elvers, e.g., in rivers which are the bait put into the pots, or by the darkness invaded by masses of elvers as is the case in when the pots are tube-like with solid walls. southern and mid-European countries and England A useful combination ci' these demands is form- (Severn).Also nets drawn by boats are used in ed by the wooden tube-like baited eel-pots with two entrances, which recently have been open water stretches, as in the Atlantic off put in use in Holland with considerable success. the Fench coast.Sctijnes a seine net is pulled through an area with elver concentration, Baited hooks are commonly used and may be e.g., in Dutch shiplocks, cf. for France: Elle attached to floats separately or with a short sideline to a common long line which may at- (1979), Gantrelle (1981); for England: Fischer tain a length of several kilometres, and Liibbert (1911), Deelder (1955). In the passive way, migrating elvers are Silver eels. Because of their restless-. induosà by a small artificial tricklet of fresh ness, stationary gear is generally used. to water, flowing along a gentle slope, to break catch silver eels. In flowing rivers the the surface and mount the slope until at the great difficulty is to sit the eels cut of end of ittbe tumble down into a basin the water without being hampered too much by (O'Leary, l932). the floating dobris, so common in autumn, and Eels are caught in many ways, adapted to without barring the flow too much, as this will the peculiar behaviour of the fish in connec- result in deterring the silver eels or tien with local oircumstances«, Here too9- near powerplants, will influence the output catching methods may be classified into an act- of themin anegative way.Very ive and a passive category. long funnel-shaped nets, which possess a high water permeability, aro the only answer to this The most important active fishing gear is problem. These nets may be fixed in a rigid the eel trawl, which is forbidden in most frame, either connected to the banks or to a freshwater areas because cf its destructive ac- weir, so that the whole flow may be forced to tien upon fish stocks. A notorious example of pass through the net, or operated from an an- a freshwater oel trawlwasthat of the IJsselmeer chored ship as occurred so often at the river (Holland), remnant of the former Zuyder Zee. Rhino (Bi1rger,1926). An adapted type is the Wilifuil sea trawling for eels occurs e..g. in so-called "Scherbrett-hamen" in use on some Holigoland Bight. Sometimes in coastal waters German rivers, This net, among the largest travis designed for other purpose, e.g. shrimp freshwater-gear in the world, has the mouth travis, are used to catch eels. kept open by a trawl-type plate put obliquely in the waterfiow. (Meyer-4aarden,1965). Another active catching method is that with use of sel spears (Went,1952; Wundach,1962) When the waterflow is small, e.g. in and eel combs, either manipulated when walking brooks, an efficient fishery may be carried or from a vessel. Anglers may catch eels with out by forcing the water over a lot of horizon- baited hook or by 'bobbing", i.e. with a mass of tal gratings, where the water f ails through strung worms, which may be surrounded by a piece and silver eéls, together with other fish and of nylon stocking. Advantage is taken of the objects, stay behind, tenacity of the eels, to hoist them above water over a container into which they are dropped. In great open-water areas silver eels are Sometimes dip-nets are also used. mainly fished with giant i'yke nets, as e.g. along the coasts cf the Western-Baltic, Small A system which has come into use in recent areas with a narrow cutlet without a heavy times is fishing with an electrified scoop net. flow are fished in a easy way: barring the Fishermen may try in this way to reach inacces- cutlet with diverging walls protruding into a sible or unfishable placee, e.g. stony or weeded cat chroom. These walls of course may be banks of canals, and catch eels here which other- built of any suitable material available, wise would have escaped. Well-known examples of this type are the catching-sites at Comacchio (LCbbert, 1908). Catching methods with long periods of fishermen's inactivityare also numerous: e.g. FIRI/S80 Anguilla anguilla (Rev.l) 53

Again another catch-method in a small canai 5.42 Seleotivity is to bar it with netting during the evening and The oatohsize of the round and slippery night (to be lowered for passing ships) and eel is very dependent on the mesh--sizo of the catch the searching silvereels with fykenets, gear. Compo-raive inveetigationc on this set out near the barring net. subject have been carried out simultaneously Along the North-Sea coasts silver eel fish.. in the IJaselmeer (Deelder, unpubliohed) with ing in the autumn has been carried out in recent sol trawla of different mesh-size, viz. 14, 17 times by pairs of trawlers, towing one net be- and 22 mm, when stretched. The results of tween them, in this way obviously getting the these trials are shown graphically in Fig. 14. necessary speed, Of course hook size, when baited hooks Beosuse of the great diversity in local aro used for eel fishing, exerts a selective circumstances under which eels and silver eels influence too, So does the size of the bait, are caught, it is virtually impossible to des- the length of the connecting lines and the cribe exactly all compositions and components size-composition of the eel stook, As a. sat- of vhich the gears exist. All material avail- ter of fact the latter item also affects catch- able and. suitable may be used and is used, not es with nets, clearly demonstrated when catches only e.g. hemp, silk, cotton and. nylon, but al- of fyke nets with same mesh-size and operated so e.g. concrete, wood, iron, reeds, etc. in two separated water-areas were compared (Fig. LS,Deolder, 1968). Except for the uso of a faint candle or kerosene light with scoop-net fishing for ely-era 5,43 Catches no practical use is made of light, nor of other enta such as echosounding, sound, etc., when Because of the secretiveness of eel f isher- fishing for this species. roen it is almost iriwossible to get reliable catch data; hence, one must conclude that statistics 5.12 .ats (cf. Table vii) are highly untrustworthy A good The already mentioned diversity in circum- exariple of this aspect was recently obtained f mn stances under which eels and silver eels are the IJsselmeer fishermen in Holland.Officially caught, exerts its influence on the boats too, about 800-9EX1 t are caught annually in this lake These may range n siso from one-person paddle- canoes, as in use e0g0 in the Nhrstva,-delta ( 180 000 ha).When, however, the Goversment con- (Yugo-Slavia), to bhe heavy trawlers, able to sidered reclamation of a certain area, the f isher- withstand stormy weather at sea. ment started in 1983 a protest and all of a sudden mentioned a total catch about2½tthies 5.2 Fishin areas higher.Considering their standard of living, In many areas fishing occurs the whole year this amount certainly was not exagerated. round; in colder areas it is interrupted by hibernation of the eel (see also 5.43). For the sake of completeness, some data about catches are referred to here,Meyer Waarden Peaks of fishing for eels occur when the (l965a; 1966; 1967) in his compilation on the eel water temperature has reached summer-values, fisheries in West Germany presented catches in Fishing for silver eels and elvers only takes place during the migration (see 3.51). several parts of the country.In total for inland waters about 1 000 t/year.Saint Paul (1977) 5,4 Fishin, operations and results estimated the catch of small eels in the German Bight about 90 t/year.Later on Aker and Koops 5.41 Effort and intensity (1979) discussed catches in the same area:220- cause of the numerous catch-methods under 270 t/year, of which by trawling nearly 100 t/ very different circumstances no uniform data can year.According to Moriarty (1981) the catch of be given about the aspects of effort and intensity of the eel fishing,the mere as data pro- the Irish Republic amounts to 150 t/year. duced by fishermen with fair certainty are fully Rasmussen and Therkiidsen (1979) calculated for a unreliable (see also 5.43). Danish brook a production of 9.3 g/m2/year.Rossi (1979) mentioned for lakes near Comacchio (Podelta) With the former trawl fishing on the a catch of 19kg/ha/year, comprising 61 animals, IJsselmeer a higher fishing intensity obviously Finally Leopold and Bnifiska (1982) discussed an resulted in a lower yield (Deelder, 1965; 1970). annual catch in Poland of 2 477 t, of which 1 789 t recreational,A special catch aspect is created by elver fishing in the areas with accumulation during the inward migration, e.g., Churchward and Hunt (1977) mention an annual catch of about 100 t in the Severn Basin,whereas Cantrelle (1981) and Ehe (1979) discussed French elver catches, resp. at the Gironde (400-800 t/year) and Loire (much more than 1 455 t/year). FIRI/S80 ngui1jajg11a (Rev.1)

20 23 30 35 40 cm

Fig. 14 X th.froquonoioo ei' IJoo1oeooo1 ocught by s.1-txawli with diffoont .00oinoi, inho ood-end 14, 17 and2 zoopootio1y thontrotohoc. Sol-longthooundod off to tho neaiot ordownwar4s. 20-z A L u£ vr meer A / I 15 I A A I III ,#4. 00 ° fi / 4' A \A D D 23 Iz.ø 30 35 0 A A A A A AA A A Fig. 15 L.ngth-frequenoies of sels oaught by fyke-nets with sane tesh-sit. (vis. 18nn when stretched), CITi in IJss.laeez and Veers. Meer. Eel-lengths rounded off to nearest os downw;: :5 u,

10 FIRI/S8O An: lIla an:uilla (Rev,1)

TABLE VEI

Eel catches (FAO Yearbook of Fishery Statistics 1981, Vol. 52)

SPECIES FISHING AIEA ESPECE ZONE NE PECHE 1970 1979 1980 1901 ESPECIE AREA DE PESCA MT 1 NT RI

EUROPEAN EEL ANGUILLA ANGUILLA ANGUILLE OEUOOPE 143 (02)002,01 ANGUILA EUROPEA OLE

ALGERIA 01 5 5 0 0 ROROCCO 01 0 0 O O

AREA TOTAL SI S O O O

TURKEY 04 283 396 224 374

AREA TOTAL 04 283 396 224 374

DENMARK 55 44 34 113 1410 FINLAND 05 60 61 63 30 FRANCE OS . . .. . 2582 SERRAR DR OP 05 634 608 595 595 hALF 05 555 665 543 530 NETHERLANDS 05 740 552 690 723 POLAND 05 670 713 504 703 SPAIN OS 60G 300 300 400 SWEDEN 05 ...... -

UREA TOTAL 05 3313 2933 5390 3124F

RISO 07 1348 543 1079 176

UREA TOTAL 07 1348 543 1079 176

DERRAIC 27 2335 1826 2141 2121F FINLAND 27 15 16 16 8 FRANCE 27 65 1083 909 335 GEEMAH DIR AP 2? 209 SON 126 114 GERRARY PG 27 319 348 330 316 IRELAND 27 76 110 75 R'. NETHERLANDS 27 143 177 187 175 NORWAY 27 347 374 387 369 POLARD 27 271 294 406 49 PORTUGAL 2? 44 25 32 33 SPAIN 27 ... 66 44 148 SWEDER 27 1076 956 1112 887 USSR 27 170 89 161 139 5K ENGLI AAL 27 19 29 53 69 0E NO IRELD 27 858 850 1000 789

AREA TOTAL 27 5947 6451 6979 5646F

R080CCO 34 2 2 5 26 SPAIR 34 - - -

AREA TOTAL 34 2 2 5 26

FRANCE 37 1875 2061 1012 1591 ITALY 37 1682 1757 1721 1750 R GR OC CA 37 - 19 0 S SPAIN 37 55 94 - - TUAIS IA 37 90 93 423 441 TUSOS LA VIA 37 56 38 44 55

AREU TOTAL 37 5758 4062 3200 3825

STER TOTAL 14601 14387 16877 13171F

Codes: 01 African inland waters 04 Asian inland waters 05 European inland waters 07 Soviet inland waters 27 NE Atlantic area 34 E Atlantic area 37 Mediterranean and Black Sea

N.B. According to these data, there should be for exarre no eel catch in West German inland waters1 and no recreational catch in Poland (cf. 5,1o.3) FIRI 580 An uilla an uilla Rev.l) 57

6 PROTECTION AD MA1iA0E?NT

6.1 Reglatory (legislative) imeasuree 6.5Artificial etookin

6.11 Limitation or reduction of It io not posciblo to track all tito otock total catch ing-progrcnct carried out with ocie ond olvors, which aro of nat1oni, intornatonal and ovom No instanoen are known of limitation. on intoroontinontal nature. E.g. in 1969,50t the efficiency of fishing units or on total of olvore wore transported from anoo to Japan2 catches (quota), Limitation on the number of which country also regularly receives elvers from fishing units (fishermen), which say be observ- the Severn (Churchyard and Hunt, 1977). ed hero anti there in inland wato-areas in mor. resulting from the urge to ensure a reasonable Tvo instances important of elver stocking in living than from the desire to protect the eel European waters might be mentioned here.Leopold population. and Bnixiska (1982) discussed stocking of the 6.12 "-teotion of portion. of Polish inland waters and concluded that this lead populations to a real yield of 4.6 kg/ha, whereas 11.7 elvers are needed to obtain 1 kg (=2 eels). The protection of portions of eel populations in onforoed in only a few areas, with According to Jagsch (1982) the Neusiedler the idea of sheltering very small eels lost See has been stocked annually since 1958; since the incomes of the eel fishermen be endangered. 1975 with 4 million elvers each year, resulting The protection may be based upon enforcing a minimum length or minimum woight. Seine in- into a catch of about 210 t/year (7 kg/ha). stances may be mentioned hors i Holland, 28 cia; This means that about 17 elvers are needed to N,Iroland,5 on(D141.6 gr); Poland, 125 gr. produce 2½ eels, weighing 1 kg together. 58 FIRI/S80 Anguilla anguilla (Rev.i)

7 EEL CULThRE

Generai Remarks region, while also from, e.g., the Dutch Considering the results obtained by eel iJsselmeer and Danish waters young eels are culture farms in southern Europe, especially in reported to be caught for eel culture purposes. Italy, which are in operation on a solid econo- 7.2 Genetic selection of stocks mic base for several years, one must conclude that problems going with eel culture have been No genetic selection can be practised mainly, or even fully, solved there, This in because of eel's biology. sharp contrast to the opinion of eel. culturists 7.3 Spawning in the northern part of Europe, which needs explanation. It might be brought about that No (artificial) spawning is hitherto must of the northern eel culture plants operate practised in eel culture. on a small scale, diminishing the chance to 7.4 Holding of stock develop into a viable corrmercial unit. For example, Anon, (1984) mentioned the setup of a Either in open-air ponds or in basins under Danish eelfann, designed to produce 45 t/year, roof, In southern Europe both types are known; and Wickins (1983) explained that in 1982 in the cooler northern Europe regions indoor Britain's five main eel producers together culture with water recycling is preferred because of energy cost reasons, marketed nearly 100 t. Gehrke and Pelka (1982) in a study for northern European eel 7.5 Management farms with expected annual productions of 20 t, calculated a return of at must 4%, Successful eel culturists are not inclined provided the average eel weight is 230-240 g, to provide data on this subject, which is con- i.e., all eels should be females, which is sidered by them to be top-secret. Apparently hard to accept. special culture policy with remarkable results might be applied, as is demunstrated by Fig. 16, Comparing the figures mentioned above showing extraordinary lengths (average: 44cm, with the annual output of Italian eel farms SD: 3.14) and weights (average: 176.6 g, of hundreds of tons each, the largest produc- SD: 38.8) of male silvereels cultured in Italy. ing annually about 800 t for a long period, the conclusion must be that there is indeed a 7.6 Foods and feeding huge difference in initial setup between the See 7.5 two European regions. A serious drawback for northern eel culturists is formed by the need 7,7Disease and parasite control to apply filters for recycled water that has See Koops and Kuhlmann (1983). Quarantine to be heated; the whole system of course to of newly arrived young animals is known to be be situated in expensive specially designed practised on eel farms. buildings, whereas the Italian farmer even can use open aír ponds (in the Italian winter pond 7.8 Harvest eels do not stop feeding for a long time if With all means suitable for the purpose; any), anyhow may refrain from heating. Besides, even suction is known to be applied. considering the vastness of the Italian eel farms, the presence of slow-growing eels might 7,9 Transport be accepted mure easil.y there than in the com- Transport of live cultured eels is carried pact northern eel, farms, thus reducing culling out in the normal. way applied for eels: e.g., losses considerably. with motor lorries supplied with aerated tanks. 7.1 Procurement of stocks

As a consequence of eel's biology, eel culture is forced to start with either glass eels (e.g., from S. France or the Severn area, cf. 5.43) or young eels. Great quantities of these are acquired from the French Mediterranean Fig. 16. 36 38 polyncnialLength frequency regression: of 297 Italian cultured male silver eels. 40 y + Bo + B1x + 42 B2x2 + 44 ... + Bxfl.(x = length; 46y = N). Interrupted line represents the 48 50 R = 0.93 52 CM 60 FIRI/S80 Anguilla anguilla (Rev.1)

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This series of documents, issued by FAO, CSIRO, INP and NMFS, contains comprehensive reviews of present knowledge on species and stocks of aquatic organisms of present or potential economic interest. The Fishery Resources and Environment Division of FAO ìs responsible for the overall coordination of the series. The primary purpose of this series is to make existing information readily available to fishery scientists according to a standard pattern, and by so doing also to draw attention to gaps in knowledge. lt is hoped that synopses in this series will be useful to other scientists initiating investigations of the species concerned or of related ones, as a means of exchange of knowledge among those already working on the species, and as the basis for comparative study of fisheries resources. They will be brought up to date from time to time as further information becomes available. The documents of this Series are issued under the following titles:

Symbol FAO Fisheries Synopsis No. FIR/S CSIRO Fisheries Synopsis No. DFO/S INP Sinopsis sobre la Pesca N° INP/S NMFS Fisheries Synopsis No. NMFS/S

Synopses in the series are compiled according to a standard outline described in Fib/Si Rev. 1 (1965). FAO, CSIRO, INP and NMFS are working to secure the cooperation of other organizations and of individual scientists in drafting synopses on species about which they have knowledge, and welcome offers of help in this task. Additions and corrections to synopses already issued will also be most welcome. Comments on individual synopses and requests for information should be addressed to the coordinators and editors of the issuing organizations, and suggestions regarding the expansion or modification of the outline to FAO:

FAO: CSIRO:

Fishery Resources and Environment Division CSIRO Division of Fisheries and Oceanography Marine Resources Service Box 21 Food and Agriculture Organization of the United Nations Cronulla, N.S.W. 2230 Via delle Terme di Caracalla Australia 00100 Rome, Italy

INP: NMFS

Instituto Nacional de Pesca Scientific Publications Office Subsecretaria de Pesca National Marine Fisheries Service, NOAA Secretaría de Pesca 1107 N.E. 45th Street Secretaría de Industria y Comercio Seattle, WA 98105, USA Carmona y Valle 101-403 México 7, D.F. Consolidated lists of species or groups covered by snyopses issued to date or in preparation will be issued from time to time.Requests for copies of synopses should be addressed to the issuring organization. The following synopses in this series have been issued since January 1983.

NMFS/S134 Synopsis onbiologicaldataofthepigfiah,Orthopristis chrysoptera - G.H. DARCY 1983

FIR/S135 Synopsis of biological data on the grass carp, Ctenopharyngodon della (Cuvier and Valenciennes, 1844) -J.V. SHIREMAN and C.R. SMITH 1983

FIR/S137 Synopsis ofbiological data on the hawksbill turtle, Eretmschelys imbricata (Linnaeus, 1766) - W.N. WITZELL 1983

FIR/S125, Vol.2 FAO Species Catalogue, Vol.2.Scombrido of the World - 11.13. COLLETTE and C.E. NAUEN 1983 FIR/S 125, Vol.3 FAO Species Catalogue, Vol.3.Cephelopods of the Wocld - C.F.E. ROPER, M.J. SWEENEY and C.E. NAUEN 1984 FIR/5139 Synopsis of biological data on the school shark, Galeorhinus australio (Maclesy, 1881) - A.M. OLSEN 1984

FIR/S8IJ(Rev.1)Synopsisofbiologicaldataontheeel,Anguillaanguilla (Linnaeus, 1758) - C.L. DEELOER 1984 NMFS/5136 Synopsis of biological data on the skipjack tuna, Katsuwonus pelamis (Linnaeua, 1758) - W.ki. MATSUMOTO, RA. SK1LLMAN and A.E. DIZON 1984

FIR/S125 Vol.4Part 1.FAO Species Catalogue, Vol.4, Part 1. Sharks of the World - L.J.V. COMPAGNO 1984

FIR/S125 Vol.4Part 2. FAO Species Catalogue, Vol.4.Sharks of the World - L.J.V. COMPAGNO in press

NMFS/S133 Synopsis of biological data on grunts, Haemulon aurolineatum and H. plumieri - G.H. DARCY in press

NMFS/S138 Synopsis of biological dstn on bl,jc crab, Callinectes sapiduo - M.3. RATI-8JUN in press M-40 ISBN 92-5-102166-X