FISHERIES RESEARCH BOARD OF CANADA
Translation Series N 2505
The European eel
by V. Kokhnenko
Original title: EvrOpeiskii ugor'
From: The European eel, : 1-108 1969
Translated by the Translation Bureau(PHY) Foreign Languages Division Department of the Secretary of State of Canada
• Department of the Environment Fisheries Research Board of Canada Biological Station • - St. Andrews, N. B.
1973 •
162 pages typescript j,,._^^. P[^IR6 1.565 DEPARTMENT OW '' .-• ŸÂRY"OF STATE SECRÉTARIAT D'ÉTAT TRANSLATION BUREAU BUREAU DES TRADUCTIONS
MULTILINGUAL SERVICES DIVISION DES SERVI&CES CAPIADA DIVISION MUMULTILINGUES
I rRA4SLhTED FROM - TRADUCTION OE INTO - EN
511 ssian i;naY^.ish ^UTNOR -- AUTEUR _^- ^,^,------
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TITL' INENr,LISH - TITRE ANGLAIS
The Llttropean- eel
TI.TLE IN FOREIGN LANGU.fGE (TRANSLITERATE FOREIGN CHARACTERS) TITRE EN LANGUE ETRANGERE ( TRANSCRIRE EN CAP.ACTÉRES ROMAINS)
Evropeisk7 ï ugor'
P.EFERENCE IN FOREIGN LANGUAGE (NAME OF BOOK OR PUBLICATION) IN FULL. TRANSLITERATE FOREIGN CHARACTERS, REFERENCE EN LANGUE ETRANGERE (NOM DU LIVRE OU PUBLICATION), AU COMPLET, TRANSCRIRE EN CARACTÉRES ROMAINS.
As above
le ERENCE IN ENGLISH - REFERENCE EN ANGLAIS k :S above
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UN El'Y.T.Z1.) TilA'„,!\1:17)N For in 1-n c.-.I/ TRt.,,.E.)UCTUZ,N NON REV1SEE Izdaterstvo "Pishchevaya promyshlennost" Inform-Ai-ion (Food Industry Press) - Moscow - 1969
• THE EUROPEAN EEL
UDC 597.555.2+669.213
S. V. Kokhnenko
Edited by Dr. Biol. Sc!. P. A. Dryagin
5Os-'2 00-10-..31
7 530•2111-5532 TABLE OF CCUrEU23
• • 2
Preface 3
The Biology and Distribution of the European Eel 5
Systematic position and distribution 5
Eorphological characteristics 13
Life cycle ,0
Différentiation of sex and sex differences 40
Age and its determinatibn 51
Habitat and food supply 57
Dimensions and growth of eels 67
The condition factor 74
The nutritive qualities of eel meat . 76
Specific properties of the blood 80
Diseases of eels 83
Eel Breeding in Inland daters 96
Stocking material and sources of its supply 96
Stocking lakes with young eels
Commercial. return and prediction of the eel catch 111
ilethods of catching eels 117
The catch of eels in the epublics of the USSR 130
The profitability of eel fishing and the outlook for its
development in the USSR 137
Note on the Bibliography 141
Bibliography
A: Translation pf references in Russian 142
B: References in languages other than Russian 152 2
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^P': ^SE?.' ^Ir^ .-. ?7c^.',- :C7 PREFACE
One of the main problems in the science of fisheries is the development
of a•sound biclogical basis for improving the icthyofauna -end . increasing the fish productivity of inland waters. An important contribution to the accom- plishment of this task is made by the introduction of valuable fish species.
The European eel -- Anguilla anguilla L. -- is very promising from this point
of view. It is an excellent food with a pleasant taste, it is tolerant of wide
changes in its external environmental conditions, and it can forage in all manner of different lakes, reservoirs, rivers, ponds, and inlets of the sea.
The biology of the eel during the period of its freshwater life has been inadequately studied despite an extensive literature (Walter, 1910; Schmidt,
1932; Frost, 1945; Bertin, 1956). Although eel breeding is a very promising branch of the fishing industry, before it can be developed certain items of information are absolutely essen-
tial; the biological characteristics of the eel.durings its life in lakes, its growth and development, its sex ratio, its ecological parameters, its range of utilizable foodbateriels, its feeding habits in relation to other species of
fish, biological breeding techniques, and other problems.
.0ver a . period of 14 years (1952-1966) the author has studied the biology of the European eel reared chiefly in White Russian waters _'the - Braslav, Narocht, Uklyanskaya, Polotsk and Vitebsk groups of lakes, and in the rivers
Neman (Niemen), 'Zapadnaya Dvina (Western Dvina), Dnepr (Dnieper) and Pripyatl
(Pripet)j, and to a lesser extent in .the Baltic States. In 1960 the opportun- ity was taken of studying eel breeding in the Adriatic. In this book, drawn frcm my own experience and frcm data in the litera- ture, I have attempted to give a more complete picture of the biology of the
eel and to present the basic facts regarding the development of eel rearing 4
in the Soviet Union.
I wish to record my gratitude to Dr. Biol. Se. Professor P. A. Dryagin
for his valuable advice wbilf'. the book was written, to Corresponding Member of the Academy of Sciences-of the Belorussian SSR I. N. Serzhanin for assist— .
ing with the collection of material and with the writing of the book, and also to my colleagues while the work was done, notably to Cand. Biol. Sci. E. A.
Borovik for help with the work, for reading through the manuscript,.and for
valuable advice.
• THE BIOLOGY AND DISTRIBUTION OF THE EUROPEAN EEL
S,y-stemwtic Position and Distribution
The European eel belongs to the( Anguillifarnesorder. ^^r^, o1-^v ^em-
bers of this order have no pelvic fins, while some forms, such as the
Myraenidae, have lost their pectoral fins also. On the basis of this fact
some authors have called this order the Apodes.
The present-day fauna includes 24 families of the Anguilliformes, num-
bering about 300 species, which consist almost entirely of tropical marine
forms. Only members of the Anguillidae family, which contains only a single
genus -- the freshwater eels (Anguillashaw.), migrate to fresh water for
forage'. I consider that the name freshwater does not correspond to the specific.
biological characteristics of the genus Anguilla for all species of eels of
this genus are connected with the sea in their origin and reproduction. Most
individuals of each speciesforage in the sea and only a few of them migrate
to fresh waters for foraging. It would be better to call them diadroinous,
but since the name freshwater has long been established in the literature the
term will be used hereafter in this book.
There is no general agreement regarding the number of species of fresh-
water eels. Gunther (170) distinguishes 25 species, Schmidt (1925) and Ege
(1939) 20 species, and Berg about 10 species. Besides the European and
American eels, this genus also contains six other species found in the Indian
Ocean and 12 species in the Pacific Ocean. Of the 12 Pacific species 7 are.
found in the waters of the Malay Archipelago and on the northern coast of
New Guinea (Schmidt., 1932).
Schmidt (1913) based his classification of the eels of Eiarope, America
and Japan on the number of vertebrae and the number of rays in the anal fin (Table 1).
TABLE 1. Meristic Features Eels
tioc.no nymeR aonaboom 5— thicno noemouos .2_ oaaamore BHA
Or-40 3 cpelnee er—ao 3 cpenoce et-
angUilla . 178-249 215 111 --119 114,7
japonica . 200-253 220 112 -- I I 9 115,8 •4. rostrata . 167-229 190 103-111 107,8
KEY: 1) Species 2) Number.of rays in anal fin 3) From - to 4) Mean 5) Number of vertebrae
The same classification has also been used by Walter (1910), Kuznetsov
(1915), Ehrenbaum (1930), Eckman (1932), Suvorov (1948), Nikorskii (1950), Kokhenko (1954, 1958), Bertin (1956) and others. Considering the close sim- ilarity between the European, American and Japanese eels both in their morpho- logical structure and in their mode of life, some workers, including Berg
0949) and Shmidt (1947) regard them as subspecies of the same species. In an article published in the journal "Nature,' Tucker (1959) concludes that A. anguilla L. and A. rostrata Le Sueur are not independent species but eccphenotypes of the same species. He considers that all the European eels die on their way to the spawning grounds in the waters of their own continental shelf and that the population is replaced by larvae of the American stock which are brought to the European shores by ocean currents. I do not accept Tucker's hypothesis for the evidence he gives is not convincing. I have given a detailed criticism of Tucker's arguments elsewhere (Kokhnenko, 1965). Eels which live in European continental waters have several names: Anguilla fluviatilis Agass (freshwater eel), Anguilla vulgaris Truton
(common eel), but the usually accepted name of the specie Ls Anguilla
anguilla L. (the European eel). Since the specific naine of the European eel reflects its geographical distribution, I shall use this term in future. In England it is called."eel," in Germany, Holland, Denmark, Iceland and Norway
."aa]," in Spain and Italy "anguilla," in France "anguille" in Finland
"ankerias" or "airokas," in Sweden "alen" or "al," in Yugoslavia "jeryla,"
in Albania "balcha," in BUlgaria "zmiorka," in Poland "megorz," in Rumania
"anghila" or "tipar," in Czechoslovakia "uhor," in White Russia and the Ukraine "vugor," in Latvia its4tie,ll in Lithuania "unguris," in the RSFSR
flugort," and in Estonia "angerias." The eels are a group of unknown origin.
So far no intermediate forms linking than with other orders of fishes have
been discovered. There are several theories regarding the origin of the freshwater eels. Let us examine some of than.
Schmidt (1932) considers that the ancestral home of the freshwater eels /6/ is the Pacific Ocean where nowadays most of their species are concentrated. eels The Atlantic eels, in his opinion, have descended fran allti› Pacific. The basis
for this conclusion is the existence at the present time of manY sPeoies of eels in the Indo-Pacific Ocean regions and their high concentration in indiv-
idual places. For example, on the anall island of Tahiti, only 33 miles long,
there are three species of freshwater eels, whereas in the whole of Europe and North Africa there is only the one European species. Eckman (1932) postulates that the European, American and Indo-Pacific
eels originated in the early Tertiary Era from eels of the Tethys Sea. The •
similarity between the Atlantic and the-Pacific Ocean faunas in his opinion
points to a commnn origin from the homogeneous fauna of the Tethys Sea. 8
• The reason for the small number of Atlantic species at the present time is not that the region lay at the periphery of the Pacific Oceanic center but that it
underwent severe climatic changes.
Eckman! s hypothesis, which examines the formation of the faunas of the
Atlantic, Indian and Pacific Oceans as a whole, in the context of their his-
torical development, carries more conviction in my opinion.
According to Berg (1955) many of the families of fish which exist at the
present time appeared during the Creta.ceous period. Presumably, therefore,
the Anguilliformes were widely represented in the Cretaceous Tethys Sea. The
gigantic Tethys Sea occupied a large part of the present continents of Europe,
Asia, Africa and Central and North America, was connected by wide cha.nnels
with the basins of the 'present Atlantic, Indian and Pacific Oceans, into which
the Anguilliformes could enter freely and gra.dually colonize (Strakhov, 1948).
Although the Anguilliformes of the Upper Cretaceous have not been adequately
studied (Lebedev, 1959), the remains of eels found in deposits of the Tethys
Sea (Eertin, 1956) and several facts concerning the geographical distribution
of other aquatic animals, lis-ted by Berg ( 1934, 1947) , confirm this hypothesis. fObb-1,0.4.5 The European eel is widely distributed: from the North Capeito the Tropic
'of Cancer, from the White Sea (or even the River Pechora) to the Black Sea in-
clusive; it is numerous on the shores of the Mediterranean, North and Baltic
Seas; it inhabits the coast of Morocco and of various islands: the Canary
Island, Azores, Madeira, British Isles and Iceland (Berg, 1949).
From the Mediterranean the eel penetrates into the rivers of Southern
Europe, especially those of Italy and the Balkan Peninsula, as well as those
of Syria and Egypt (the Nile) ; through the Aegean Sea, Dardanelles, Sea of
Marmara and the Bosphorus it enters the Black Sea. From the North Sea it
passes through the straits of the Skagerrak and Kattegat into the Bal-tic Sea, /7/ 9
from which it enters the Gulfs of Finland and Bothnia. The extreme latitildes of distribution of A. anguilla are from 23 to 72°N. The Atlantic coast of America is inhabited by the American eel, which is found in the rivers of North America and, less frequently, in the rivers of Mexico and the Isthmus of Panama. Of the total number of eels 98% are caught from rivers flowing into the Atlantic and only 2% from rivers flowing into the Gulf of Mexico (Bertin, 1956). Acccrding to Jensen (1937) this species is also found on the West Coast of Greenland up to latitude 62° N y and it also inhabits the fresh waters of the islands of Bermuda. Anguilla 0 rostrata thus ranges between latitudes 5 and 62 N.
The Japanese eel is found on the Pacific coast of Asia. Anguilla japonica ranges between latitudes 20 and 44 0 N.
Besides this group (European plus American plus Japanese) of eels which inhabit the temperate latitudes of the northern hemisphere, an Indo-Pacific camplex is found in the southern hemisphere. Eels living in the southern hemisphere are called tropical or Indo-Pacific, and they are subdivided into Africano-Nalagasy, Indo-Malayan, Australian and Polynesian. The tropical freshwater eels are characterized by many different species living in the same habitat. For example, five species live.in New Caledonia,. four species in Australia, and three species on the island of Tahiti. Eels of the tropical zone differ greatly from the eels of the temperate -h-opicaj. - zone. The/males reach a length of 80 am and a body weight of 2 kg, while the females (according to Schmidt, 1932) attain a length of 2 m, whereas males of reack Avâs.iihui, the European eel, according to Smolian (1920) 1- - _ . length of 51 cm and body weight of 200-250 g. Tropical eels are of different colours and sometimes speckled. In individUal species (A. bicolor, A. obscura, A. australis) the dorsal fin is shortened, and in some of them it may even be shorter than the anal fin.
Because of the absence of seasonal fluctuations of temperature in the tropical zone, almost continuous reproduction of tropical eels may occur
throughout the years 9:71'=n:teel . For this reason larvae of different sizes can be found at the same time at the spawning grounds. The spawning grounds of tropical eels are close to the shores.
iiew.Quee, all the freshwater eels are very similar in their reproductive biology. Tropical freshwater eels, like eels of the temperate zone, become
silvery in the adult state and leave the fresh waters for the sea to spawn. They also reproduce in waters with a constant temperature. The larval stage is very short, and according to Jespersen (1923) it is 2-3 months for A. bicolor. The larva develops rapidly although it is much smaller in size than
the larva of the European eel. Larvae of the tropical eels, like those of larvae of
ineictivi - e , , eels from the temperate zones, are carried awuy by the current and ..aee-eenze4p-ï /8/ dpie4 into ' , glass" eels which go in search of fresh waters. , (1e.vorc4c .heo CuArrevrt Just as the Gulf Stream and the Kureiwnrisseminaià- the larvae of the
European, American and Japanese eels, the currents of the tropical zone of the
Indian and Pacific Oceans carry larvae of the tropical eels to the shores of
the Malay Archipelago, Australia, New Zealand, New Caledonia and the other
islands of Oceania. All freshwater eels, whether from the tropical or the temperate zone, are
adapted to the flow of the current. The sites where they reproduce are charac-
terized by specific physicochemical conditions of the surface currents and the 4rktr deep countercurrents. The.eakele. carry pelagic larvae to the foraging places while the latter return the producers to the spawning grounds.
Where such currents do not exist there are no eels, despite favourable conditions for foraging. For example, not a single species of fresh*nrater eel
can reach the east coast of South Ameri.ca, although the conditions for eels to
forage there are.no worse than on the east coast of North America, where the km.erican eel is widely distributed. For this same reason there are no fresh- water eels on the west coast of the South China Sea or on the west coast of
South Africa from the Gulf of Guinea to the Cape of Good Hope.
In 1874-1£3$2 the Gulf of California was stocked with young eels with the
aim of obtaining a progeny from them. However, the last fully grown eels were
caught in 1$94 without leaving a progeny, for no suitable spawning ground could be found on the Pacific Coast although carp, sardines and sa7mon were readily
acclimatized there.
Some non-Russian authors do not indicate precisely enough the Eastern limits of the habitat of the European eel. Schmidt (1909), for example, denies
that this eel can be found in the White and Black Seas, Walter ( 1910) considers
that the Eastern limit of its distribution is formed by the Baltic and wW'le Mediterranean Seas, 44-4&se Ehrenbaum (1930) places it on the north coast of
Norway, the Baltic and Black Seas, and so on. Schmidt attempts to explain the
absence of eels in the Black Sea by the presence of hydrogen sulphide at a depth
of 137 m. These arguments have been partially disproved by the Rumanian ichthyologist Antipa (1909), who concluded from the incidence of
eels in the Danube that they could reach the Black Sea from the Mediterranean.
The camparatively short distance for migration of the eel from.the Aegean to
the Black Sea cannot play an important role, and since the young eels migrate in the surface layers of the water the hydrogen sulphide cannot present al
obstacle to their entry into the Black Sea.
Berg (1916) gave many examples to disprove ScYmidt's conclusions. " After
the lapse of 16 years Schmidt (1925) agreed that the eel is found in the White .and Black Seas, but he considered that it penetrates into the Black Sea ony /9/ • via artificial water systems from the Baltic Sea. This is incorrect because
we know that eels were found in the Black Sea basin as long ago as at the end of the 18th century (dildenstâdt,l cited by Berg, 1916), and the water systems
joining the Baltic and Black Seas were not constructed until the beginning of the 19th century.
From the Baltic Sea and its gulfs the eel enters all the rivers draining into it (Kherm and Dement'eva, 1949; Sakowicz, 1952; Andriyashev, 1954; Kokhnenko, 1954; 19571:1, 1958, 1962b; 2hukov, 1965; Naumov, 1957; Voronin,
1957). Along the River Neva the eel reaches Lakes Ladoga and Onega and the River Volkhov, via the River Narva it reaches Lake Chudskoe (Peipus), and then
Lake Pskov, where it was observed by Soldatov (1938) and Petrov (1947). Along
the canals the eel reaches the system of the River Volga, down to its mouth,
as reported by Kessler (1870), Partsman (1870), Varpakhovskii (1898), Safgeeva,
Lebedev and Mitropollskii (1909), Sabaneev (1911) and Berg (1916). The eel
is four d on the Human Coast, and occasionally reaches the White Sea where single specimens have been caught in the Northern Dvina, Vychegda and Sysola
rivers.(Lepekhin, 1780; Kessler, 1865; Konstantinov and Sorokin, 1960).
Exceptionally, the eel is found in the lower reaches of the River Pechora
(Pallas, 1809; Berg, 1916).
The eel is found rarely in the Black Sea, but throughout its extent. It has been found in Odessa, Sevastopol', Kerch' (Berg, 1916); Kutaisi, and in
the River Rioni (Kokhocheshvili, 1941), and through the Isthmus of Kerch' it
reaches the Sea of AZOV and the rivers which drain into it -- the Don and
Kuban' (Pengo, 1872; Maisldi, 1950), and it also enters the River Dnepr (Dnieper) and its tributaries as far as Mogilev, Mbzyr1 and Pinsk (Kessler, 1864; Beling, 1914; Berg, 1916; Sharleman', 1954; Kokhnenko, 1954; Penyazi, 1957). The limits of distribution of the European eel in Eastern Europe are thus much wider than those given by non-Russian authors. In connection with the artificial stocking of the water systems of the Soviet Union with young eels, the boundaries of its habitat in the very near future will be shifted much further to the East. .For instance, in 1966 it was already possible to find eels up to 1 ni in length in the Caspian Sea, which they could have reached either from the reservoirs of the Orenburg Region along the River Ural or from
Lake Seliger, via the Volga water system. These watercourses were stocked with
"glass" eels in 1960.
Morphological Characteristics
The body of the eel is long and snake-like, more or less ciradlar in the
anterior part, but laterally compressed from the anal orifice toward the tail.
Pelvic fins are absent. The dorsal, caudal and anal fins form a frill-like band which extends over more than half (55% on the ventral aspect, 6'7% on the /10/ dorsal) of the length of the fish. .The rays of all the fins are covered by
skin.. In the Overwhelming majority of eels the caudal fin is protocercal and it is very rare to find an eel with a hanocercal fin. The pectoral fins are wide but short. The shoulder girdle is campletely separate from the skull as
the result of disappearance of the post-temporale. Because of this feature of the shoulder girdle the fins can be drawn into the trunk or to the head if
necessary, facilitating free movement of the eel in mud. The shape of the body and arrangement of the unpaired fins of the eel have evidently undergone little
change during evolution of the species. They correspond to itp mode of life and are adaptive in character. The swim bladder is spindle-shaped and connected with the intestine. Inside the bladder on the dorsal aspect there are twm "red bodies" whose function is that ora gas 'gland. The function of the.swim Ira p c co k vt&ro i o4 bladder in the eel, just as in other fishes is that of .
çeZ2.1=U-Ite.:1-4g The red bodies are very powerfully developed, probably in connection with the mode of life and specific features of the spawning migration of the eel. The scales are very small and hidden in the skin. The eel's head varies in shape but inmost cases is almost conical, slightly flat- Jo tenee-and rigidly fixed to the spinal column. It merges so gradually with the trunk that the two can be distinguished only by the gill slits. The total length of the eel's body is 7.5-9 times the length of its head. This figure, given by Berg, is Confirmed by my own measurements. However, the statement iAa 45y9a / made by Berg (1949) that.the distance-between the beginning of4 (see Fig. 1)
and upward, very much so in some individuals. The lips are On the jaws and vamerine bones there are smar conical teeth, curved towurd the pharynx and with the appearance of thick bristles. The velutinous teeth, which are D,x /lac much smaller than the eaxi1.1.arzy.., are found on the superior and inferior pharyn- geal bones. The tongue is and free. The branchial arches are very strongly curved and they extend along the head in the form of a letter V. This arrangement enables the pharynx to be greatly dilated when a large prey is swallowed. On their outer aspect the arches carry the gill processes, which. - 15 -
attain their greatest length at the point where the arches bend. Ile ells
F,re,covered by the skin-like branchiostegel membrane, which teminates in a small slit at the base of the pectoral fins. A relatively large gil1 cavity is thus formed.
Jr
il
•
Fig. 1. Scheme of measurement of eels. 1: ab -- pr -- greatest height of body; aq -- anterodorsal distance; af -- e...nteanal ‘thedorealpfh distance; qf -- distance from beginning of, e to beginning of'; - lee-tee4- .distance from end of snout to anal orifice; eg -- length ofie; qge' -- length • , 4- eciney,: 64. ,,eki 2 r- of .0;7 hb -- length of head; ac A ,v4; h -- length oW ad -- length of length of snout; mn -- height of head. 11: ab -- width of head (through middle of eyes); ft -- width of forehead; cd -- distance between res;
hp -- distance between anterior nares (4.ubes); rm -- diameter of eye; gq -- greatest body width.
When attempts are made in the.literature (Schmidt, Walter, Ehrenbaum, Ege, Berg, Suvorov, Pravdin, Nikol'skii and others) to determine the system-
atic position of the eel, mainly those features which can be expressed numerically are used and little or no attention is paid to formative features.
To establish the taxonomic position of the European eel more completely I prop- ose the following scheme for meaeurement (Fig. 1). Besides the features recommended by Pravdin (1939, 1966), this.schame is based upon other important features, particularly in ei tr-41 determination of the narrowness or wideness of the eel's head: 1) the distance between the anterior nares, 2)
the distance between the posterior nares; 3) the circumference of the head through the middle of the eyes, 4) the width of the head through the middle of the eyes, etc. The eels studied were taken from the White Russian reservoirs stocked in 1928-1939 and in 1956-1964, from the Baltic Sea and the Gulf of erland (Kurshskii Zaliv), from Albanian waters, as inell as "glass" eels im- ported from France and England. All the material collected was subjected to statistical analysis in the usual way (Pravdin, 1939, 1951, 1966; Rokitskii, 1961, 1964); indices were calculated as percentages of the body length and length of the head.
No differences were found in the numerical features for eels from differ- ent waters and belonging to different age groups, and in this respect our results do not differ from those of other workers (Table 2). Counting the number of vertebrae in 371 specimens showd variation frcm 111 to 119 with a /12/ mean number of 115. According to Schmidt (1913) and Berg (1949) the mean number of vertebrae of the European eel is 114.7 with identical extremes of variation of 110 and 119.
The colour of eels changes with their age and depends on the character of the body of water or aquarium in -which they live. In addition, other con- ditions being equal, individual variations are found in colouring so that in the same body of water or aquarium eels of different colours inay be seen. Theymay be olive-green, golden-silver, or very rarely, as Walter (1910) points - 17 -
out, speckled.
1
TABLE 2. Nimlerical Indices of Eels h'd _--_-^_
ItHTepaTypHbie Hawn âanxc+e naxeae ^ittc.te- n ir cP7a. cpés- 6f oTA0 nec )T-10 Hee
i-103S0}IKO6 371 1i0^ ^115,1 114,7 (Schmidt, 1913)^
OTaepcrilii B 6oxonoi't jw- u}n1 19 87- 104,2 (5epr,.1949) ; 110 ia â 3 .Tlytteli B P 339 15-21 16,6 17.4 (Bepr, 1949) ti Jlywit a D 160 230- 249,7 245- (Ehrenbaum, 1930) 278 275 3 Jlpte}^ B C 776 9-12 10,8 7-12 (Bépr, 1949)
j Jly4eït B A . 156 170-- 212,0 176-=' 215,0 (Schmidt, 1913) 235 249 i+ )ita6epHb1x' AyWAl 211 8-13 10,57 10,8 93^8Î
KEŸ: 1) Vertebrae. 2) Openings in lateral line. 3) Rays in.
From - to. 4) Bra.nchiostegal rays. 5) Our observations. 6)
7) Mean. $) Data in literature. 9) Source. 10) Berg, 1949.
11) About.
The eel larva is transparent. Pigmentation appears for the first time in "glass" elvers which are beginning to enter coastal waters. In "glass" eels introduced into White Russia in May 1956 pigment cells were present on the dorsum in the anterior part of the trunk and close to the tail fin, while the remainder of the skin was completely transparent. In eels caught frôm
Lake Drivyata on 2$ June, 1956, i.e., two months after the lake had been stocked, the wlaa-le dorsal region was pign;ented. The number of pigment cells was considerably increased ^`^-t^^r^---==^-d T' •j l, ' --' 1`+ ^a • The cells were like snowflakes in shape but they were dark brown in col.our. The number of pigment cells was much greater in eels caught in the same lake in October. At this time the eels in the ponds had a deeper colouring and the distance be- , tween the pigment cells was so small that in individual cases the processes of • the different cells almost touched one another. Eels from ponds and lakes differed only slightly in colour, and as a rule /13/ their dorsal region and sides were brownish-green in colour while their ventral region was white.
In eels below the age of sexual maturity the colour of the dorsal region is dark greenish or dark brown and even black, while the sides are various shades of yellow and the ventral aspect yellow or white. Depending onwhich particular colour is predominant, the eels are described as yellow or greea.
As the eels grow older they change in colour from yellow or green to become pale, when they are called silver. Usually in downstream-migrant or silver eels the dorsal region is dark brown or black in colour, the flank is greyish- white and the ventral region white. The whole body of these eels has a metallic gleam instead of the dull appearance of the yellow and green eels.
The eel's skin is comparatively thick and strong. rt covers not only the bcdy but also the fins, together with the rays, protecting the fish from various forms of harmful mechanical and toxic influences and even from poisons
.dissolved in the water.
The eel's skin is covered with a comparatively thick layer of slime, produced by special goblet-shaped cells. The contents of the goblet cells of eels have a thread-like structure, unlike the granular structure found in other fishes (Puchkov, 1941). If placed in water the tangles of threads break up and the threads themselves swell, thus forming a swollen mass of Slime.
The more strongly the eel's body is pressed, the more slime is secreted. This can clearly be observed if the eel leaves the trap or net thrôugh the mesh of the net webbing. The slime is a protective adaptation of the skin. It • protects the sldn and scales of the -eel from meohanical injury and from drying • and it also maes the eel slippery, so that it can surmount obstacles and,es- ---e--- t.) cape from traps. 4INIg=t;ez3i -taz,on)e-The expression "slippery as an
eeriusedr Even a powerful man cannot hold an eel in his hands. The eelts scale is cycloid, transparent, elongated and oval in shape,
and slightly compressed in the middle. Berg (1949) stated that its length is
2-2.5 mm and its width 0.6-0.7 mm. However, subsequent work has shown that
these are by no means the limits of size of the eelts scale. In my collection
there is a scale 8 mm long and 2.5 mm wide, and in eels of the older age groups
in general the scales are much larger than the figure given by Berg.
In their structure, shape, size and arrangement the scales of eelsduiffer
very considerably from those of other fishes. The structure of the scale is as follows: tiny cylindrical tubercles rise above the surface of the basal
plaque and are separated from each other by small incisions or grooves, forming
concentric, aval rows more or less parallel to the edges of the scale. The
earlier, inner rings (1-3) usually have 2-4 rows of cylindrical plaques, while
the later rings.consist of many rows. Several such rows are separated by a
deeper groove . to form rings, which are usually regarded as growth or annual
rings, although often instead of a complete ring there is only part of it at /14/
both ends or at only one end of the scale. Sometimes the annual rings are eetAmnewmge not laid down every- year, especially if ',fiat conditions are unfavourable.
The trunk, head and fins are cavered with scales. On the back and flanks
the scales are arranged in zig-zag rows, rather like a parquet floor (Fig. 2).
Each such "celle or group usually contain from 3 to 7 scales, and sometimes a
curved row will have up to 20 scales. On the ventral surface the scales are
arranged in parallel rows along the body. Characteristically the scales of eels do not overlap as they do in other fishes, but they lie free*" of each other and are èmbedded in the ski:n. This di: tinctive arrangement of the scales
and their comparatively small size are adaptive properties; the lateral flexi-
ciliby of the eel's body is increased and, consequently, the locomotor function
0
Fig. 2. Arrangement of a scale: a) yellow eel, age 0+, length 22 cm, body
weight 19 g; b) silver eel, . age 7+, length 67 cm, weight $CO g; c) individual
scales of an eel which had lived over 25 yeb.rs in fresh water (natural size:
length 7 mm, width 2.3 mm) . ie distributed'more or lees unifoxmly over the whole body. As à result of
this the eel has the highest motive efficiency (0.83) and the greatest (0.53)
reduced step (Aleev, 1963). This enables the eel to make more economical use of its energy reserves during its long migrations. The protective properties
of the scales evidently play a less important role in eels than in other fish.
Some workers (Puchkov, 1941) state that the eel's scales are incompletely / 1 5/ developed, but they do not explain why this should be so.
To discover what morphological changes take place in eels with age inves-
tigations were carried out on glass eels (Kokhnenko and Borovik, 1957c).
Comparison of these statistics with:the corresponding figures for older
eels (Kokhnenko, 1955) shows (Table 3) that the overwhelming majority of forma-
tive features increase with the eel's age. For example, the anteanal and /16/ antedorsal distances and the distance frau the end of the snout to the anus are on the average 3-4% greater in the older age groups than in "glass" eels. However, the dorsal and anal fins in the older age groups are shorter by the same amount.
Glass eels differ from adults in the more slender shape of their body, as reflected in the lower indices of the circumference, heightl and thickness
àf their body and also the smaller number of weight units per unit length of the body (atout 0.5 for the glass eel, on the average about 18 units for adults).
The relative length of the eel's head shows little variation with age, its height and width decrease appreciably, while the width of the forehead, on the other hand, increases.
In view of reports in the literature (Bellini, 1907, Walter, 1910) that breadnosed and sharp-nosed forms of eels can be distinguished as early as in theeàtage dIgM(glass eell I have analyeed my- own material from this point of view. Variance series of head width indices for glass eels as percentages '
- 29 -
of head lehgth are gl.ven below.
TABLE 3. Ccmparison of some Measurements for "Glass" and Eels
3 2.- c L2 ,4- KoeetiJa --- 111)1131ml; . B03paCT n 3NI !wimple cEtwo M-1-nt t pseaa
npolkeuTax ASIUUW Tema
'2_ 1-IZ11160.1b11111ii o6xpar CTemoniumuir 99 7,5-16,5 13,59±0,15 32,6 , Tema Bapocmuil 308 14,05-22,05 19,13 ± 0,08 '3 Haii6ombutasi Tompoitta CUKOOBILRUMfl 100 2,4-4,0 3,13±0,04 24,8 Tema Ibpoemwit 308 2,3-6,8 4,37+0,03 11aii6ombwan .131,1COTa CTUJI0BURHWil 100 3,0-5,8 4,35±0,09 17,3 Tema B3p0C.Iblft 308 3,05-8,03 5,91+0,04 ..-- AltTeLtopcambaoe pac- CTeKAOLIHRUNI1 100 23,0-36,0 27,83+0,21 11,1 : cToaulle B3p0Cablel 308 25,05-39,05 30,72+0,15 iLefloTeattambooe pac- CTeNnOBURHIa 100 34,0-44,5 38,56+0,17 22,9 CTOnlifle B3p0CeMil 108 39,05-47,05 42,92+0,09 PaCCT0511-111e OT D ,Ro A CTOKJOBIUHWft 99 6,6-15,2 11,73±0,18 6,1 B3pOCALA 306 10,05-17,05 12,9+0,03 Sl'accTonune OT KOHUfl CTOMOBHAHMft 96 32,0-42,5 36,45±0,18 18,0 puma ,11,0 aoyca- B3pOCAUft 308 36,05-44,03 39,79+0,06 q,U.nrilla P CTOMOBIUfflà 76 2,5-5,9 3,18±0,07 12,2 B3p000Mft 308 2,8-5,8 4,16+ 0,03 D CTeKa013W1Hblii 100 66,0-76,0 71,73+ 0.19 18,9 Bapocmuil 318 59,05-73,05 67,36 ± 0,1 D,mitua /1 CTeKnOWIRliMû 100 50,0-64,0 39,95+0,23 17,4 133pOCAlat 315 31,05-60,05 55,77±0,08 11.1inia C CUKAOBWMfi 30 0,3-2,5 1,2±0,01 5,0 B3p0C1blii 315 0,5-2,3 1,25±0,01 roaoabt / 0 CTOKAOBH.RUblii 100 9 ,8-14, 2 11,65+0,10 2,27 B3pOCAlat 320 9,3-14,3 11,411-0,05
Ii g n p0 I euTax a JIIIIIbI r o.ao B bi '01 rt e- t.35 0,L) IL) ID C Ce?
J1ittna pbuta I2- Creluroauouri 98 11,1-28,5 18,67±0,33 2,11 Bapocmiiii 324 14,05-24,05 19,39±0,09 BbICOTB r0J1013br CTeKnoalunufi 97 16,7-31,0 24,01+0,29 11,3 Bapocomfi 311 15,05-27,05 20,37±0,13 Ulupitna romoebt CTemommuurt 05 14,6-33,5 25,73+0,34 11,9 B3p0e.ablâ 313 16,05-30,05 21,42-L0,12 Illupiuta .n6a / CTeKAOBIUTHWil 94 5,4-20,0 11,21-1'0,30 18,2 B3pocablii 326 12,05--23,05 17,05+0,1 Ruamerp masa fLo CremoBwantri 97 5,2--12,4 8,98±0,15 1,23 Bnoc;11.4 323 5,05-14,05 9,19-±-0,09
KEY: 1) In percent of body length. 2) Greatest circumference of body.
3) Greatest thickness of body. 4) Greatest height of body. 5) Antedorsal distance. 6) Anteanal distance. , 7) Distance fram D to A. 8) Distance from end of snout to anus. 9) Length of. 10) Length of head. 11) In
percent of head length. 12) Length of snout. 13) Height of head.
14) Width of head. 15) Width of forehead. 16) Dianeter of eye.
17) Glass. 18) Adult. 19) Variations of empirical series. I - 23 -
Fig. 3. Variance series of head width indices: 1) glass eels; ,_ii iL:J. t. E" .
;i '^,5 ^27,5 20.51 31,5 33's 113,5 1915 °1.5 I GIa 5ses 1 1551 ' -I- I ~ I^^° - I - 1 I _ 1. I I 1 '1 rru1„ber ^f 1 3 6 12 20 2G 1 ï 6 ^ C ases ^
5
^5 11,5 i3r5 15,5 -17,5 19,5 Z1,5 Z3,5 3,5 7,5 Knaccbr
0 Key: 1) Incidence 2) Ces ,^0a-0t Gw40" ,t,4 6'-f
It cannot be concluded from these results that there is.any differentia-
tior. of gla.ss eels into broad-nosed and sharp-nosed forms. This state of
affairs is illustrated more clearly in Fig. 3, where the data of the varialce /17/
series for head width are plotted for adult and glass eels. The same result
height of the head, width of the is obtained with respect to otrer features:
forehead and length of the snout. -
Variance series for the shape of the head of eels obtained from different
waters show that the sharp-nosed and broad-nosed individuals behave as extreme
variants ( 7'able G.) .
- -
TABLE 4. Variance Series of Certainlvlorphological Indices of Eels, %
BapHauminal vu / n
IIAHHa ro.noBw KO Bc9ii Annme 2- 9,0 -- 9,5- 10,0 -- 10,5 -- 11,0 -- 11,5 - 12,0 -- 12,5 -- 13,0 -- 13,5 -- 14,0 -- 14,3. 11,41-0,05 320 1 19 70 71 75 39 22 ' 13 .5 4 1 . LUnpuna roaonu « RAOHe roaonu 3 • i 15,5- 16,5- 17,5 -- 18,5 -- 19,5 -- 20,5 -- 21,5-22,5 . -23,5-24,5-25,5-26,5-27..5 24,421-0,18 341 3 12 18. 30 , 54 54 49 43 20 14 _11 3 • . LUnpuna a6 a K .a.anne rononu Lt. 11,5 - 12,5 - 13,5 - 14,5 - 15,5 -- 16,5 - 17,5-18,5-19,5-20,5-21,5-22,5-23,5 47,01f0,1 .326, • 2 7 23 42 59 60 62 37 27 3 2 2 BmcoTa roaonm K namie rOJOBW 5- . 14,5- 15,5-16,5 - 17,5-18,5-19,5-20,5 -21,5 -22,5- 23,5 24,5 - 25,5-26.5-27.5 20,36i±0,.13 31.1 1 4 22 36 45 63 49 36 29 16 7 '-3 5 . 1.1011113 pmaa K ,uutlie r0â0131.1 G 13,5 -- 14,5 - 15,5 -- 16,5 -- 17,5 -- 18,5 - 19,5,- 20,5 -- 21,5 -- 22,5 - 23,5 -- 24,5 324 1 6 ' 8 31' 56 73 67 41 33 7 1 • 7 PaCCTORHHO MOHC,gy 3aaHOMO HOCO3MMil orEsepcTugmll K eaHHO roaonU 10,5 -- 11,5 - 12,5 - 13,5 - 14,5 - 15,5 - 16,5 - 17,5 - 18,5 - 19,5 . 15 ,45±...0, 9 29& _ 5 25 44. / 74 73 46 20 10 1 PaccTonnne f mew.ay nepeAnumn 11000 . ONMO . OTBOpOTHAMH K ,pinne rozonu 4,5 - 5,5 - 6,5 -- 7,5 - 8,5'- 9,5 •-• 10,5 - 11,5. - 12,5 8,36,1-0,09 243 • - 2 20 45 71 ' 57. 36 ' .9 .. ,3 . • . - ., _ ___ - •
KEY: 1) Variance series.. 2) Length of head to total length. 3) Width of head to length of head. 4) Width of forehead to length of head. 5) Height of head to length of head. 6) Length of snout
to length of head. 7) Distance between posterior nares to length
of head. . 8) Distance between anterior nares to length of head.
The main difference observed between the sharp-nosed and broad-nosed eels is that the broad-nosed individuals have a bide snout, thicker and bider lips, a bigger mouth and more powerfully developed muscles of mastication
(temporalis muscle). The skull of the broad-nosed eel is longer, bider and lower than that of the sharp-nosed eel. The head of the broad-nosed eel is
n.:11,1rlys loner than that of the sharp-nosed eel. 1 • ,à22, ■. ■2 ' 2 - 25 -
Such marked individual variation in the shape of the head and size of
the mouth is observed in the European eel that in ,extreme variants of the species it is always possible to distinguish two forms, one broad-nosed and
the other sharp-nosed (Fig. 4), although usually individuals with an inter-
mediate type of character are predominant.
•
Fig. 4. Differences in shape of the eel's head: a) downstream migrant eels; b) extreme and average individuals from Fig. la; c) • of. sharp-nosed and broad-nosed eels (afterlgalter). The- absence of taxonomic differences between these two forms is proved
by the fact that all the morphological features fall into one variance series
and also by the fact that sharp-nosed, broad=nosed and irr!-^ --_--rmedia,te forms live
together, when the last of these groups is numerically predominant. I have
examined this problem in more detail earlier (Kokhnenko, 1955, 1958, 1959).
Life Cycle
The study of the life.cycle of the eel is particularly interesting for
y it differs in many biological features from other species of fish.
During its lifetime the eel performs two migrations: anadromous, when
the eel larva.e approach the continental shores from the oceatz and, after
metamorphosis, some enter fresh water while others remain in the sea for feed-
ing, and catad-romous, when the eels, on reaching a certain size and stage of
sexual maturity, migrate back into the ocean for.spawning.
In most cases eels begin to leave their freshwater habitats for the sea
when they reach stage II-III of sexual maturity (on a five-point scale) . How
ever, not all individuals of the same age migrate simultaneously, Nordqvist
and-Alm (1920) state that the nearer the eel to the sea the sooner it starts
its spawning migration. This view is supported by my own observations. For
example, among migrating eels from the inland waters of White Russia individuals
weighing 2.5 kg are often found, and sometimes specimens exceeding 3 kg have
been observed., whereas the mass of migrating eels caught in the coastal waters
of the Baltic States averages 0.7 kg and very rarely exceeds 1.5 kg.
Commercial trapping and tagging have shown that eels entering the Baltic
'Sea continue their journey westw-.ixd. In large -numbers they follow the sukrna-
rine valleys along the coast of Sweden.â.nd pass through the straits of the
Store Baelt (Groat Belt), Li11.e Baelt (Little Belt) and Oresund (Sound), the
Kattegat and Skagerrak, and into the North Sea. • Ehrenbaum (1930) points
out that migrating eels in the Bal bic Sea do not aIways keep to great depths /20/
but are sometimes found- in the surface waters alSo (3-5 m).
At the beginning of the 20th century, Swedish, Norwegian and Finnish scientists determined the speed of the migrating eels in the Baltic Sea by
tagging. Eels were tagged off the coast of Finland. Some tagged individuals were caught near the coast of Denmark, 1200-1400 km from the starting point.
Trybom (1905) calculated from these results that the mean speed of movement of the eels was 15 km per day, and it sometimes reached as much as 36-50 km per day.
Some of the tagged eels covered the same distance in 17-30 days less than the others. Similar experiments in whichnigrating eels were tagged were carried
out in 1937 - 1939 in Estonian waters. On 25 August, 1938 a batch of tagged
eels wa released on the Viimsi Peninsula, and some of them were caught on
12 September of the same year off the island of Gottland, 220 km from the starting point, so that their speed was about 13 km per day. It should be
noted that the spawning migration of eels, especially in the Baltic Sea, is
associated with considerable expenditure of energy and, consequently, with
much loss of weight. For example, eels weighing 700-1450 g lost 75-150 g, or more than 10% of their mass, after a journey lasting 20-93 days. In the North Sea, just as in thellediterranean, traces of the migrating
eels are lost and their subsequent path is unknown. Admittedly we have in-'
formation (Ehrenbaum, 1930) relating to the discovery of a large female eel-
in the stomach of a sperm whale caught near the Azores. Grassi and. . . . Calandruccio (1897) found eels in the stomachs of swordfish caught in the
Gulf of Messina. These eels had enlarged eyes and dark pectoral fins. This • is the only information we have of the discovery of migrating sexually .
mature eels in the Atlantic Ocean and Mediterranean Sea. - 28 -
Migrations of the European eel have attracted the attention of many in-
vestigators. The spawning migrations have proved a particularly difficult
problem: how can sexually mnture eels find their spawning ground and what do 4 they use for er,emébee?
Many hypotheses have been put forward to explain this problem but they
have not always been in accordance with the facts and sometimes they have con-
tradicted them. For exemple, Schmidt (1923) postulates that the European eel
finds its spawning grounds as the result of a migration instinct acquired dur-
ing the larval stage. This hypothesis cannot - be accepted for the conditions of migration of the larvae differ radically from the conditions of migration
of the adult eels. The larvae migrate in the surface layers of water while
the adult eels migrate in the deep layers, which differ sharply in temperature
and salinity fram the surface layers. Consequently, even if the larvae did
acquire information of some sort during migration, they coUld not remember the /21/
way to the spawning grounds taken by adult eels, for the adults,are under the
influence of completely different conditions.
According to Eckman (1932) eels are guided during their spawning migra-
tions by temperature and salinity. From whatever point on the coast of Europe 'or North America the eel set out, if it moved in the direction of a constantly
rising temperature and salinity, as Eckman considers, it must by the shortest
route reach the region of the Sargasso Sea, i.e., the region of the highest
temperature and salinity in the Atlantic. However, if.we accept that Eckman
is right and that sexually mature eels swim in the direction of an ever- increasing temperature or salinity, or of both together, this cannot explain why eels from the Mediterranean sea should go to the Sargasso Sea when the
temperature and salinity in the Mediterranean are higher than in the Atlantic
Ocean. Eckman assumes that on the way to thé Atlantic the Mediterranean eels are guided by the deep current which runs from the Mediterranean,Sea into thé • Atlantic Ocean, and once in the Atlantic they are guided by the same factors
as eels from the northern seas. This results in a very complex system of ori-
entation with physiological readjustments of the eel's hormonal system at
certain stages of the journey, and it emphasizes the artificiality of EclQnan's
hypothesis and casts doubts on it. I have examined the hypotheses on the
spawning migrations of the European eel in more detail elsewhere (Kokhnenko,
1958, 1959a, 1962b, 1965b).
After analysing the hyd.rological and hydrochemical conditions along the N
migration routes and the energy expenditure of eels during migration in the
B altic, in 195$ I put forward a new hypothesis according to which the chief
factor determining the direction of the spawning migration of European eels
is the current. Both Mediterranean eels and eels living in the basin of the sk-4.z2 &e+AU_ ^tu northern seas currents. This feature of the behavior
of eels is exhibited throughout their journey from the feeding grounds to the cav»,."^r^. spawning grounds. The deep current -- the-em-t^-Gulf Stream, flowing to the
south-west, not only guides the eels to the spawning grounds but also helps
them to cover the vast distance. It must be remembered that because of the
lower temperatures and higher salinity at a depth of 1500-1$00 m, the metabolic
rate of the eels is considerably reduced and their energy reserves are utilized
more economically. If the eels had to swim, as was hitherto supposed, against
the currents or even in still water, their energy reserves would be exhausted
long before they reached the spawning grounds.
Adaptation of the European eels to currents I regard not as.a specific
case for this particular species but as a facility belonging to all freshwater
eels, both Atlantic 'and Indo-Pacific, which also utilize a deep countercurrent
for the return of the adult eels to the spawning grounds and a warm surface /22/ current for delivering their larvae to the feeding grounds.
• In the vast expanses of the Pacific and Atlantic Oceans the spawning
grounds of the eel, it will be noted, are restricted only by current factors.
In regions in which a suitable combination of these currents is absent, despite the identity of the physicochemical conditions no freshwater eels are to be found. This is the only way of explaining the absence of freshwater eels on
long stretches of the western seaboard of America and Africa, the east coast of South America, the South China Sea, and elsewhere.
The spawning grounds of the eel must therefore have the following combina-
tion of external environmental conditions: the presence of a deep current run-
ning from the continental land mass to the spawning region, where it rises to the surface, and a warm surface current in the opposite direction, returning
toward the continental land mass; the presence of certain values of the tem-
perature and salinity. It is because these factors are combined in the Sargasso Sea that it is the spawning ground for European and American eels.
Examination of the migrations of the European eel frem the standpoint of
evolution reveals two remarkable features. The first is that the eel, as the
result of migration of its larvae, ranges over bide stretches of the ocean. This emphasizes the bide adaptability of the eel to external environmental con-
ditions, -which must ultimately tend to increase the'numbers of the species. Conversely, the second feature is the conservatism of the sexually mature indi-
viduals, bbich adhere so rigidly to one very localized spawning ground, thereby
•restricting the population of the species.
An explanation of the principle of migration of larvae of the European
eel was given by Schmidt (1923) and is not in question although the duration of the migration bhich he gave (three years) requires more acCurate specifica-
tion. The larva of the European eel undertakes its long journey (about
4000-7000 km) passively, through the aid of the Gulf Stream. According to
data in the literature (Schmidt, Hjort, Ehrenbaum, etc.) most.larvae are found
in the surface layers of water (to a depth of 50 m). Consequently, the spped of their migration must be approximately equal to the rate of flow of the water in the surface layers. Rough calculation of the time of passive migration of the larvae on the basis of the rate of flow of the Gulf Stream (allowing for
slowing down as it comes closer to the coasts of Europe and as it rises from
the depth of the ocean) shows that the journey from the spawning grounds
(Sargasso Sea) to the coasts of Western Europe must take the larva 380-400 days. Even if we add 100 days to this figure to allow for inaccuracies of calculation
and for possible deviation's we only obtain 500 days and hot 3 years, the figure
given by Schmidt. • The metamorphosis of larvae of the European eel, according to Schmidt (1923) lasts about a year. He bases this figure on the fact that in the litto- /23/
rai zone of Europe (to a depth not exceeding 1000 m)* larvae are found at
different stages of development during the summer period. Larvae of the
American eel, Schmidt states, take 9-10 months to develop while metamorphosis
lasts only 1-2 months. Schmidt explains this fact by the proximity of the
spawning grounds to the feeding grounds. If, however, we take into account Schmidt's (1906) conclusion that eel larvae are exclusively marine animais and
7.-7Dtnote * The 1000 meter depth line or continental shelf runs along the coastal zone
to the south of Iceland and the Far& Islands, and then to the South-West of
the British Isles, France and Spain. The temperature at this depth does not 0 descend below 9 C throughout the year.
■•■ that their meternorphosis begins only when they reach the littoral zone, where the salinity of the water is reduced, the distance from the final habitat of the European eel (the contir.^y:;al shelf) to the regions of mass concentration of "glass" eels must be about the same as for larvae of the American and
Pacific eels. According to Schmidt, spawning of the European eel begins in early spring and ends in midsummer, so that the larvae hatch out at different times. Furthermore, according to the observations of Ecknan (1932) larvae of
the European eel are not held at the spawning ground but drift toward the shores
of Europe. They must therefore reach the shorès of Europe at different times.
Tnlhy, then, must they pass through metamorphosis at the same time if they reached
the littoral zone of Europe at different times? The larvae unquestionably meta-
morphose at different times and the glass eels enter the river mouths of the
European coast at different times. For example, they reach Spain in November-
January, France and England in February-May, Germany and Denmark in April-June,
and the Baltic later still. The length of stay of the glass eels in fresh
water roughly corresponds to the duration of spa*nming. For that reason Schmidtts
arguments are evidently insufficient to determine the duration of the period of
metamorphosis in larvae of the European eel.
In addition, authors differ in the figure they give for the duration of
metamorphosis. For example, Schmidt (1923) and Shmidt (1947) state that meta-
morphcsis of the Eu.roFea:1 eel larva lasts about one year; Suvorov ( 191a.$) and
Berg (1949) say about six months. This difference in the estimated duration
of metamorphosis indicates that insufficient study has been made of this prob-
lem. Many biologists have studied the European eel, but a leading place in
these investigations must be awarded to the Danish biologist Johann Schmidt.
Hov,rever, his study of the life cycle of the European eel needs to be continued for the following reasons.
First, Schmidt drew a number of hasty conclusions which have since been
accepted in the literature as proved. For instance, his conclusions regarding /24/
the distribution of larvae in the Atlantic Ocean in relation to their size and
the duration of their migration, and also regarding spawning migrations have been accepted by sninent scientific investigators. I consider that these con- clusions rest on an insufficiently scientific basis and require substantial verification, for the reasons given below: % 1) Schmidt ignored the duration of spawning of the eel, which last and in his scheme he did not show the difference between the length of the larvae fram the different times of spawning and concluded that heterogeneity of the larvae is explained by their belonging to different generations, born in different regions. Nevertheless these differences are probably due in fact to differences in the time of arrival of the parent fish at the spawning grounds and in the time of spawning. With this pattern of spawning, several generations of larvae may be found in the same year. Schmidt did not take this into account and he therefore considered that the larvae of the European eel grad extremely sladly by comparison with the American eel; 2) Schmidt did.not allaw for the actual rate of flow of the Gulf Stream and for the différence in the times of arrival of the young eels near the shores of the. different European countries, and he thus artificially prolonged the duration of the larval period of the European eel. Ne.also unneceséarily restricted the regions of spawning of the European eel, for during subsequent years small ,larvae were caught outside the area bounded by his lines; 3) Schmidt produced no biological evidence to explain the duration (up to one year) of metamorphosis of the European eel larva into the glass eel, although he knew that this period in the American . eel is very short. • Second, many aspects of the biology of the eel still await explanation. For example, we have rn information On the state Of the sex glands pi female and males in stages III, IV, V, and VI of maturity under natural conditions: on the sex composition of the spawning population, on the aubryonic period, the character of spawning, or death of the brood stock after spawning. The duration of the larval period and of metamorphosis and the conditions of life and migration of the larvae have been inadecivately studied. The causes of the different types of migration of females and males, the times and conditions of the migrations of the brood stock, and other problems remain unsolved. From analysis of the available information and from our data on the biology of the European eel its life cycle can be divided into three character- istic periods, which differ not only in the morphological and biological changes in the eel, but also in its relationship to the external environment. Each of these peaods in turn can be subdivided into separate phases of development, which also are characterized by specific features. The first period is that of morphogenesis, characterized'by considerable changes in the shape and structure of the organs. Three phases of development of the European eel are distinguish, /25/ able in this period: 1) eMbryonic (ovum); 2) larval; 3) metamorphosis. The embryonic phase includes fertilization of the ovum and development of the embryo.and it ends with hatching of the larva. The ovum of the eel is pela- gic and it is considered to be laid at depths of not more than 400 m in places where the deep waters rise to the surface. The laying down of the vertebrae and myomeres is completed .in this phase. The larval phase begins with the time of hatching and continues until the beginning of metamorphosis, i.e., it lasts approximately 1.5-2 years. This long duration of the larval phase of the European eel compared with that of the e're°4"' . American eel (9 months) and of the two-coloured Indo-Malay el (3 months) is caused by the Wile much greater distance from the spawning ground to the feeding • ground and., consequently, by the longer time taken to cemplete this journey. There are herefore corresponding differences in the mean sizes of the larvae of these three species of eel.,: European 75 mm, American 65 mm, Indo-Malay 55 mm. The external shape of the larva is distinctive and it does not resemble the shape of the adult eel. Its body is highly compressed from the sides, pointed at the head and tail, and its shape is like that of a w'l.llow 1ea,f. ^ Having hatched out in '' " -- the larvae rise towards the surface Ri and lead an entirely pelagic mode of life. By day they descend to a depth of 50-100 m, but at night they rise to the surface (Sc'runidt, 1932). This is evidently linked with the vertical migration of the plankton, which is the main food of the larvae. In the currents of the Gulf Stream, which flows from south-west to north-east, they drift toward the shores of Europe and reach the Nlediterranean Sea. The passive movement of the larvae is also facilitated by their distinctive body shape. They react positively to the direction of the current, light and salinity. Eel larvae are purely marine animals and they <4A^4_wl never live in fresh or ,_o „ y waters. They likewise are not found even in the North and Baltic Seas and English Channel. The phase of metamorphosis is an intermediate stage between the first period and the second. On reaching the continental shelf, probably under the _Y=qof the less salty waters the larva begins to change gradually into a it^ht^of nthe nbody is reduced, its shape changes from leaf-like to round, and it i nly in the tail -^^ . During conversion • Fig. 5. Conversion of larva into glass eel (after Johan Schmidt). It is characteristic that the larvae do not reach the continental shallows, andYthey do not enter the Ehglish Channel or the North and Baltic Seas. After comPlete metamorphosis of the leptocephali into glass eels the latter make for the coastal waters. The narrower the continental shallowe, the sooner they /26/ reachd the shore. For example, glass eels in October swim up to the coasts of Portugal and Northern Spain; in November4December they reach the coasts of the Bey of Biscay and Valentia Island, off the south-west coast of Ireland. In January glass eels appear off the French coast near the towns of Pauillac, Rochefort and St. Nazaire. At this time they are appearing along the whole of the western coast of Ireland. In February glass eels penetrate into the Irish Sea and English Channel. In March, haVing gone round the north of Scotland or - 37 - through the English Channel they enter the North -Sea. In April-May the eels penetrate into the Baltic. Finally, in the sumMer (June-August), the eels which are now pigmented:05-30 cm in lengtreach the gulfs of the Baltic Sea. Larvae of the European eel enter the Mediterranean Sea from the 4 1\rpcte Atlantic Ocean -_-Fraerthe Straits of Gibraltar and travel eastwards as far as Cyprus. However, most of them go no further than the Straits of Messina. Metamorphosis of the larvae in the Mediterranean Sea takes place, as it does In sur faC -e- uo cc-(e'3 in the Atlantic, alsbeFer1 e-deep-,ea,t7ers, and exclusively glass eels reach billp coastal waters. The times of their arrival range from November to April. In these regions the times at which the glass eels approach the shore depend on the character of the winter. If winter is prolonged, sometimes their approach is delayed by as much as a month. The number of glass eels reaching a parti- cular shore is directly dependent on the Pater levelein the Atlantic. The higher theç ater level, the more glass eels reach the coastal waters. For this reason, different numbers of glass eels reach the same places in differ- ent years. Migration of the glass eels in the sea is observed both by day and by night. By night they keep closer to the surface than by day. It is postlilated /27/ that during migrations the glass eels use the incoming tides and keep in the surface layers of the water, whereas during outgoing tides they descend to the bettom and may even burrow into the sand. No connection.has yet been found between the temperature and intensity of migrations of the glass eels, but it has been shown that the lower tempera- ture limit lies within the region 4-5 °C. .It has also been noted that glass InictneÀ eels enter the jent-ernal water systems in greatest intensity when the tempera- ture difference between the sea and fresh water is least (March-April). or) The t e ofglass eels towa.rd light probably depends on 'its strength. P_n51a-dl . For example, eels caught in the rivers Loire and Severn ara repelled by day- light and by bright electric ?.'tght. Nevertheless, fishermen when catching glass eels in these rivers use paraffin lamps or a torch to attract thein,-kc ^t. It can therefore be postulated that there is a definite relationship between the intensity of the light and the eel's activity. Glass eels have been shown to be capable of detecting fresh water brought by rivers into the sea even at long distances from the mouth of the river. Even a low concentration of fresh water induces increaséd activity in the eels. If, however, the flow of fresh water is accompanied by turbulence, this stimu- lates the migration activity of the glass eels and serves to guide them during their migration toward the shore. Finally, glass eels migrating from the sea into fresh water develop a positive response to solid underwater objects (stones, underwater vegetation, etc.), which serve as a refuge for them. Young eels begin to enter Etzropean rivers in mild winters as early as in February, but as a rule this takes place in March, and they are most num- erous in April. Sometimes the entry of the eels continues until May 15-25th. Large numbers of glass eels enter the rivers Loire, Gironde, Garonne and Rhône (France) and Severn (England). The second period covers the life of the eels in a sexually immature state in fresh water and in the sea, corresponding to the period of growth Tk \\5 in the life cycle of vertebrates ( Severtsev, 1939) •T4i@- period of growth contains only one phase ( juvenile) which lasts from the end of metamorphosis until the onset of sexual maturity. The phases of semzal dévelopment of the second period will be examined below. Using incoming tides the glass eels reach the coastal zone and while -- 39 — some make for the freshwater (against the current), others remain in the ' i n -&-c,5/,_ tAictfer Put- e---15 qne coastal region. At this stage they are repelled by light. ilmost entirely /28/ c■ females ateftwee'imIdeettleWee*Yt, while in brackish wuter they are mainly males. During artificial stocking of reservoirs with glass eels the percent- age of males discovered rises. Sexually immature eels are adapted to different conditions of life: to differences in the chemical composition of the water, differences in its transparency and colour, differences in the type of food. They live freely in oligotrophic, mesotrophic and eutrophic bodies of wuter, and even in the Baltic and North Seas eels of both sexes and of different age groups are found. I have caught females of all ages in the lagoons of the Adriatic where the salinity of the water reaches 48 parts per thousand. At the end of this period the colour of the eel changes from yellow or green, with a matt appearance, to a silvery colour with a metallic glean, evidence of the onset of sexual maturity. According to anolian (1920) male eels attain sexual maturity at the end of 6-7 years, or at the earliest four years after metamorphosis into glass eels, and in fenales at the age of 9-10 years, at the earliest five years . after their metamorphosis. . Migration of eels from the feeding grounds in the Atlantic to the sites of reproduction, spawning and death of the brood stock cover the third period. This period corresponds to that of sexual maturity or to the fully.grown state as described by. Rass (1948) for other species of fish. For this period it is possible to distinguish an adult phase, which . includes the time of the spawning migration and the process of spawning it — pict,-h-yors. self. According toVpreliminary calcUlations, allowing for the speed of move- • ment.of the eels during the spawning migration and the rate of flow of the • CC) (.1 rt4 -e-- \ deep current.of the ae41-gulf streanre—the migration takes sexually mature eels can accomplish distances of 4000-5000 km in 150-200 days. During this time the,eels react-positively to the direction of the curreht and to the salinity but negatively to light. The eels dârken in colour and their eyes increase both in volume and in dianeter (a characteristic feature ^ of deep water fish). During migration of the eels not only do the sex products ^ ripen, but profound physiological changes also take place: demineralization V2 of the bone and muscle tissues and degeneration of the digestive organs, which reach their greatest limit after spa7n,ming. The hypothesis has therefore been put forrvvard that after spawning the eels die in the same way as the conger eel, for there can be no return to the normal state after such profound physiological changes. This hypothesis is conf?r^_ed by experiments (Fontaine, 1937, 1965) con- ducted on male and female European eels wrd.ch were kept under artificial con- ditions after hypophyseal injections until maturation of the sex products. After discharging their milt and eggs all the spawning fish died within 12-24 h. Differentiation of sex and sex differences /29/ The problem of sex determina.tiôn in the eel has been studied for a long time but since no ripe eel eggs had been found and the initial stage of development of the eel (larva) was unknovan it was thought for a long time that eels come into the world in a different way from other fish. Fantastic hypotheses with no scientific basis were put forward. For example it was considered for a long time that eels are viviparous fish on for• the grounds that parasitic roundworms which were taken a-& young eels, were found in them. At the beginning of the 1$.th century it was considered that eels are born from a viviparousfish -- the eelpout or viviparous blenny, and in Gexma.ny this fish is called the Aalmutter ( mother of eels) . The Italian naturalist Mondiri (1777) first discovered and described-the ovaries of the female eel, but since male eels were unknown at that time some investigators considered tl-,é1,5 eels are hermaphrodites. It was only about 100 years later that the Austrian biologist Syrski (1874), investigating anall eels (about 40 cm) near Trieste, found testes in them. Since then it has been established that eels reproduce just like all other fishes. After Syrskils discovery of the male gonads, a vigorous search was made for male eels in various waters of the European continent including the Baltic, North and Mediterranean seas and their elf and bays. •These investigations (Hermes, 1893; Peterson, 1895; Seligo, 1906; Tribom, 1905; Schmidt, 1906; Walter, 1910; Grassi, 1919; Marcus, 1919; Smolian, 1920; Hornyold, 1922; Ehrenbaum, 1930; Schiemenz, 1935; Bertin, 1956; DIAncona, 1957) showed that eels have no external sexual dimorphism, except that all male eels are much smaller than females. Usually males are found up to a length of 40 cm, rarely 48 an and exceptionally 51 cm (Sibolds, cited by Walter, 1910). The maximum mass of male eels is 200-250 g (Smolian, 1920), whereas females reach a length of 130 cm and a mass of 4 kg, or may sometimes be even larger. Consequently, most of the eels longer than 51 am are females. The smallest length of males * during the spawning migration is 29 an and of females 42 an (Peterson, 1908). In 1961, however, with a Narochanka trap I caught a migrating female 41 an long weighing 103 g.. Since eels have no external sex differences, the.sex of individuals less than 51 am long can be determined only by dissection. Walter (1910) and Ehrenbaum (1930) state that it is difficult to determine the sex of eels less than 26 cm long, and in eels shorter than 20 cm it is impossible even micro- scopically, although DIAncona (1957) found differences between the sex glands of individuals 15.5 cm long. In the ,course ,cf an investigation.of-eivers from White Russian waters /3)/ I caught an eel 25 cm long which grew for 6 months in Truikont pond. Mean- . while an eel 37 cm long, caught from the same pond during the second year of life in fresh water still did not possess differentiated sex organs. An eel 42 cm long in which the sax organs likewise had not differentiated was caught in Lake Narocht. In the early stage of development thé ovaries are lobnlar in shape, like the testes, and only later do they become folded. This must be renembered . of sex in young indiViduals. The sex organs of during visual determination eels in the early stage of development are called Syrskils organs, and the name applies equally to the testes and avaries. As a rule, however, it is easy to determine the sex of most eels over 30 cm long by dissection for the •1. sex organs are already méll defined. If the belly of the female is opened, ovaries can be seen on both sides of the alimentary tract, lying next to it along the whole length of the body cavity in the form of bands (Fig. 6). The ovaries also extend into the caudal part of the body, and each of them bifurcates (from the anus toward the tail) and they dedrease considerably in width. Walter and Ehrenbaum shoméd that the width of the ovaries is 1-2 cm, but in eels mbich I have studied I havé found ovaries up to 3 cm in width. The relative length of the ovaries (compared with the body lengtb) averages 35%. In their shape the ovaries resemble a mesentery. On the outer side they are amooth, but on the inner side they are folded, and their free edge is considerably thicker. They are white or sometimes slightly greyish in /3V colour and they contain large numbers of tiny eggs mith glistening granule. Eels are very fertile fishes. Mather (cited byWalter) found 9 million eggs • in an eel weighing 2.4 kg. The testes of males are paired and are arranged like the.ovaries, but 0 they consist of very narrow, pale opaque bands. Each band contains about 50 arched lobules. The vas deferens of the testis opens into the anus. The ovaries have no oviducts. ?,daiter (1910) points out that the ripe eggs first enter the body cavity, from which they reach the anus through the two genital pores. •t Fi9. 6. Int.ernal organs of the eel: 1) liver; 2) stomach; 3) valve between stomach and intestine; 4) intestine; 5) ovary; 6) anus; 7) part of the eel' s ovary. So far nobody has observed ripe eggs of the European eel under natural conditions. Admittedly after prolonged investigations Fontaine (1965), by means of pituitary injections under laboratory conditions, obtained ripe eggs which were laid in.small batches. The round egg (0.93-1.4 mm in diameter) contained a large quantity of yolk and a few fat droplets. For sever:ll yec:rs I have investigated the ovaries of eels of different ages grôwn in water differing in their food supply. In the case of eels introdt.rcer.i in 19:-,8-19-39 and caught in 1954, despite their long period of life in fresh wtér .(more;than 15years after the last introduction), the largest diameter of the eggs found was 0.31 mm (Table 5). TABLE 5. Mean Diameter of Eggs of Eels Caught in 1954 fram White Russian Waters, mm te Ihmem i KimeGiaing if n GpacJancKne ompa 2— 0,11--0,31 0,219 128 P. flpyfiRa 3 0,17-0,30 0,224 44 Di TerepKn LI- , 0,07--0,19 0,124 9 D3, flAmpecbt 5— 0,05-0,19 0,106 14 KEY: 1) Water 2) Braslav lakes 3) River Druika 4) Lake Teterki 5) Lake Plyussy 6) Variations These results show that the mean size of eel& eggs from the dystrophic lakes Teterki and Plyussy is much analler than that of eggs from the eutrophic Braslav lakes and River Druika, although they were of the same age. If growth of the eels is retarded, the sex products evidently develop more slowly. Characteristically in eels recently introduced, but caught in 1961-1964, i.e., 20-25 years after the last introduction, the diameter of the eggs mus not increased. Since 1956 I have made yearly observations on sex formation and the development of sex products in elvers introduced in the spring of the same year into different types of muter. These investigations showed that in waters sex is largely not differentiated in eels less than White Ruesian24 am long, i.e., they are in the stage of juvenile development, or what is called the Stage of juvenile hermaphroditism. Thé gonads at this stage are penetrated by a thick network of blood capillaries, favouring their rapid i:rowth .and development. In some individual s grown in a pond and attaining a leiv;th of 25 cm six months after introduction, lobular organs could be seen even with the unaided eye, and examination of histological specimens under the microscope clearly revealed single female sex cells, much larger than the surrounding undifferentiated cells. The oocytes were not absolutely circular in shape and they differed in size. They were covered by a thin membrane and each one was clearly separate. The cytoplasm was granular in structure, The nucleus was large, round or aval in shape, and occupied more than half of the oocyte. Small nucleoli could be seen at the periphery of the nucleus, with chromatin rods in the center. The smaller oocytes possessed one or more vacuoles. A similar pattern of gonad development is observed in eels of the sane size but caught 2-3 years after introduction from lakes Navyato, Drivyaty, Myastro, NaroCh', and others. Consequently, development of the gonads in elvers is directly dependent on the size of the individual and not on its age. However, a further increase in the weight and length of the individual is not accompanied by a proportionate increase in the size of the sex cells, although the gonads become considerably larger. For example, the development of eggs taken from an eel 46.2 am long and weighing 193 g, grown in a pond for two years, was 0.030-0.090 mm (mean 0.063 mm), while eggs from an eel 82 an long weighing 930 gl which had lived in the pond for five years, measured 0.045-0.090 mm (mean 0.065 mm). . The size of the eggs (meauuredunder the microscope by means of an ocular micrometer) differs both for eels of the same age and of different size and also for eels of the same size but of different age (Table 6). No change in the diameter of eels' eggs is observed from one season to -another (Table 7). - 46 - 'Females whose eggs exeeed .0.2 mm in diameter are found at different seasons of the year. The possible explanation is that when their eggs reach a diameter of 0.2-0.3 mm the cels migrate downstream, and if no migration takes place growth of the eggs ceases. The further growth and maturation of the eggs are evidently resumed at a later stage of the migration journey -- when the eel reaches Atlantic waters. The development of the eggs in any ovary takes place asynchronously.' . . "I'Ar:LE 6. Mean Diameter of Eggs from Eels taken fran White Russian Waters an al-lov\-- . 'Ndirtit.,..-- 2) I Cti, ek ee- C f A 9e-- V KoAeGatmg, Cpemill. Ritamesp ma. 1303piCt .Adad .WAt .it.« S ...... •.w...... —.. ■.. ...... 0+ 0,0159-0,0265 0,018 -0,007 2 1+ 0,030-0,084 0,046 0,034 6 2+ 0,026-4,093 0,054 0,038 8 3+ 0,030-0,105 0,068 0,033 22 44- - 0,037-0,125 0,069 0,032 11 5+ 0,090-0,225 0,142 6 6+ 0,105-0,180 0,134 2 7+ . 0,079-0,254 0,170 0,066 9 84- 0,147-0,231 0,167 5 15+ 0,170-0,310 0,224 172 25+ 0,079-0,260 0,231 - 0,074 44' TABLE 7. Changes in Diameter of Eels' Eggs in Different Seasons --... M o•-%-t k_ Kae6amiiii, Cpe.nntri, tt blecsiu .V.11 1 Mail .. Mc` . ,..,. .. 0,04-0,30 0,19 71 OKTsi6pb Pe-A-° I;er. .. 0,06-0,30 0,20 68 KEY: 1) Variation, mm 2) Mean, mm My results for the development and size of eels! eggs do not agree with those published by Benecke ( 1$$1) . He considered that the eggs of éels.migrat- ing to the spawning grounds enlarge from August or September, and that whereas /33/ their mean diameter for all eels was previously 0.09 mm in September it was 0.10, in October 0.16, and in November 0.18-0.23 mm. It is not Imown for how long the growth of the eggs ceases. It must be assumed that this period depends on the actual ecological conditions encountered and, in certain cases, it may be a very long time. For example, eggs more than 0.2 mm in diameter are observed both in individuals during spawning migrations at the age of 7-9 years and in eels which have lived in fresh water for more than 20 years. As well as large eggs (0.2-0.3 mm) at the stage of accumulation of yolk, small oocytes (0.01-0.1 mm) may also be found, either in the phase of protoplasmic growth or, in some cases, in the stage of trophoplasmic growth (Fig. 7). The latter do •not count for more than 10-15% of the total. I have already stated that male eels in most cases live in salt water: in the sea, in bays and estuaries, and in the mouths of rivers draining into them, whereas females penetrate into fresh water. However, as one goes in an easterly direction away from the Atlantic Ocean one finds considerably fewer /34/ males until they eventually disappear altogether, even in saltwater. Ehrenbaun (1930), for example, points out that the zone of distribution of male eels in the Baltic is confined to Southern Sweden and North-Eastern Germany. In the eastern part of the Baltic Sea, in its gulfs and in the estuaries of the Western Dvina and Neman, no males are found (Tribcm, 1905; Kokhnenko, 195$). In European rivers reached by both female"and male eels their relative per- centages in different parts of the river vary considerably. For example, in the lower reaches of the be up to 95% of males are found, whereas in the /35/ middle part-of its course males are much fewer (not more than 25%), and in the upper stretches of the Elbe they are absent altogether (Bruhl, 1909; - 48 - 1916). Approximately the same phenomenon is ohuerved in the rivers of Denmark (Peterson, 1895; Tesch, 1928), France (Bertin, 1956), Eneand ,Frost, 1946) and Italy (D'Ancona, 1957). - _ • • • '‘" 'à • • kf " ^4•1;,: ":;,,, e • ‘ 4„...',i% , 'ach. >11, • < - _ t . • •.• S' ».4 •■V "ee• e".)° 3t, -----,- 40111 te), Niaree 4 111,e . . e.inmo Fig. 7. Transverse section through ovaries of an eel (magnification 15 x 8): ee( a)Alength 50.2 an, weight 242 g, age 1+ (Truikont pond); blilength 87 cm, ce,\ weight 930 g, age 8+ (Lake Narocht); g/length 107.5 cm, mees 1730 g, age more than 25 years (Lake Narochl). In 1953 - 1954 I dissected about 12n0 eels introduced previously (1928- 11› 1939), five of them less than 50 cm in length. All proved to be females with clearly defined sex organs (Kckhnenko, 1958). Judeng from this material, no ma1F eels are :foùnd in White Russian waters. An investigation of eels from the same waters introduced in 1956, however, showed that some males do exi.st. For expmple, in October 1960, i.e., five years after stocking, I found 12 migrating eels, caught with a Narcchanka eel trap, of which 9 were females with well-marked gonads, 2 were males, and 1 had no differentiated sex:organs. The length of the eels varied from 45 to 67 cm and their weight from 10$ to 573 g. Appro,dmately the same relative porportion of males was found in sub- sequent years ('1961-1,61a.) . The size of the smallest migrating eels was: males Az 39 cm long and 86 g in weight, females 41 cm ând 103 g respectively. Similar figures are given by Machenis (1963). Of the 30 migrating eels, introduced in 1956 and caught in a stream flooring from Lake Vyevich (Lithuania), which he investigated, 9 were males and 21 females. The fact that male eels are found in fresh water, and that they can live 0 and develop in it is not in any doubt. Males are particularly numerous in inland waters when stocked with glass eels. Schiemenz ('1935), for instance, points out that in individual batches of elvers males account for up to 50% of the total. However, the infrequency with which adult males are found in fresh water has prompted. some investigators to postulate that sex in the eel is unstable, and that elvers introduced into inland waters give rise only to females, which does not correspond to the truth. Observations of the growth of elvers in Lake Lukoml.'skoe showed that some individuals leave the lake during the first years.after introduction. This lake was first stocked with glass eels in 1962, and by the autumn of 1963 elvers 15-25 cm in length migrating dovunstream were caught with fine-meshed ($-10 mm) traps in the River j,ukomka flowing from the lake. Elvers were also observed leaving the Narochanskaya lakes, which have repeatedly been stocked with glass eels. However, since the netting of the eel trap sited. on the River Narochanka had an 1S-mm mesh, all the small migrating eels passed through it, leaving only mucous rings on the net. Probably most of the males and the fast-maturing females begin to migrate from inlar' ,.,raters in the /36/ second year after introduction, and not in the 5th-7th year as was hitherto , considezeed. The migrating elvers, because of their very small size, were not held back by the commercial trapping equipment and thus remained unobserved. This fact goes some way toward explaining the rare discovery of adult males in inland waters. Many investigators have studied sex in the eel because this problem is not only of theoretical but also of great practical importance, especially in artificial stocking of inland waters with young eels. So far there is no general agreement on the matter and several contradictory interpretations have been given. Some workers (Grassi, 1919; Tesch, 1928; Hornyold, 1931; Rodolico, 1938; D'Ancona, 192)1 , 1943, 1954; Bertin, 1956) produce evidence for the view that sex in the eel is unstable and that its formation depends on external environmental factors, i.e., that it takes place phenotypically. Others (Schiemenz, 1935) consider that sex in the eel is established in the embryonic stage of development and remains stable throughout life. His argu- ment in support of this view is that in enclosed German lakes, stocked, in 1921 and in 1924 with glass eels, large numbers (sometimes more than 50%) of males were found. The following experiment was carried out: in the lower Elbe in autumn, when most elvers are males, several hundreds of elvers were caught and transferred to ponds. After the eels had remained in the ponds for 3 years it was found that 95% of them were males, i.e., their sex ratio was the same as in the lower Elbe. On this basis, as Schiemenz points out, it can justifiably be concluded that sex in the eel is determined genotypically. There is insufficient evidence either way to give a final answer to the •question of whether sex:is determined genotypically or phenotypically in the eel. Conclusive evidence in this respect would be the presence of different chromosomes in eels of one sex, as is the case in primatos, but unfortunately all 38 chromosomes of both male and female eels are the same. Age and its Determination No precise information is yet available on the life span of the eel, for its subsequent fate after the spawning migration is unknown. According to reports in the literature, an eel has lived in an aquarium for 37 years (France), 55 years (Italy) and 88 years (Sweden). Ehrenbaum (1930) considers that eels can live to the same age in enclosed bodies of water, with no access to the sea. In the waters of White Russia eels (a-Z-been caue)which were introduced in prewar yearsee4: 23-30 years after their introduction. There is likewise no unanimity regarding the determination of an eel's age from its scales. For exnmple, Gemzge (1908), Ehrenbaum and Marukawa (1914), Marcus (1919), Tesch (1928), Suvorov (1948), VoIf and Smisek (1955) and others assert that scales appear on the eel when it attains a length of 17-18 mm, in about the third year of life in freshwater. They therefore consider that if age is determined by reference to the scales, two or three years more sholad be added. Hempel and Nerecheimer (1914) fould scales on eels 15-18 am long in the second year of their life in freshwater. Opuszynski (1963), -wilo investiga- ted eels in the River Sapina (Poland), found scales on individuals 14, 15 and 16 cm long, whereas.there were no scales on an eel 18 am long. Nordqvist and Alm (1920) . state that scales appear on eels in Swedish waters in the 4th year. Frost (1945) states that scales can appear at any age from 1 to 5 years in fresh water. Ighen determining the age of eels by the scales, different authorities add different numbers of years (from 2 to 5) to the number of .rings obtained on the scales. • Besides annual rings, incomplete or interrupted rings of grawth, re- sembling in shape the - peak of a cap, are observed at the ends of the long axis of the scale. These structures are considered to be - incomplete annual growth rings. Some workers believe that when determining age the "caps" on both ends of the scale should be taken as one year, and alternate caps as two separate years. Marcus (1919) postulated that these caps are formed in years of poor growth, when no complete ring is fonned. However, my investigations have shown that even if scales are formed during the first year of life of an eel in fresh waterl scales are unreliable as a means of determining age. Mistakes may arise as both under- and overestimations of the age. Observations have shown that in young eels the caps can be formed in years of good growth also. Both a complete and an interrupted ring-cep can appear in the same year, Determination of age an the basis of scales in this case gives too high a figure. Under unfavourable conditions, as experiments have shawm, annual rings are not formed. In that case the estimated age will be too law. Moreover, scales taken from the same eel may have different numbers of rings. Sometimes, especially in individual“rom older age groups the differ- ence between the minimal and maximal numbers of rings may reach eight. A similar state of affairs has been described by Rasmussen (1952). My own observations on scale development in elvers have shown that scales are not all laid down at the same time, but gradually over a period of years, as a result of which different numbers of rings are found on the same individual. Conse- • quently, if the age of an eel is determined by examination of its scales at least ten scales must be examined and the age must be judged from the largest number of rings. -53- Ehrenbaum and Marukawu (1914) suggested that the age of an eel could be determined from its otoliths. They state that age can be determined more accurately from the otolith;', because they appear in the larval stage whereas . scales appear only in the stage of the yellow or green eel. In fact, the otolith of the glass eel has only its elver ring despite the fact that the eass eel is probably more than two years old. Later, as a ràle, the annual growth of the young eel can be estimated from its otoliths. The increase in size of the fish is directly proportional to the increase in size of the oto- lith. For example, during a period of rapid growth of eels in a pond for 3 years there was correspondingly rapid growth of the otoliths. An eel, which had spent the summer in a pond and had reached a length of 22 am and a weight of 15.2 g wus placed in an aquarium where, in one year, its weight re') increased by. 3 g and its length by 1 cm. ..rrespondl to the growth of the fish, the otoliths also increased in size: during the first year by 0.6 mm but during the seccnd year by only 0.1 nu, the total length of the otolith being 1 mm. On otoliths of eels which had lived from 2 to 6 years in lakes, rings of growth were laid down le- correspond to each passing year. However, the number of rings on the otolith does not always correspond to the number of years lived by the eel after its metamorphosis into a eass eel. In 1960 glass eels were placed in an aquarium, and some were given food in excess, especially in summer, while the others were kept on a semistarvation diet. By May 1962, i.e., after two years, as was to be expected they had grown to different sizes, the largest was 32 cm long and weighed 26 g, the smallest was 9.5 cm long and -weighed 1.08 g. 'Iwo rings were present on the. otoliths of the first eel and one ring on its scales, whereas only the elver ring was present on the otoliths of the second and it had no scales. In eels of the older age groups the annual growth of the otoliths is sharply reduced, and the outer rings are a.lmost corr,pletely merged and diffi- cult to distinguish. The number of rings on the scale and on the otolitl-., usually does not correspond to the nun..bE:r of years lived in the case of eels living under unfavourable conditions. For example, eels introduced in 193$--1939 into Lake Teterki, when caught in 1953 had only 5-9 rings on their scales and 7-11 rings on their otoliths. A similar ex^;mple is given by Wr^nds c.^ 053 • He e /( rr^ c.^ .sF b^` sta^tes. ,t,ha.t in 1929 elvers were introduced into a._c-lay pit deficient in food q,.70 At c,- ,__--- ^ supplies. From 1929 to 1936 the .f.ood`supply was supplemented, but this was or then stopped. In 195 ^hey were caught at the age of 25 years but the scales had only 8-11 rings. The difference observed between the number of rings on /39/ the scales and the number on the otoliths of eels increases with age. To discover the nature of the error when the age of an eel is determined from its scales and otoliths, eels of the same age but grown under different ecological conditions were investigated. In May 1956, glass eels were intro- duced into lakes, and for the purpose of the experiment into a pond and aquar- ^ro(A_^ -A- iurri. The eels grew at different rates, and by the end of the vo-^ petiod ( October 1956) they had reached the following size: in the aquarium -- length 10-15 cm, weight 1-2.2 g; lake -- length 12.2-14.5 cm, weight 1.9-3.6 g; in the pond -- length 20-24.$ cm, mass 11-26.1 g. On careful inspection of the eel's skin under the microscope, no scales were found in those caught in the lakes and aquarium. A11. the eels grown for five months ( from 17 May to 15th October) in the pond had scales. These experiments showed that scales appear primarily in the part of the body be- tween the origin of the dorsal and anal fins, above and below the lateral line. In my experiments the scales on all eight eels were arranged near the lateral line only and none were . found on the dorsal or ventral parts of the body, whereas in adult eels scales cover the whole body, even the head and fins. Scales with 2, 4-, 6, 8 and 10 rows of plaquettes could be observed. This indicates that scales with fewer plaquettes wsre formed later than those with more rows. Consequently, scales are laid down at different times, not only on different parts of the eel's body, but also in the same part. It must also be noted that often scales of the same individual with two annual rings wsre the same size as, or even were smaller than e scales with only one ring. It thus follows that growth of the scales does not always strictly re- flect growth of the eel, as it does in other fish, and for this reason it is impossible to determine the rate of growth of eels by back calculation. By the end of the second summer (1957) the scales of the largest eel (lengthr50 cm, wsight 242 g) from the pond wsre now 2.0-2.1 mm in length and they had 10 to 11 rows of plaquettes. Just as in the first year, scales of different sizes wsre observed on the same part of the skin. Side by side with scales having 11 rows of plaquettes wsre others with only four or five rows. Further from the diddle of the body the scales became smaller. On the head and tail parts of the body they were smallest of all. The extent to which the eel's body was covered with scales differed: the larger the eel the more camplete the covering of scales. Howsver, even in the largest eel, the formation of scales was not yet complete. The caudal and pectoral fins msre completely free fram scales, while on the anal and dorsal fins only a few scales wsre present. Of the 65 eels grown in the aquaria, by the end of the second year scales had appeared on only three, while on eels which had lived one year in the pond . - and one year in the aquarium, the size of the scales and the character of their arrangement remained the same as before their transfer to the aquarium. This can be explained by the slow,growth of the eels. The time at which the eel's scales are laid. down is directly dependent or, the ecelogical condition., i°ather than on the number of years which the eel has lived in fresh water. Observations showed that glass eels grown in a carp pend had scales only 4-5 months after introduction. Eels grohm in eutrophic lakes (Drivyaty, Myastro, Batorino, Osveiskoe) became covered with scales only at the end of the second year, while eels in mesotrophic lakes (Narocht, Strusto) became covered at the end of the third year, when their N, length was 1$-20 cm or more. Consequently, the "scaleless period" is not constant in its duration. During the determination of age, a number of remarkable biological f ea- tures which call for further investigation are thus exhibited: a) considerable fluctuations in the number of annual rings on the scales not only on eels of 0 the same age group, but also in the same eel, probably attributable to dif- ferences in the time of laying down of the scales; b) the number of rings on the otoliths is greater than the number on the scales, possibly due to the fact that the scales are laid down later than the otoliths; c) the number of annual rings, even on the otoliths, does not agree with the number of years which the eel has actually lived, presumably because of cessation of the eel's growth through lack of food, and also in the later stages, possibly with the onset of old age. It can accordingly be concluded from these experiments that the method of determining the age of an eel from its sc2les and otoliths is inaccurate. The result given may be either an overestimate or an underestimate of the real age. If age is determined from the scales at least ten scales must be chosen in a part of the body near the lateral line between the origin of the dorsàl and anall fins. The age must be taken from the largest number of rings - 57 -. on the scale and it must be verified by reference to the otolith. Habitat and Food Supply Because of its considerable powers of tolerance the eel can live in all types of water. In White Russia it is found in rivers and in mesotrophic, eutrophic and even dystrophic lakes. The eurytrophic nature, or as it is usually called, the high ecological valency of the eel, enables it to colonize extensive areas and thus to ensure preservation of the species. It can live not only in different types of fresh water, but also in salt water. However, eels growmuch quicker in the mesotrophic and eutrophic Braslav and Narochanskaya group of lakes than in the dystrophic Lake Teterki. In their mode of life eels can be described as nocturnal and bottom fish, and in the daytime they spend longer in the ground than above it. Their habitat varies with age. Young eels in the first years of life in freshwater as a rule keep to the muddy zone near the bank, overgrannwith vegetation, where safe shelter from enemies and an abundant food supply (small crustaceans, chironomid larvae, etc.) can be obtained. Young eels do not burrow so deeply into the ground as older eels. The latter migrate from the zone nearest the shore to deeper muddy regions of the river or lake, often covered with debris. The eels burrow into the ground to a depth of up to 80 cm, but Schiemenz (1910) states that he has found eels even at depths of 1.5 m. The ground at the bottom of the river or lake is not only a hiding place, but also evidently a pasture for in its surface layers the eels can eat benthic fonns of life. The eels catch their food mainly at night on the surface of the bottom or in the lower layers of water. They SbliM over the, whole extent of the water, reaching the region nearest its bank where they penetrate into the parts overgroWn with reeds, rushes, and other plants, where caddis-flies and other food objects -58- congregate, and in 5.0 doing they frequently are caught in eeteàleeêâky-hidden nets. They can also be easily caught on hooks both in deep water and near skoreL.._ tilyetT.IgieFk. Eels avoid places Uth a hard, rocky bottom, and this must be remembered when they are introduced into bodies of water. The eel moves in a snake-like fashion, comparatively slowly, but if danger arises it burrows rapidly into the ground or makes for its hiding place. Eels can live for a long time (up to several days) without water, es- pecially in a damp place. I have seen living eels three days after being caught. They were packed in a basket with grass, sprinkled over with amall pieces of ice, and they were kept at a temperature of about 00 C. If newly caught eels are dropped into grass, especially if there is a dew or after rai , they can move about in it. Theymove on dry land just as they do in water, like a snake. However, on dry land eels move only for short distances measured in tens of meters, and not in any specific direction. Eels can move about readily on wet gravel and pebbles, but as stated in the literature, they cannot negotiate a dam. On sand or dry,grass eels quickly dry up and cease to move. In the popular scientific literature ofillast century and the beginning D rt e, 41-1 of this tee statement can be found that eels visit pea fields by night where pea leaves and pods are to be found. This story about eels visiting pea fields was accepted by Sybold (1882) without adequate verification and, on no good grounds whatsoever it found its place in the Western, and later in the Russian poplilar ichthyological literature and became widespread among the general public and, in particular, among fishermen. Tbis story has not been confirmed by scientific investigations. The ability of eels to live for a comparatively long time out of water can be explained by their predominantly cutaneous respiration (in this period) as a proportion of the total. For example,.,Krogh (1904) states thàt 60% of the total respiration is cutaneous; according to Strel'tsova (1963) cutaneous CtN- respiration in eels (at $-11 ° C) account* for up to $0-$8.5 of the total, while on the average it is 32% of the total, compared with 17$ in-the crucian carp, 11.2% in two-year-old carp, 6.01% in the burbot, 5•8% in the roach, 5.`ff. in the perch and 3.21o in the Ladoga whitefish. Eels can breathe with their gills for a long time when out of water. The gill apparatus is so designed that the oxygen of the air can be brea.tl-Bd N to a certain extent when the eel is on dry land. The gill cavities terminate in small narrow slits, which remain closed while the eel sucks in air and very often are covered additionally by the pectoral fins. In this way the moisture can be kept longer in the gills and the gas exchange maintained. Fishes which can live only within narrow limits of variation of water salinity are called stenohal.ine; those which can live within a wide range of variations of salinity and can pass freely from fresh water to salt or vice versa are called euryhaline. The European eel belongs to this second category. When an eel moves from fresh water to salt water the following physiolo- gic^%l and biochemical processes take place: chlorides enter the eel's blood stream from the sea water, increasing its osmotic pressure; the concentration of bicarbonates and lipoproteins falls, lowering the osmotic pressure. Since these processes take place simultaneously and have opposite effects on the osmotic pressure, the total osmotic pressure of the eel's blood serum remains at about its initial level, i.e., it is in equilibrium. Osmoregulation also takes place with the aid of chloride-secreting cells, which were discovered by Keys (1931, 1933), Keys and Willmer (1932) and Sch7_ipper (1933) in the branchial epithelium of eels and later in the other species of fish. It was found that if fish were kept in a hypertoni.c medium active excretion of the excess of salt takes place by means of these cells. Further investigations (Krogh, 1937; Black, 1951, 1957) showed that these cells in fishes can funetion in opposite directions. In fresh water, fish extract the salts they need by means of the chloride-secreting cells even if they are present in only very weak concentrations. The eells diet is very varied and consists of mollusks, insect larvae, crustaceans, fishes and other aquatic organisms. Eels are thus rightly called /43/ euryphagous. Eels feed only during warm weather, mainly at night, and by day they burrow in the gedafflftd, with only their head above, IgetTf a prey should come close during the day the eel will leave its hiding place and endeavour to catch it. For this reason, an eel can be caught on bait sanetimes in the dgytime, especially after hibernation. Schiemenz (1910) states that eels seek their food with the aid of their olfactory organs and their visual organs play only a secondary role. The opposite is evidently true by day, for eels can • bu sometimes be caught, although only occasionally, trollin% erimemee tntensive feeding of eels begins in May and continues until September. With the onset of the first frosts (October-November) the eel stops feeding. It does not feed in winter but, having burrowed into the soft ground, hibernates. Solitary specimens probably also remain in motion in winter, for eels have been caught in January or February, not only by seines, but also by nets. Dissection of these eels shows that as a rule their stanachs are empty. With the first cold spells of autumn, it is rare for food to be found in the stamachs of silver eels. The stomach usually shrinks considerably and shortens, the anus becomes appreciably.analler, and its walls become firm and a black ring -is forMed around it. These features show the fishermen that the season of el catching at an end, With the Coming of the warm weather, the eels become grèedy after their hibernation and they hunt around vigorously for food. They take it indiscriminately, filling their stomach and intestine so tightly that their walls become as thin as cigarette paper. I.have found jirtje..‘ as many as 80 caddis-fly larvae at a time in the stomachs oefeels in May. Some of them were swallowed complete with their "houses.tt An extremely varied diet is found in the stomachs of eels: insect larvae, mollusks, fishes, crabs, plant residues and debris. - Representatives of more than 30 species of animals belonging to the following systematic groups were found in the stomachs of eels caught in e_s White Russian waters: oligochaete worms, leeches, mollusks, may-54, stone te5- --(ttes f4r, dragony, caddis-rIK and chironmid larvae, lower and higher crustaceans, fishes (perch, ocean perch, spiny loach, rudd, roach, bleak, etc.). ZWZI9me„. it is very difficult to establish the principal or the favourite food of eels. • The composition of the eelts diet and the predominance of certain forms of animals in it depend on the age of the eel, the fauna of the water in which it is grown and, finally, the season of the year. Glass eels introduced into lakes feed during the first two years mainly on lower crustaceans and amnia larvae of various insects, but sometimes algae earl:also be found in their alimentary tract. In Lake Drivyaty on 28 June 1956, i.e., three months after introduction of elvers, ten young eels with a mean length of 88 mm and mean weight of 1430 mg were caught. Analysis of the contents of the alimentary tracts of these eels (Kokhenko and Borovik (1957a) showed that they fed mainly on small benthic forms and very rarely on plankton. For axample chironomid larvae were found in nine stanachs, oligochaetes in five, may-fls larvae in four, Asellus aquaticus in two, lower crustaceans (benthos) in two, algae in two, lower crustaceans (plankton) in one, and debris in one stomach. From one to five groups of food objects could be found in the - 62 - same alimentary tract, indicating the oMnivorous hature of eels even during their first year of life in fresh water. In the second year of life in ponds there is no particUlar change in the diet of young eels, although remnants of caddis-fly larvae, which were not found during the first year, were identified in individual stomachs. It must be pointed out that remnants of fish were discovered in the stomach of an eel 26 cm long, weighing 25 g, caught from a pond, during the year of its introduction. The possibility cannot be ruled out that certain fast- growing eels in lakes may eat the young of other fish even in the second year »of life. As a rijle, however, eels in lakes begin to eat fish only in the third year of life in fresh water (Table 8), and this at once considerably increases their rate of growth. TABLE 8. Composition of the Diet of Three-Year Old Eels from Lakes Drivyaty, Myastro and Novyato (incidence in percent) 12.. il 10 . 06umii ■ 111111U1 ..upmurrbi Mambo. Home() acTpe tae- MOCTII JlittutilKii xiipoliomita 3 4 28 J1iniiiit 1:11 apyritx pacekombix 3 1 18 PaliooGpanble . ..., . . . 5 21 ‘, Ilepnit . . r . . . . . 2 11 Pu6a ...... le . . . 4 2 1 24 .aeTPIIT 7 1 3 flyerble we..tyRKII . .g" . . ., . 4. 21 Note. 14 fish were examined from Lake Drivyaty, 5 from Lake Mysstro and 9 fish from Lake Novyato. KEY: 1) Components of food 2) Chironomid larvae 3) Larvae of other insects 4) Crustaceans 5) Worms 6) Fish 7) D:Lris 8) StomaChs empty 9) DrPryaty 1C) Mytr,o 11) Novyato 12) Total incidence ,r,,.t:.,.t tn^t eels are carnivores was observed by other investigators Schiemenz, 1910; Wundsch, 1916; Ehrenbaun, 1930; Frost, Murina, J F., . :orovik, 1954; i:^:khnenko, 1954, 1955, 1957, 195$; but, th^:: description applied to eels of older age groups. It is possible i;!:nt the earlier change to feeding on fish by eels is the result of their more rn;v il Croc-,-th in our waters. This conclusion c an be drawn if the figures given t,,- Frost (1945-1946) for the rate of growth of eels in Lake Winderil.ere, where their maln food consists of mollusks (75%) and insect larvae (i'), while fish account for only 0.51o, are compared with those 'obtained in White Russia. Eels in Lake Windennere grow slow7..y, as they do in Lake Teterki, where they feed mainly on insect larvae. No great differences are observed in the food of eels aged 3-4, 5-6 and 7--$ years, and only as the eel increases in size does it become capable of ^ eating larger food objects. The data on the food of eels aged up to 8 years and over 13 years in Lake Drivyaty are compared in Table 9. The diet of the former is more varied and, judging from their incidence, fish, chironomid and caddis-fly larvae and crustaceans occupy an important place in it, whereas the food of older eels consists mainly of mollusks and there is a much higher incidence of empty stomachs. In my opinion the changes taking place in the range of diet of the young and old eels are due not only to differences in their habitat, but also to the fact that the former are greedier. Before g years of age eels live in all parts of the lake, and being greedy they consume all edible objects which they find in the water, whereas older eels become adapted to the deep zone and,. consequently, to a narrower range of /40/ dietary components. Even in the season of intensive feeding it is common to find such eels with empty stomachs. """ 64 - The composition of the eel's diet depends both qualitatively and quantitatively on the character and composition of the fauna in the water TABLE 9. Food of Eels of Different Ages in Lake Drivyaty --Ii Ro8aer '7 ierapute 13 ACT I 2' . KONIflOtiellTIÀ maw' . Berpevae- 9i BcTpe- ocrpeue- 94 sme- mom, 43eMOCTI4 MOCTb 4aCMOCTI1 ,.■..... ^I— .71(1,1{1111M pyilefitutKoil•* . . . 11 30,5 13 28,8 ' xlipottomuo, 1-1- 12 33,3 2 4,4 e-rpetto3 .s--- • 1 2,8 3 6,6 rioaeiloK. . . Ce ..... 5 13,9 - — - ilecuililoK. '7 .1 2,8 — — 11poilne ilaceKomble '3 9 25,0 — — ci Petutort paK . • - — 2 4,4 11pol-tile paKoo6puilble / c• 6 16,4 1 2,2 %pall . • .. .il . .... 1. 2,8 ' 2 4,4 n1151BKii . - -. . .. . ( 77 .... 1 2,8 2 • 4,4 Bo:meal-in-:a ...... 13.... . — — 2 4,4 Alo.notocKit . 14: 3 8,3 16 35,5 is, Pbioa 16 44,4 9 20,0 1,1Kpa pb15 . _. . /G..... — — 3 6,6 PacrtyrurbiloeTb . . .t -7 . . : — — 8 • 17,7 .tleTpkyr .... ) e ... 1 ' 2,8 7 15,5 Ilyerbie :1:e.iy,uRit 19 4 11,1 8 17,7 Note. 36 specimens exanined were under 8 years old, 45 specimens were over • 13 years old. KEY: 1) Components of food 2) Larvae of 3) Caddis-flies 4) Chironcmids 5) Dragon-flies 6) May-flies 7) Stone flies 8) Other insects 9) Crayfish 10) Other crustaceans 11) Worms 12) Leeches 13) Nanatomorphs 14) Mollusks 15) Fish 16) Fish eggs. 17) Plants 18) Debris 19) Stomachs empty 20) Under 8 years 21) Over 13 years 22) Tr...71fIcnc 23) Incidence in % in which it lives. Eels find a more varied diet in deep eutrophic lakes such as Lake Drivyaty, and a less varied diet, consisting mainly of chironomid and caddis-fly larvae, in sha1.lovr, eutrophic lakes, with certain dystrophic charac- teristics, such as Lake No-v.yato and Plyussy, and a very restricted diet in the dystrophic, marshy Lake Teterki.. Because of the lack of food in Lake Tete.rki the eels are sometimes forced to eat longhorn beetles which have a thick chitinous skin and which therefore remain undigested in the alimentary tract. No sharp changes are observed in the composition of the eel's diet in different seasons, but there is nevertheless a tendency for the amount of insect larvae and crustaceans eaten to increase in the spring and summer and the number of fish eaten to increase towards the end of summer and in the autumn. The reason is that in spring and summer insect larvae are available for consumption as food on the largest scale, but by the end of summer their numbers are considerably reduced. Empty stomachs are fo und much more often in autumn, probably because Of the reduction in the quantity of readily available food (insect larvae) and the fall in temperature. The principal objects of the eel's diet in White Russian lakes are mollusks, which account for between $ and 50% of the total, caddis-fly larvae ( from 16 to 50%) , chironomid larvae ( from 2 to $0%) and fish ( from 7 to 4Li.ô) . Chironomid larvae and crustaceans predominate in the diet of young eels, and they are a less frequent component of the diet of older eels. Ehrenbaum (1930) and certain other biologists state that eels prey on the eggs of other fish and devour them greedily. I have specially -investi-' gated the stomachs of more than 100 eels caught with both hoop- and drag-M ts at spawning grounds and during the spawning of bream. Eggs were found in only four stomachs ( ?, 11, 103 and, in one case over 1000 eggs). No eggs were found in the other Stomachs even though the eels had fed. This suggests that eggs are an additional article of diet, presumably accidental, for eels. Eels prey on the bleak (inland waters of White Russia), snelt, Baltic herring and Baltic lake smelt (GUI' of Kurland), but it is not the eggs, but the fishes themselves, which they hunt and which they devour together with the eggs. In East German inland.waters, as M iller (1962) points out, the favourite -food of ;eels is the crayfish Cambarus affinis. Eels compete for food to a varying degree with bream, ide, silver bream, tench, ruffe, roach and perch. HoWever, the competition is not acute because the eel is an omnivorous fish. It also feeds on large mollusks, and when burrowing in the mud it eats insect larvae and worms, which are not readily accessible for other fish. Finally, it devours perch, ruffe and other small fish, thereby increasing the content of the more valuable eel meat and ridding the water systems of economically useless fish. Walter (1910), Hornyold (1922), Ehrenbaum (1930) and other workers have described cannibalism among eels on the grounds that a large eel can easi]y be caught on a hook if a small eel is used .as bait. I have examined about 2000 stanachs of adult eels from Lakes Drityaty, Snudy and Narochl, caught in 1956, 1957, 1960 and 1961 after the lakes had been stocked with glass eels. Small eels were not found in any of these stomachs. This couild be explained by the law density of stocking with eels or by the inadequate number of stomachs examined, for as experiments in aquaria have shown, cannibalism does occur among eels. In the ,spring of 1958 glass eels were introduced into all aquaria containing two-year-old eels grown in a pond. Several days later the glass eels in two aquaria had been eaten. For verification a'fresh stipply of glass eels was added and these were again eaten by the two-year-olds, • despite the fact that the aquaria contained plenty of rms. . In other aquaria the glass eels remained unmolested, even though in two of the uaria the two-year-o7d eels were given insufficient amounts of food. Cannibàlismin eels was thus confirmed by my eyperiments, although it was not observed in all aquaria. • Dimensions and Growth of Eels There is no shortage of information in the literature of the dimensions of eels. Sabaneev (1911), for instance, reports that in 1786 an eel more than 1 sazhene (7 feet) in length and weighing about 2 poods ( 80 lb.) was caught in the River Babe. Brem (1931) and Berg (1949) state that eels attain a length of 1.5 m and a weight of 6 kg, and occasionally a length of 2 m and a weight of 8 kg. Suvorov (1948) accepts that eels may grow to a length of gl› 1.5 m and a weight of 4-6 kg. Walter (1910) gives the largest size of eels as 127 cm in length and 4-6 kg in weighte but he is not certain whsther these may not have been Conger eels, for the information was obtained from nelrepapers. In White Russian waters eels introduced in 1928-1939 had grown in 1954 to a mean length of 94.8 cm and weight 1.5 kg, and individual eels,were 119 am long and they weighed 3.6 kg. Eels 113 am in length and 2.8 kg in weight are found fairly frequently. The .length and weight of eels are not always directly proportional. For example, eels of the same length (101 cm) weighed 1320 g and 1905 g. There are differences of opinion at the present time regarding the rate of growth of eels. Some workers consider that eels are slow-growing fishes, whlie others state that they grow quickly. Marcus (1919), for example, gives details of the mean rate of growth of eels in freshwater in Northern Germany: in the first year they weighed 1 g, in the second 3 g, in the third 13 g, in the fourth 27 . g, in the fifth 31 „g, in the sixth 46 g, in the seventh 65 , g, in the eighth 110 g and in the ninth year 227 g. Judging.from these figures eels grow very slowly, for the annual increase in weight until the seventh year was less than 20 g, and only in the 9th year was an increase of 117 g obtained. Similar data on growth of the eel are given by Wundsch (1916) and Ehrenbaum (1930) for the lower Elbe, by Frost (1946) for English waters, and by Tesch (1928) for Dutch waters. Wundsch (1953) states that eels introduced in a high density into a pond with a poor supply of food gave a mean annual gain in weight over 25 years of 5-21 g, their weight varying frcm 120 to 535 g. In Lake Teterki (Kokhnenko, 1955), also with a poor food supply, during a period of 14 years eels grew on the average to 350 g, i.e., their mean annual gain in weight was 25 g. Whsn, however, these same eels were transferred to the wUsyany" pond, with a good food supply, in the three summer months they gave a mean increase in weight of 250 g, while one specimen gained in weight by 360 g. Walter (1910) observed that glass eels placed in freshwater grew on the average during the first year to 20 g and to a length of 25 cm, in the second year to.260 g and a length of 52 cm, and during the third year to 500 g or more, and concluded that eels grow rapidly. Dryagin (1953) states that the mean annual increase in weight of eels may be from 0.2 to 0.49 kg. According to one report in the literature, during the fourth year of its stay in a pond in Italy with added food an eel weighed 1250 g. Although these figures for the growth of eels are contradictory, they reflect the true picture, which is that under different conditions eels exhibit a wide range of variation. For 8 years I have made observations on the'growth of eels introduced in 1956 at the glass stage and reared in White Russian inland waters with - 69 - different characteristics. My investigations showed that eels grcw at different rates in different types of White Russian waters (Table 10). The fastest growth was observed in the "Truikont" carp pond at the "Shemetovo" fish hatchery, good growth was obtained in the eutrophic lakes Batorino, Drivyaty, and others, and an intermediate rate of growth was obtained in the mesotrophic lake Narochl (Fig. 8). TABLE 10. Growth of Elvers Introduced in 1956 into White Russian Waters 2_ .a.1HFla, CAC .Ç'. Mace* , z roz ,r . A0Ba Br..3pzer nBN Koae6amisi cpeamig wo.ieGiliiii ? cpeAnzei KaptHassii npYA «T p y 11 K 0 T» 1956 ' 20,0-24,8 22,2 14,4--26,1 17,3 10 1957 39,9 50,0--242,0 117,9 18 1958 2+ 46,5- 71,0 59,0 149,0--564,0 - 330,0 7 1959 3+ 64,0-70,0 67,0 430,0-635,0 558,3 3 1960 4+ 76,0-82,0 78,0 930,0-1445;0 1187,5 03. Rpntuurm (3Brpo(pnoe, ray 6oxoe) 7 1956 0- 12,0--14,5 13,1 1,9--3,6 2,5 ' 16 1937 1-7 16,0--25,6 19,9 15,0-25,0 19,1 21 1958 2+ 22,5-50,0 38,9 19,5--314,0 126,5 13 1959 3+ 29,5-62,0 43,4 36,8--500,0 148,7 32 1960 37,0-80,0 50,5 65,0--884,0 234,0 19 1961 5 49,0-83,0 59,1 203,0--1060,0 377,0 24 1062 6+ 65,0--87,0 68,3 285,0--1120,0 538,0 40 1963 58,0-93,0 73,0 350,0--1470,0 673,6 87 03. Hapoqb (me3oTpodnioe) 1939 3+ 36,0-43,3 39,1 61,5--121,0 88,8 4 1960 • 4+ 41,0--46,0 43;1 117,0--157,0 139,1 6 1961 5+ 48,0--72,0 37,4 150,0-500,0 319,0 12 1063 7+ 54,0-70,6 61,9 1 202,0-755,0 392,4 15 KEY: 1) Year wten caught 2) Lenth, cm 3) Variations _• 4) Mean 5) Weight, g 6) "Truikont" carp . pond 7) Lake Drivrity (eutrophic, deep) 8) Lake Narochl, (mesotrophic) Comparison of the figures for grcmth of eels in White Russian waters with the corre:spor.dng figures for 1tdcctcrn covntries (Table 11) shows that in White Russia the eel grows just as well as in Italy, Czechoslovakia, Poland, and France and slightly better than in Fr:,-land, etc. Admit `^ecil.y the slow growth of eels in EnE,.7.ish waters can be e.xplained by the large ntmibers of eels which reach them in the natural way. For example, eels arrive in rivers such as the Lcire ( Frr.nce) , Severn (England) and lower Elbe (West Germany) in enor- mous numbers, as Marcus pointed out as long ago as in 19'19,. An increa.se in the stocking density of the eel retards its grovrth. For exaaple, Lakes Myastro and Drivyaty, which are of the same type, have approximdtely the same /4G/ Gi e^S l'^ ^ kn^h^^ of food supply and during the first three years in these lakes, when the stocking density was equal, eels grew almost at the same rate^ but when the stocking density in the first lake was doubled, the rate of growth of the eels slowed down. 400 Fiâ. $. Growth of ce] :. .'i.n different White Russian waters: 1) Truikont pond; 2) Drivyaty; L,.) L^il•.e Naroch' - 71 - If the growth of eels is examined, at each âge group starting from the glass eel considerable variations can be observed in the length and weight of tle eels, not only if 'reared in waters of different types, but also if reared in the same body of water. The wide amplitude of variations within the age group observed in all waters indicates that eels of the same generation grow unequally and attain a commercially useful size at different times, so that the period when they begin to be commercially useful stretches over several years. This must be taken into account when the catch of.eels is predicted. TABLE 11. Growth of Eels (mean length) in Various European Waters, cm /50/ Pr Bo3pact J. • Bonoemu Aigrop 0+ 1 1+ 1 2+ 1 3+ 4+ 5+ 0+ 7+ t-Lor- Beaopyccim 2- npyii TpyiiKoirr 3 22,2 39,9 59,0 67,0 78,0 Hann! gainible ;.27 oaepa: fiounTo 95,6 -- -- 51,3 69,8 To ›Ke C)cpericHoe LP -- 44,8 45,2 - » ii,plinsubi '7 14,1 28,2 ------Citygbi 13,1 19,9 38,9 43,4 50,5 59,1- 68,5 73,0 Tpo -7 -- 25,0 36,4 44,7 -- -- 67,7 fiapoulh 22,0 33,4 41,4 44,8 55,6 56,5 59,9 » BaTopinio 39,1 43,1 57,4 61,9 Anraus 12_ oa. i3unaepmepe !3 . .• 9,0 15,4 19,5 22,7 96,4 32,2 38,4 40,8 Frost 1945 - 28,3 32,1 33,8 Marcus, 1919 p. CeuepH /1.-r 13,5 13,5 17,5 24,7 27,0 Peua. kaape, IlmaHrnia iS- 33,5 32,6 35,6 38,5 41,0 Marcus, 1919 - Flpy.ed KaMaq4110, I1Ta0119 1.Ce 24,6 38,2 65,0 -- -- 79,0 Bellini, 1910 26,0 31,5 36,5 41,0 46,0 Hempel u. blerecheirner, Jlarylibt thaaHn 17 .10,9 15,5 21,0 1914 rlpyR 14xo, Ihpanium . 23,2 25,4 27,5 -- -- 45,5 47,0 Flornyold, 1928 Kamm flon- ;Le-Pyro, 1>pamuni . -- 31,4 36,7 42,4 51,0 -- 1-lornyold, 1930- 1-111aHlasi 9.11,6a, repmanHu , 9 12 15 19 26 34 39 .45 Ehrenbaurn u. Marukawa, 1913 CeuepHoil repmaillui . 9,0 12,0. 21,0 28,0 29,0 33,0 37,0 42,0 Marcus, 1919 PeHa Baanb, l'o.maitanH . 21,5 25,6 31,7 Tesch, 1928 „ • MeXOCJIMIKHU 22 29,1 35,1 46,0 - -- Volf F., Smisek, 1955 39,0 49,4 48,4 Thurow, 1959 Baxmilcuoe mope . - 34,4 (14111cHn5 3JUth. . 2.5 -- 31,1 40,2 56,7 Alumni, 1939 Kypemdi aaoHu . 7-fd 38,3 47,0 52,6 61,4 Maluouac, 1959 .:2- 11 • _ - KEY: 1) Location 2) White hussia 3).Truikont pond 4) Lakes: 5) Novyato 6) Osveiskoe Table 11 ( contirted) 7) Drivyaty $) Snudy 9) Myastro 10) Naroch' 11) Batorino 12) England 13) Lake Windermere 'lli.) River Severn 15) Ireland, River Clare ^ 16) Italy, Ccmacchio ponds 17) Italy, lagoons 18) France, Etang de Thau 19) France, Pont de Rousty canal 20) Germany, lower be 21) Northern Germany 22) Holland, River Waal 23) Czechoslovakia 24.) Baltic Sea 25) Gulf of Finland 26) Gùlf of Kurland 27) Kokhnenko 2$) Mikhin, 1939 29) Manyuka.s, 1959 Observations lasting $ years proved insufficient to allow a complete picture to be obtained of the duration of growth of eels in White Russian in- land waters, for growth continues until the eels are older. It was therefore necessary to use figures for growth of eels introduced even longer ago than 15 1/ this, and an investigation was therefore carried out .9 years after the last introduction of elvers into the Braslav lakes (Table 12). Accordingly, by ot ct--1^ plotting the , - s from Tables 11 and 12 on a graph curves characterizing the mean growth in length and weight of eels over a period of 23 years in Lake Drivyaty are obtained (Fig. 9). In all the White Russian waters eels grow considerably in length starting from the first year of life in fresh water, and grow -th ccntinues up to $ or 9 years and then falls off appreciably. For example, whereas after the first 9 years the mean length of the eels was $3 cm, with an annual increase in length of 9.2. cm, during the next 14 yea-rs the total increase in length was only 14 cm, or 1 cm per annzi. The increase in weight begins to rise rapidly after the second year in ponds, after the third year in - 73 - • eutrophie lakes, - and after the fourth year in mesotrophic lakes and it con- tinues until 13-15 years, uten it falls.off appreciably. The results for the growth of eels in length and weight thus entirely obey the general rüle established by Berdichevskii (1964) for other species of fish. TABLE 12. Size of Eels (introduced in 1928-1939) from Lake Drivyaty Limita, ci Macca, Fo a eu:torta 3 . Ko.ielianust cpeanga cume6anng cpe.inso 1948 60-105 83,4 300-1900 1064,0 78 1954 63-113 90,4 503-2370 1545,3 264 1962 78-115 97,5 800-2730 1553,5 89 KEY: 1) Year when caught 2) Length, cm 3) Variations 0 Mean 5) Wedght, g 16'0 1600 f10 1200 bD 800 W n 4 1..) a 3 70 15 20 Age Boecin years 9. Growth of eels in Lake Drivyaty: 1) length, cm; g) weight, g Considering thi.tt the productive growth of eels is observed only until ne of 15 years, cc=ercial fishinF. must be so organizecrat eels live • .5d - 74 - in our White Russian weers not more than this period. This will ensure the most economical usé of the f:od supplies in the waters. The Condition Factor Many workers consider that the condition factor, usually denoted by the letter K, depends on the degree of accumulation of fat. However, although the growth of fishes in length an1 weight and the accumulation of fat are inter- connected processes (Vasnetscv, 1953). , the dynamics of the growth in weight, or of the condition factor calculated by Fulton's method, does not always coincide with the dynamics of fat deposition. The increase in weight of the fish takes place not only through the deposition of fat, but also through an increase in the protein content, development of the skeleton, uptake of water by the muscles, and other prooesses which follow different courses in males /53/ and females and which vary with the season and with the age of the fish. All • these circumstances must be taken into account when the condition factor is calculated. On the basis of the generally accepted equation (7.100 the coefficient of proportionality was calculated for eels of all sizes start- ing from glass.eels. However, since the changes in the mean value of K for consecutive size classes with intervals of 1 cm were very snail, all indivi duals from 11 to 90 cm were subdivided into classes with a 5-cm interval of length, those from 91 to 100 cm were considered as a single group with an interval of 10 cm, and all those exceeding 101 cm in. length alSo were com- bined into one group (Table 13). - 75 - TABLE 13. Condition Facbor of Eels in White Russian Waters 2- Cpelmic Macca, Cpeamaa Oritotucitite aannd, CA OT-A0 macca. z A' Q 7-8,5 __ 0,27-0,51 0,389 0.0790 0,0490 200 9--10 '9,3 1,6--1,75 1,13 0,1318 0,1189 6 11-15 12,65 1,i5-6,6 3,2 • 0,1456 0,2461 35 16-20 18,12 4,19--11,9 8,6 0,1475 0,4777 12 ' 21-25 23,07 10,5-25,5 16,9 0,1388 0,7347 14 . 20-30 27,62 21,7-46,0 31,3 0,1426 1,1178 21 31-85 33,42 52.0-75,0 64,35 0,1730 1,9500 19 36-40 33,41 77,3--114,2 87,68 0 .1598 2,3074 22 41-45 43,C0 92,2-148,1 125,2 0,1575 2,9116 42 46--50 47,87 128-217 171,7 0,1553 3,5770 49 51-55 53,21 184,4--251 210,7 . 0,1415 3,9717 28 56-60 58,15 257,6--376 316,3 0,1621 5,4534 39 61-65 63,08 367,8-493,7 421,7 0,1687 6,6936 57 66--70 : 68,00 414--627 523,9 0,1666 7,7044 60 71-75 72,72 470-920 706,3 0,1815 9,6712 25 76-80 78,32 700-1065 824,5 0,1737 10,5705 16 81-85 83,77 700--1280 1007,8 0,1763 12,1421 35. 86-90 88,44 820--1720 1222,3 0,1794 13,8898 82 91--100 95,54 1089-1897,9 1525,8 0,1752 15,9710 253 101-115 105,67 1838-2323,5 2097,3 0,1665 19,4194 123 • KEY: 1) Length, an 2) Mean length, cm 3) W..:ight, ge from-to 4) Mean weight, g 5) Ratio Q/L3 The mnallest value of the factor (0.079) corresponded to the glass eels, and thereafter K rosegradually to reach its maximum (0.181) in the group measuring 7175 an. The marked changes in condition in eels measuring from 31 to 35 am in length will be noted, for K increased by 0.031 compared with the previous group (26-30 am), and also in eels measuring 56-60 cm, where K increased by 0.021 compared with its value for eels measuring 51-55 an. The ranaining changes were very amall (0.01). In the first case the reason is /5111 evidently that sex differentiation is still continuing in most of the eels, iddle in the second case most eels are in process of,change . from the yellow- to the-silver stage. However, this hypothesis requires further confirmation. Since the difference between the mean minimal and maximal values of K (Table 13) 'is cotilparativ°ly small and does not `xc,2^.:_1 C.05, an a'..tempt was made to d.eternine a general mean conditioned factor for ad.l eels measuring from 9 to 115 cm in length, and this has a value of 0.165. The difference betweeri the general mean value of K and its minimal, value of 0.01 S-, wbile the difference between the genera,l mean and maximal values of K is 0.020. It sho:).ld be noted that the mean value of K is also 0.165 for eels 29-73 cm long from the Kieler Bucht (Bay of Kiel) (Thurow, 1959). There is thus every rea- son to accept the value of 0.165 as the general mean condition factor for eels of all sizes. It reflects the actual ratio between the weight and length of "_ all eel populations. So far.as individual deviations are concerned, especi- ally at the extreme li.mits in each size group, they cancel out when the mean factor is calculated. s The Nutritive Qualities of Eel Meat Eels are a high-quality food product. They can be eaten smoked) fried, stewed and piclcled, but they have the most delicate flavour and are tastiest when smoked. Of the total annual catch the Belglaveybprom Organization markets $5-90% in the smoked form, î0-15/ chilled and a fraction of 1% frozen. •Eéls have a sufficiently high fat content for them to be cooked in their own fat, and indeed sometimes the fat content is excessive. The fat content of eels depends on their age and the character of the water in which they live. Young eels have.less fat than old. Eels living in waters with a richer food supply are fatter, but in every case the fat content of the eel is greater than that of the white fish or any other species of fish (Table 14). Eel meat contains not only large quantities of easily assimilable fats and proteins, but also a wide range of mineral elements (Na, K, Ca, Mg, Fe, P) C.,L. etc.) (Table 15) and vitamins .(vitamin A, aneurin, riboClavin, etc.), which are essential to man and, in particular, to children. 'In some countries Can- ned products prepared from young eels are recommended as a ranedial diet for children suffering from various severe illnesses. TABLE 14- Biochemical Composition of Eels and other Fish, % 1 .... ;7 I ( O " 13 114" c-be , K Bill pu6bs fienot *up Baara 3ona 2o6Ha n :ac.TicB, ? AnTop nacTb purs YrOPI, ir..2-1 . 10.7- 22-3253,2--- 1,03- 75-792665- Hanna "muffle' /5 14,9 55,3 1,29 3300 Nropb '7' . . 14,78 22,22 62,16 0,84 76 2670 n. Ilepcoa9 /9 Y-roph 1:7- e- 1 . . 13,5 28,3 57 - Mc. Cance3 . Cur . . . 18,32 1,53 69,13 1,82 65 890 IT. Ilepcoa t ? .TIOCOCI) . '1 . . 20,0- 10,7-- 64,0- 1,2- JI. 140.11bC011 2.0 21,1 13,3 67,0 1.4 neut . 1.-h. . 16,1- 2,5- 69,2- 1,0- » 21,8 14,0 78,0 1,5 Cyaa . 5- . , 15,5-0,4- 76,2-1,1- » 21,9 0,8 81,5 1,5 • . , oxoao 0,5- oaoao OKOJIO . » 18,5 0,7 79,5 1,3 Bpac.-nuicKue o3epa. 2 (01111CN.Ili1 311.111B. 9 3 Boaoemm Aumuu. KEY: 1) Species of fish 2) Whitefish 3) Salmon 4) tream 5) Pike-perch 6) Perch '7) Braslav lakss 8) Gulf of Finland 9) Inland waters of England 10) Protein 11) Fat 12) Moisture 13) Ash 14) Edible part 15) Calorific content . 16) Author 17) Kokhnenko 18) Persov 19) Iollson Another valuaUe property or the eel as a food is that •ts meat has no bones among the muscles. The whole trunk from head to tail inclusive is /re- bounded only by the spinal column with its small spinous processes attached , ))// to the vertebrae. On the average about 70% of the eel is edible, while in autumn, when its condition is best, in some cases up to 79% of the total , weight of the fish is edible. The edible part of the whitefish is 65%, and for other species of fish it is much less. TABLE 15. Biochemical Composition of Eels at Different Stages of Growth and Development (after McCance, 1 944) 7 ,___):,..t. tnef e. e. e. ! • ,à ‹.., 4' c Yoph %8 gcD 8 o o o a 2. - =0 UZ -J.; z • CTCMOBIUllbln . 1 . 81,8 12,6 2,19 67 230 51:-) 31,0 4,00 440 55,4 2,02 . • 76,5 19,0 2,50 110 302 610 28,0 1,80 475 52,0 3,04 YKCJI1Blii . . 70,8 17,2 10,60 129 247 411 23,1 1,33 419 92,6 2,75 Cepe6plicruil . et 59,7 13,6 23,20 92 207 323 18,0 1,83 313 79,0 2,51 KEY: 1) Glass (elvers) 2) Stock (snigs) 3) Yellow 4) Silver 5) Water content, g/100 g 6) Protein, g/100 g. 7) Fat, g/100 g 8) mg/100 g Figures are given in Table 16 for the weights of certain organs of the eel as a percentage of the total body weight. It will be seen that the head, skin with fins, and the internal organs account for on the average 25% of the total body weight of the eel. The relative weight of the head, of all /56/ the viscera, (liver, kidneys, alimentary tract, swim bladder, heart, panereas), except the gonads, is'much greater in young eels than in older eels.' The relative percents of the main components of the various tissues . and OranS of the eel are inconstant and Vary with age. For example, the p.frentai;e of protein in the mu';cle5 of young eels, including yellow eels, lies between 16 and and the percentage of fat between 3 and 11%, whereas the corresponding fi^urr^c^ for silver eels are protein 13-15f and fat 26-3C,t. Consequently, with increasin^; age there is a marked decrease in the percentage of protein and water in the body, but on the other hand there is an increase in the fat content, a7.th.ci.lgh the absolute weight of all the components increases with age. TABLE 16. Weight of Certain Parts of the Body and Organs of Eels as a N Percentage of the Body Weight I'bnona . ^. . . . .-' . . 28 5-13 8,87 Kox KEY: 1) Organ, part of body 2) Head 3) Skin with fins 4) Viscera 5) Including 6) Alimentary tract ?) Gonads $) Liver 9) Kidneys 10) Swim bladder 11) Heart 12) Pancreas 13) From-to 14) Mean These results of biochemical analyses point to the high nutritive quali- ties of eels. Young eels, including yellow, have a relatively high content of protein and vitamins and can be used in dietetics and in the canning industry. When eels reach a length of 50-55 cm and a weight of 200-350 g in most cases they become silver in colOur. At about this time the fat content of the eel rises sharply, and with further. growth it increased even more. Consequently it is best to catch eels in inland wate7-s when their length reaches at least 50 - 55 cm. .p_lpecific Properties of the Blood It is stated in the literature (Walter, 1910; Sabaneev, 1911; Puchkov, 1941; Suvorov, 1948) that the eel's blood contains a poison with an action sim- ilar to that of snake venom or even of curare. The toxicity of the eel's blood serum ha s frequently been investigated /57/ (Richet and Hericourt, 1897; Camus and Gley, 1912; Phisalix, 1922; Duval, 1925; Delage, 1939; Bertin, 1956) from various points of ViEW. It has been shown that if 0.1-0.3 cm3 of the blood serum of an eel is in- jected subcutaneously into a rabbit death occurs within 2.5 min, while dogs die 4.5 min after injection of 0.5 an3 into a subcutaneous vein of the hind limb. Immediately after the injection the pulse and respiration rates rise, spaans and generalized convulsions occur, and the intestine is involuntarily emptied. In 1954 I studied the toxicity of eells blood on two dogs. For 3 days I injected the serum of an eel intravenously into the animals: on the first 3 3 dsy 0 1 5 - 1.0 cm and on the following days 1.5-2 am . After the injection the dogs became generally excited and restless and they howled and vomited. In- voluntary defaecation took place evidently on account of increased peristalsis. Inhibition soon followed (the dogs lay down, did not respond to external stim- uli, and developed convulsions and rapid respiration). The dogs remained in a comatose state for 20-50 min. This was followed by slightimprovement, although the dogs refused to eat. The subsequent injections were tolerated better, and after three injections the animals still remained'alive. .Although the statement that dogs die 4-5 min after injection of 0.5 an3 eel's blood serum w.as not confirmc,d by my e.xperiments,; the other symptoiY'ls and si.gns re- putedly caused by the poison were in fact observed. Investigations especially by French workers have shown that eel's blood may vary in its toxic action because the toxin consists of several components. The neurotoxic action, or poisoning of the nervous system of the res- piratory and cardiovascular centers, is produced by a neurotoxin. If the poison is taken by mouth, no neurotoxic effect is observed. Eel's blood serum has a cytotoxic action, causing destruction of cells. As Bertin ('1956) points out, lysis of neurons may perhaps be the cause of the neurotoxic action. The experiments of PettLt (1g9$) showed that the blood serum causes very rapid changes in the tissues of the kidneys. The urinary tubules undergo specific crystalline or granular degeneration of their walls, their lumen becomes obstruced, and blood appears in the urine, i.e., destruction of the red blood cells takes place wi.th the liberation of hemoglobin. Conse- quently, eelts blood serum contains a haemolysin which destroys red blood cells. The essence of the anticoagulant action is that after injection of ee?ts blood serum into a rabbit the animall' s blood is prevented from clotting. In that case the eelt s serum acts, as Delezenne ( 1$97) showed, not on the rabbit's/5,53-ill blocd but on its liver, stimulating the production of anticoagulating diastase by the liver, with an action similar to that of peptone. In. an investigation of the toxic action of eel' s blood serum. Phisalix ( i896) showed that if heated above 56° C for 15 min the serum cor.ipletely loses its toxicity. After destruction of the toxins in this way the serum exhibits an antitoxic effect, opposite in direction to its former toxic properties. If snake venom is mixed with the antitoxic serum, it is neutralized by it. The antitoxic serum, injected into guinea pigs, led to a more or less permanent immunity to snake bite, i.e., the antitoxic serum behaves as a natural antivenom vaccine.(Ketalnikov, 1939). Phisalix (1922) showed by his experi- ments that the antitoxic serum ottained from eel's blood can also neutralize rabies vaccine. Heated serui, mixed with a preparation of rabies vaccine, can be injected into a rabbit through a trephine hole -without harmful effect. If, however, the serum is injected alone it gives the animal a relative immun- ity against rabies, i.e., it does not stop the development of the disease but merely retards it. On the basis of these experiments Phisalix concludes that eel blood serum contains at least two antitoxins or an antigen-antitoxin against venom and an-antitoxin against rabies.' Probably these antitoxins have different biochemical properties and are inactivated separately by heat. In a study of the spread of sarcoma in cold-b]ooded vertebrates Peyron (1939) found that this tumour wras rare or absent altogether in eels, toads, vipers, and other snakes with a toxic internal medium. Considering that eel blood serum neutralizes snake venam, and even causes an animal into which the venam is injected to develop a general and cytological immunity against the venom, the possibility cannot be rUled out that eel blood antiserum may have a negative action on the development of malignant tumours. Extremely small doses of cobra venom are known to stabilize malignant tumours and to make than painless, in the same way as eel blood serum neutralizes the development of rabies.vaccine. However, this hypothesis requires detailed and. comprehensive experimental investigation. Soon after the properties of the eel's blood serum had been discovered it became clear that they accounted for the eel's exceptional tenacity for life, its resistance to starvation especially during spawning migrations, and its ability to adapt to different ecological conditions. Eel's blood serum thus contains both toxins and antitoxins. The former stimulate the nervous system and evidently play an important role in general /59/ -83- metabolism, i.e., their toxic function is entirely tonic. -The latter have a protective function against internal poisons and venoms entering the body from the externnl environnent, i.e., they play a bactericidal 'role. Diseases of Eels There is now an extensive literature on diseases of eels. Parasites of eels have been described by Soviet authors: Skryabin (1923), Dogelt -(1936), Goreglyad (1955), Kokhnenko r Borovik and Gorovaya (1959), Borovik and Kokhnenko (1961) and others. The foreign bibliography also includes scores of titles. The most important are the series by the famous German ichthyolo- gist Ehrenbaum (1930) and the famous biologist Schaperclaus (1954). After 1954 a few other papers devoted to reports of cases of eels affected with It cauliflower disease", inflammation of the swim bladder and various other conditions have appeared in the foreign literature. Although it was not part of my purpose to investigate diseases of eels, which is a field for the ichthyopathologist, in the course Of a study of the biology of eels'in natural waters and also in aquaria I have often encountered diseases of eels unavoidably. For example, during investigations of the contents of the alimentary tract, parasites have been discovered besides food Objects. In eels with ichthyophthiriasis and dropsy in uaria careful obser- vations have been made on the course of the disease, on the behaviour of the eels, and on pathological changes in the outer coverings and the viscera produced by.the disease. Since the eel population of the internal waters of the USSR increases year by. year I consider that a short accoUnt of diseases of eels in this book will be quite appropriate. To give a general picture of the diseases of eels, they are listed in Table 17 together with their pathogenic agents. - 81, - TABLE 17. Diseases of Eels and Their Agents /60/ ■•■■•••■■■■■■...... ■•■•■■■■■•••■••••■■••. 11o315yeureAb, eumnalcuturt ' 3a6oicealime • / a6u.lenatute He3apa3nbie 3a6o.1euaiu1: upocry.tta; palm. nchaytielinbie OT XHIIIIIHKOB H Ept04KOB: ,xo6poKa4e;roietutbie onyx°- . irexallitgecxue XIINTWICZKIIC 110- Bmiiienasi >ita6p; 6o:te3t-la cepaita; aocnanerme nepepm.h..aerule liegem'. 110.10ablX. IlO0,1SI lchthyobronema gneclini (Suriarikov). Camalanus truncatus Camalanus lacustris (Loega). Philometra sanguinea (Rud). Spiroptera conoura (Linstow). 85 - Table 17 (Continued) Bo36yaRre1b. obInbBarouüiif 3a6o.] eoaxre i saUoaesatiue Acanthocephala: Echinoncynchus salmonis Pséudoeéhinorhynchus clat•ula :lcanthocephalus lucii (AIüller). Acanthocephalus anguilla (iilüller). Pomphorhçnchus laevis (Müller). Neoechinorhynchus rutili (Müller). Corynosoma cemenue (larva) Atollusca: Anodonta sp. Unio sp. Glochidium . Crustacea: Ergasilus siboldi (Nord.). Ergasilus gibbus (Nord.). Argulus foliaceus Lernaea cyprinacea K^.1^`^X: 1) Disease 2) Noninfectious diseases: catarrh, wounds received from carnivores and hooks; benign tumours, mechanical and chemical injuries to the gills, diseases of the heart; inflammation and degeneration of the liver and sex products; curvature of the spine, etc. 3) "Cauliflower" disease 4) Ascites 5) Virus and bacterial diseases 6) Eel dropsy in salt water -- eel plague, red-spot disease 7) Eel drop,.-,y in fresh water $) Fungus diseases 9) Protozoal diseases 10) Trypanosomiasis 11) Coccidiosis 12) Ichthyophthiriasis 13) Trichodiria.sis 14) Helmintri.c diseases 15) Agent causing disease 16) Agent not identified The noninfectious diseases include ascites and "cauliflower disease." In "cauliflower disease" a ,distinctive tumour appears on the head, sometimes larger than the head itself, and shaped like a cauliflower. The tumour usually Is brownish red in colotir and is covered with red spots and lines. Most fre- quently the tumour consists of two parts: one part on the upper and the other on the lower jaw. The tumour grows so rapidly that the eel cannot take its food and dies. According to Polish, Swedish, Danish and German investigators "cauliflower disease" sometimes flares up with considerable severity. Consequently, an in- /6 1/ fectious factor may also be concerned in its pathogenesis. It is found in eels living in salt .water. I saw eels with such tumours in 1953 in the Gulf of Kurland. Of the bacterial diseases of eels which can break out in inland fresh waters, the fresh water form of dropsy is highly dangerous. This is an acute infectious disease which causes death of eels on a large scale. In the fresh inland waters of Western Europe eel dropsy has frequently been observed. In the Soviet Union the only account of it is that given by Goreglyad (1955),'who found eels with features Of dropsy in catches of fish obtained from -Lake Narechl. Two opposite views are'held at the present time on the nature of the agent of dropsy. Sch4erclaus (1953) and others consider that the agent of red-spot disease of carp is the bacterium Pseudomonas ounctata f. sacroviensis, for it is always .found in this disease. On the other hand, Goncharov (1949) and others have shown that the agent of red-spot disease of carp and other fish is a filter-passing virus, but its properties have not yet been adequately studied. - Remembering the possibility of variation of microorganisms, it can be postulated that the bacterial agent of red-spot disease of carp (P,Seudomonas ounctata) can change into a filter-passing form. .Sch^perclaus (1951a.) states that the external features of drpps,y in eels /62/ - are as follows: 1) a spotted or red body, fins, and anus; 2) whitish or bluish glistening spots appa^.^'ing as a result of loss of the upper mucous layer of the skin; 3) open round, whitish ulcers exposing the muscles; 4) lumps (tubercles) varying in size from a pea to a walnut; sores, espec- ia.lly on the head in the spring; 5) blindness. However, eels may be found sick but without any external features of disease. Such fish appear lethargic and they die comparatively quickly. This state of affairs arises if the pathogenic agerit is of high virulence, so that the disease runs a rapid course. Observations on sick eels in aquaria have shown that the features des- cribed by Schaperclaus develop gradually if the bacteria are of low virulence. It is stated in the literature that only adult eels are susceptible to dropsy, but I have observed the disease in eels two or three years old. Older eels were transferred to the laboratory from the "Shemetovo" carp hatchery, where they had lived one year and five months. Externally the eels appeared perfectly healthy and they were well nourished. They varied in length from 36 to 1. .3 cm and in weight from 76 to 101i. g. All five eels were placed in an ordinary where they spent the winter satisfactorily. In spring, i.e., in May 1958) glass eels were introduced with these two-year-old eels. One year later, i.e. in October 195$, all the eels were weighed and measured. The results showed that the glass eels had hardly grown at all, while the old eels had all grown thinner; they had lost 10-20 g in weight. One morewi.nter passed, at the end of which dropsy broke out in the By this time there were. five eels in the aged 2 years 9 months and three eels aged 10 months. The first signs of the disease among the older eels were observed on 2J\larch- . 1959. Before this date the behaviour of the eels had been normal. From the first day of the disease until death of the sick eels, systematic observations were made. Microbiological tests were carried out in the laboratory of Academician of the Academy of Sciences of the Belorussian SSR Kh. S. Goreglyad, who shaned that the agent of dropsy, Pseudomonas punctata.f. sacroviensis, was present in the blood of a sick eel. The results of observations lasting 33 days of the sick eels showed that the course of the disease was as followe. To begin with the eels ceased to burrow in the ground, their movements became sluggish, their body lost its resilience, and considerably less mucus was present on the skin. The sides and back were covered with whitish or bluish spots with a metallic lustre and irregular in shape. An appreciable area of erythema was present at the base of the fins, more marked near the anal fin. Later, on the ventral aspect of the eel and near the anus red spots of various shapes appeared. A fen days later the epidermis broke over the blue and white spots and began to peel. The upper layer of skin began to die and the spots became white in colour. Sometimes they were surrounded by red lines of demarcation with a dotted struc- ture. Destruction of the corium and ulcer formation then began (Fig. 10). White ulcers were formed at the ends of the upper and lower jawe of one of the eels. With time the ulcers became deeper to expose the muscles. In two eels large areas of the tail part of the body lost the whDle of their skin cover. At the beginning of the disease the eels ate their food but their appetite subsequently- was lost and eventually they ceased to eat altogether. Respiration was slowed butremained regillar. The rhythm then became irreelar, spasms deVeloped, and after 1-2 days the eel died. Two eels which had died from dropsy-were dissected but no pathological changes were found in the viscera. AU that was observed was an intense red- dening of the posterior part of the intestine. The disease in young eels followed a different course. :hese eels, kept in the same bath as the larger 3-year-old eels, shaded no manifestations of the disease whatever and were transferred to another aquarium. They became ill almost 10 months later, at 41,,■••• Fig. 10. Ulcers and spots on the body of an eel with dropsy. the age of 1 year and 7 months. To begin with these eels also ceased to burraw in the ground, but unlike the first two, their behaviour was quiet: their movements were jerky, their gill pouches were strongly inflated, and spasms travelled periodically along the body. The eels then began to lose their move- ment coordination. After 15 days convulsions developed and the eel's movement became sluggish. Some of them, as a result of rupture of the gall bladder, developed a large greenish swelling under the skin in the region of the liver. At autopsy degenerative changes were found in the liver, gall ipladder and intestine. Next day the other eels died. Neither ulcers nor spots were found on their body. Only one of them had dark red lumps or tubercles on its head. Cases of drop;y occurring in our aquaria caused me to question the specificity of the infectious principle of Pseudamonas punctata f. sacroviensis. It is difficult to imagine that this bacterium was present in the external medium of our aquaria, for the ground (gravel) was washed and steamed before it was scattered into the aquarium, and the water wus well water fran the public supply. Most probably the infectious principle was contained in the eels themselves before they-were transferred into the aquaria. Since the eels had been transferred directly into the laboratory and into the Shemetovo hatchery they could have been infected either in the sea, as glass eels, or in the carp ponds of the hatchery. However, dropsy among eels in the sea iS -caused by the bacterium Vibrio angullarum, which cannot develop properly or live fora long time in fresh water. The possibility of the transfer of the infectiOus prin- ciple by glass eels was thus ruled out. Dropsy in carp is caused by P. punctata f. ascitea, which is regarded as specific for the carp. However, since the disease occurred only in the aquarium which contained eels brought from the carp ponds, presumably the original form was P. punctata f. ascitea. Once it had-become adapted to the new species (eel) the organism changed its relation to certain media, and this led some investigators to distinguish it as a special form, Pseudomonas punctata f. sacroviensis. These bypotheses require further verification and clarification. It is noteworthy that in 1962, examination of 37 7-year-old eels caught in Lake Myastro revealed red spots around the anus of two individuals, although no large-scale mortality was observed among the eels. Of the protozoans parasitic on eels the most dangerous is the infusorian Ichthrphthirus multifiliis. For example, Wolf (1958) states that in some waters of Czechoslovakia many eels died in the course of 20 years fram ichthyophtlLiriasis. I observed ichthyophthiriasis under laboratory conditions in the:spring of 1-956,.195$ and 1962; in 195$ most of the fish in the aquaria were affected by this disease, which continued into the summer (Koklu7en'.Lo, Borovik and Gorovaya, 1959). On 10 May 1958 200 glass eels were brought into the labo.ra.tory for .futur,: stocking Of the e.xperimental ponds. While in the labo.ratory the eels were temporaril.y placed in a large porcelain vessel with acapacit;lr of about 50 litres, the water in which was changed daily ( canpletely or partly) The eels were fed on water fleas bred specially in separate It aquaria. The outbreak of ichthyophtlv.riasis began on 17th May. By this time there were 2.00 glass eels and 11 2-year-old eels as well as carp and tench in the laboratory. Ichthyophthiriasis was first found in nine eels in the porcelain vessel. With each day the number of infected fish increased and the infection became more severe. Some eels bec^!ne a.lmos•c completely covered with infusor- ians in the course of time. They ceased to take food or to react to external stLnulation and their movements became sluggish. As a result of this disease the fish died .on a large scale. More than 100 glass eels died from the 1st /65/ until the 4th of June alone. On 19 May 1962 62 glass eels were brought into the laboratory and kept temporarily in two 25-litre aquaria. On 9 June i.e., 20 days later, the infusorians were found on several eels. The infested eels were immediately transferred to a separate aquarium, but despite this preventive measure next day the number of eels infested with ichthyophthiriasis was considerably greater. Knowing the course of the disease in glass eels and its results from the experiment in the previous years, no attempt was made to treat the infested eels in a dilute solution of potassium permanganate or by keeping them in running water, because this preventive measure had been shown to be ineffective. It was decided to test the action of malachite green solution on the infusorian. The surviving 45 eels were transferred to three glass aquaria, 15 eels to each. Iwo aquaria were experimental and onethe control. . A solution of malachite green was prepared in twn crystallization tanks; the concentration in the first tank was 1:10 000 and in the second tank 1:20 000. The exposure to the 1:10 000 solution was 15 sec and to the 1:20 000 solution 20 min. The eels were bathed on 13 and 14 June. - Eels from the experimental aquaria were placed in gauze bags in which they were innersed in the solution for the required time. After bathing the eels were washed with a jet of tap water for a few seconds and then replaced in the aquarium which had been filled with fresh water. All the eels were free from Ichthuphthirius on the 5th day after bathing and they began to fatten and to behave normally. AU the eels in the control group died within 15 days. Fifty glass eels kept in the neighbouring laboratory also died from ichthyoph- , . thiriasis. Bathing eels infested with ichthyophthiriasis in malachite green solution thus gives good results. Admittedly this method cannot be recommended for the • control of ichthyophthiriasis in natural waters, where it would be impossible to use it, but in order to eliminate ichthyophthiriasis anon g eels kept in aquaria or even in small ponds it is perfectly suitable. I have only once caught an eel with a severe degree of ichthyophthiriasis in the natural waters of White Russia, fram Lake Niyastro. This eel was aged 3 years and 2 months (counting its life in fresh water). Infestation of eels with Ichthvophthirius in natural -waters can easily be understood because the infusorian is found on other fish in Russian waters. It is difficUlt to say how glass eels can be infected in aquaria. I examined many glass eels imported from France and England but they were all free from ichbhyophthiriasis. However, in some of the boxes, among "the glass eels there /66/ were one or two pigmented eels, 12-16 cm long, and Ichthyophthirius was found on two of them. These pigmented eels were evidently the source of infection of the glass eels placed in the aquaria. According to the available information, 31 species of parasitic helminths are found in eels. • Of the ten species of digenetic trematodes (Tranatoidea) nine have been found in the intestine of the eel and the larva of one species in the lens of the eel's eye. The species most frequently found, according to Ehrenbaum (1930), is Hemiurus appendictilatus. Soviet investigators have not reported this species as a parasite for eels. I have found one specimen of Hemiurus se. in the intestine or an eel caue; in Lake Ellnya. Unfortunately the parasite was slightly damaged and it was therefore impossible to determine the species to which.it belonged. Larvae of the tranatodes of the Strigeidea order, with the systematic naine Diplostomulum s2:2than,um, are dangerous to eels. They settle in the lens of the fishIs eye and cause blindness. This species has been found in one eel in White Ruesia (identified by G. K. Petrushevskii). No monogenetic tranatodes have been found in eels caught in the inland waters of the Soviet Union. V. A. Doger writes to the effect that he found three specimens.of Gyrodactylus sp. in tlinro small (61-71 mm long) eels sent to hiM from Naples. The eels were caught in the Regi Lagni Canal about 20 km from the sea. Tapeworms are found in the eel's intestines. Species such as Bothriocephalus claviceps and Proteocephalus macrOcephalus are,specific and usual parasites for the eel (Dogel', 1936). Eels are particularly intensively infested with tapeworms of the first of these species. Dogell, for example, states that of 15 .eels from the Nevskaya Guba (Neva Bay) 11 were infested with Bothriocephalus claviceps (up to 20 worms from each eel) . In 1953 and 19551a. T examined the ali,mentary tracts of about 2000 eels. Taper,vorms were found in 10% of the eels. Some had as many as five of the parasites. The greatest length of the parasite was 70 cm. Examination of the contents of the alimentary tracts of $7 young eels (age 3+ and 4+) cauLht in Lakes Drivyaty, Novyato, E1' nya and Myastro, re- vealed a high intensity of infestation with Bothriocephalus claviceps and P:coteocepalus macrocephs.lus: 11 eels were infested with tapeworms of these species (Borovik and Kokhnenko, 1961). The nematode fauna of eels consists of nine species, four of which are ascarids and five spirurids. Only two species, Camallanus lacustris and ^ Rnaphidascaris acus have so far been found in White Russia. Individuals of the first species are found frequently in the sta.nachs of young eels, sometimes /67/ as many as a score of wor:ns to each eel. Rhap.^v..dascaris acus has been found in only two eels. Besides the species mentioned above, hair-worms (Gordius sp.) have frequently been found in the intestine of eels. Their role in fish is not yet clear. Some workers consider that hair-worms are parasites while others are doubtful on the basis that hair-worms can live freely in inland waters. Hoôlra)nns are particularly common in eels. According to EhrenbaLUn (1930), $0-9Vo of eels in the lower Elbe are infested with Acanthocephalus anguillae. In White Russian waters this parasite has also been found among eels in the system of the Braslav Lakes (Borovik and Kokhnenko, 1961), but in smaller numbers. Dog' el (1936) isolated another species of hookworm, Acanthocephalus lucti, in the intestine of an eel from the Nevskaya Guba. The gills of es are infested .with,parasitic.làrVae of the:Anadonta . and freshwater mussels (Unio), the larval stages of which are called glochidia. The discovery of glochidia on the gills of an eel in waters of the USSR has been reported by - Dogiel (1936). Of the parasitic crustaceans, Ergasilus - sibbldi and Ergasilus gibbus are found on the gills or eels. Doglel states that 40% of the eels from the Nevskaya Gilba which he studied had Ergasilus gibbus on their gills. The parasitic fauna of eels living in the waters of the Soviet Union has so far received little study. The information given in the Soviet litera- ture is scanty and fragmentary. As a valuable fish the eel deserves the attention of ichthyopathologists, more especially because the eel population Of the inland waters of the USSR inereases in scale year by year. 1h addition, the study of the parasitic fauna is of great interest from the point of view of zoological geography. 96 _ • L:^, iii.'.;!: rIi;G IN 1:11l.,rlit7 ;.ïrl.'. tS Stocki n - 1 r-at^y'ln l and Sources of ' ts •`3Lil.li)l17 It is soi`d (;;urtr-•lrlet, ]^?62) that the ennu.al 'catch o'!' glass eels in France 1.:i about Gl;l1 •Toi^'i.l'ic to;,7i, i.e., about 2XIO 9 eels. These eels could be used to stock- about 20 million hectares of internal viaters every year. I IIo^,ever, in -1-^'rance itself only •abotit 20 metric tons of eels, i.e., about 60 millian individuals, - are used to stock inland waters (rivers and lakes) or ex- ported for this purpose to other European countries. Most of the glass eels cau, lit (about 5100 niei;ric tons) are consumed by the local population, Mhile some are exportcd canned in oil or frozen to Spain and to North and South America, • and some is actually used for feedinm pigs. The natural st?p,^ly of younT eels reachin;_; the inland waters depends on their distance from the PAlantic Ocean. The further east a body of water is from the l)lace of i'.èet<:.^.o"ï>hosis of the eel larvae, the teL7er elVers \TTi 11 ?^efl.ch it. Since the eastern limit of spread of the :âuropean eel is a line passing throu-;h the Soviet Union, Russian inland waters receive far ferrer youn-- eels than those of ':,este-rn :surope. The eels which arrive in the Baltic Sea and' Gulf of Finland do so much later and in comparatively small numbers, but the .eels themselves are larger. For instance, young eels entering the River Vistula are 10-12 cm long, those entering the .2-i-ver Neman are 15-25 cm long, and those entering the Western Dvina are 20-40 cm long. Smaller specimens are rarely seen. Although eels reach the Frisches Tïaff and the Gulfs of f:urland, Riga. 0 and Finls.nd, where they are of commercial importance, naturally and in consid- erable numbers, the nur.iber of eels reacliin^• the basins of the rivers which flow into tlicsc> :;u1fs is very nn)ch sm,,ller. For a number of years so few eels - 07 - • errived in the rivers and le:en of.the Ï2,altic States, White l'!ussia snd Leningrad .legion by the natural route that they were of no commercial importance. Only a;.:er these inland waters had been stocked with young eels did their numbers incresse sinificantly. Glass eels are cau.7ht with a special eel net made of I:apron rauze with a mesh of 2 mn. The diameter of the net is SO cm. It is mounted , on a \n pole 3-4 metres long. The fisherman, who stands on the river bank or5em. a boat floatir with the current or ridin at anchor, plunges the net into the LYi pu..)co.--0L5 water in the direction of the current, with the opening e,t the at the depth freçluented by yung eels. *:rhen in the net the younri: eels arc held all the time against the current, so that they give the impression of being sta- tiOhary. Every 15-20 Mih, or every 5-10 min if the catch is large, the net is lifted and the elvers emptied into a pail one-nuarter filled with vnter. To prevent the eels from dying, not more than 1.5-2 kg of plass eels, i.e. not more than 5C0-0000 individuals, are placed in one pail. The eels thus cau:7ht are poured from the pail into specially made pots, previously set in the curr- ent, which are intended for-keeping them for a short time. The catchinfr is more effective if the surface of the water is illuminated by a flare or by a • special lantern known as a "bat," which gives a feeble light and .acts as a et,ArLP decoy for glass eels. The brighter light of an electric iq.,e,e;I:gmp frightens them away. To increase the size of the catch of [- lass eels and the productivity - of'fishing, French fishermen have begun to use motor boats with a displacement of 1- metric tons. Two large eel nets (120 cm in diameter), mounted on poles 3-4 metres long so that they can be lowered to the required depth, are drawn by two lines like a trawl behind the boat. The speed of the boat must be slow, 11, not more than 6 km/h. In the middle of the boat is a tank of water in which about a hundredweiht of eels can be kept for several hours. To prevent rub- - 98 - bish from entering the tank when the eels are thrown_in from the nets, it ip covered with wire net-ring with a 4-6 mm mesh, slightly' bent inward. The glass eels go throurrh the holes into the tank while the rubbish stays on top. When the tank is full of eels, they are emptied out and transferred into pots. The eels can be kept in pots for several days until suf-f--lei-ent have been'caught for disposal. The stocking material can be transported in different ways. Previ- ously the glass eelS were carried in large metal tubs -or basins filled with water. However, this method proved to be cumbersome and inefficient, for the mortality among the young eels during transportation was high, of the order of , sometimes, actually 1.0M. Nowadays glass eels are transported with- -4- out. water, in a moist environment in wooden boxes made.specially for the pur- pose. These boxes are of two types, French and English, of slightly different design. Fig. 11. The French type of box for transporting eel.s. A tray rests on the top of the open box. The French type of box (Fig. 11)consists of a stack of separate wooden racks or trays. When the trays are placed one'above thejApther they - .. 99 f or:m a box men.sur:ing Sô c ni lonr; and .44 cm ^^^ide. Thé height depezids on -^he number of trays (most boxes have 12 t;.°ays). The heiÛh.+, of each tray is 3 cm. /70/ The bottom tray is covered with thin boards and serves as the base, with 9 to I 12 si-iial1 holes in it to allow water to run out. The other 1 1 trays are lined with coarse linen fabrïc, 'which does not shrink with the damp and which thus allovis the water from the me].ting ice to run out and at the same time provides ventilation. Each tray is divided into four equal sections by wooden cross pieces. On the ends of the bottom tray two metal rods, with a notch at the ends, are fixed, and the remaining trays and . the wooden lid, which have holes N at the ends co:rrespondin.- to them, are fitted on these rods. Wing nuts are threaded on the ends of the rods and when these are tightened they hold the :;. trays securely together. Of the 11 trays lined with fabric the topmost one, and if the temnerature is relativèly h3.,-,,h ( 18-25°C) one in the middle as well, are filled with small pieces of ice (up to 5 cm). The ice melts gradually, keep- in^ the interior of' all the trays moist and at a cômparatively low temperature (4-12°C). The English type of box is about the same size as that just described (length 85 cm, width 50 cm, height 40 cm), but it is better made. It is con- structed from closely fitting boards. Two 3-cm boards are fixed inside the box'at each corner to form a corner-piece restricting horizontal nxrvemènt of ' the trays. As a result an air space is formed between the walls of the box and the trays, so that the latter can be taken in and out freely. At the bot- tom of the box there is a tray with a wooden cross-piece and without cloth. On it rest eight more trays, ÿined with cloth and designed to hold the eels, and these are covered by the ninth (top) rack, covered with thin (1 cm) boards which are not so close-fitting. This tray is lined with a soft,1thin (0.7 cm) layer of spongy material (foam rubber). Small pieces (not larger than 5 cm)i. of ice are placed on this cover. The spongy material. allows a unif orm distri- • - 100 - bution of water from the Melting'ice throughout the tray. The box lid is made of similar boards to the box itself and its cross-pieces fit under the outer- most board, which is fixed securely to the top of the box. The box is closed by a lock. Two handles are provided at its ends. The English type of box is more convenient for the transportation of young eels. The ice lasts longer in it and a lower temperature can be maintained. In addition, it is easier to re- - ,... move the trays with the young eels from the boxes. _ . Before transportation the 'eass eels are placed on hair sieves in order to remove the excess of mucus, after which they are packed into the box- es. The young eels are placed on the trays lined with cloth which is first moistened with water. Depending on the temperature conditions from 1.4 to . 2.2 kg cif glass eels, the number of which will depend - on their size, is placed on each tray. As an illustration, Volf-Smisek (1955) states that 1 kg of 0 . young eels imported from France contains 2300-3000 eels, from England 3000-4000 eels, and from Italy 6000-7000 eels. The French load the trays and, consequently, the boxes with young eels more heavily than the English. A temperature between 4 and 12° C is regarded as the most favourable for transportation of young eels. High (over 20 ° C) and low (below 0°C) temp- eratures lead to a high mortality among the glass eels. Young eels can be carried by all forms of transport: by rail, road or air. The results of transportation depend almost entirely on its organiza- tion. - Young eels are imported into the USSR at the present time mainly by: (Dor+3, air to ecrimeia-ffl-mt,a,e4e-to the breeding stations, from which they are taken by helicopters or lorries to the waters to' be stocked.' Their time in transit-, including'transshipment and distribution, varies from 24 to 48 :11›: . , When young eels are-transported by air, specially made tin- trays are - 101 - placed under the boxes to catch the water from the melting ice. If there are many boxes they are paced one above the other and a tin tray is placed under the bottom box only. If the water to be stocked with eels is a long way from t51.1reelrt" the aeeed-i-eme and the boxes containing the eels will have to spend several hours on the lorry, they must be covered with tarpaulins to protect them from the wind, rain, frost or sunlight, for all these factors may increase the mortality .among the yoUng eels. 'When the eels arrive 'at the transshipment aerodrome the top trays must be inspected to make sure that they still contain ice. If there is only a very little ice or none at all, more must be added. Tests of the effective- edi.rre ness of soaking:the eels in the boxes at the transshipment alpieedeomee for a short time showed that such soaking does mot reduce the mortality rate. How- • ever, when the eels are discharged into the river or lake, etc., without having been soaked they are more mobile and they burrow into the mud at the bottom more quickly. I therefore consider that it is unnecessary to wet the eels briefly, for it brings them out of their state of anabiosis. Indeed it is undesirable, for this procedure actually weakens the eels. /72/ - 102 - In Poland and West Germany there are distributing «?,.. u stations in which glass eels can be kept for up to two weele,?.ss' N 1.4 e c 0 eje c.„ These stations are at Bydgosz.ci e Gdynia and Hamburg. „J1. 4e;') At Bydgoszcz., 11 wooden basins, each measuringe-' 1.2 x 0.8 x 0.8 m have been installed in an enclosed sptceS:1/41 o _ •. As water supply and drainage from the basins .ere independent operations, it is possible to control the replacement of water and maintain the requisite water tempera- ture in them. The Hamburg distributing station is much larger and better organized. Many more young eels, intended not only for stocking the inland bodies of water in West Germany but also for export, pass through it. It is not necessary to build distributing stations in the Soviet Union, since in our country 111, the stocking is done in accordance with a plan. Bulk deliveries of the material to be stocked are made by air to previously designated sites. The boxes containing the young eels are loaded from the shore into boats and placed in a strictly horizontal position (from 2 to )4 boxes are loaded into a boat). They are then delivered to the place where the young are to be released. In each lake the sectors for releasing the elvers are designated in advance; usually they are located in the shore zone at depths of 2 to 5 m. These sectors should be dispersed in such a way that they are in keeping with the biological peculiarities of the young eels which, immediately after being released attempt to conceal themselves in the underwater vegetation or to burrow into the mud. It is therefore desirable to release glass eels - 102 , at sites that have a muddy bottom, or a covering of underwater vegetation. This will enable the young to escape from predators and to locate food in the form of minute chironomids and various species of crustacea. In these sectors, prior to introducing the eels it is desirable to arrange for intensive eradication of the predators, especially pike and perch. Where underwater vegetation is lacking, the submergence of bundles of straw or reeds is recommended, as these will assure the eels of a safe refuge. Sites with a hard stony bottom are unsuitable for stocking. As soon as the boat arrives at the designated lake site the fishermen open one or two of the boxes, remove the upper tray containing the ice and make a start on releasing the young. Since the effect of the low temperatures is to cause the elvers in the trays to be in an almost anabiotic state, it is suggested that the opened box be primed with lake water by means of a watering pot. This will instantly set the eels in motion, not only in the upper trays but also in the lower ones. Following this, the tray containing the eels should be removed and placed on the surface of the water, raised a little to one side. The eels will gradually begin to leave the tray and will then rapidly sink to the bottom of the lake. After 3 to 5 minutes the tray will be free of young. Dead eels, however, will remain in the /73 tray, which means that an allowance can be made for wastage. A fisherman can release all of the eels from one box in the space of 30 to 40 minutes. Dumping of the eels from the tray is not recommended. - 103 - The stocking of inland waters with young eels is done in the daytime during the ice-free period, in calm, cloudy weather at temperatures of 8 - 12°C. If it is at all windy it is necessary for a man to remain at the oars and hold the boat steady against the wind. When working at night it is necessary to have an electric lamp or "bat" lantern in order to light up the boxes and trays. In practice, the releasing of the young must often be done upon the arrival of the material to be stocked, without waiting for favourable conditions. In such instances it is necessary to provide for a temporary holding station. Containers (such as vats, casks or tubs) with a total capacity of about 10 cu. m., must be erected in a suitable place and arrangements made for replacing the water by means of siphons. So as to prevent the eels from escaping the water level should be 10 cm below the edges of the containers. The temperature of the water in them should approximate to that of the lower layers of the lake water into which it is proposed to introduce the young. After being temporarily held in the vats and tanks the young eels are delivered to the stocking site. If they are being released beneath ice, prior drilling of holes through the ice is recommended. Sometimes it happens that stocking is done when the shore zone situated 200 -- 300 m from the water's edge is free of ice. In such cases, the elvers are released in muddy places in this zone, near the edge of the ice and at depths of 2 - 3 m. - 103 a - THE STOCKING OF LAKES WITH YOUNG • In some Western European countries the practice of stocking inland waters with elvers was begun as early as the latter part of the nineteenth century. In 1907, on the basis of an agreement between Germany and Britain, the Germans acquired the right to catch young eels near the mouth of the River Severn, on the southwest coast of England. For this purpose they set up their own station there, which remained in operation from 1907 to 1914 and later from 1924 to 1939. Simultaneously, a station was set up at Hamburg for the reception and distribution of these elvers. Between 1906 and 1939 about 142 million glass eels passed through the Hamburg distribution station, of which 102 million were introduced into the inland waters of Germany and roughly 40 million were exported to other European countries (Kokhnenko, 1958). Between 1940 and 1945, stocking with young eels was virtually discontinued. The programme was reinstituted and expanded following the end of the War. For example, according to the data of Krec and Zakrzewski (1962), in the People's Republic of Poland `0'3 ot ^ SS .. :lesides glass eels the Poles release several millions of larger eels svery year /74/, into their inland vraters. They subdivide the larger eels into .'immigrant" and. "stocking" eels. The immigrant eels are those caught in rivers ( when minrating upstream). These eels vary in length from 7 to 26 cm (mean 11 cm) and in weight from 0.3 to 21 g (mean 4 g). Stocking eels have lived f or 5 to 6 years in fresh water. Their length varies from 28 to 40 cm ( mean 30 cm) and their weight frvm.44 to 130 g ( mean 60 g). The Germans use eels 25-47 cm ( mean.29- 31 cm) in lenôth . and with a mean weight of 30-40 g for stocking. . Much progress with the stocking of inland viaters with young eels has been madnce the }^arin East :^erma^ According to figures given by Miller (1962), bétweesx 1950 and 1960 a total of 9 595 000 stocking eels and 55 573 000 glass eels.s;ras introduced. TABLE 18. Distribution of Eels Used for Stocking amonô.the Union 1:e publics,: im thousands ü^ v^^OWv 40^. ^ Republic Year roa Total Pecny6nyxa Bcero 1936 1958 1959 1960 1961 1962 1963 l 1964 1965 .1 PC(I)CP 200 1950 3158 1598 1200 2000 10455- 2 YCCP 1000 1000 202 1210 800 1000 5212:., 3 BCCP 3600 1680 3458 4390 3400 2000 3000 21523"` 4 J1aTpaïtcsaa CCP . 3569 2874 1915 1000 1000 '10335 5 J1liroochan CCP 334 2428 2340 1000 2500 2850 11452 6 8croxci 1) 21.SFSR 2) Ukraine 3) 'White Russia ^^ - 5) Lithus,nis 6) Estonia 7) Karelian ASSIt • . - 105 - Internal waters are stocke with young eels in Itnly, France, West Germany and other :uropean countries also, despite the fact that the natural arrival of eels in the rivers and lakes of these countries is on a much larger scale that in ::ussia. In the 1,37k :. a start was :alacie un stocking with young eels in 1956. Between 1056 and 1965 about 65 million glass eels were imported from France and lUisland. The younj eels were released into z.n.ny lakes in the union republics (Table 18), with a total area of a-LJout 170 CC° ha. All the lakes are eutrophic (shallow and deep), mesotiophic and oligotrophic, and suitable for eel raisin;. Stocking with eels is not limited by the area and Cepth Gf the lakes. Lakes and gulfs with an area of between 5 and 50 000 ha or more, and with a maximum depth of 2 to 50 m or more, are used for stocking with young eels. ITystrophic lakes with a high content of humic acids in the water, and in which winter Lill is observed s7stematically or periodically amonl fish, /75/ should not be used for stocking with eels. They can be used for this purpose only if the water is aerated. Lakes which are well warmed in suArJar are more favourable for glass eels than those which are not. For stocking with young eels it is best to éhoose lakes or lake syst- ems connected by channels and with a common outflow. This enables ::.any eels to - 106 . - . . . . be raised over a. wide area, their industrial exploitation can be concentrated, •-,, and the . eels migrating for spawning cari be daught more easily by,means of traps built in the Aver flowing out of the lake. To obtain a larger and more stable stock of eels for commercial exploitation, the lakes must.be stocked systematically. In White Russia eels have been introduced into separate large lakes: Lukomrskoe, Osveiskoe, Neshcherdo, Myader, Ezerische, Boginskoe, losvido, etc. - and into the Parochanskaya, Drivyatskaya, Uklyanskaya, Ushachskaya and Svirskaya • lake systems. Altogether in aite Russia 35 lakes with a total area of 46 308 ha. have been stocked, and about 27 000 ha of the total more than once. In Lithuania twenty lake groups have been stocked, including 131•1akes measuring from 3 to 2400 ha, with a total area of 37 427 ha. In Latvia 10 lakes With a total,area of 13 763 ha have been stocked, four of these lakes repeatedly. In Estonia 14.1akes with a total area of 25 600 ha have been stocked, six of them repeatedly. The . stocking density in Lakes Raigstvere and Kuuremaa in . Estonia was 1000 eels per hectare and in Lake•Elistvere.4393 eels per hectare. Two systema of lakes have been stocked with eels in the Ukraine; Shatskaya with an area of about 6000 ha and Rovenskaya with an area of about 400 ha. In addition, 85 000 eels were introduced into . Kalebanykskoe Reservoir. In.the RSFSR young eels have been introduced into some Waters in Leningrad, . • Pskov, Novgorod, Kalinin and Orenburg Regions. •.Because of the'lack of stocking material it has not yet been possible to introduce eels. into all the lakes of the USSR. Rivers and reservoirs must oboiou,i> . . ev-eeretrtly be stocked with eels as a secondary measure. Despite the fact that eel raising has been undertaken for a long time, there are still no generally accepted norms for stocking with•young eels. For example, Walter (1910) suggests introducing eels 25 cm in length at the rate of 100 eels per hectare over a period of six years, or 15-18 eels per hectare annually. -- 107 r About the same norm is given by Schg.perclaus (1949). He states that the lose of .eels introduced is 50-6e, so that in order to obtain a production of 5 kg/ha 14-1 9 eels per hectare must be introduced annually. Much of this loss is - evidently accounted for by eels which are no caught on their way to the sea during the spawning migration because of the lack of permanently operating eel . if these traps were available the loss would be traps. Presumably considerably reduced. Ehrenbaum (1930) gives the following norms for stocking in a growth -period of six years: stocking eels (snigs) 120 per hectare, glass eels 400 per •hectare. In Poland the official instruction for establishment of lake fisheries stipulate's the following annual norms for eel introduction (mean length 11 cm, weight 4 g), in fish per hectare.: into waters for breeding eisco 60, breaM 260, pike-perch 260-300, tench-lpike 40. Li my opinion the following factors must be taken into consideration when •stocking norms'for young eels are established:- a) the type of eel to be • introduced into the water glass or stocking eels; b) the type or character of the water and its food supply; e) the composition of the iehthyofauna; •Ci.) the eel productivity required. Since the commercial return from stocking with glass eels is much. less (20-3e) than with A--.D (40-6e) the stocking norm for glass eels, other things being equal, will 'se considerably higher than forjÉige) Observations from 1956 to 1963 on young eels of the same -generation but grown in different types of water in White Russia showed that during the first years of: life the eels grow better in shallow eutrophic lakes with a maximum depth ,of. 6 metres and in carp pond à with an abUndance-of food and'warmed through- , 'out in summer. In eutrophie lakes with a maximum depth of 15 metres young eels ' ,grow more slowly. Inmesotrophic lakes with a maximum:depth 6f 25 metres or mere, young eels grow :even slower: than in deepeutrophie lakes. In the study of - .-.108 - the habitat of newly introduced young eels it was found, that ^^ûring the first 2-3 years most of them were concentratec_ in muddy reT,ior.s of the littoral zone, overgrot,n with moss and other un .er rater vegetati on, not exceeding 5-6 metres in depth. in winter they remair in the same zone and burrow into the mud. Not until the 3r'--4th year, with the change to feeding on larger objects, includinù small fish, do the young eels sprea-' over the whole of the body of water. Adult eels also avoid places with a hard or stony bottom. In mesotrophic anO deep eutrophic lakes, in which the young eels occupy an even smaller area, the actual density in places suitable for eels to live is in fa.ct much greater than the theo.retical stocking c'-ensity for the v:hole area of the lake. For example, in the deep eutrophic lake Drivyaty with a total area of 3.;28 ha the zone suitable for young eels to live in (from 0.2 to 6 metres in `d-epth) is about 50;' (Borovik, 1954). Consequently, if the calculated stocking density.in 1956 was 200 eels per hectare, the actual density in the region suit- able for habitation by young eels would be 400 per hectare. In mesotrophic lakes the zone between 5 and 6 metres in ci.epth is relatively smaller: in Lake Naroch' about 33%b, in Lake Strusto 26i. In shallow eutrophic lakes with a maximum depth of 5-6 metres young eels spread throughout almost the whole extent of the rater during the first year, so that _the calculated stocking density •agrees with the calculations for the whole area of thelake. For shallow eutrophie lakes the calculated stocking density must be about twice that for deep eutrophic lakes and about three or four times greater than that for mesotrophic lakes. In the inland waters of the USSR eels live successfully alongside various ,groups of ichthyôfauna and the.density of stocking with eels does not affect the nu_nbers-of other valuable specie s of fish,. although to some extent they compete for food with bream and pond carp, as benthophagous fish, and -s•:ith' the pike-pe_rch, as a pre:^ator. However, the, catch of bream in Lakes P-:yastro, - 109 - Batorino and Drivyaty, where the eel stoCking density is 500 per hectare, haS Imt been reduced. A ,similar example can be given for the inland waters of East an in'creased density of stocking with eels, aimed at catches of Germany, where - between 5 and 15 kg/ha, had no adverse effect on the commercial catches of bream and pike-perch. In some lakes, admittedly, 3-4 years after introduction-of:the young eels the ruffe -popilation showed a 'substantial decrease through having been eaten by eels. For this reason the stocking norms for young eels in waters with an abundance of ruffe, bleak, perch and roach can,be increased, for these fish form part of the diet of eels and they thus increase .their potential food supply. For waters reserved for a special purpose, in which wild and pond carp, peled, cisco, and other valuable fish are raised, the stocking norms for young eels mut be reduced. -Conversely, in waters where eels are to be the chief. commercial fish, such as in Lake Konventer in East Germany, where the annual eel catch is 15-60 kg/ha, the stocking norms of young eels must be increased. With the above facts in mind, I (Kokhnenko, 1958) have recomMended the following norms foristocking White Russian waters withyoung eels (Table 19). - These stocking florins can be applied to the waters of other republics of the • Soviet Union. •From results obtained by Borovik (1954) and Savina (1957) and from other data of the White Russian Fisheries Research Institute, the following lakes-can be classified in this way: lakes for cisco , raising (mesotrophic) Drivyaty, Richa, Volos, MYadel', earocht, etc., for bream and pike-perch raising (deep eutrophic) Privyaty, Bogino, Kyastro, Lukomrskoe, etc.; bream raiSing (shallow.eutrophic) --'Batorino, Lisno, Yeshcherdo and many others. . • • 1 regard eels in White Russian waters as benthophagous and carnivorous. • For this reason, when stocking norms for young eels are drawn Up it is essential to take intoaccount the principal valuable' species offish with which the,eels may compete for food (bream, carp, pike-perch) as well as the unimportant fish - 110 - :an': .'i'nr l.i^'F t0 .crl^=.' ^?:'^L`_7^: for f00 The tJai 7.^ble f oo.' ..w,... p.r e._^1' O.'_ t_ for =-=-- • E"_^^Ci''1, nL;r' nC'^ slc7 ..7 e=- tal_?:1 i^t0 cCCOIi . - ► 7-PcOi1Cta::+ a.^_ they 4!ill rZ'Ob971j be c:.?.riZeC: after the fir`.` Of t:C r'3tt?=` ..^t. 1'O L_ ^cc'! c. ftC a Ct2Jle C0--"'-erCial ^ t^C_ CF- .,1 j'_^P1" ^.^ ., -r.'`a•i„ :i ' c C_ a'U l^- °''1,_ n* r,.• 1Cr.- Cr a^ rt^r-*.:1± cf II ' /LI Crcu:+oBHiHUi+ yropb noc,.wmak yrop. u rHU 03epHOrO I L TKn o3epa ao3aacrea na 5-6 He 5-6 eSKeroIHO neT^ 7 eKerOSNO A!T IL ^3 tile30Tp04)H0e ^- Nin} WAO- Boe -7 60-100 300 10-15 40-60 98Tpo(PHoe rm•6oKoe leuleao- CyAa4be jr 100-200 500 20-2-7) 80-100 100-140- 3BTpo(bHoe me.11(Oe Le Jleuionoc 150- 250 600-750 ,).)-3J 3BTpo(^1IOe c npi:3HaxaNtH 11HHeao, AHCTpo^HKaüHH llly4be 10 120-150 600 37) 140 • .'.: J fCT ,J :.^._ . 1) 1^ i C of lake 2) °^^l^vï ii1 C ?) 7:ee,, e,a troi^i:Zc 4) 11a, lo ? eut ro -,lhic 5) ûL_t=o^ .i (, _t^ c-o-_e c feature^:, r) ;-^rj e of fi,:'-,l bre~ C 7) Ci . i}-o-_ zrch BreaM 1 ^) - -ch, 1^.. 1 ^ I r11Q^^ ?G^ 1?\ __n 11^^^J 1) Cver 15-6 ^T•'^r^ 14) rtOC?;i:'-^; ^acj,. r. e t_:o-- of .,toc^_i_ the :,at ^-- o►it I, -las^ or : toc'_ci r_^ ee1. -- ï.a^. a'.'ar1,^. Fir^.l ii`- - .J ± n n'^ir _ J ' el?l' ha- the fo1-1C't.rI^^r.... a"^rrr}^.J ' ^ ^ ttil?ir nuT.^er ner unit mass (`:Pr 1i^) _s 50-100 t1^P; ^reater ti^^^.a^ fOr _tOcki:i- e-1:. P_!1: t::e^r are only t`,ric2 t0 three tiL_eS tore Cx^eI1C1V^^; ;eCOII,', -lacs: E'C1_ 0 Cqri be t_^'1:^2Mrte",. in 1rr^C ntmbers (1.5 to _ LT^1117.O1'1 eei" rer aircraft) in a — 111 mois vironment t 4 7.. e> (U2 tl 2-5 .:7 PY) rn'7 0= e :27-ste ." "L:t"c :' • no-c 4 1f.o ::+ce72:in: 1 • • • -v.., rin. of br'n,:j2.: in ve .r.icu: %-pe- of p-trn.'..dtic which ca_nct 'ce guarantee' .1- tocking eel' are intro'uce:. Th a .- -:. 7tE. _e of S.:.trz u-inj :ztnc:dn- eel5 into a uatr zyFte- i t1.- 5.t commercial pre uot-to,1 can 'CP b-ought forwa,-.1 by 9-4 years, an 1 the corimercial return ie -uoh _Jrce-q. er u2 to 4 0-'50 rith ?0-30, obtainei by stockin:. If rit eElY (e otoc:,:ing eels are int ,m-'uce' it i important to note where an .7 at that tii of yoar the stockin7 eels were caught, for they ma y inclue a high percentage of males which, because of their small size (naxinum length not more than 51 cm, weight 250-300 g), are unsuitable for comnero -!al fiching. Co:ou-erc 4 a1 7:eturn Pre7ict 4 on of the Mel Catch 179/ Cne of the most important practical problems in the develorment of the biologcal bards of eto*ing internal waters with eels iF the '1.etermination of the commercial return. Knorinz the commercial return the economic efficiency of ztoc7(..ine7 a - >ater syr‘tem with youn,7 eels can be estir:.ate 1_, i.e., t'le profit- ability of eel raising in the internal waters can be assessel. 2actors which can influence the commercial return must be examined. These are principally tvc in number: a) natural mortality, includinz eath of the nevly introuced. younn- eels from preators, end trapping of irrUvieuals before reaching commercial size, and b) leparture of eels to the sea for spawning. The natural mortality affect all age a-roups of eels, but it falls as the eels grow older. sels can live up to 0 years and for this reason the number of eels which die from old age in Russian waters is so small that it ic of no practical ri7nificance. 7ortality arion.. /71as eel:> . after introuction .en.' of youn7 eels in the years after intr.e.uction likewise is probably small by comparison with the inortality among other speciec of fish. Young eels show exceptionally hizh viability - 11 .2 - (even during sharp changes cf external environmental factors such as temperature, salinity, etc.). Ifthere is the Eli::,:htest risk of danger eels of all age grouf2s burrow into the ground. Only in cases. of winter kill or infectious disease is a mortalitY of 100 of the newly introduced eels reached. Although there are recorte in the literature that young eels are eaten by predators, the effect of predators on the population of introduced eels is probably small. This view is confirmed by the fact that when I examined several thousand carnivorous fish (pike, perch, pike-perch, and also eels) from waters stocked with youne; eels, none of the latter were found. in the stomachs of the predators. Catching young eels before they reach the commercial size may affect the population of the stock, esT:ecially in waters which are a favourite haunt of amateur anglers, e,sr. Lakes Kyastro, Parocht, 1)rivyaty, etc. Young eels, when they roach a length of 30 cri or .f.ore, are readily lured by worm baits. For this reason, a single angler in the course of 2-3 h may catch 10-20 eels, especially • near the shore of the lake where the young eels are usually concentrated- - 7atural mortality, death from predators a .oe.. catching before reaching the commercialsize do of course reduce the commercial return, but the principal loss of eels (hiring the period between introduction and commercial catching, in My opinion, is that due to eels going to the sea for spawning. Temporary eel nets placed across the path of the spawning migration catch by no means all the migrating eels. This can be confirmed by the fact that in the Rivers Narochanka and Druika, which are on the migration pathways, eels migrating down- stream can be caught below the eel traps. Eels enter the sea in particularly large numbers from the Gulf of Kurland and the Frisches Haff, where the channels are free from traps and where there are no obstacles in the path of the migrating eels. For this reason, by organizing-more efficient trapping of eels during the spawning migration (by means. of eel traps, barriers of light, and other — 113 — tyles of train: equirment) the col catch ca._ 1 (_, consi(3 -alably increase', with a corre:le.I'L.„; ir- - c in the cz..ercial. rcturn. This is 'ifficult to carry cut in strai - or oms but it is perfectly practicable in rivers flowing from It is very imortant to esta'cli:h optimal colercial r7inensions for eels. cr,=rcial ez172 a"Fpt ,-- in the fishery rules cf the republics -'_iffer r.£n -7 or e7.9mpl ,, , in -!-.1-1e eiory rules of Latvia, Lithuania, an Estonia an". for the Kalinin-l.ra': Region the minimum size of the 4 F. c 4 'n 4.1 1e rIC.14 n -Cit is 50 cm an:n. in "hit ,, Russe.a C ce. In some 7e-c-ts ,-n 7w-opee.: im-"N -aters enter in large nuben: by the natural route, there are in :.eneral no size restrictions for eels caught. For snelc, in este-,,n Germany it is the rule to ':istinguish eels for catching purely by their variety. The first variety is the term given to eels with a merin 1 ,eight of 320 g, the eecon2 to eels weighing 180 g, ani the thin' to eels weighing 90 g; the thir variety thus includes all eels below 40 cm in length. In my opinion it is unfrofitable to catch eels 45 cm or less in length in waters stocked with glass eels. The results of chemical analysis show that eels 45 cm or less in length an with a mean weight of 200-250 g cortain 10-12 fat whereas eels 50 cm or more in length contain from 22 to 32 of fat. Furthermore, the annual increane in weight of eels, once they have reache" the lenjth of 50 cm, is at least 100 g. I therefore consi:ler that the minimum commercial si-se for eels in waters stocked with youre eels must be established at not less than 50 cm. The only exceptions are mi-rrating eelo cau-,ht in eel traps on the pathway of the spawning migration. Euch eels can be caught regardless of their size, for they are trying to leave the lakee for the sea. 7any of them are male, whose len-th hare.ly ever exceeds 51 cm. Alen preicting the catch of eels allowance must be made not only for the overall commercial return, but also for the length of tine that eels of the saine - 114 - t;^11:'- ,,::enCratio r, occur in t_la Catch, as a rrti.ult of =. 1fi'ere1'_C :e^^ - in tllE:• t 'l-;c 01- ,, r • ïï.'atWO_7 1?1^0^ i11^c.^ 1T VE':>t]_c^.'^nsio have shoT:i1 ' J^11 t< t eel:. c a ^ ^ ^ ^ ^ r in the catch a t in,.•i- ro_^.:-:uc ^• ^.^ 1i' the a_.:^. tii:e r^^.c1' 1 acom mco^r..^^e^.^ercia l ^ ::i^^ ^.y ifrt're21t t1..:8^,'. S0'=C __ ^. l.ïr1 _U!lti rE!aCh t'ïe CO:zi::erCi'.'-.l ^._ ;e in the 7th-8th ;,rear of lif c, ûilt .:G.-.t of the`.: in the Pt-1^1 0t'It year c:f ter i lïtT'O.]uC :ion. j t^. i ^, • .^ („ L t .^.. 17,e I;L ;rCte t^.0;?n- f8 1;f C ha?^'a C'f;eri^'^icall;}T, in ;'iV^ ::.L1a1C on r^ c:Chi31 tt>1COT.11^h e rC1 a1 wtreaT'- 1.nt0 the ^-i _t tix.le£^, probably as a, re:=ult of c_ifferercesin tl^%^ir of sexual .. ^:VE lO^ .c?1?t. The _ trio.' Of their co!C1_e7.'c1.al exploitation alC the "er? Od of do'.in,-i:r Cü.til• mi-rationt1.o1? lasts C.Z^i)roYiI:l^.tel^7 1 0-15 yeç.r::. Or Pt sometimes even. longer. 'Eels of the same generation thus stay in a particular water for 17-25 years, rT'_erea:; the rapid increase in :•rei;_;ht of the eels is îiniti^•he: a'.,2roxi;:ate1y 15 year^ =•f-ter intro•.'uction. Conseyuently it is un_esir- aiJle fOr the ac is to le:lGin in the ?7attr lo^ï^e1' than 15 Jear^. (after intro:;.uction), for it is not econo..iically ^^rofi ta^ale. They must be cau^ht before attaining tilis a-;e. On tt?O ba '::? :' G_^ the ex1.°tiT'_ss i rLOi',^^.ti cïl on ^.toC^ïing with youll`; eelç: an-- J' ' °^- • - _ , z.' a ^`. ^ ?dur -.^y,.1:-.^ on :'lvi.r ro:.^:l :6n i---,.o • c? ^^z I, o'__ the lE'._`th of the _T •Z`10'" i•Ihich eels of the Saï1z generatio_1 are Cau^;ht CO 1I2erClally, and also on the results Of comm ercial catches of eels in ï,ïaters recently stocke.--, 1 have atte?:_i;t4c. to pre ict tl:e catches of eels frO^;l those ilttro:^.uce1.- in 1956,. 11-358, 1960, 1;6?, 196:, 1064 an,:. 1^165. The "olloT•,in,; data serve;; as the basis for the prediction: a) the number of eels intro:iuce:I in the particular year; b) the commercial return of 10',.. (in Tïhite Rus;ia curino- the previous years it was 5•7;^7) c) the first eels t•:ere caught co.nmercially 7 yeara after intro^luction; d) eels of a particular generation were foun6 in the commercial catch over a period of 1C) zT'c'.arE ; r?) the _E?an initial uei:_'.:1t of the e.°.Z.L. Cau;ht was 0.5 iï.:; an-'. the final wei.7,ht 1.4 iLg. It is a>sU;Tel^ that, on reaching the commercial size, the , •r. ^ l'. ^:L^,•}lt of t't1r^_ eelc.v^2.Châ;.Cï^ cUCCc,cen• ^.. ^;.^.V3 YE'.^tr on the aV^^'I'Lï-,e by 100 ^,, and for thic reason the total t, ci.,jht f rom ;;re<^,r to yec.r; f) the - 115 - Catch year by year is distributed as fol•oys: in the seventh year 2, the eighth year ninth 5, tenth 1 0(;. eleventh 15% twelfth 2e, thirteenth 20, fourteenth 15;::;, fifteenth 8, and in the sixteenth year after introduction e of the total catch is derived from the eels introducea in one particular year. As an example let us calculate the expected total catch of eels from one -earticular stockin9; in the subsequent years. In 1956 3 600 000 glass eels were introduce into aus.sian waters. dith a commercial return of 10, Ç.uring the 10 years after their first appearance in commercial production the catch is 360 06 0 adult eal• The eels intro:_uced in this -3articular year first began to be fis'ne commercially in 1962 (Table 23). The results in Table 20 show that the total catch of eels derives from those introrl.uced in 1956 is 5648 centners. The cmculatem weight of an eel is 1 kg. ---.„ - 1-..- I! Z. "3 .5-- 1 14'g e— lipoueuir Kononecroo Cpeaon2 FOI •61,1083 Boapacy or o6tuero yrpeii, urr. , xacca oruoro BW1013, 4 . , auxosa YU*, NZ • i 11110311.1MIMM.MMEMI■ere 1962 7 2 7200 0,5 37 1963 8 • 3 10800 0,6 65 1964 9 5 • 18000 0,7 126 . 1965 10 10 36000 0,8 . 288 1966 ' 11 15 54000 0,9 . 486 • 1967 12 20 ' 72000 1,0 720 1968 13. 90 72000 1,1 792 1969 14 13 46800 1,2 560 1970 15 8 28800 1,3 374 . 1971 16 4 14400 1,4 200 - " ' Phoro — 100 360000 — 3698 -re., E-c•- ( TABLiU 20. Calculation of Catch of Eels in Consecutive Years (introduced in 1956) KEY: 1) Year of catch 2) Percentage of total catch 5) iftimber of eels 4) near' weight of one eel, kg 5) Catch, centpers To calculate the total catches of eels from those introhIced in the particular year I propose the formula given below n D b Y --177 uhere y is the expecte total catch of e(1:-; - 116 - n the number of f2lass eels introduce6; ip the commercial'return, in of the number of eels introduced; b is the calculated mean weight of one eel. 1_600 x 10 x 1 . 3636 centners. y = 100 The catch of eels as a percentage of those introduced in one particular year can be calculated for subsequent years by the equation y = 1)- n x 100 ' • where yx is the catch of eels during the year; p is the Cœmlercial return; n is the yield of eels (percent of the yield over 10 years) excepte d for the particular year concerne; b is the mean weight of one sel in that year. Since the total catch (y) is made up of the catches for successive years (y7 + y8 + yg and so on), when the catch for a certain year "y" is calculated the number corresponding to the age of the eel in years after its introduction is substituted for "x" in the index. The hypothetical catches for the years probably do not always agree with the actual catches. Deviations on the side of a decrease or increase in the catches must be expected depending on the hydrological and meteorological conditions of the year and also on the intensi451> -ieligireW of fiShing. AS an example, let us calculate the catch of eels introduced in 1956 when they have obtained the age of 11 years (in 1966): .560 000 x 15 x 0.9 Y = 486 centners. - 11 100 Similar calmilations were made for all stockings of Ùhite Russian waters with glass eels, including that in 1965. The total 'catch of eels was made up of the following: from the 1956 stocking 3648 centners, 1958 -- 404 centners, 1960 -- 3528 Centners, 1962 -- 4460 centners, 1 063 -- 3450 centners, 1964_— 2028 centners, and the 1965 stocking 3042 centners. The total catch from all - 117 - stock4 nss 'erao 20 5.10 centnerz, worth a total of 4 500 GOO rcuble. The cozt of the stocl:in!s material wao only CO 750 roubles, i.e., the value of the comnercial yi el:. is more than 50 times &rester than the cott of the stockinG raterial. If the commercie )-eturn is not 10, but 20 or 30;.; as Schaperclaus stator, the rrofit obtsine' fro:: eel raisins is increase corresporC.inly. This is a realistic :re'Liction,- for the renults on T:ihich it is bas e '. are reliable. Jaice thio preiction is ma-:e . for the first tiLe it cannot clairs to be very accurate. Eroerienca uith its a2plication to prouction conUtions uill probably result in th s introuction cf oubstantial corrections, so that in the future more accurate pre:lotions of the eel catch can be obtainec:. Iethoi's of Catchin7 2 el o Various types of equipnent are use to catch eels, both passive an:- active. The passive forms inclue: ira-nets, ordinary hoop-nets, h000-nsts of the bottensarn* type, hoom-nets of the Ion--.rashkin oystem, bow nets, set lines with hoolo:„ ro',i with lines an, hook:, attache:. an,i various types of eel tracs; the active tyleo inclu'e trap nets, bean trawls an -; electric traps. Eels are alFc caujht -17ith the ai: of a barrier of lisht in conjunction uith various types of traps. t 1 trap of the Konrashkin apilidmme -i-h tcri:: for a scoop or stake net. - 113 - - ra_. un ^,)- nt tJr of all are U`'°% fo"c_z,rc._^^._ c`n t' r_': ,-r • :: 1.^ . `?:. _, i. e. , b^fo-^F F.1 ac'tzall;: ir the cut^ ^" iT_... i.c'_:=.. =:;Y'_-•_^ , ri...____..2 rrC.-. ^51 ttaL:J _^..:1^^^^^r CC _ CT^1VE:' i.rl thelc '^i^lll ^ O^ U.'C hOC;-.E'^ t`- .^^_ I r tht. e -raC:1l _].11 the-1 :o?ty.her 1th net "all" C,^ r"etre: 1Q:^- ÛT_'. littoral zone for a of L;^' to OP Cc:Qt be U_d . t0 :et tilîc CCLUI._:. to t- :.'v o^ L G Y c.. LC-. Cc :^!U ^ C'C J_'_ _ • best C . c. CL c_... ^: ïc` an ^^^O' n ^ï , ÜZ'î P - c'1:i n a o:^-', __ -.. Cl c°1 . an C':_c' i:!E" Gr2 can 'i72 Cûll^i1^ rlgr ^ in ^ic .r-:- • :oon netc of the botten„arn type. The fi.:herr:en of L.;tor.ia catch cel: :rith rIo: ifiec: hoop net-, of a àottell 'arl: ty.^e(Ti^.. I -J) 'e::cribe' by ;=âr'.i an,, _-Iajulai.: (11557) in their paper "^a?cricnce of eel catchi.n:: ira fiE^hery collectivec." The eelc enter the trap throu2h the 1 1.ï t i_:1 ±^• t) ^ i n, Dut t bof^u r: into tile ,,o t of the 'i100 > T'• ' 1:• `le_ e ûut,ler- accor,-in;-l^r rece^^.ell - that the top ^u. port of the inlet be rai., ^^ . • ` a