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Some early printed books are hard to OCR-process correctly and the text may contain errors, so one should always visually compare it with the ima- ges to determine what is correct. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 C M Odamål, K vill* an, Bohuslän Hällristning Rock carving Flakar* Irån Bronz* aga bronsåldern fisherman MEDDELANDE från HAVSFISKELABORATORIET • LYSEKIL OBSERVATIONS ON GLASS EELS IN THE SKAGERAK AND KATTEGAT fcy Amin Lindquist Juli 1976 Observations cm Glass Eels in the Skagerak and Kattegat by Annin Lindquist Institute of Marine Research 1*53 00 Lysekil, Sweden Abstract Elvers are found in the plankton of the Skagerak and Kattegat from January to April, with a peak in February-March. Thereafter they enter the fresh water. The pathways of immigration have been studied since the end of the 1960’s. Immigration takes place as a drift with the Jutland current. By its speed the current acts as a ”jet stream”, carrying elvers (and other plankton organisms) with it from the North Sea. During the night elvers rise to the surface; during daytime they are supposed to be close to the bottom. Fluctuations in the abundance have been considerable from cruise to cruise. 21. Introduction The landings of eel in the Skagerak, the Kattegat, and the Baltic are considerable. According to the preliminary statistics of ICES in 1971*, they amounted to 4 056 ton. To this figure must be added the eel caught in fresh waters. Variations in the catches of eel have been great, and to ascertain the causes it was considered of value to have an assessment of the quantities of immigrating glass eel or elvers. There was no work in this field be­ fore in the region. This question gave rise to the present work. All our eels originate from larvae drifting fron north of Scotland into the North Sea and Skagerak, from where they drift farther into the Baltic. Little information is available on how this drift occurs: occasionally glass eels have been found from February to April, 6-9 cm in length. Along the Danish North Sea and Skagerak coasts A.C. JOHANSEN caught elvers during March and April. ”Während dieser Monate leben sie nachts pelagisch an der Oberfläche, am Tage näher zum Boden” (NORDQVIST 192U). ÖSTERGREN found elvers in the Gullmarf jord from the middle of February to the beginning of June. Until April they are typical elvers (NORDQVIST, op.cit., p. l4). In the Sound, elvers have been found either hidden amongst Zostera or in the harbour of Landskrona (NORDQVIST, op. cit., p. 13 and 19). LÖNNBERG (1903) found elvers in June. VALLIN (1975) reports about the contents of the intestine of elvers fron the Kattegat and the Sound in 1922 and 1923. These old finds, which were made during plankton studies with entirely different objectives, give little information about the occurrence of elvers in time and space. Our task was first to find a method for catching elvers in a quantitative way as satisfactory as possible and then to study fluctuations frcm year to year. 2. Methodological investigations Since the end of the 1960*s we have tried to catch elvers with different nets in the open sea. In the Gullmarfjord experiments were carried out for the first time in 1969 to attract them with light. From these and la­ ter experiments some information was gained about the occurrence of el­ vers along the coast, Tab. 1 and Fig. 1. - Most of the elvers were ob­ served during the darkest hours of the night. 3The work with underwater light was time consuming. As will be seen from Fig. 1 and Tab. 1 only one station could be visited during the night. The experiments did not give any information about the vertical distribu­ tion of the elvers. During work with underwater television, which followed then, elvers were observed only in the surface layer. Horizontal and ver­ tical hauls with plankton nets during daylight at the same stations as the night work, gave no catches of elvers. We tried, therefore, to find sane means of straining large quantities of water. Numerous trials were made with a high speed plankton sampler, type Gulf III, but no glass eels were caught. We then decided to try another gear, made to catch larger species of plankton, the Isaacs-Kidd -Midwater Trawl (IKMWT). This is described in Fig. 2. The mesh size in the cone was 0.5 cm bar, and when the net was stretched the meshes were almost completely closed. Hie trawl was used at different depths1 according to the depth of and just above the thermocline. Since nothing was known with certainty about the ver­ tical distribution of elvers, this was considered to be a good approach particularly as the trials were made at station after station continu­ ously day and night. The results were totally negative, for no elvers were obtained, March 1972. Later in 1972, in April, tows with the IKMWT at the surface during the darkest hours (2100-0300) were for the first time successful and gave an indication on the immigration routes of elvers, Fig. 3. When using the word "surface" here, it should be borne in mind, that this, according to the construction of the IKMWT means about the top three meters. Since 1972 the investigations have been carried out with an IKMWT, fishing at the surface during the night, avoiding twilight and dawn. The number of elvers caught was mostly between 0 and 15 per half-hour haul with a speed of the vessel of about 2-3 knots. Elvers do not swim very rapidly, as can be observed around a. lamp at the surface or in an aquarium. The towing speed is therefore quite sufficient. 1) The depths were determined by means of wire length and angle or at a depth of> 25 m by a wireless netsonde. UThis method has recently been tested in connection with 0-group surveys by Ackefors and Hagström (1975)* The authors found - in general - that fish larvae in the Skagerak are closer to the surface than in the North Sea. Horizontal hauls are, in comparison to oblique hauls, somewhat easier to perform and therefore permit visits to a greater number of stations, which is of importance in our investigations. 3. Results The results from experiments with underwater lights in April 1970 were given in Fig. 1. Later results, obtained with IKMWT, from 1972 onwards, are given in Fig. 3-8. The fishing experiments show that elvers occur along the north coast of Denmark and in the waters between Denmark and Sweden and along the Swedish coast. The maps must be interpreted by the permanent current system of the area: the Jutland and the Baltic currents (cf. LINDQUIST 1963). The Jutland current flows eastward from the surface to the bottom and has a velocity of at least one knot (Swedish pilots* manuals). The Baltic current follows the coast of Sweden and flows always to the north (SVANSSON 1975). The immigration of elvers apparently takes place by drift with the Jut­ land current. As this current is a permanent one, the situation can be used for the calculation of the number of immigrants. This is exempli­ fied in Fig. 3 where an area of 20 x 20 nautical miles is inserted into the map at a point where the current, so to say, is going to loose its natural boundary to the south. A distance of 20 nautical miles to the north fron Denmark is considered to cover the main section of the Jut­ land current. A current velocity of 1-2 knots implies that in 2k hours the elvers can drift away from the 20 x 20 n.m. square and be replaced by others from the west. For all investigated years calculations along this principle give the following results: April 1972 2.6 10^ elvers/day March 1973 0.03 10° April 197Î+ 0.03 106 February 1975 0.3 106 April 1975 0.1 106 February 1976 0.5 106 5These figures are, of course, very uncertain and are only presented to show which order of magnitude the fluctuations do have. The figure are, after all, minimum figures, for seme elvers no doubt pass thrcu^i the meshes in the front part of the IKMWT. During daytime elvers are not very active. Observations from aquaria have been confirmed by observations from the North Sea (DEELDER i960). These experiences indicate immigration takes place during night only, that means with a speed of 1-2 knots 6 to 12 nautical miles a night, which corresponds to 11-22 km/night. It is therefore possible, that only lA to 1/2 of the a.m. number of elvers actually immigrate. There is, at the moment no way to prove this. In any event, the immigration to the North Sea is considered to be in the order of 8 km/2*+ hours (TESCH 1973). This means that the Jutland current is much faster and virtually acts as a ”jet stream” for elvers (and possibly many other plankton forms). Regarding the time of immigration there are some other observations. In the Brofjord (HALLBÄCK 197*+ ) elvers were fished from February to March/ April with the a.m., method but using a smaller IKMWT. The trout (Salmo trutta ) in the same fjord consume a great deal of elvers (unpigmented), and trout stomachs are filled with glass eels during March and April (WINSTRÖM 197*+) Fig. 9» The migration into fresh water starts during April/May. In a trawl haul at the end of January 197*+ in the southern Kattegat ("Lysegrund”) with the Swedish research vessel Thetis, a smn11 glass eel was found in the catch; presumably it originated from a vomi­ ting fish. NYBELIN reported a finding of glass eel in the stomach of Raja clavata (February 1921, trawl station ”Soten", in NORDQVIST, op. cit., p. 1*+). The yearly fluctuations in the abundance of elvers are certainly depen­ dant on many factors. Fluctuations tend also to be greater when the num­ ber of individuals is small. JENSEN (1961) and TESCH (1973) give details on the fluctuations in the occurrence of elvers along the European coasts. JENSEN compared differences in barometric pressure between F an <6 and Skagen (as an expression of west wind frequency) with the eel fishery 1912-1959. He found a positive relation between increased west wind and a greater number of immigrating elvers. There is a time difference of 7 years bet­ ween elvers and eel fishery. 6Summary 1. Glass eels drift with the Jutland current into the Skagerak. As the current is a rather strong one this results in a greater mean speed for elver immigration into this area than into the North Sea proper. 2. Elvers have been caught from February to April. The peak of immi­ gration is probably during February/March. 3. During the night elvers ascend to the surface and can be success­ fully fished with an Isaacs-Kidd -Midwater Trawl. 4. Preliminary calculations about the number of elvers show great fluctuations from one cruise to another. 7 REFERENCES Ackefors, Haas & Hagström, Olle, 1975 •* A methodological study of catching 0-groups fish (larvae) and small fish in the North Sea area by using Isaacs-Kidd Midwater Trawl. - ICES C.M. 1975, H:30, 6 pp., 3 tabs., 13 figs., (mimeo). Ask, L., Berntsson, K.-E. & Lindquist, A., 1971: Älyngel i Skagerrak. - Medd. fr. Havsilskelab., Lysekil, nr 101, 3 sid., 3 fig. (mimeo). Deelder, C.L., I960: Ergebnisse der holländischen Untersuchtaigen über den Glasaalzug. - Arch, Fischereiwiss. ll(l):l-80. Hallbäck, Hans, Hagström, Olle & Winström, Kjell, 197ks Fiskeribiologiska undersökningar i Brof jorden. - Medd. fr. Havsfiskelab., nr 175, 45 sid., 77 fig., 68 tab. Hagström, Olle, 1976 (unpublished): On the results of the young herring survey in the Skagerak, February 1976. Jensen, A.J.C., 1961: Die Schwankungen des Aalbestandes und der Aal­ fischerei in den dänischen Küstengewässem. - Z. Fischerei 10 (N.F.): 635-641. Lindquist, Amin 1963: Skarpsillen - ett fiskerihydrografiskt exempel. - Zool. Revy (4):8l-89. ,1972: Eel larvae in the Skagerak. - Medd. fr. Havs­ fiskelab., Iysekil, nr 127, 5 sid., 3 fig. (mimeo). Nordqvist, 0: Ueber das Vorkommen von Glasaalen vor und an den Küsten Schwedens. See NORDQVIST & VALLIN. Nordqvist, 0 & Vallin, Sten, 1924: Untersuchungen über Aalbrut, ihre Einwanderung in die Ostsee und längs der Ostseeküste Schwedens. - Wiss. Meeresunters. Helgoland, Abt, Helgo­ land 15(2), Nr. 11:1-32. Svansson, Artur, 1975: Physical and chemical oceanography of the Ska­ gerak and the Kattegat. I. Open Sea Conditions. - Reports Fish, Bd. Sweden, Inst. Mar. Res. No. 1, 88 pp. ' Tesch, Friedrich-Wilhelm, 1973: Der Aal. - Paul Parey; Hamburg & Berlin, 306 pp. Vallin, Sten, 1975: Fish larvae and other organisms in the Sound and the Baltic Sea during the years 1912-1926 (1927). - Medd. fr. Havsfiskelab., Lysekil, No. 178, 3 pp + 49 ”appendices” (mimeo). Winström, Kjell, 1974: Havsöringens näringsval i Brofjorden, Februari 1973 - September 1974. - 3~betygsarbete, Zool. institu­ tionen, Stockholms universitet. Ta b. 1. N um be r o f gla ss .,e el s w hi ch ha s b ee n o bs er ve d a t th e. su rfa ce rou nd an un de rw at er lam p. The l am p h ad an ef fe ct of 50 0 W and ha ng ab t, a ha lf m et re bel ow the su rfa ce . The ob se rv er 's e ye s w er e a bt . 2 m ab ov e th e s ur fa ce . Fo r st at io ns see Fi g. 1, In fo rm at io n r eg ar di ng 19 69 an d 1 97 0 fr om AS K , BE BN TS SO N & LI N D Q U IS T 1 97 1. CO CO rH m co m w w i cm co H -sr<0 o1— 4 Os 1 I I I I I I I VO ot— 4 rH CO (O CM rH CM CM rH rH ITv rH in Ito•Ö S ß h-p TJ 48 Vi • O CO es ‘»-a 32 V« I CM 4 US rH 4 4 t~ 8 e !8* ^ 4> 4 J* O »m ««-} l~<< I I I I I I I CM CM P ■s .0 to rH •rl • 01M u ■■at « A! (0 ■ Hl£< Ov VO in rH ri 4 \o n va h m i iin ro m co t"- rH 0 • CO rH VO rH vo CM CM 1 CM-3- tf\ rH CM 0 ITV• VO rH r-f rH -4 rHCO CO in4 00 CM f\ CM CMt— VO xsuo «U 8 « ot- in ovvo vo rH O, I I I I rH H I I I O » CM in CM "E o '£u Vp(0o« o •-4 CM OvM> CM CMI CM I I in rH CM o CO rS r~\ r~\ r~t o>- VO cSo in ovo § •iHprtpW p 5 O a 0 0 0 0 0 0 0CO CO on co 00 on on co 0 H CM on 0 rH CM on 4 CM CM CM CM 0 O 0 0 0 1 1 1 1 1 1 1 1 1 O O O 0 0 C 0 0 aCO CO CO on CO CO on on co 8t 8 £3 N cr>CM 8 i H CO fcs ■a I N IV 1970 THETIS Fig. 1. Ntimber of elvers observed at the water surface, cf. Tab. 1. From LINDQUIST 1972. in st itu te of Mi S- 45 3 0 0 LY Si CM e>M*u. Ë 8z u_ IV 1972 THETIS io Fig. 3. Humber of elvers fished during 1/2 hour surface hauls with the IKMWT. Modified fro® LIKDQUIST 1972. Ill 1973 THETIS ig* 4» lumber of elvers fished during 1/2 hour surface hauls with the XXMf*3 IV 1974 THETIS Number of elvers fished during 1/2 hour surface hauls with the ÏKMWT r- o Nu mb er o f el re ra f is he d du ri ng 1 /2 h ou r su rf ac e an d ob li qu e ha ul s wi th t he I KM VT , Fr om  CK EF OR S an d KA GS TR ÖM 1 97 5, Fi g. 7 . Nu mb er o f el ve rs f is he d du ri ng 1 /2 h ou r su rf a. e ha ul s wi th t he FU. 8. Humber of elvers fished during 1/2 hour oblique end surface hauls with WT. With kind permission from OLUE HAGSTRÖM, unpublished. 100 90 80 70 60 50 40 30 20 10 0 Fig. Proa * no samples Q. FMAMJ JASONDJ FMAMJ J AS 1973 1974 Abundance of elvers in stomachs of trout in the Brofjord. 'INSTRÖM 197*».