SHORT COMMUNICATION

Rearing and release experiments with Baltic cod

Jana Pickova1 & P-O Larsson2

1Department of Food Science, Swedish University of Agricultural Sciences, Uppsala, Sweden2Institute of Marine Research, Lysekil, Sweden

Correspondence: J Pickova, Department of Food Science, Swedish University of Agricultural Sciences, PO Box 7051, S-750 07 Uppsala,

Sweden. E-mail: jana.pickova@lmv.slu.se

Baltic cod was reared at an experimental rearing sta-tion at the Swedish west coast with the purpose of (i)development of intensive rearing techniques applic-able for Baltic cod and for (ii) the release of youngcod in enhancement experiments.The Baltic stocks have been suggested to show

adaptations to the brackish environment (salinity5^20 ppt) (Segerstr�le 1965; Bagge, Thurow, Ste¡en-sen & Bay 1994). Reproduction is possible at salinitydown to ca11ppt (Westin & Nissling1991). Baltic cod(Gadus morhua) has shown strong abundance varia-tions during the previous century. The Baltic cod po-pulation is assessed as two separate stocks: theeastern Baltic stock and the smaller western stock(Brander 1994). A strong decline in the populationsize from the mid-1980s until today is partly causedby poor management resulting in over¢shing. Thishas reduced the recruitment of cod to the northernpart (Gulf of Bothnia) of the Baltic Sea to almost zero,with severe e¡ects on the local ¢shery.Disorders in Baltic cod reproduction have been ob-

served by screening themorphologyof early egg clea-vages (Pickova, Dutta, Larsson & Kiessling 1997;Kj�rsvik, Pedersen & Wold1999) and in terms of low-er hatching rates compared with other cod stocks(Pickova et al.1997; —kerman & Balk1998). In general,Baltic ¢sh have been shown to su¡er with respect toreproduction success, as discussed by Norrgren,Amco¡, B˛rjesson & Larsson (1998), caused bya highxenobiotic load in the Baltic Sea. However, rearing ofBaltic cod has not been reported earlier. In rearingexperiments, the quality of eggs in terms of hatchingrates was shown to be lower in Baltic cod eggs thanthose of Skagerrak (Pickova & Larsson1992; Pickova

et al. 1997) and the Barents Sea (—kerman & Balk1998).The spawning ¢sh were adapted to higher salinity

using a mixture of tap water and marine deep water(approximately 32 ppt) during a period of 2 weeks,starting at approximately 17 ppt, where the salinitywas kept stable at 27^28 ppt. The spawning cod, 20¢shes of mixed sex, were kept in tanks of 9m3. Ferti-lized eggs were collected at the surface in the tankusing a net (500 mm) and were incubated in marinewater. The egg batches were incubated separately foreach day, and the quality of eggs, in terms of cell clea-vage symmetry and hatching rate, were observed asdescribed in a previous study (Pickova et al.1997). It isknown that coldwater marine ¢sh larvae have nutri-tional requirements including highly unsaturatedlong-chain fatty acids (HUFA), such as 20:4 n-6 (ara-chidonic fatty acid), 20:5 n-3 (eicosapentaenoic fattyacid; EPA) and 22:6 n-3 (docosahexaenoic fatty acids;DHA) (Watanabe1982; Pickova et al.1997). Therefore,the rearing system included algae (Isochrysis galbana)in a poly-culturewith a copepod (Tisbe holothuriae), toensure a high amount of HUFA and rotifer cultures(Brachionus plicatilis) as well as supplementationwith newly hatched enriched Artemia. The survivalof Baltic cod larvae was low. A high mortalityappeared around day 19 and later at weaning andon feeding formulated diets. Approximately 1% ofthe larvae survived to the stage of fully metamor-phosed and weaned ¢ngerlings (44 cm). Experi-ments with di¡erent salinities showed that Balticcod had the same survival rate in much highersalinity than is normally prevailing in the BalticSea.The survival from egg to releasewas not optimal.

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Approximately1%,1000 ¢sh, reached the critical sizeof 3^4 cm. After attaining this size, there was almostno mortality, and no cannibalistic behaviour wasobserved. The ¢ngerlings were kept till they reacheda size when tagging was possible. The cod juveniles,around 400, which were not transported for releasein the Bothnian Sea, were kept at the rearing site toserve as future broodstock ¢sh in the approachingseasons.Carlin tags were used for the tagging of cod juve-

niles above12.5 cm in total length. The number of re-leased juveniles was totally 570, of which 320 weretagged. Of these, 24 died in the next few days follow-ing the tagging procedure. The remaining ¢sh and250 untagged cod were transported to the BothnianSea for release. A few ¢sh lost their tags during thetransportation, resulting in a total of 284 tagged codbeing released.Totally, nine recaptures of tagged cod were re-

ported. They are listed inTable1.The ¢rst two reportscontained only the tag, which is typical when the ¢shis undersized (o35 cm for cod in the Baltic Sea).The enhancement experiment could not be fully

evaluated, since all ¢sheries targeting cod had ceasedin the Gulf of Bothnia (due to lack of natural recruit-ment) when the released cod grew up.The recapturesobtained were mainly by gears (gillnets) set for troutand white¢sh (coregonids) at the coast. Consideringthis and a reporting rate most probably far less than100%, we conclude that the reported recapture rate,3.2%, indicates a good survival of the released codjuveniles.The growth rate of the recaptured ¢sh was high

compared with the average growth for Baltic cod.Weconclude that the rearing system showed that therearing of Baltic cod has potential, but more develop-mentalwork is needed. Considering that the ¢shwere

released in such a large water bodyand that all direc-ted cod ¢shery had ceased in the area, we concludethat the quality of the released ¢shwas good.

References

—kermanG. & Balk L. (1998) Descriptive studies of mortalityandmorphological disorders in early life stages of cod andsalmon originating from Baltic Sea. In: Early Life StageMortality Syndrome in Fishes of the Great Lakes and BalticSea (ed. by B.-E. Bengtsson, C. Hill & S. Nellbrink).Ameri-can Fisheries Society Symposium 21, 41^46.

Bagge O.,Thurow F., Ste¡ensen E. & Bay J. (1994) The Balticcod. In: The Baltic Sea (ed. by A. Voipio), pp. 312^319.Elsevier, Amsterdam,The Netherlands.

Brander K. (Ed.) (1994) Spawning and life history informationfor North Atlantic cod stocks. ICES Coop, Report No. 205.ICES, Copenhagen, Denmark.

Kj�rsvik E., Pedersen B.H. & Wold P.-A. (1999) Egg qualityand endoparasitic infection in eggs of Baltic cod, Gadusmorhua L. In: Nordic Research Cooperation on ReproductiveDisturbances in Fish, (ed. by B.-E. Bengtsson, C. Hill &S. Nellbrink),Vol.530, pp.143^160. Nordic Council of Min-isters,TemaNord.

Norrgren L., Amco¡ P., B˛rjesson H. & Larsson P.-O. (1998)Reproductive disturbances in Baltic ¢sh: a review. In:Early Life Stage Mortality Syndrome in Fishes of the GreatLakes and Baltic Sea (ed. by G. McDonald, J.D. Fitzimons &D.C. Honey¢eld). American Fisheries Society Symposium,(Bethesda, MD, USA) 21,8^17.

Pickova J., Dutta P., Larsson P.-O. & Kiessling A. (1997) Earlyembryonic cleavage pattern, hatching success and egg-li-pid fatty acid composition: Comparison between two codstocks (Gadus morhua). Canadian Journal of Fisheries andAquatic Sciences 54, 2410^2416.

Pickova J. & Larsson P.-O. (1992) Rearing experiments withcod: comparison between Baltic cod and Skagerrak coastalcod. ICES Doc. C.M. 1992/F:12. ICES, Copenhagen, Den-mark.

Table1 Recaptures of tagged Baltic cod produced in1993 and released in the Bothnian Sea in October1993

Date Total length at tagging (mm) Place of recapture Type of fishery Length (cm) Weight (g)

1995 142 Only the tag sent in – – –

1995 120 Only the tag sent in – – –

26 May 1995 132 Finnish coast, Bothnian Sea White fish net 38.0 400

8 June 1995 133 Finnish coast, Bothnian Sea Gillnet 35.4 470

2 July 1995 125 Swedish coast, Bothnian Sea White fish net 32 300

31 July 1995 151 Swedish coast, Bothnian Sea Gillnet 41 500

October 1995 140 Swedish coast, Bothnian Sea Gillnet 40 –

4 April 1996 147 Aland Sea, 601060N, 181540E Gillnet, 130 mm 50.0 1265� 132 Gillnet 65 –

�Probably1997. Reported that year with no information except for length and gear.

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Segerstr�le S.G. (1965) On the salinity conditions of thesouth coast of Finland since 1950, with comments onsome remarkable hydrographical and biological phenom-ena in the Baltic area during this period. CommentationesBiologicqe 28,5^28.

Watanabe T. (1982) Lipid nutrition in ¢sh. Comparative Bio-chemistry and Physiology 73B,3^15.

Westin L. & Nissling A. (1991) E¡ects of salinity on sperma-tozoa mortality, percentage of fertilized eggs and egg

development of Baltic cod (Gadus morhua), and implica-tions for cod stock £uctuations in the Baltic. Marine Biol-ogy108,5^9.

Keywords: algae, cod, intensive culture, larvae,plankton, cod release

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