THIS PAPER NOT TO BE CITED WITHOUT PRIOR REFERENCE TO THE AUTHOR

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1 THIS PAPER NOT TO BE CITED WITHOUT PRIOR REFERENCE TO THE AUTHOR International Council for the Exploration of the Sea CH 1979/F:17 Mariculture Co~ittee REARING LARVAL COD (Gadus morhua 1.) ON CU1TURED mods B R Hm'lell Ministry of Agriculture,Fi~heries and,food, Fisheries 1aboratory, 1owestoft, NR33 OHT, England SUMMARY Coo larvae were reared to a length of about 3 cm with a survival of 5-7%. The larvae were fed first on rotifers (Bpaahionus) and then on 2-day grown Artemia. Cultures of the unicellular algae Isoahrysis gazbana and Pavlov~. lutherii were added to the tanks each day. Freshly hatched Artemia naup~ii were, found to b~,ap inadequate food for larval cod. INTRODUCTION " Att~mptsto discove~ a reliable technique for rearing larval cod have failed despite a long history of interest in the rearing of this and other marine species. Studies of larval growth under controlled (1aurence, 1978~ and semi-controlled (Ellersten et az, in press) conditions have been possible using collected natural plankton ~s food, but the variable availability and natur~ of vlankton reduces the repeatability and the reliability of such rearing methods. The use of cultured foods has met with limited success. When first able to ~eed, co~ larvae,are about 5 mm lons and are too small to ingest Artemia nauplii. Howev~r, even when larval cod have been grown to a size at which they can still t~ke,!lauplii" this diet has proved to be inadequate to sustain the larvae throughout the,remainder of ~he larvalstaßes (Dannevig and Dannevig, 1950). This has also been the experience at this laboratory. In 1976 larval cod \'lere rear:ed to a l~ngth of 15 mm on adiet of rotifers (Braahionus pzicatizis) but died within a week of transfer to a diet of Artemia nauplii (1 Emerson, pers. ~comm.). In :t:hi~.. trial., a unicellular alga (Isoahrysis gazbanaj was added to the tan!<s when the larv?-e',w~re feeding on rotifers, following the technique developed for rearing ~afval turbot (Howeil, in press). Artemia nauplii are also inadequate as food for larval turbot but are improved vlhen fed for two days before use and offered to the larvae with Isoahrysis galbana (Howeil, unpublished data). The possibility that Artemia nauplii might similarly be improved as food for cod larvae has been tested. The results of this experiment and of two rearing trials are described in this paper. _-" 1

2 MATERIALS AND Egg supply METHODS Naturally-spqw~ed eggs were obtained trom a captive stock of mature cod and incubated in mesh bags suspended in tanks of running seawater at a temperao ture of 8.0 ± 1.0 C. Food production Unicellular algae and rotifers wereogrown by methods similar to those described by Howell (1973). Artemia nauplii were hatched trom eggs of San Francisco origin which were incubated for 24 h at 2S o C. Artemia metanauplii were produced by feeding nauplii ~o excess on Isoc~jsis galbana for 2 days.,,,, l ',. -. Short-ter"m 0 feedingoexperiment'... " Six 30'cm diameter black plastic tanks containing 12 1 of sea water were each stocked with 50 larvae which had been grown to a length of about 9 mm on ~ rotifers. Rotifers were fed to the larvae in one pair of tanks, nauplii to those in a second pair and metanauplii to those in the third pair. were added daily to excess. All foods A mixture of the unicellular algae Pavlova lutheri and Isoahrysis galbcma ''las added to the rotifers and metanauplii diets in' ;" sufficient quantity tb prövide an excess in the tanks after 24 h. 0 The tanks were gently aerated and thewater was refreshed daily by siphoning out 70% of the volume and replacing with fresh seawater. This procedure also removed most of the excess food. Temperature varied from C (mean 11.6 C). Rearing through metamorphosis Two rearing trials were carried out in black painted 60 x 30 x 60 cm fibreglass tanks stocked with one and two thousand yolk sac larvae, respectively. The larvae were fed on rotifers for 30 days and were then gradually transferred toa diet of metanauplii. the tanks' each day. A mixture of Isoahrysis and Pavlova was added to The tanks were gently aerated and water was continuously exchanged at an initial rate of 12 l/h. This was gradually increased to SO l/h. o 0 Water temperatures were controlled at 10.0 ± 1.5 C. Surviving larvae were transferred to clean tanks every days. Mortalities could not be closely followed during the early stages and so survival data are limited to counts on the occasions when larvae were transferred. Mortalities during the later stages were recorded daily. RESULTS Short term feeding trial After about a week, the mortjrity rate among larvae fed nauplii was much higher than that of fish fed on either rotifers or metanauplii (Figure 1). 2

3 Although the latter two diets supported similar survival, 25% of the larvae fed on rotifers had developed skeletal deformities indicating that prolonged feeding on this food was detrimental~ After 16 days the mean survival of larvae which had been fed on rotif~rs and metanauplii was.68 and 66% respectively, whereas that of larvae fed on nauplii was'only 15%. Rearing through metamorphosis The survival rates of larvae were sirnilar in both trials, 5-7% of the initial yolk-sac larvae survived to day 72 (Figure 2). The mortality pattern was typical of that of other marine fish, the greatest losses occurring during the early stages, probably because many of the larvae failed to start feeding. The mean length of the survivors was 3.2 ± 0.2 cm. These readily accepted a semi-moist artificial diet prepared from a standard trout food with added lugworm (ArenicoZa sp.). CONCLUSIONS The trials have confirmed the inadequacy of Artemia nauplii as a food for larval cod. Larvae were reared through metamorphosis, however, on a diet sequence of rotifers followed by 2-day grown Artemia, with daily additions to the tanks of the unicellular algae Isoohrysi8 and PavZova. The importance of adding algae to tanks of larval cod has not been established, though when turbot larvae are fed on rotifers these algae appear to have an important nutritional effect (Scott and Middleton, in press). These preliminary trials have indicated that there is abasie similarity between the requirements of cod and turbot larvae under laboratory conditions. The slightly larger size of early cod larvae may, however, prove to be an advantage and eventually allow them to be reared with greater ease than those of the turbot. REFERENCES Dannevig, A. & Dannevig, G., Factors affecting the survival of fish larvae. J. Cons. int. Explor. Mer, 16: Ellersten, B., Moksness, E., Solemdal, P., Tilseth, S., Westgard, T. and ~iestad, V., in press. Growth and survival of three larval populations of cod (GadU8 morhua L.) in an enclosure. 2nd International Symposium on the Early Life History of Fish, Woods Hole, USA, 2-5 April Howell, B. R., Marine fish culture in Britain VIII. A marine rotifer ~achionus pzicatitis Muller, and the larvae of the mussei, f~titus edutis L., as foods for larval flatfish. J. Cons. int Explor. Mer, 35:

4 ..~...".'"'..-. ':.' Howell,'B. R., in press. Experiments on the rearing of larval turbot, ScophthaZmua maximusl. Aquaculture. Laurence, G. C., Comparativegrowth, respiration and delayed feeding abilities of larval cod-(gadus mo rhua} and haddock (MeZanogrammus aeglefinus) as influenced by temperature during laboratory studies. Mar. Biol., 50: 1-7. Scott, A. P. and Middleton, C. M., in press. Unicellular algae as a food for turbot larv'ae'- otter' importance of dietary long-chain polyunsaturated fatty acids. AquacultUre. :. j'. ~. ;'T\'."~....., ;' 4

5 f J.~ 50 > t.:. :=J Vl.40 '~ ~\. 30 e 20 o 0 Rotifers. Arfemia nauphi e Arfemia metanaupui Day of the experi ment 15 Figure 1 The survival of cod larvae on various diets

6 .. '... I~ ~ =60 t:s >.- t40.:.: Arfemia >.~' metanaup~i i ", " ".> ',' ::J lj'). 20, 10 20;"30 40,' 50 Days after htitching Figure 2, The survival of cod larvae'during two rearing trials. " ~ ~ ~,~ ". \',~.., c., ;'~ t,,' ".',...'. ~"," <,,,".- ". :-.. '~, "'.. : ; ;" \,. '-.,i. "