REPRODUCTION OF COD, GADUS MORHUA (ACTINOPTERYGII: GADIFORMES: GADIDAE), FROM THE GDAŃSK DEEP (BALTIC SEA) UNDER CONTROLLED CONDITIONS.

Background. The stocks of cod, Gadus morhua Linnaeus, 1758, have been declining within recent decades and the Baltic Sea is one of the most affected areas. The latter body of water with its low salinity, pollution, and vast areas of demersal hypoxia is particularly vulnerable. Areas of natural spawning are scarce and the reproductive output of cod has been irregular and more and more limited. One of the proposed solutions for this problem might be an artificial reproduction. One of such areas has been the Gdańsk Deep, but the number of potential brood fish is relatively low, which makes the eggs and sperm poorly accessible. The aim of the presently reported study was to conduct spawning and egg incubation under controlled conditions to obtain viable hatchlings and to monitor the embryonic development of cod originating from the area of the Gdańsk Deep. Materials and methods. The parental fish specimens were obtained during commercial trawl catches. The eggs were fertilized artificially at the field laboratory using filtered brackish sea water with salinity of 15%, and then were incubated at a constant temperature of 8 ± 0.5°C. Eggs and larvae development, and larvae beginning exogenous feeding were observed under a stereo microscope, and photographed regularly to determine the sizes of the eggs and larvae and to document the results of the experiment. Results. The mean percentage of fertilization of eggs was 70%. Hatching began 236 h after fertilization, and the newly hatched larvae comprised 30% of the initial stocking density. The mean length of the larvae was 3.95 mm. The cod larvae began exogenous feeding before the yolk sac was fully absorbed. Conclusion. Artificial reproduction with gametes obtained from cod spawning in the Gdańsk Deep is possible, and the fertilization rates obtained were similar to those achieved with cod from the Bornholm and Gotland basins. Therefore, our results also show that cod spawning in the Gdańsk Deep can be suggested as a model species for further research. [ABSTRACT FROM AUTHOR]