Offspring from farmed cod (Gadus morhua L.) spawning in netpens: documentation of larval survival, recruitment to spawningstock, and successful reproduction

No abstracts are to be cited without prior reference to the author.

Marine fish in culture, such as Atlantic cod, will mature in the seawater cage farms and thereby spawn and spread fertilized eggs into the surrounding water. The present study investigates the fate of such eggs originating from coastal Atlantic cod, kept in net pens during the spawning season in the small fjord system Heimarkspollen in Austevoll on the west coast of Norway. The fjord is known to be a spawning ground for wild cod. Use of a genetically marked farmed cod strain made it possible to identify the origin of collected cod larvae, juveniles, and adults in the wild environment. Eggs were released in the spawning seasons from 2006 to 2008 in Heimarkspollen, and a sampling program was initiated to monitor the occurrence of genetic marked cod at various life stages. Cod larger than 30 cm were tagged by T-Floy after muscle biopsy sampling, and then released back to the fjord. Substantial fractions of genetically marked cod larvae (6-36%) were collected in Heimarkspollen during the three spawning seasons. However, recovery of genetically marked juvenile cod within the expected size ranges was low (0.4-5.8% of catches). Largest genetically marked cod caught from Heimarkspollen in 2011 was 62 cm, which is well into the size of mature coastal cod. In the spawning season 2011, genetically marked cod larvae (2.4%) were again found, documenting recruitment to the spawning population in the area and successful reproduction. Interbreeding between cod of farmed origin and wild cod has so far not been indicated.