Migration, growth and mortality of released reared and wild cod (Gadus morhua L.) in Malangen, northern Norway

Abstract

In order to evaluate the potential of cod enhancement in a north Norwegian fjord, 4842 hatchery-reared cod juveniles (14-23 cm in length) were tagged and released into Stålvikbotn in 1988, a 10 km2 area sill fjord in Northern Norway. Wild cod were tagged to serve as a reference group for the hatchery-reared group. At the time of release, the size of the reared juvenile cod was about the same as that of the 1-year-older wild cod. The released fish showed a similar migration, growth and mortality pattern as wild fish of similar size. Both wild and released cod were mainly recaptured in Stålvikbotn. Large cod, short-horn sculpin and harp seal predated on the released reared juveniles. The recaptured reared cod were longer than average at release, indicating size-dependent predation mortality. The natural mortality rate (M) was estimated to be about 0.5 year-1 for cod longer than 30 cm and about 1.1 year-1 for both small wild and reared cod (14-30 cm). The fishing mortality rates were low, about 0.2 year-1 for cod longer than 30 cm. Only 2.4% of the reared cod were recaptured, and the yield was about 100 g per released cod, indicating low feasibility of release of small juveniles.