The effect of time, size and sex on recapture rates and yield after river releases of Salmo salar smolts

Abstract In each of the years 1988 and 1989, about 15 000 individually tagged Baltic salmon juveniles were released into the Umealven (Ume river) in northern Sweden. Two release groups, sexually immature fish and previously mature males, of varying sizes were released at five different times during the natural period for seaward migration for Baltic salmon smolts in this area. Analyses of 1069 adult recoveries caught in the commercial sea fishery from 29 863 releases in 1988 and 1989 were undertaken. An average 7–8 times higher total recapture rate for releases of immature fish than releases of previously mature males was found, suggesting a post-release mortality related to the maturity type of fish after river release. In 1988, the highest recapture rates were obtained for immature fish (18.7%) and previously mature males (3.0%) released in late May. Larger fish had a higher recapture rate at all release dates. Fish released in 1989 showed about the same overall ratio in total recapture rate between immature fish and early mature males although the total number of recoveries was lower in 1989. The two maturity types of released fish had a similar growth pattern in the sea. Average weight at catch for releases of immature smolts and previously mature males was 4.0 kg. There was no relationship between early sexual maturation and grilsing (fish returning after 1 sea-winter). At optimal time of release (27 May), the yields (kg/1000 smolts released) from releases of immature smolts and previously mature males were 748 kg and 120 kg, respectively. This difference in yield between releases of the two maturity types of fish is suggested to be due to a low smolting rate among the previously mature males. If the ultimate goal of river stockings of Baltic salmon is a high survival to adulthood and a maximal yield, the proportion of early mature males in the stocking material has to be controlled. The annual loss in production value by releasing a large proportion of non-migratory previously mature males in Swedish rivers is suggested to be 400 tonnes having a present catch value of c. 16 million Swedish krona (at a salmon price of 40 Kr/kg). Alternative smolt management strategies are discussed.