Fin erosion and injuries in relation to adult recapture rates in cultured smolts of Atlantic salmon and brown trout

The implications of fin erosion and other injuries that are directly or indirectly caused by the hatchery environment have long been debated. Fin condition has been regarded as an indicator of welfare in fish farms, but until now there has been little evidence that eroded fins have negative effects on survival after stocking in the wild. Based on over 40 years of tagging and recapture data, we show that Atlantic salmon (Salmo salar) with dorsal fin erosion and brown trout (Salmo trutta) with any kind of injuries had lower recapture rates than fish without injuries. In salmon, precocious mature males had a lower probability of being recaptured compared with immature fish. Data from a hatchery monitoring program indicated that the degree of fin erosion on the dorsal fin in salmon and on the caudal fin in trout was correlated with the number of other injuries. We conclude that fin erosion and other injuries may reduce the probability of survival after release. All actions in the hatcheries to reduce fin erosion and other injuries will most likely be positive for the long-term outcome of the stocking programs.