Reduced Fitness of Atlantic Salmon Released in the Wild after One Generation of Captive-Breeding

Salmonids rank among the most socio-economically valuable fishes and the most targeted species by stocking with hatchery-reared individuals. We used molecular parentage analysis to assess the reproductive success of wild- and hatchery-born Atlantic salmon over three consecutive years in a small river. Yearly restocking in this river follows a single generation of captive breeding. Among the adults returning to the river to spawn, between 11% and 41% each year were born in hatchery. Their relative reproductive success (RRS) was nearly half that of wild-born fish (0.55). RRS varied with life stage, being 0.71 for fish released at the fry stage and 0.42 for fish released as smolt. The lower reproductive success of salmon released as smolt was partly mediated by modification of the proportion of single-sea-winter/multi-sea-winter fish. Overall, our results suggest that hatchery rearing altered the life-history strategies of these fish at the cost of their reproductive success. Our results warn on the negative demographic and evolutionary effects of repeated stocking for the population and support the adoption of more natural rearing conditions for captive juveniles and their release at a younger stage such as unfed fry.