Effects of Rearing Density and Structural Complexity on the Pre‐ and Postrelease Performance of Atlantic Salmon

Abstract Hatchery fish released for supplementation purposes often have difficulties adapting to wild conditions and, therefore, perform poorly in the wild. This can, at least partly, be explained by differences between hatchery and wild conditions. Our objective was to evaluate the effects of rearing density and structural complexity on the culture and postrelease performance of age‐0 Atlantic salmon Salmo salar . By using a 2 × 2 factorial design, we manipulated density (standard density and one‐third of standard density) and structure (standard rearing tanks with and without added rocks and plastic plants) in a conventional hatchery. After 3 months of rearing, 300 fish/treatment were individually tagged with passive integrated transponders, released in a nearby stream, and recaptured in late autumn (November) and summer (June). Fish not released were retained in the hatchery until smolting occurred during the next spring. In the stream, Atlantic salmon reared at reduced density grew faster during the first period after release, but there was no difference in June. The treatments had no effect on postrelease survival estimated by the recapture rates. In the hatchery, fish kept at low density with structure grew faster than conventionally reared fish. At smoltification, fish kept at low density had higher levels of insulin‐like growth factor I than those reared at standard density. Independent of size, fish kept at low density were more silvery (smolt‐like) and had a lower mortality rate than fish reared at high density. There was also a density effect on dorsal fin damage; Atlantic salmon at reduced density had less‐damaged fins than those at standard density. These results collectively indicate that reduced rearing density may be more important than structural complexity for improving postrelease performance of juvenile Atlantic salmon.