Adult recaptures of farmed Atlantic salmon post-smolts allowed to escape during summer

The influence of time of escape on survival to adulthood was tested in farmed Atlantic salmon Salmo salar smolt and post-smolts. Escape events were simulated in a small fjord in western Norway during and after the natural period of smolt migration by releasing 6 groups of individually tagged smolts and post-smolts (similar to 2000 fish in each group) every second week from 27 May to 5 August 2005. With the exception of a lower return rate of the first released group (0.31%), the recapture rates were very similar (0.85 to 1.05%) and did not vary significantly with time of release. Of the 100 recaptures, 57% were 1-sea-winter (1SW) salmon, 25% were 2SW and 18% returned as 3SW salmon. Release date did not influence the sea age but affected the weights of the recaptured adults moderately. Fifty-four percent of the fish were recaptured in the vicinity of the release site, most of them in the freshwater effluent from a hydropower plant. The rest were spread along the coast of Norway and in rivers (26% of the distant recaptures) over distances of 100s of kilometres. The present study shows that farmed salmon escaped during their first summer in sea cages-after the natural time for smolt migration in spring-are still capable of adopting the marine migratory pattern of their wild conspecifics. This suggests that escapes at this time of the year are a hazard to the conservation of wild salmon populations.