Abstract
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 (~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 recap- tures, 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 moder- ately. 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 con- specifics. This suggests that escapes at this time of the year are a hazard to the conservation of wild salmon populations.
Highlights
Atlantic salmon farming has grown rapidly since the mid-1960s, and has been linked to declines in natural populations (Ford & Myers 2008)
Recapture rates and the sea age of the 6 groups released on various dates during the summer were very similar to each other, except for a lower recapture rate of the fish released in May (Fig. 1)
The similarity in survival and sea age among release groups observed in the present study indicates that the conditions in sea did not change significantly from June to August 2005, and the reduction in the size of the recaptured fish with time of release was very low compared to the 20% weight loss observed in a sea-ranching experiment in which the time of release ranged from May to early June (Skilbrei & Wennevik 2006)
Summary
Atlantic salmon farming has grown rapidly since the mid-1960s, and has been linked to declines in natural populations (Ford & Myers 2008). The incidence of escaped farmed salmon tends to be higher in the vicinity of concentrations of fish farms (Fiske et al 2006). Farmed salmon that escape from cages have become a problem both for the fish-farming industry itself and for the conservation of wild stocks. Most Atlantic salmon populations have a welldefined migratory cycle: the majority of the young salmon leave the river as smolts in spring when they are 1 to 5 yr old and migrate to their feeding grounds in the open sea, where they remain for 1 yr (1-sea-winter fish, 1SW) or longer (2 to 3SW) before returning to their home river to spawn (Youngson & Hay 1996). Post-smolt is the life stage from the time the smolt departs from the river until the end of its first winter at sea
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