Salmon ranching experiments in the River Imsa: Effect of timing of Atlantic salmon ( Salmo salar) smolt migration on survival to adults

Abstract

We tested the hypothesis that survival of Atlantic salmon adapted to seawater in the spring does not change with time of release. This was done through successive releases of tagged smolts and post-smolts in the estuary of the River Imsa and at a sea locality 4 km outside the Research Station for Freshwater Fish at Ims, S.W. Norway, where the fish were reared. Based on tag returns it was demonstrated that survival was highest for fish released in the spring, at the same time natural smolts leave the River Imsa. In autumn and winter the survival to adults was poor. Our results suggest that there is a window of migration which gradually closes in late summer. We hypothesise that behaviour of the fish will change with time of release and that this will result in heavy predation at times other than spring.