Effects of release timing on migration behaviour and survival of hatchery-reared Atlantic salmon smolts in a regulated river

In this study, 221 two-year-old hatchery-reared salmon, Salmo salar, smolts were tagged with radio transmitters over a period of three consecutive years and released in the river in groups of 20-21 fish in various dates between late April and early June. Tagged smolts were tracked during their downstream migration in the lower 36-km stretch of the regulated River Oulujoki, with the focus on the effects of release date, water temperature and river flow on migration behaviour and survival. The results indicate that release timing and river temperature have profound effects on the initiation of migration, swimming speed and survival of released S. salar smolts. Smolts released early in the spring in cold waters ceased migration after brief downstream movement and were vulnerable to predation, whereas the migration speed and survival rates increased markedly for smolts released later in the spring.