Effect of hatchery rearing and environmental factors on the survival, growth and migration of Atlantic salmon in the Baltic Sea

The endangered Baltic salmon (Salmo salar) stock in the Simojoki river has mainly been managed by stocking hatchery-reared smolts, but parr were also used between the years 1986 and 2007 together with tagged wild smolts. The parr were semi-wild compared to hatchery-reared smolts, as they experienced the river conditions for at least a year before descent to the sea. We studied year- and environment-related changes in the life-history traits of these three groups differing in their source. The salmon originating from reared smolts had a larger smolt size compared to semi-wild or wild salmon. Wild salmon caught during the second winter were smaller than fish in the other source groups, but at greater sea ages the three groups did not differ in size. The annual survival of the wild salmon was higher than that of the reared source groups. Wild salmon were captured more often in the Main Basin, far from their home river, as opposed to semi-wild or reared salmon that remained near the home river in the Bothnian Sea. A higher proportion of wild salmon were multi-sea-winter fish on the return migration (57%) compared to reared and semi-wild salmon (32%). Low survival was associated with a low recruitment of young herring (Clupea harengus membras) and the mean catch weight of salmon in all source groups, but the reasons for the long-term decreasing trend in survival remain unclear.

► The semi-wild salmon smolts released as parr was compared to wild and reared smolts.
► Wild and reared smolts survived better than reared ones in the sea.
► The wild salmon had the longer feeding distance than that of semi-wild and reared salmon.
► Low survival was associated with a low recruitment of prey fish and the mean catch weight of salmon in all source groups.