DIFFERENCES IN LIFE-HISTORY AND MIGRATORY BEHAVIOR BETWEEN WILD AND HATCHERY-REARED ATLANTIC SALMON IN NATURE

Hatchery smolts reared from crosses of wild Atlantic salmon of the River Imsa stock were released annually at the mouth of the River Imsa, 1981-1987. In the river, the smolts were released in May at the same time of year as wild smolts were migrating to sea. In the sea, wild salmon survived more than twice as well as hatchery fish, but sea growth did not differ significantly between the two groups. After 1 year in the sea, most surviving fish in both categories returned to coastal Norway as sexually maturing adults. Smaller numbers returned as multiple-sea-winter spawners. Wild fish ascended the River Imsa earlier in the season than did hatchery fish. Furthermore, wild fish strayed less than hatchery fish to rivers other than the River Imsa. Within the river, hatchery fish moved about more, and a higher proportion was injured during the spawning period than in wild fish. Moreover, hatchery fish stayed for a shorter period of time in the river than wild fish, and a larger proportion of the hatchery fish returned to sea without having spawned. Average river survival was high (> 60%) for both groups.