Increased Recapture Rate of Adult Atlantic Salmon Released as Smolts into Large Shoals of Wild Smolts in the River Orkla, Norway

Abstract

Carlin-tagged, hatchery-reared smolts of Atlantic salmon Salmo salar were released in the regulated River Orkla at the same time of year and same site from 1984 through 1988. Recapture rates of adult Atlantic salmon varied considerably (from 0.7 to 6.8% of number of smolts released). Survival of released hatchery-reared smolts increased when they were released into large shoals of migrating wild smolts. Wild Atlantic salmon smolts generally aggregate into large schools when migrating out of a river, although the size of schools varies between years. When the hatchery-reared smolts were stocked within lesser shoals, survival was lower than in years with larger shoals of wild smolts. The magnitude of water discharge, as well as the direction of change in the magnitude, greatly influences the initiation of smolt migration and shoal formation in the Orkla. Artificial manipulation of water discharges throughout May can influence formation of large shoals of wild smolts and is a means of achieving optimal survival for wild and hatchery-reared smolts in the River Orkla.