Low survival of hatchery-released Atlantic salmon smolts during initial river and fjord migration

Even though release strategies have been improved, recapture rates of hatchery-reared Atlantic salmon as adults have been low in the River Eira, Norway. To evaluate whether loss of fish occurs in the river immediately after release, in the early marine phase in the fjord, or during the subsequent feeding migration at sea, 20 smolts were equipped with acoustic transmitters. The survival and movement pattern of the smolts were monitored along the 9-km-long river and during the first 37 km of the fjord migration. Only 25% of the smolts survived from release in the upper part of the river and until passing the fjord site 37 km from the river mouth. The within-river loss (15%, 3 of 20 smolts) was smaller than the marine mortality (71%, 12 of 17 smolts). The marine mortality was largest in the inner part of the fjord, with 41% mortality during the first 3.6 km, 40% mortality during the next 6.0 km, and only 17% mortality during the last 27.5 km (i.e., 11.4%, 6.7% and 0.6% mortality per km, respectively). The data suggested that at least 9 of the 12 smolts lost in the fjord were eaten by predatory fish (i.e. 45% of the fish released in the river were lost to fish predators). Hence, our results indicate that a considerable proportion of hatchery-reared smolts released in rivers might be lost due to predation before they actually leave the fjords.