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Stocking location and predation by marine fishes affect survival of hatchery-reared Atlantic salmon smolts


Eva B. Thorstad, Norwegian Institute for Nature Research, N-7485 Trondheim, Norway (e-mail:


Abstract  Release strategies of hatchery-reared Atlantic salmon, Salmo salar L., smolts were compared by studying survival and migration of smolts (= 99) and their predators (Atlantic cod, Gadus morhua L., = 8; and saithe, Pollachius virens (L.), = 2) during the first 37 km of the marine migration using acoustic telemetry. Survivorship was higher in smolts released at the river mouth (30%) compared with smolts released in the river (12%). This was likely due to mortality or reduced migratory behaviour in fresh water. The marine mortality was 37% during the first 2 km after leaving the river (at least 25% mortality because of predation from marine fishes), and total marine mortality over 37 km was 68%. Detection-depth data were useful for evaluating whether the tagged smolts were alive or predated; mortality during the first 2 km of outward migration would have been underestimated at 26% instead of 37% without the analysis of depth detection. Transmitters from consumed post-smolts remained in predators for up to 47 days (average 29 days).