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Migration motivation of cultured Atlantic salmon, Salmo salar L., smolts in relation to size, time of release and acclimatization period
- SKILBREI, O.T., HOLM, M., JØRSTAD, K.E., HANDELAND, S.A.
- Aquaculture research 1994 v.25 no.S2 pp. 65-77
- Salmo salar, acclimation, body length, estuaries, migratory behavior, motivation, net pens, predation, predators, rearing, salinity, smolts, stomach, tanks, Norway
- Within the framework of a national sea‐ranching programme in Norway a field study of marine release of smolts was conducted in Selsto Bay, west Norway. In March/early April 42 000 cultured Atlantic salmon, Salmo salar L., 10–20cm in body length were divided into two size groups consisting of fish smaller and larger than approximately 14–15 cm and reared in eight tanks, four of each size category. The eight groups were transferred to net pens located in a small marine bay where artificial estuarine salinity gradients had been established. Prior to release from net pens between 13 May and 3 June the fish were exposed to an acclimatization period of either 3 or 7 days. The migratory behaviour of the released smolts was studied by underwater video recordings and divers observations. Fast‐migrating schools followed the seabed at 8–10m depth, leaving the bay. Non‐migrating fish formed large groups close to the seabed (4–7 m depth), or smaller groups close to surface, in the inner part of the bay. Migratory motivation was higher for the large‐size groups and increased during the release period for both size categories. Increasing the acclimatization period from 3 to 7 days may have stimulated the migration of small fish early in the release period but the effect of time of release seemed to be more important for the rest of the experiment. During the release period only small smolts were found in the stomachs of gill‐netted predators in the bay. The data indicated that smolt size and time of release strongly influenced the migratory motivation of cultured smolts and that the release strategy may affect predation in the release area.