Restocking the River Akerselv, Oslo with Atlantic salmon smolts Salmo salar L. of different stocks
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Abstract
Sea ranched hatchery-reared smolts of Atlantic salmon originating from the Rivers Imsa and Lone in Norway and Neva in USSR were released in the River Akerselv, in 1985 and 1987. Both 1+ and 2+ smolts were released into the river which had been empty of fish due to heavy pollution from the middle of the 19th century to the beginning of the l980ies. The Norwegian salmon migrated to sea and left the Oslofjord shortly after release. The Neva salmon, on the other hand, stayed in the Oslofjord during the summer after release, and many turned up as maturing adults the first autumn after release. Few recaptures were made the following summer as grilse. The reported recapture-rate varied between 7.1 and 19.1% of the number of smolts released. Few fish strayed to other rivers; those that did entered nearby rivers. Mean weight of grilse produced from 2+ smolts, reflecting differences in size at release. Reported yields per 1000 smolts released varied between 77.5 and 138.4 kg, it was lowest for 2+ Lone smolts and highest for 2+ Neva smolts. These figures are gross underestimates when taking tagging and handling mortality and non-reported tags into consideration.
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