Manipulations of feed ration and rearing density : effects on river migration performance of Atlantic salmon smolt

Abstract: The out-migration of Atlantic salmon smolts was studied at the lower part of the Umeå River (River Umeälven) from the Norrfors dam towards the sea (Bothnian Bay) in northern Sweden. Two-year old hatchery-reared Atlantic salmon (n=250) were tagged with acoustic transmitters and released downstream of the Norrfors dam and monitored while migrating downstream. The aim of this study was to determine the effects of feed restriction and rearing density on the migratory behaviour of Atlantic salmon smolts in the lower part of the flow-controlled river Umeälven in northern Sweden. Twenty acoustic receivers were deployed along the river section to cover all possible migration routes. Hatchery-reared Atlantic salmon smolts from five treatment groups were tagged using acoustic transmitters and released in late May 2012. A majority of the smolts migrated to the sea within a week after release. The smolts showed different migration speeds at different parts of the river. However, the average migration speed was similar for all treatment groups except for the strongest feed restricted treatment group that was significantly slower. The total mortality was higher in the old river channel downstream the dam to the Confluence area. No effects of any treatments could be detected on either migration speed or mortality.