Stress-reducing methods for releases of Atlantic salmon (Salmo salar) smolts in Norway

Abstract

One-year-old, hatchery-reared Atlantic salmon ( Salmo salar ) smolts were Carlin-tagged to test the effects of light sedation and salt water addition during transporting as well as releasing methods on migratory behavior and marine survival in northern Norway. A total of 1600 salmon were tagged for the migratory behavior study in the Halselva River. The fish were transported in water containing either 1 g l −1 metomidate or 11 g l −1 balanced sea salt (Instant Ocean), or a combination of the two. The groups were then allowed to recover for 2 days in net pens in the river, after which they were permitted to migrate freely from the net pens located 1 km upstream from the fish trap. These groups were compared to the untreated groups, which were released directly into the river at the same releasing site at the same time. The results showed that brackish water with metomidate as transport medium, and/or recovery after transport led to improved recaptures of smolts compared to untreated controls. In addition, two groups of approximately 3000 Carlin-tagged Atlantic salmon were transported in 1998 by helicopter from the research station to the nearby Alta River (1-h transport time, including loading) and either released directly into the river or kept in a net pen in the river for 7 days before release. The same releases were performed in 1999. Recaptures of adult salmon in the Alta River were 0.43% and 1.43% (1998 releases) and 1.23% and 1.89% (1999 releases) for smolts released either directly or allowed 7 days to recover in the river before release, respectively. These results shows that the use of stress-reducing methods, especially recovery after transport, can improve both the migratory behavior and marine survival of smolt releases.