Stocking of brown trout, Salmo trutta L.: effects of acclimatization

One-year-old hatchery brown trout, Salmo trutta L., (n = 16 520) from a sea -run, local brood stock were marked and released (scatter-planted) into the River Laisalven in northern Sweden. Eight different groups were created us ing Alcian blue and Visible Implant Elastomer tags. Half the fish were kept in small enclosures in four stocking areas for 6 days before release. The other half were released just after transportation. To evaluate the effect of acclimatization on post-stocking performance, the areas were electric fi shed 2 months later. During the electric fishing survey, a higher number of the acclimatized hatchery fish were recaptured than those released immedia tely. The growth rate of stocked fish differed significantly between stocki ng areas and fish held in enclosures grew more than those released directly . The rate of recapture of hatchery fish varied between stocking areas (6.4 -17.4%). Movements of juveniles within and between the stocking areas were low, and only 3.6% of the recaptured fish were found in an area not origina lly stocked. These results showed that acclimatization of fish before relea se increases the number and size at recapture within a stocking area.