RECAPTURE RATES OF HATCHERY-REARED ATLANTIC SALMON (SALMO-SALAR) RELATED TO SMOLT DEVELOPMENT AND TIME OF RELEASE

Groups of 2-year-old Atlantic salmon (Salmo salar) were released inter mittently from the middle of April to late June 1986 into the River In gdalselva in Central Norway. Their marine survival was estimated from return of tags. Their smolt status was assessed by morphological chara cterization (condition factor, smolt colouration), determination of gi ll Na/K-ATPase activity and seawater tolerance challenge tests (surviv al in water of 40 parts per thousand salinity, mortality and ionic reg ulation in water of 33 parts per thousand salinity). Return rates were highest for fish released during a period from late April to mid-May which coincided with the time of highest seawater tolerance of smolts. Prior to this period the fish were incompletely smolted, and in late May-June the fish had poor seawater tolerance. Recapture rates were po sitively correlated to survival after 1, 3 or 6 days in water of 40 pa rts per thousand salinity, and negatively correlated to plasma Cl- con centration after 1 day in water of 33 parts per thousand salinity. The re were no correlations between recapture rates and smolt colouration, condition factor, or gill Na/K-ATPase activity. The condition factor decreased slowly during the experiment. A complete, silvery smolt colo uration was not attained until late May, whereas gill Na/K-ATPase acti vity remained high until mid-June when recapture rates were low.