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 intermittently from the middle of April to late June 1986 into the River Ingdalselva in Central Norway. Their marine survival was estimated from return of tags. Their smolt status was assessed by morphological characterization (condition factor, smolt colouration), determination of gill Na/K-ATPase activity and seawater tolerance challenge tests (survival in water of 40 parts per thousand salinity, mortality and ionic regulation 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 positively correlated to survival after 1, 3 or 6 days in water of 40 parts per thousand salinity, and negatively correlated to plasma Cl- concentration after 1 day in water of 33 parts per thousand salinity. There 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 colouration was not attained until late May, whereas gill Na/K-ATPase activity remained high until mid-June when recapture rates were low.