Effects of pre-release acclimatization period on salmon return rates

Abstract

42.000 1+smolts of Atlantic salmon were size graded into two size groups and released during three weeks in spring 1993; one group of small and one of large smolts on each of the four release days. Approximately 25% of the fish had been individually tagged using Carlin-tags. The fish were transferred to net pens in a small marine bay in which estuarine salinity gradients had been established by a tarpaulin barrier. The smolts were held in the net pens for three days prior to the first and third release day and seven days before the second and fourth release day. Total recoveries of Carlin-tagged grilse in 1994 showed higher return rates for smolts acclimatized for three days compared with seven days. The difference was small for large smolts, but significant for small smolts, as return rate for the three days acclimatization was twice that of seven days acclimatization. The retum of precocious mature males, males that were mature prior to release, were clearly higher than that of other smolts. Possible causes for these differences are discussed.