Effect of stocking time on yield and location of recapture in two forms of brown trout (Salmo trutta) when stocked in respect to migration activity

Abstract

Tagged 2-year-old trout smolts, Salmo trutta, were stocked into River Isojoki in 1996, 1997 and 1998 (a sea trout strain), and Lake Konnevesi in 1997 and 1998 (a brown trout strain) in April, May or June–July. Stocking dates were determined in respect to the migration activity of 100 PIT-tagged fish held in the laboratory. Migration activity was relatively similar in both trout forms each year. In April, movement activity was low; in May, movement activity of the PIT-tagged fish increased and in June–July, movement had ceased. There was a general tendency for lower migration activity in maturing males than for immature fish, but differences were not statistically significant every year. In sea trout, yield varied widely between years within a stocking time and between stocking times, being least in June–July (54.3 kg/1000 fish) and 77.3 and 86.3 kg/1000 fish in April and May, respectively. Brown trout yield was higher from stockings in June–July (181 and 312 kg/1000 fish) than from those in April or May (between 87 and 117 kg/1000 fish). Brown trout stocked in April or May tended to migrate downstream from the Lake Konnevesi more than did trout stocked in June–July.