Seawater tolerance and downstream migration in hatchery-reared and wild brown trout

Seawater tolerance and downstream migration were examined in 2-year old hat chery-reared brown trout, Salmo trutta L., and compared with wild downstrea m migrating trout in the spring 1994 and 1995, using a fish trap in the Riv er Halselva, northern Norway. At release, seawater challenge tests (72 h, 3 4 parts per thousand) showed that about half of the variance in seawater to lerance of hatchery-reared fish was explained by fish size. On average, 34% of the released trout migrated downstream, whereas 44-51% had acceptable s eawater tolerance at release (seawater challenge plasma chloride less than or equal to 160 mmol l(-1)). The migration tendency in hatchery-reared fish increased with increasing fish size at release. Migrating hatchery-reared fishes and first-time migrants of wild trout showed a well-developed hypoos moregulatory capacity. Downstream migration in this brown trout stock there fore appears to be associated with a well-developed seawater tolerance. (C) 1998 Elsevier Science B.V. All rights reserved.