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

Abstract

Seawater tolerance and downstream migration were examined in 2-year old hatchery-reared brown trout, Salmo trutta L., and compared with wild downstream migrating trout in the spring 1994 and 1995, using a fish trap in the River Halselva, northern Norway. At release, seawater challenge tests (72 h, 34‰) showed that about half of the variance in seawater tolerance of hatchery-reared fish was explained by fish size. On average, 34% of the released trout migrated downstream, whereas 44–51% had acceptable seawater tolerance at release (seawater challenge plasma chloride ≤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 hypoosmoregulatory capacity. Downstream migration in this brown trout stock therefore appears to be associated with a well-developed seawater tolerance.