The survival of stocked Atlantic salmon smolts during sea run and the timing of migration in the river Simojoki, northern Finland

The migration of Atlantic salmon smolts in a river phase was studied in 1996–1999 in the Simojoki, a river flowing into the northernmost part of the Gulf of Bothnia, northern Finland. The influence of migration distance on the recapture rate of stocked smolts was evaluated and the timing of their migration compared with that of wild smolts. We assumed that the recapture rate would indicate the relative survival of stocked smolts during their migration from different parts of the river. A log-linear model was used to describe the number of fish recaptured with a smolt trap at the river mouth. The recapture rate of marked smolts stocked in different rapids did not depend on the migration distance (30–174 km) but varied randomly. The stocked smolts migrated at the same time as the wild smolts in 1996–1998 but not in 1999. The correlation between the daily catch of wild and released smolts ranged from 67% to 95% in stocking groups released from 30 to 100 km above the smolt trap. Generally, no correlation was found with smolts released only 11 km above the trap. We concluded that the migration distance of stocked smolts did not have a significant effect on the survival of sea-running smolts in the Simojoki and that hatchery-rearing had not altered the migration ability of stocked smolts, which migrated simultaneously with wild smolts.