Handling of wild Atlantic salmon smolts reduced marine survival more than hydropower turbine passage

Atlantic salmon smolts are sensitive to disturbance during their migration and negative effects experienced in freshwater may carry over into the marine environment. In this study, smolts were PIT-tagged and detected during their subsequent spawning migration. A generalized linear model fitted to the return data predicted a return probability of 0.091 +/- 0.0061 (+/- SE) for smolt that was released downstream of a hydropower dam. The additional effect of migrating through a Kaplan turbine associated with hydroelectric generation did not significantly reduce return rates, while the additional effect of being handled twice in a trap was significant and lowered the predicted return probability to 0.057 +/- 0.0079 (+/- SE; model predictions from logistic regression). Catch-mark-recapture methods such as those using physical recapture of fish should be applied with great care to avoid multiple handling stress that may lead to reduced marine survival and biasing estimates intended to help monitor population statuses. Moreover, similar return rates in turbine and bypass migrating smolts emphasize that more knowledge is needed when evaluating mitigation actions for migratory fish at hydropower plants.

Automated summary

Atlantic salmon smolts are affected by disruptions during migration, which may have negative impacts on their survival in the marine environment. This study used PIT tagging to track smolts and found that those released downstream of a hydropower dam had a return probability of 0.091 +/- 0.0061 (+/- SE). The migration through a Kaplan turbine did not significantly reduce return rates, but being handled twice in a trap significantly lowered the return probability to 0.057 +/- 0.0079 (+/- SE). Therefore, caution should be exercised when using catch-mark-recapture methods to avoid stress-induced decreases in marine survival and biased population status estimates.