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.