Migratory passage structures at hydropower plants as potential physiological and behavioural selective agents

Anthropogenic activities affect fish populations worldwide. River dams have profound impacts on ecosystems by changing habitats and hindering migration. In an effort to counteract such effects a diversity of mitigation measures has been installed at hydroelectric power plants. Still, not all individuals in a population use these measures, creating potentially strong selection processes at hydroelectric power plants. Especially, during migration, when fish can get heavily delayed or pass through a hydropower turbine, facing increased mortality compared to those using a safe bypass route. In this study, we quantify migration route choices of descending wild PIT-tagged Atlantic salmon smolts released upstream from a hydroelectric plant. We demonstrate how only a few meters’ displacement of bypass canals can have a large impact on the fish guidance efficiency. The proportion of fish using the bypasses increased when water was released in surface gates closer to the turbine intake. During a period of low fish guidance efficiency, we observed two different smolt migratory strategies. While some individuals spent little time in the forebay before migrating through the turbine tunnel, others remained in the forebay. We suggest that these groups represent different behavioural types, and that sub-optimal mitigation measures at hydropower intakes may therefore induce strong selection on salmon behavioural traits. The ultimate outcome of these selection mechanisms is discussed in light of potential trade-offs between turbine-migration mortality costs and optimal sea-entrance timing survival benefits.