Behaviour and mortality of downstream migrating Atlantic salmon smolts at a small power station with multiple migration routes

Salmon smolts were released upstream of a run-of-river hydropower site and recaptured downstream for inspection. Descending fish behaviour through three possible migration routes (turbines, fishway, spillway) was analysed using telemetry, fyke nets and diving. Tagged smolts did not follow the main water flow; over 70% used the fishway, which received only about 10% of the flow. The turbines received about 80% of the water, but <25% of the smolts. Smolts were not fully stopped from entering the turbines by the 25-mm bar racks. Mortality of smolts passing through the Kaplan turbines was at minimum 36%. No mortality was found in fish moving through the fishway or spillway. This shows that small and fast-rotating Kaplan turbines can cause relatively high mortality. No mortality in alternative migration routes resulted in a total mortality for descending smolts at the hydropower station of 8.5%, emphasising the importance of providing functional alternative migration routes.