Migration losses of Atlantic salmon (Salmo salar L.) smolts at a hydropower station area in River Åbyälven, Northern Sweden

A large number of rivers in northern Sweden have hydropower developments that cause negative effects on both up- and downstream migrations of anadromous species like Atlantic salmon. So far, most attention has focused on the hindrances of adult fish during their upstream spawning migration. However, since turbines in power stations cause losses on downstream passing fish, the focus on negative effects on smolts has increased. The aim of this study was to compare three different causes of losses of salmon smolts passing downstream through a power station area in the flow-controlled River Åbyälven in northern Sweden. A total of 61 wild salmon smolts were caught and radio-tagged in the River Åbyälven during their downstream migration in June 2009. The salmon smolts were released at three locations, 1.1 km upstream from the power station, in the turbine intake and in the upper part of a fishway, acting both for up- and downstream fish passage. In this study, 59% of the radio-tagged smolts released upstream from the power station remained stationary close to their release location during the entire study period. It is believed that most of these smolts became victims of predation. About 35% of the radio tagged smolts that descended downstream via the turbine were judged as dead after passage, whereas the losses of smolts passing downstream through the fishway was 33% (explained by lethal injuries). The overall mortality of smolts passing the power station reservoir, the turbine and the fishway in the river was estimated to be about 94%. Taking these results into account, the dammed up area with its predators, seems to be what poses the greatest threat to the smolts during their downstream migration. The mortality during passage through the fishway and the mortality during passage through the turbine showed little difference. It seems that it is almost as dangerous for the smolts to take the route through the fishway as to take the route through the turbine