Downstream migration of brown trout (Salmo trutta) smolts past hydropowerplants in the river Emån

Abstract: The passage efficiency of downstream migrating brown trout (Salmo trutta) smolts past two hydropower plants in the River Emån, southeastern Sweden, was evaluated by telemetry. From 26 April to 17 May 2005, 40 wild brown trout smolts were radio-tagged, and their positions were recorded daily until 14 June. Approximately 45% of the individuals passed both power plants, and 15% made it to the sea, some 28 km downstream of the power plants. About 20% of the individuals ceased migrating, residing in a lotic habitat. Mortality due to predation was 33%, and the predation rate was higher in the power plant dams than it was downstream of the power plants. Turbine-induced mortality was greater in the upper power plant (40%) than in the lower one (13%). Fish that chose the alternative routes, through the gates, swam past the upper dam faster than fish that swam through the turbines. The results confirm previous work in the river, namely that measures need to be taken to improve passage efficiency past the power plants, particularly passage of the upper power plant. Such measures are of crucial importance if the nature-like bypass channels at the power plants in Finsjö are going to have any long-term effects on the brown trout population.