Optimising Atlantic salmon smolt survival by use of hydropower simulation modelling in a regulated river

Studies on downstream passage of diadromous fishes in northern Europe are rare, and in Norway, the problem is strongly linked to demands for efficient hydropower production. The current study explored mitigating measures using a hydropower simulation model for Atlantic salmon, Salmo salar L., smolt migration. Migration pattern and route choice at a hydropower intake for 22 years were described for a river in southern Norway, based on simulated data for discharge and water temperature. Subsequently, the potential for controlling the migration pattern and smolt routing past the intake by altering release patterns from the reservoirs was tested. Modelling of a general annual increase in bypass discharge from 3 to 15 m³ s^?1 increased average bypass migration from 30 to 43% at the cost of 14 € per fish. Individual release schedules from reservoirs for selected years indicated that bypass rates could be increased to 80% at an average cost of 4.5 € per fish and 2.2 € in the best years. Mitigating measures presumably depend on the specific site, but the methods developed in this study represent a general technique for evaluating increased smolt survival past hydropower intakes.