Effect of trash diverters and overhead cover on downstream migrating brown trout smolts

Power plant dams constitute barriers for downstream migration by smolts. The purpose of this study was to measure guidance efficiency of existing trash diverters and the use of overhead cover in combination with trash diverters to guide brown trout (Salmo trutta L.) smolts away from turbine intakes into trash spillway gates at two power plants in the Emån River, southern Sweden. A total of 44 trout smolts were caught, radio-tagged, released at the two power plants and tracked daily for six weeks. The trash diverter at the lower power plant had a significant guiding effect, as the proportion of smolt that entered the spillway gate was significantly greater than the relative proportion of water that flowed through the gate (52% vs 17%). In contrast, there was no evidence of a guidance effect at upper Finsjö, where the proportion of smolts that entered the spillway gate did not differ significantly from the relative proportion of water that flowed through the gate (0% vs 10%). The lack of a guidance effect at upper Finsjö could not be explained. The effect of overhead cover was tested at the upper power plant as illumination from outdoor, overhead lamps at the power station was believed to attract smolts to the turbine intake. This was accomplishing by setting up and removing a tarpaulin placed between the trash deflector and the turbine intake approximately every 2–5 days for about one month, so that 52.6% of the time the tarpaulin was in place and 47.4% of the time it was not. The presence of the tarpaulin reduced turbine passage, as 31% of the smolts swam through the trash spillway gate instead of the turbines when the tarpaulin was in place, whereas all smolts entered the turbines when no tarpaulin was used. For fish that passed through the turbines, mortality was higher at the upper power plant, equipped with two twin-Francis turbines, than at the lower one, equipped with a single Kaplan turbine.