| Summary: | Abstract: To mitigate negative effects of hydropower production on riverine fish populations, new technological solutions need to be found to increase downstream fish passage. In this study, downstream passage of Atlantic salmon smolt was documented at the power plant Unkelmühle (River Sieg, Germany). Here, a novel approach is used, combining a 10 mm vertical trashrack with surface bypasses and a vertical slot pass for fish protection. Fish migrating through surface bypasses can be diverted into a concrete basin for monitoring purposes. A complex radio telemetry system was used to cover all potential migration corridors. In spring 2014, 78 salmon smolt were tagged with radio transmitters and released about 9 km upstream the power station. Additionally, another 800 smolt were dye marked and released at the same site. An intense monitoring program during the following weeks provided, amongst others, migration time, routes and speed. Catches in the monitoring basin showed that salmon was the dominant species migrating through the surface bypass (more than 60%), followed by nase, barbel and brown trout. Smolt were mainly caught after dusk and migration could be related to water discharge and temperature with wild smolt migrating slightly later than the stocked ones. Telemetry data showed that most smolt migrated with the main current towards the turbines, but were prevented from going through the turbines by the trashrack and used the surface bypasses instead. Migration speed for smolt differed according to the river stretch fish swam through (natural river stretch, reservoir and power plant). Studies at Unkelmühle as well as at two other power plants with different technological approaches for fish protection will continue to compare solutions and concepts that enable downstream fish passage.
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