Downstream migration of European eel at the hydro-power plant Unkelmuhle, two methods, same result?

The hydropower plant Unkelmuhle (River Sieg) has been equipped with a vertically oriented 10 mm narrow-spaced low-sloping rack. Bypasses were installed for salmon smolts (surface- orientated) and for eel (bottom-orientated) similar to several hydropower installations in Europe. The efficiency of these fish protection installations was investigated by monitoring the bottom bypasses with fish traps and simultaneously by using radiotelemetry with silver eel released upstream of the power plant. Between 96-100% and 92-100% of the eels successfully passed the powerplant and continued downstream migration in 2014 and 2015, respectively, indicating that eel can be efficiently protected and guided downstream at relatively small hydropower installations. Furthermore, none of the tagged eels passed the rack and the turbines. More than 90 % of the tagged eels used surface corridors available during flood directly at the weir or in the vicinity of the powerhouse (surface bypasses or vertical slot fish pass). Few silver eels (untagged and radio-tagged eels) used the bottom- oriented bypass near the rack. Only results of the radio-telemetry study finally show the low overall importance of these bypasses for eel.