Downstream guidance performance of a bubble curtain and a net barrier for the European eel, Anguilla anguilla, in an experimental flume

Populations of the European eel ( Anguilla anguilla) , a critically endangered species, have been severely impacted by migration barriers, as losses due to turbine-induced mortality can be substantial. To prevent eels from entering turbines, effective guidance systems are needed to redirect downstream-migrating eels towards safer alternative passage routes. Although physical guidance screens may have very high guidance efficiencies, these generally come with high construction and maintenance costs and are difficult to scale up to large rivers. Behavioural guidance systems are typically less costly, but have often been ineffective. Hence, more work is needed to identify more effective behavioural solutions or physical barriers that are less costly to upscale. In this study, we assessed the performance of a physical net barrier (23 mm mesh size) and a behavioural bubble curtain guidance solution, for downstream-migrating eels and compared these with a guidance-free control at four different water velocities (0.1, 0.4, 0.7 and 1 m/s) in a large experimental flume using PIT-telemetry and video. The overall passage rate with the net barrier was 68 % higher than during the control treatment, whereas there was no significant difference between the bubble curtain and the control. We also found an effect of eel body size, where larger eels were less likely to enter the bypass than smaller eels. Velocity did not influence passage rate. Video data, in addition, revealed that guidance along the barrier was greater, and passes through the barrier fewer, for the net barrier than for the bubble curtain and the control. The results suggest that net guidance solution for downstream guidance of eels should be explored further, whereas the bubble curtain does not appear appropriate for eel guidance.