Improving European Silver Eel ( Anguilla anguilla ) downstream migration by undershot sluice gate management at a small-scale hydropower plant

Hydropower plants have been linked with high mortality and passage impairments during Silver Eel (Anguilla anguilla) downstream migration, but there is still a lack of effective and economically viable management options for safe power plant passage. This study used an Adaptive Resolution Imaging Sonar (ARIS) to investigate how undershot sluice gate management at a small-scale hydropower plant affects Silver Eel behavior during downstream migration. Not a single eel out of 1323 counts used the eel bypass system, which is currently considered a technical standard. Instead, Silver Eels approached the opening of an undershot sluice gate and effectively used this corridor during their downstream migration. The opening size of the undershot sluice gate and the resulting higher current velocities in front of this corridor were identified as the most important triggers. Migration occurred primarily at night and peaked with rising discharge. This study suggests that undershot sluice gates can be used as a cost-effective downstream migration pathway and should be operated at night on rising discharge during the peak migration period for eels.