Assessment of silver eel (Anguilla anguilla) route selection at a water-regulating weir using an acoustic camera

Riverine dams and weirs present silver eels with multiple migration routes during their downstream migrations. In relation to dams, understanding route selection is important for estimating silver eel escapement, especially where mortality rates vary among routes. On the lower River Shannon, Ireland, a water-regulating weir presents silver eels with two potential migration routes, the natural river channel and a headrace canal leading to a hydropower station. Route selection at the weir was investigated using a dual-frequency identification sonar (DIDSON) and replicated mark-recapture experiments. The proportion of eels migrating via the headrace canal was strongly related to the proportion of total flow diverted to that route (R2 = 0.827, P < 0.001), suggesting that eel route selection was passive. The active swimming speeds of eels (median = 0.16 m s-1), observed with the acoustic camera, did not greatly exceed water velocity and further suggested that eels were largely carried by the current. Knowledge of the relationship among flow, water velocity and eel route selection highlighted the potential to strategically alter hydropower-generating protocols to maximise silver eel passage via safer routes, and enhance survival.