Management of European Eel Spawner Escapement in an Irish River Used for Hydroelectricity Generation

The Irish Eel Management Plan (EMP) contributes to recovery of the European eel spawning stock by conservation measures that include closure of commercial fisheries and mitigation of hydropower impacts. The conservation strategy for the hydropower-regulated River Shannon (mean annual discharge 186m³s^-1) is largely focused on silver eel trap and transport (T & T). Annual monitoring of this conservation action provides information on silver eel population structure and dynamics. Together with mark-recapture estimation of population size and acoustic telemetry, these data contribute to long-term studies on: silver eel production rates, downstream route selection, turbine passage mortality and estimation of spawner escapement biomass. In the 2009–2012 migration seasons, mean annual silver eel production was 65.2 t and mean escapement was 58.1 t (89% of production). The proportion of non-captured silver eels migrating via the hydropower dam varied (56.8–98.9%). Regulating weir spillage, which facilitated migration via the safe river by-pass, and variation in river discharge affected route selection by eels. Hydropower dam mortality varied annually from 5.5–9.1 t. EMP –specified targets for T & T (30% of spawner production) were exceeded in each season. The contributions of fishery closure, T & T and spillage, to improved spawner escapement were evaluated.