The impact of an Archimedes screw hydropower turbine on fish migration in a lowland river

There has been an increase in the abundance of small hydropower (< 10 MW) installations. These tend to be 'run-of-river', thereby reducing or averting the need for impoundment and water storage, and so are considered to have lower environmental impact. The Archimedes screw turbine (AST) has been described as 'fish friendly' based on magnitude of observed first order impacts, i.e. low rates of direct damage and mortality due to blade strike. However, potential second order effects, such as altered fish behaviour prior to or after passage, and consequences for long-term survival and fitness, have been largely ignored. This two-year study employed acoustic and Passive Integrated Transponder (PIT) telemetry and dual frequency imaging sonar to investigate the influence of an AST and associated fish passes (Larinier super-active baffle pass and upstream eel ladders) on the movement patterns and behaviour of five species of potadromous fish and European eel (Anguilla anguilla). Seaward-migrating adult (silver) eels successfully descended the AST, but under high flows most individuals took an alternative route via an overshot weir which resulted in faster passage rates. There was no immediate silver eel mortality as a result of passage through the turbine and no effect on subsequent migration behaviour through the lower freshwater catchment and estuary. Some eels were delayed at the complex and exhibited frequent rejection and milling on their approach to the AST and fish passes. Upstream passage rate (%) at the Larinier pass varied among species with highest for dace (81%) and the lowest for roach (10%). Fish that aggregated up- and downstream of the AST rapidly dispersed on turbine start-up. Although ASTs can be considered a potential downstream passage route for both eels and potadromous species, adult eels milled and rejected at the upstream entrance, while other fish exhibited startle response on turbine start-up. Potential delay to seaward migration of silver eel and the energetic costs of observed behaviours should be considered during future impact assessments, particularly where there is no alternative migration route or there are multiple such facilities in a watercourse.