The influence of passive wedge-wire screen aperture and flow velocity on juvenile European eel exclusion, impingement and passage

The European eel (Anguilla anguilla) is a critically endangered catadromous fish. The decline has partly been attributed to water management infrastructure that abstract water from rivers for potable and industrial water supply, irrigation, hydroelectric power generation and flood defence; eels can be impinged on weedscreens and trashracks and entrained in pumps and turbines. The Eel Regulations (England and Wales) 2009 stipulates measures are required to provide safe (upstream and downstream) passage of eels past such hazardous intakes. Preventing impingement and entrainment of upstream migrating (glass eel and elver) and river-resident (yellow) juvenile eels at hazardous intakes may require fine-mesh aperture screens and low approach velocities due to eels' small size and relatively poor swimming capacity but quantitative evidence is lacking. Here, passive wedge-wire screen aperture (1, 2, 3 and 5 mm) and depth-averaged flow velocities (0, 0.1, 0.15 and 0.2 m.s(-1)) both influenced the fate (i.e., impingement or passage) and behaviour (i.e., migratory separation or behavioural avoidance) of two size classes of juvenile eels (60-80 mm glass eels and 100-160 mm elvers) in an experimental flume. One and 2 mm aperture screens were required to physically exclude 60-80 mm and 100-160 mm, respectively. Up to 90% and 100% of the 60-80 mm and 100-160-mm size class eels were impinged at 0.2 m.s(-1) depth-averaged flow velocity, which also positively influence number of screen contacts per eel and time to eel fate (from first contact). A small proportion of 60-80 mm eels (9.2%) did not approach the screen due to migratory separation (i.e., positive rheotaxis) and eels narrower than the screen aperture did not always pass through the screen, and thus other biological or hydraulic processes may also influence screen passage. It is hoped that these findings help improve screening guidance for regulators, key stakeholders and water abstraction managers to further improve protective measures required for critically endangered eels.