Quantifying the movement and behaviour of migratory European eel (Anguilla anguilla) in relation to physical and hydrodynamic conditions associated with riverine structures

Abstract

Anthropogenic structures such as dams, weirs, sluices, and hydropower facilities fragment river networks and restrict the movement of aquatic biota. The critically endangered European eel (Anguilla anguilla) migrates between marine and freshwater habitats and has undergone severe population decline. Barriers to migration are one of the negative impacts to be addressed for compliance with the EC Council Regulation for recovery of eel stocks. This thesis examines measures to reduce the effects of riverine structures on eel and improve passage facilities for both juvenile upstream and adult downstream migrating lifestages of this comparatively understudied species. The influence of turbulent attraction flow on eels ascending passage facilities was quantified at an intertidal weir. Plunging flow resulted in a two-fold increase in the number of eels using a pass. The behavioural mechanisms underlying this attraction, and wider questions of how eels respond to elevated water velocity and turbulent conditions found at barriers and fish passes were further investigated within a field flume. Eels showed a similarly strong attraction to turbulent areas, though adopted an energy conservation strategy by adjusting swim path to reduce the magnitude of velocity and turbulence encountered. Compensatory swimming speed was also used to reduce exposure to energetically expensive environments. Management recommendations are made to optimise the attraction of eels to pass facilities, yet ensure hydrodynamic conditions within the pass do not deter ascent. Legislative drivers also stipulate targets for seaward escapement of adult spawner stock. The impacts of multiple low head barriers and water abstraction intakes on route choice, delay, entrainment and escapement were quantified in a heavily regulated sub-catchment using telemetry. Entrainment loss at a single abstraction point was the biggest cause of reduced escapement, and was influenced by pumping regimes and management of intertidal structures. Delays at some structures were substantial (up to 68.5days), and reflected water management practices and environmental conditions. Sub-metre positioning telemetry allowed detailed behaviour of adult eel to be further quantified in relation to physical and hydrodynamic features at a hydropower intake. There was predominance of milling and thigmotactic behaviours at lower velocities (0.15 – 0.71 m s-1), whereas rejection occurred on encountering the higher water velocities and abrupt velocity gradients associated with flow constriction near the intake entrance. Information presented has implications for wider catchment management and highlights the potential to reduce barrier impacts through manipulation of structures and abstraction regimes. Quantifying eel behaviour in response to physical and hydrodynamic environments will aid the development of attraction, guidance and passage technologies.

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ORCID for P.S. Kemp: orcid.org/0000-0003-4470-0589

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