Fish passage solution: European eel kinematics and behaviour in shear layer turbulent flows

High velocity barriers pose a risk to upstream migrating European eels (Anguilla anguilla, Linnaeus) as the flow can be too fast for them to swim against.These barriers delay or even prevent migration, potentially exacerbating population declines of this critically endangered species.Eel tiles are an emerging solution for this application, already successfully deployed to increase passage at gravity barriers.Here, eel tiles mounted to the bed of an open channel recirculating flume were assessed in terms of eel passage, behaviour and kinematics relative to movement in the absence of the tiles.The tiles effectively increased passage and allowed the eels to rest without the need to swim back downstream.The tiles also reduced the amount of energy needed to travel upstream.For the first time eel swimming was examined in a flow with multiple shear layers and turbulent structures of varying lengthscale.Swimming kinematics were analysed for complex turbulent flows and revealed a new swimming gait in the shear layer beside the tile.By allowing the eels to continuously move upstream, the tiles potentially decrease predation and infection risk at resting hotspots.Overall, the tiles were effective in helping eels pass upstream in an experimental flume.