Evaluation of eel tile hydrodynamics and passage of a non-target shoaling species, the three spined stickleback (Gasterosteus aculeatus)

Culverts are high velocity barriers that pose a challenge for fish navigation. The high velocities generated within culverts have the potential to halt migrations and fragment habitats. Passage solutions for small, river resident, freshwater fish are scarce and eel tiles present a possible multi-species solution. In this study, moulded tiles designed to promote eel passage were mounted in a recirculating open channel flume, we quantified the associated hydrodynamics and assessed whether a sentinel species, the three spined stickleback (Gasterosteus aculeatus) could navigate fixed flow conditions in the presence and absence of tiles, either alone or in shoals of three fish. The tiles produced a large reduction in streamwise velocity within them as well as around them and did so consistently for all flow conditions. The vertical and horizontal Reynolds shear stresses produced by the tiles was similar to canopy flow turbulence but the turbulent structures were not as discrete. The hydrodynamics of the tiles appear appropriate for fish passage due to the induced slow flow and moderate turbulence. The tiles significantly decreased impingement on the downstream flow straightener and exhaustion. Shoaling significantly increased passage, but in harsher local velocity conditions fish in shoals did not maintain cohesion. The tiles benefitted the fish by providing lower flow velocities but produced turbulence that in some cases destabilised the swimming fish. Despite this the tiles improved the swimming ability of minor species in areas where they would have otherwise become quickly exhausted.