Slow and steady? The effect of flow over eel tile and its instabilities on eel kinematics*

Guiding elver towards fish passes and helping them migrate past sections of high velocity flow is a challenging problem. The European eel (Anguilla anguilla) has experienced a population decline due to migration barriers in rivers and this has contributed to them being included on the IUCN critically endangered list. Eel tiles are a potential solution to some of these issues and they have been shown to be an effective substrate for elver to negotiate other types of obstructions such as weirs and low head dams. This experimental study examined the effect of the tiles on the swimming behaviour in a an open channel flume. The eel behaviour was quantified and analysed to extract key parameters for passage and kinematics, and the flow field was analysed using 2D Particle Image Velocimetry (PIV). The tile protrusions produced lower velocities within and immediately above the protrusion layer where turbulent structures formed as a result of instabilities between layers of flow travelling at different speeds. This structure may have the ability to destabilise fish. The eels seemed to favour the low velocity zones offered by the protrusions, and their kinematics were analysed to find the effect of this modified flow on their gait. Overall these tiles could represent a low cost and effective solution to some of the problems facing eels and may also help other species.