The effect of weir crest width and discharge on passage performance of a potamodromous cyprinid

Abstract

Contrary to the vast literature on dams, much less is known about the effects of small weirs on fish movements. This study aims to evaluate the upstream and downstream passage of the Iberian barbel (Luciobarbus bocagei), when facing a small broad-crested experimental weir with varying widths of the weir crest (W) and discharges (Q). Nine configurations (W×Q) were tested. Fish were released downstream of the weir and upstream and downstream passages were monitored. Hydrodynamics were characterised with a flow probe and a three-dimensional acoustic Doppler velocimeter (ADV). Fish negotiated all configurations, registering a total of 273 upstream passages and 192 downstream passages. The number of fish that approached, attempted and successfully ascended and descended the weir varied among configurations, emphasising the complex influence of factors on passage performance. Discharge was preponderant on both passages, proving that fish movements may be hampered by increasing discharges. However, the width of the weir crest only significantly influenced downstream passages, showing that strong velocity gradients experienced on narrower widths of the weir crest may discourage downstream movements. Results from the present study are expected to be useful to identify potential migration obstacles and may be applicable to other cyprinid species.