The Effect of Slope and Ramp Length on Upstream Passage Performance of Potamodromous Cyprinids Negotiating Low-head Ramped Weirs

River fragmentation by small engineered structures, far more numerous than dams, has led to severe declines or local extinctions of many fish populations, by blocking upstream movements for reproduction, feeding and refuge needs. Low-head ramped weirs are one of the most common instream obstacles to longitudinal fish movements. Fish passability of these structures, where water passes over but does not generate a waterfall, is primarily related to ramp length and slope, but their relative contribution has seldom been assessed. This study aims to assess the upstream passage performance of a medium-size potamodromous cyprinid, the Iberian barbel (Luciobarbus bocagei), negotiating an experimental low-head ramped weir with varying ramp length (L), slope (S), and discharge (Q). A total of 4 configurations were initially tested contemplating different combinations of L (1.50, 3.00 m) and S (10, 20, 30 %), with a constant Q of 110 L.s^-1. The configuration with the lowest number of successful upstream passages was then assessed for an additional Q of 55 L.s^-1. Each configuration tested had 4 replicates performed with schools of 5 fish (n = 100 fish). Fish movements (number of times fish approach the ramp, number of attempts to negotiate it, and number of successful upstream passages) were monitored by direct observation and recorded by a video camera. Water velocity along the ramp, as well as upstream and downstream of the ramp, was characterized with a flow probe. Results revealed that both factors L and S, as well as Q, influenced passage performance of Iberian barbel. The number of successful upstream passages decreased with increased L (Mann–Whitney U-test, Z = 15.5, P = 0.04) and S (Z = 0.5, P = 0.04). Although reduction in Q contributed to the presence of a smaller water column on the ramp, which could have hindered fish passage, the number of upstream successful passages increased significantly (Z = 0.5, P = 0.04), indicating that hydrodynamic conditions downstream and over the ramp may have influenced passage performance. These results can be useful for designing appropriate passage structures for low-head instream obstacles. Future research should also consider the retrofitting of low-head ramped weirs to provide improved fish passage and restore connectivity at these small barriers.