Passage through a fishway entrance at different velocities—results from flume experiments with small non‐salmonids

A challenge in the design of fishways especially for large rivers is the trade‐off between attraction and passability of the entrance. High flow velocities in the entrance slot generate a strong attraction flow and tend to have better attraction efficiency for upstream migration. However, these velocities may also prevent small‐bodied species or juvenile fish from entering the fishway. With our experiment, we reproduced fish swimming behaviour and passage of a fishway entrance for small‐bodied fish under realistic hydraulic conditions. At an entrance slot with 0.4 m width four velocities 0.8, 1.2, 1.5 and 1.8 ms−1 were investigated. In total, 326 juvenile roach (Rutilus rutilus (Linnaeus, 1758)), gudgeons (Gobio gobio (Linnaeus, 1758)) and spirlins (Alburnoides bipunctatus (Bloch, 1782)) were tested. The passage rates of the three tested species were altogether higher than would have been predicted from reported swimming capacities. They increased from gudgeons to roach and (significantly) to spirlins and we could show how passage rates of the three species decreased with increasing flow velocities, suggesting species‐specific critical slot velocities. Still, these velocities are lower than those currently proposed to generate sufficient attraction flow, which may have implications for fishway design.