Author(s): Lina Focht; Patrick Heneka; Martin Henning; Christian Noss; Jochen Aberle

Linked Author(s): Martin Henning, Jochen Aberle

Keywords: Vertical slot fishway (VSF); Fish passage; Potamodromous cyprinids; Shear layer; Track analysis

Abstract: Vertical slot fishways (VSF) are common in Europe, but their design demands certain swimming speeds that may exceed the performance of small potamodromous cyprinids. We examined fine-scale swimming characteristics of juvenile roach (Rutilus rutilus) in the jet downstream of a vertical slot in a prototype-scale flume. Video tracking was used to reconstruct trajectories in a downstream region of interest and relate them to time-averaged hydraulics. The jet was defined as flow region where the velocity magnitude was larger than 0.2 m s⁻¹; tracks were classified as “Passing” if tracks crossed the slot profile and “Failing” otherwise. Out of 343 video sequences, 61 tracks entered the jet and only 7 passed. Roach approached from low-velocity zones and did not follow the jet as attraction flow. Tracks of “Passing” fish entered the jet much closer to the slot (mean jet entry position 0.16 m vs. 0.91 m downstream slot position ) and at minimal jet exposure time (mean 0.54 s vs. 1.14 s), while swimming faster within the jet. Peak swimming speeds exceeded laboratory capacity estimates, suggesting roach performance is underestimated under simplified flow conditions.

DOI: https://doi.org/10.64697/iahr.proc.ise2026.abs.212

Year: 2026