Flow hydrodynamics drive effective fish attraction behaviour into slotted fishway entrances

Effective fishways rely on attracting fish, utilising the natural rheotactic behaviour of fish to orient into an attraction flow near the entrance. Despite the critical importance of attraction, understanding of the hydrodynamics of vertical slot entrances in relation to fish behaviour remains poor. Herein, hydrodynamic measurements of flows at slotted fishway entrances were experimented with two different designs, two velocities, three water depths, and two fish species, silver perch (Bidyanus bidyanus) and Australian bass (Percalates novemaculeata). Fish behaviours were tracked in relation to hydrodynamic measures of three-dimensional velocity and turbulent kinetic energy (TKE). There were distinct differences in the attraction flow between entrance designs, irrespective of velocity and water depth. A plain slotted entrance produced a more symmetric flow in the centre of the flume, causing fish to approach the entrance by skirting the core of the attraction jet flow and areas of high turbulence. In contrast, streamlined slotted entrance design resulted in an asymmetric attraction flow which guided fish along the wingwall towards the slotted entrance, improving attraction for both species. There were clear patterns in swimming trajectories for silver perch, swimming along the sidewalls of the observation zone towards the entrance, but Australian bass were less predictable, using random routes on their way to the slotted entrance. Both species preferred areas of low turbulence (TKE < 0.02 m² / s²). This work has important implications for design of vertical slotted entrance systems.