Relationships among the fish passage efficiency, fish swimming behavior, and hydraulic properties in a vertical‐slot fishway

Abstract Three‐dimensional hydrodynamic numerical simulation and laboratory fish‐passage experiment with juvenile grass carp ( Ctenopharyngodon idella ) were used to systematically estimate hydrodynamic parameters, fish passage efficiency, fish swimming behavior, and fish swimming trajectories in a vertical‐slot fishway (VSF) under five different flow discharges. The spatial‐mean time‐averaged velocity magnitude along the slot section determined passage success rate. The absolute value of the time‐averaged Reynolds‐shear‐stress component in the X – Y direction played a decisive role in selecting the swimming path. In addition, fish energy expenditure along the mean‐50% swimming route indicated that higher mainstream velocity did not necessarily signify larger energy consumption, because successfully‐migrating fish selected the relatively lower velocity zone during the migration process. Our findings for fish swimming behavior in response to velocity and turbulence can be used to develop high‐efficiency fishways for grass carps or other species with similar traits.