Modelling experiments reveal fish swimming strategies in the vertical slot fishway: Insights from swimming behaviours, kinematics and trajectory energy benefits

The vertical slot fishway (VSF) has been widely installed to help fish pass hydropower dams, mitigating migration barriers from dam construction. Current research on VSF construction has not fully incorporated the fundamental principles of fish swimming kinematics and the quantification of trajectory characteristics, which is of great significance for optimizing existing VSFs and guiding future development. This study investigated the swimming behaviours, kinematics and trajectory characteristics of Procypris rabaudi (P.R.), Onychostoma sima (O.S.) and Squalidus argentatus (S.A.) in a VSF. Kinematics metrics included tail-beat frequency, angle, amplitude, midline kinematics and wavelength. The results demonstrated that the passage rate for the three fish species (8-10 cm body length) decreased as the outlet velocity increased. Analysis of trajectory hydraulic complexity revealed that swimming along side walls and baffles provided greater energy benefits. Furthermore, fish exhibited the lowest tail-beat frequency in recirculation zones, whereas the midline oscillation amplitude and wavelength progressively decreased from recirculation zones to the mainstream and slots. Long-term retention behaviours were also observed to adversely affect passage efficiency. This study thereby enhances understanding of fish swimming strategies and trajectory characteristics in VSFs.