Effect of flow velocity variation on individual behaviour and collective cohesion of two fish species

Proper recognition of fish swimming and collective behaviours hold great significance in the development of fish conservation. The influence mechanism of hydrodynamic parameters on fish collective and swimming behaviours remains incompletely established. This study investigated the individual and collective behaviours of juvenile Hemibarbus maculatus Bleeker and Dabry's sturgeon (Acipenser dabryanus) under various flow velocities [≈1-7 body length (BL/s)] from laboratory experiments and video analysis. Fish individual behaviours were quantified using tail-beat kinematics, and five collective metrics were used to evaluate fish collective behaviour. The results demonstrated that increasing flow velocity had a significant impact on the tail-beat movement of fish individuals, and collective behaviour had the potential to reduce swimming energy expenditure of fish individual in shoals. Meanwhile, the two species exhibited significant collective differences, and enhancing flow velocity can promote fish collective effects for both species with an upper limit. Various configurations and formations were adopted by fish shoals to cope with different flow velocities and optimize swimming efficiency. The results of this work contribute to fish conservation and a deeper understanding of collective behaviour in fish.