Effect of Schooling Behavior on Upstream Migration of Juvenile Grass Carp and Silver Carp

The construction of dams impedes energy exchange and material circulation in rivers, and the operation of hydropower stations negatively impacts the function of river ecosystems. Countries across the globe have implemented various fish protection countermeasures and conducted many associated hydraulic and ecological studies to mitigate the impact of hydropower development on fish survival, maintain the abundance and diversity of fish populations, and restore riverine and lacustrine fish migration routes and habitats. Remarkably, wild fish generally migrate in groups, whereas most contemporary studies on fish passage facilities focus on individual fish. Hence, the behavioral characteristics of fish schools are worth investigating. Given that schooling behavior is a pervasive feature of fish communities and plays an essential role in dealing with potential risks, improving self-adaptation, expanding resilience, and enhancing population sustainability, this study concentrated on investigating its effect on upstream fish migration and decoding the internal mechanism. We conducted an experiment targeting grass carp (Ctenopharyngodon idella) and silver carp (Hypophthalmichthys molitrix), two economically important freshwater fish species in China, on a non-uniform flow ground with low turbulence. The experiment quantified the impact of fish schooling on their ability to overcome flow barriers by examining the ascending sustainability and swimming performance of five-fish groups and one-fish groups at three flow velocity levels (0.25–0.50 m/s, 0.30–0.60 m/s, and 0.35–0.70 m/s) utilizing a novel index system. The new index system employed nondimensionalized ascending sustainability, first-attempt endurance, and first-attempt ascending energy consumption to indicate the fish's persistent ascending ability, ascending efficiency, and ascending energy cost, respectively. Finally, the ascending trajectory of fish was investigated simultaneously to determine the distribution of trajectory and the distribution of the hydrodynamic force factor at the trajectory points. It could be concluded that (1) the influence of schooling behavior on ascending fish behavior was related to the ascending sustainability of individual fish. Schooling behavior significantly increased the ascending sustainability while producing no specific impact on the ascending efficiency at low-level individual ascending sustainability. Inversely, schooling behavior significantly decreased the ascending efficiency while having no particular effect on high-level individual ascending sustainability. Moreover, the contribution of schooling behavior to the ascending sustainability of grass carp varied with the flow velocity, as the ascending sustainability of the five-fish groups was significantly higher (P = 0.030) and lower (P = 0.048) than the one-fish group at the velocity levels of 0.30–0.60 m/s and 0.35–0.70 m/s, respectively. In contrast, schooling behavior holistically improved the upward swimming of silver carp, significantly increasing the ascending sustainability of the five-fish groups at velocity levels of 0.30–0.60 m/s (P = 0.004) and 0.35–0.70 m/s (P < 0.001). (2) The endurance of the first attempt in the juvenile grass carp group was significantly higher than that of the single fish at the velocity levels of 0.25–0.50 m/s (P < 0.001) and 0.35–0.70 m/s (P = 0.005), while the endurance of the first attempt in the juvenile silver carp group was significantly higher than that of the single fish at the velocity levels of 0.25–0.50 m/s (P < 0.001) and 0.30–0.60 m/s (P = 0.005). The endurance of the first attempt in juvenile silver carp decreased significantly in schools (P < 0.001) solely at the velocity level of 0.35–0.70 m/s. In addition, the flow velocity generally increased the first-attempt endurance and cumulative energy consumption of individual and grouped fish. However, since the burst-coast swimming mode forced on juvenile silver carp in high-velocity conditions significantly improved ascending efficiency, the endurance of the first attempt initially increased and then decreased with the flow velocity. (3) Schooling behavior enabled grass carp to swim with less energy and significantly lowered their energy cost at the velocity level of 0.25–0.50 m/s (P < 0.001), whereas it augmented the accumulated energy consumption of five-fish groups in silver carp and significantly increased their energy cost at the velocity level of 0.25–0.50 m/s (P = 0.050). (4) Collectively, juvenile silver carp could find an ideal ascending trajectory more rapidly than juvenile grass carp. The ascending trajectory of grass carp tended to concentrate at first and then disperse with increasing velocity, whereas the trajectory of silver carp tended to concentrate with increasing velocity. In brief, schooling is an unsubstituted behavior in the upstream migration of fish migrants, motivating and inhibiting the fish's upstream movement performance with its primary effect on locomotion in terms of energy consumption, visual response, and the ability to overcome flow barriers. The findings could improve the design of fish protection measures and provide specific recommendations for the operation of fish passage facilities. For example, when silver carp migrate through a fishway, the additional light source can adequately promote schooling behavior and improve their capacity to overcome flow barriers. When grass carp migrate through a fish passage, the additional light source in the rest pond of the fishway can effectively promote schooling behavior, thereby preventing the return of fish owing to a lack of ascending sustainability. Considering that the hydrodynamic environment of natural waters and the size of fish groups are highly complex and variable, the schooling behavior characteristics of fish groups in different water flow environments should be investigated in future research to enrich the database for the planning and implementation of fish protection engineering measures.