Impact of hydrodynamic-schooling interaction on the ascending behaviour of juvenile Hypophthalmichthys molitrix

Elucidating the hydrodynamic-fish motion response relationship during the ascending migration of schooling fish is fundamental for designing fish passage in hydropower infrastructure. The current study quantified the upstream movement patterns of individual and group (ten individuals) juvenile Hypophthalmichthys molitrix, an economically significant fish species in China, under the combined hydrodynamic-schooling influence in non-fatigued flow fields with varying turbulence levels. We concluded that: ① In the low-turbulence flow field, individual juvenile H. molitrix achieved a 100% passing rate with an average time expenditure of (40±27) s, exhibiting uniform movement distribution without a clear velocity preference. ② Within the locally high-turbulence flow field, individual juvenile H. molitrix tended to linger in areas of lower velocity and turbulence, utilising the Kármán vortex street to minimise instantaneous energy cost, leading to a significantly increased time expenditure (84±54) s, P<0.001). ③ The swimming trajectories of juvenile H. molitrix in school were relatively concentrated, with the attraction and guidance effect between neighbour fish promoting their linear formations along the primary flow. ④ In locally high-turbulence flow field, the inter-individual repulsion effect of juvenile H. molitrix in school hindered their effective avoidance of high-speed, high-turbulence flowing areas, causing impeded upstream movement, with passing rate dropping significantly to 54% (P<0.05) and time expenditure extending to (88±12) s. The study indicates that the movement distribution of juvenile H. molitrix during upstream migration is primarily influenced by the combined effects of flow velocity and turbulence intensity, with a tendency to avoid high-turbulence areas. During schooling, the attraction and guidance between individuals help optimize the upstream path and reduce energy expenditure, while the repulsion effect may decrease migration efficiency and hinder the full utilization of favorable water flow conditions by individual fish. The findings offer theoretical foundations and data support for optimising the design and enhancing the operational efficiency of fish passage by regulating fish school size and developing characteristic flow fields.