The Effect of ‘Hydraulic‐Visual’ Interaction on the Intensity of Fish Schooling in the Upstream Movement of Juvenile Silver Carp

ABSTRACT Understanding how hydraulic and visual factors jointly regulate fish schooling is critical for improving fish passage performance. Using juvenile silver carp ( Hypophthalmichthys molitrix ) as a model organism, we investigated how schooling intensity responds to the combined effects of flow velocity, turbulence kinetic energy (TKE) and visual guidance cues from obstacles. The experiment was conducted in a controlled flume with three flow conditions at a fixed fish group size (five individuals). By integrating simulated flow fields with the spatial distribution of fish schooling intensity and swimming energy expenditure, relationships among flow characteristics, energetic demand and schooling behaviour during fish upstream movement were quantitatively analysed. We concluded that (1) increasing flow velocity constrained the fish's lateral exploration and prompted them to ascend predominantly along the main flow current; (2) in relatively mild flow conditions, fish schooling intensity was barely affected by TKE, whereas in obstacle areas where high velocity co‐occurred with low turbulence, they preferred to select high‐velocity paths as efficient ascending passages near obstacles; (3) increase in short‐term or cumulative energy expenditure alone was not sufficient to drive an increase in fish schooling, whereas the surge in energy requirements under conditions of high‐velocity barriers correlated with enhanced fish schooling intensity; (4) after passing velocity barriers, fish adopted a short‐burst acceleration strategy in low‐velocity, low‐turbulence regions upstream of obstacles to avoid reforming dense schools, indicating an active tendency to minimise prolonged energy costs when possible; and (5) obstacles provided salient visual cues that enabled fish to avoid collisions and adjust their swimming direction during rapid upstream movement. Overall, these findings demonstrate that schooling intensity is a dynamic behavioural adjustment shaped by the interplay between hydraulic conditions, associated energetic costs and visual information. The results provide mechanistic insight into collective upstream migration and offer practical guidance for the hydraulic and structural optimisation of fish passages.