Abstract: Water flow, light, and individual interaction are the key factors affecting fish-pass efficiency. Clarifying the comprehensive influence of those three factors on fish movement characteristics is of great significance for facilitating fish passage over the water power complex. In this study, we examined rheotaxis, upstream swimming activeness, and swimming stability of juvenile bighead carp under different illuminances, water flows, and schooling sizes, quantifying their non-fatigue swimming behavior under the combined effects of the three variables. The rheotaxis of juvenile bighead carp significantly increased with flow velocity with the maximal increment from 0.15 to 0.25 m·s^-1 (P<0.001 for the three light treatments). The variation of rheotaxis with light differed with flow velocity, and the rheotaxis value significantly augmented in bright (10 lx and 100 lx) groups. In addition, light effectively eliminated the backward swimming trend of independent fish individual, and the interaction of clustering individuals significantly promoted the group’s upstream motion. The flow velocities effectively enhanced the swimming stability of juvenile bighead carp. Light only significantly improved the stability of group treatments, with their stability values at each flow velocity increasing significantly with illuminance (P<0.05 for flow velocities from 0.15 to 0.65 m·s^-1). In contrast, the interaction of clustering individuals compromised fish’s stability in dark conditions. Based on the findings, the fishpass can be optimized by mediating water flow, light environment, and grouping conditions at the entrance of the fishway and the upstream intake of the downstream route, thereby improving fish migration over the dam.

Key words: Aristichthys nobilis, fish migration, behavioral strategy, schooling individual interaction, fish passage facility