Impact of light environment on the behavioral characteristics and upstream swimming activeness of juvenile Hypophthalmichthys molitrix

This study investigates the effects of phototaxis and behavioral characteristics of fish under different light conditions on their upstream swimming activeness, with the goal of facilitating fish migration past dams and improving the efficiency of fish passage facilities in China. The experiment used juvenile Hypophthalmichthys molitrix, a key economic fish species in China, as the test subject. By analyzing their phototaxis, stress degree, activeness, and light-oriented activeness under varying light wavelengths and illuminance levels, we assessed how these factors influence upstream swimming activeness at different flow velocities. ① Under red light, the preferred illuminance range for H. molitrix was 3.268-5.444 lx; under blue light, the acceptable illuminance range was 0.033-10.511 lx; under green light, the acceptable and preferred illuminance ranges were 0.367-74.489 lx and 3.776-9.833 lx, respectively; under white light, no distinct preference or acceptable illuminance range was observed. ② The stress degree of H. molitrix under red and green light initially increased and then decreased with rising illuminance, whereas under blue light, it increased monotonically. The activeness of H. molitrix under red and green light increased steadily with illuminance, while under blue light, it exhibited a fluctuating upward trend. ③ The upstream swimming tendency of H. molitrix was primarily influenced by light preference and stress degree. When stress degree was low, upstream swimming activeness increased with higher light-oriented activeness; conversely, when stress degree was high, upstream swimming activeness decreased as light-oriented activeness increased. To optimize fish passage efficiency: a 10 lx red light source at the facility entrance can attract fish. Shaded areas inside the passage, combined with intermittent 1 000 lx green light sources, can promote sustained upstream movement. Near hazardous turbine intake zones, a combination of 1 000 lx blue light and a 0.35 m/s flow velocity can deter fish from approaching. This study provides theoretical and data-driven support for optimizing fish protection engineering in various fish passage scenarios.