In Situ Study on the Influence of a Dark Environment on the Upstream Behaviors of Plateau Fishes in Fishways: A Pilot Study

To help fish to bypass dams and other human-made barriers, some fishways have ingeniously incorporated extended tunnel sections. This innovative design not only optimizes the overall structure of the fishway but also significantly reduces disturbances to the surrounding ecosystem. However, the potential challenges posed by long tunnel sections to fish upstream migration remain insufficiently studied and poorly understood. This study conducted in situ experiments utilizing a passive-integrated-transponder (PIT) system to quantitatively assess the effects of dark and natural light environments on the upstream migration behavior of plateau-endemic fishes (Schizothorax macropogon, Schizothorax waltoni, and Schizothorax oconnori) in a vertical-slot fishway. A 655 m section of the fishway was selected for the experiment, with shading cloth used to simulate the dark environment (DE) of tunnel sections, and its removal serving as the natural light environment (NE). The results showed that in the DE, the upstream behaviors of S. macropogon, S. waltoni, and S. oconnori were not hindered. The entry efficiency at the experimental segment (Ee) of all three species exceeded 65% in the DE, which was higher than that in the NE. The passage efficiency (Ep) of S. macropogon and S. waltoni showed no significant difference between the DE and NE, whereas S. oconnori exhibited a significant difference, with an overall Ep of 0% in the NE and 75.0% in the DE. Additionally, the DE caused a temporary disruption to the diel migration rhythms of the three species. The transit speeds (St) of S. macropogon and S. waltoni were both elevated in the DE, with S. waltoni showing a particularly significant increase; its average St in the DE (0.080 m/s) was much higher than in the NE (0.021 m/s). Ridge regression analysis further indicated that the DE was the primary factor influencing the St and had a positive effect on upstream behavior. Moreover, differences in the upstream migration performances of different species under varying light conditions highlighted species-specific sensitivity to light. This study offers key insights for fish passage design in canyon hydropower projects and highlights the potential of tunnel-type fishways in restoring river connectivity.