Identifying the optimal flow conditions of a fishway with two entrances for endemic fishes at a high-altitude hydropower station in the Tibetan Himalaya, China

Operational rules of the hydropower plant and hydrological conditions can lead to significant fluctuations of water levels downstream the dam and affect the efficiency of fishway entrance. Designing a fishway with two entrances to accommodate the fluctuations in downstream water levels can help fish successful upstream migration. This study aims at identifying the operating modes and optimal flow conditions of fishway with two entrances considering of fish swimming performance and hydraulics conditions at fishway entrances. Hence, fish swimming tests related to four endemic species including Schizothorax oconnori, Schizothorax waltoni, Ptychobarbus dipogon and Schizopygopsis younghusbandi in XH River to identify five behavioral zones (non-rheotactic, induction, preference, burst, and barrier zone) of fish by coupling of hydrodynamic conditions under seven different dam operational scenarios. It is found that the induction zone (0.1–0.4 m/s) and preference zone (0.4–1.3 m/s) for four endemic fish species, the potential migration routes of fish and potential management rules of two entrances have been proposed. The results suggested that #1 entrance of fishway can be operated in scenarios A2, and #2 entrance of fishway can be operated in scenarios A1 and B2, while both two entrances of the fishway needed to be operated in scenarios B1, B3 and C. This study highlights the importance reference for regulating fishway with two entrances at the investigated case study in the Tibetan Himalaya, and proposes a methodology of the fishway entrances for other high-altitude hydropower stations to improve fishway efficiency.