The Combined Effects of River Flow and Water Quality on Freshwater Fish Population Dynamics

The aquatic ecosystems of the rivers maintain aquatic biodiversity, and freshwater fish are important bioindicators for the ecological health of a river. Compound stress on fish populations all over the world has been caused by hydrological changes and water quality degradation due to intensive human activities such as dam regulation and pollutant discharge. This study focused on the lower reaches of the Yellow River, and investigated the independent and interactive impacts of river flow and water quality on the dynamics of freshwater fish populations based on multiple-year field monitoring data from 2014 to 2023. Flow variation accounted for 21% of the temporal variation in fish community and was found to have a significant influence on fish spawning cues, habitat availability, and physiological metabolism of the fish, while water quality accounted for 15% of the temporal variation in fish community and was the most limiting factor in the most heavily polluted river reach. Low flow conditions increased risk of pollutant accumulation, flood conditions increased both pollutant accumulations and introduced non-point source pollution risks; there were non-additive coupled interactions between flow and water quality. Benthic resident fish had higher tolerance to composite stress while pelagic and migratory fish had higher sensitivity to hydrological/water quality co-stress. Single factor regulation would hardly be enough to restore fish communities, coordinated regulation of flow regime and water environment was needed. This paper offers scientific evidence for watershed ecological controls and fish biodiversity conservation under various human disturbances.