Responses of the whole life cycle of typical fish species in the upper Yangtze River to hydrological regime alterations and their ecological flow requirements.

Driven by anthropogenic disturbances, including cascade hydropower development, navigation channel regulation, and sand extraction, the hydrological processes and thermal regimes of the National Nature Reserve for Rare and Endemic Fishes in the upper reaches of the Yangtze River (hereafter referred to as the Reserve) have undergone profound alterations. These alterations have substantially degraded fish habitat conditions and generated persistent adverse effects on the completion of fish life-history processes. Therefore, elucidating the ecological response mechanisms of representative fish species to hydrological regime changes and accurately quantifying their habitat requirements across different life-history stages are crucial for effective fish population restoration and habitat rehabilitation in protected river reaches. In this study, the Four Major Chinese Carps were selected as target species, and a physically based coupled model was developed to establish an integrated framework for evaluating habitat suitability throughout their entire life cycle. According to species-specific life-history characteristics, the life cycle was divided into three key stages: spawning, foraging, and overwintering periods. By incorporating the distinct habitat preferences and ecological requirements of each stage, the corresponding ecological flow requirements of the Reserve were quantified for different seasonal periods. The results revealed that habitat suitability for Four Major Chinese Carps was highest during the foraging period, followed by the spawning period, and lowest during the overwintering period. This variation suggests that the current hydrodynamic conditions in the studied river reach are relatively favorable for habitat maintenance and population survival during the foraging period, whereas greater ecological constraints occur during overwintering. To optimize habitat conditions for Four Major Chinese Carps, an ecological flow regime of 4800-6000 m3/s is recommended during the spawning period (April-July), with appropriately timed flood pulses between April and June to trigger spawning behavior. During the foraging period (August-November), flows of 9600-12,000 m3/s should be maintained to support foraging and growth. During the overwintering period (December-March), when natural runoff is insufficient to meet habitat demands, adaptive regulation of upstream reservoirs should be implemented where feasible to enhance downstream discharge and improve overwintering habitat conditions in the large mountainous river ecosystems of the Reserve. The life-cycle habitat suitability assessment framework developed in this study, together with the stage-specific ecological flow requirements identified, provides a scientific basis for reservoir operation strategies, fish habitat evaluation, and targeted ecological restoration planning in regulated river systems.