Deciphering Hydrothermal Cues for the Wintering Migration of Endemic Fish in Cryospheric Headwaters: A 3‐Year Monitoring on the Tibetan Plateau

ABSTRACT The hydrological drivers triggering overwintering migration of fish in the headwaters of the Yangtze River, characterized by winter bottom‐up freezing, remain poorly understood. This study investigated the hydrological dynamics influencing the overwintering migration of Schizopygopsis microcephalus , a key endemic species. Combining ultrasonic telemetry data with continuous, hourly water temperature and depth monitoring at an identified overwintering ground (October 2020–June 2023), we employed statistical analyses (including Mann–Kendall [M‐K] test, wavelet transform, generalized linear mixed model, receiver operating characteristic analysis and decision trees) to identify migration triggers. Results revealed significant diurnal thermal fluctuations at the overwintering site. During the overwintering period, the daily temperature difference ranged from 0.2°C to 16.1°C, with a dominant 36‐h periodicity in both temperature and depth. Returning periods exhibited temperatures of 0°C–8.7°C and large daily fluctuations (1.1°C–8.4°C), while leaving periods showed 0°C–16.9°C and fluctuations of 2.3°C–13.9°C. Distinct hydrological thresholds governed migration phases: both decreasing temperature (< 0.9°C) and depth (> 0.81 m) triggered autumn returning, while increasing temperature (> 4.6°C) and depth (> 0.81 m) initiated spring departure. The synergy between water temperature and depth is the primary driver of S. microcephalus overwintering migration. This study provides key data for the hydrological demands of the overwintering migration of important fish species in Yangtze headwaters and serves as a basis for overwintering response to climate change.