Dams are considered to be a serious threat to migratory fish on a global scale. Most of the world's rivers have been dammed, including those containing hotspots of fish diversity [1-3]. The mechanism by which dams affect fish is unclear and has often led to an underestimation of their cumulative and far-reaching adverse effects. Remedial measures of fish rescue for existing dams, including fish passage facilities and restocking, are insufficient or inefficient [1]. Although small and old dams are increasingly being removed, it is clearly unrealistic to demolish large, multi-section dams [4]. Hence, designing environmental flow to rescue fish is a complex challenge [5] and requires an in-depth understanding of the mechanism. Here, we propose an essential theory to reveal the relationship between dams and fish and discover novel insights into the migration behaviors, gonadal degeneration, and population dynamics of the Chinese sturgeon. We show that since 1981, the Gezhouba Dam has reduced the migration distance by 1,175 km, resulting in gonadal development being delayed by 37 days, resulting in the effective breeding population size and environmental capacity of the new spawning ground respectively reduced to 24.1% and 6.5% of the original. Even worse, subsequently built dams, particularly the Three Gorges Dam and Xiluodu Dam, have further reduced the effective breeding quantity to 0%-4.5% by elevating the water temperature to inhibit breeding activity during the breeding season. The cumulative effect of the cascade dams has led to an ongoing decline in adult abundances in the Yangtze River and the sea.