Seeking the chance of escaping from the population depression spiral for Chinese sturgeon (Acipenser sinensis)

Chinese sturgeon (Acipenser sinensis) are the flagship species of the Yangtze River aquatic ecosystem and the indicator species for indicating the degradation of the Yangtze River aquatic ecosystem in recent decades. The Gezhou Dam, constructed in 1981, blocked the previous spawning migration route of A. sinensis. The previous spawning fields located in the section from the upstream of the Yangtze River to the downstream of the Jinsha River (as long as more than 600 km) were squeezed into a new spawning field located in the section under the Gezhou Dam (about 4 km). It greatly affected the entire spawning migration process of A. sinensis. Subsequently, a series of river engineering projects further impacted on the natural spawning activities of A. sinensis. Although central government and local governments have taken a series of protective measures, the natural population of A. sinensis continued to decline. Natural spawning activity of A. sinensis in the wild has been interrupted in 2013, and there has been no natural spawning detected for 8 consecutive years from 2017 to 2024, making the A. sinensis natural population critically endangered. Why does A. sinensis fall in such an endangered situation? What are the key influencing factors? There has been much debate about this in the academic community. We proposed that A. sinensis may have fallen into a population depression spiral after the Gezhou Dam blocked the previous spawning migration route of A. sinensis, and the subsequent human activity disturbance further pushed A. sinensis towards extinction. Based on the life cycle of A. sinensis, we constructed a simplified parametric model to estimate the dynamic of the natural population of A. sinensis and to test this hypothesis and explore the possibility of rescuing A. sinensis from the population depression spiral. The results showed that: ① the river engineering projects that were constructed in the spawning field under the Gezhou Dam had a small contribution to the current failure of A. sinensis natural spawning (1 year ahead). ② The temperature hysteresis that was caused by the Three Gorges reservoir impounding led to A. sinensis natural spawning failure 15 years ahead. ③ Artificial propagation and release that were implemented over the past 40 years had little contribution towards keeping the A. sinensis natural spawning (delaying the failure of A. sinensis natural spawning for 1 year), but had some contributions on the spawning migration population (postponing the disappear time of spawning migration population by 31 years). ④ Assuming that water temperature regulation and spawning ground restoration can be completed in 2025 and 300 000 Chinese sturgeon juveniles are released each year, natural spawning may be reappeared in 2025, but may failures in both 2029 and 2030, and then natural spawning would be restored once again. However, natural spawning and breeding population will heavily rely on artificial release. If artificial release was stopped, natural spawning will fail again in approximately 33 years later, with a complete loss of spawning migration population after 41 years. ⑤ Assuming that water temperature regulation and spawning ground restoration can be completed in 2025, A. sinensis breeding population can adapt to the environmental changes caused by Gezhou Dam, with fertilization rate, egg production recovered, and gonadal development rhythm adapted to the Gezhou Dam, then 300 000 A. sinensis juveniles only need to be released from now to 2027, and A. sinensis can form a small-scale sustainable population. However, the size of breeding population is positively correlated with the duration of artificial release and the number of released A. sinensis individuals, but there is an inherent ceiling (♀217, ♂197) which was mainly determined by the ecological capacity of spawning ground (♀90). Moreover, there is a minimum value (♀14) for the ecological capacity of spawning ground. If the ecological capacity of spawning ground is lower than the minimum value, the sustainable wild population of A. sinensis would not appear. It is recommended to carry out the water temperature regulation and spawning ground restoration under Gezhou Dam as soon as possible, and continue to release 300 000 A. sinensis juveniles per year. What needs to be emphasized is that more released A. sinensis juveniles potentially mean higher possibility of the emergence of A. sinensis breeding individuals that can adapt to the environmental changes caused by Gezhou Dam. Of course, it is suggested that all released A. sinensis juveniles should be genetically designed for adapting to the environmental changes caused by Gezhou Dam according to adaptive genetic management.