Potential hydrological regime requirements for spawning success of the Chinese sturgeon Acipenser sinensis in its present spawning ground of the Yangtze River

Reservoir impoundment often alters the hydrological regimes of the river and consequently leads to the extinction of some sensitive species. This study analysed the spawning response of a critically endangered (CR) species, Chinese sturgeon Acipenser sinensis, under the changing hydrological regimes caused by the Three Gorges Reservoir (TGR). Based on our statistic analysis, we inferred that the long duration (>100 days) and large magnitude (>1.8 * 10(6) m(3)/s) of high flow pulse in the flood season and low accumulated water temperature in October (<660 degrees C) and November (<540 degrees C) were key hydrological regimes to induce the spawning success of Chinese sturgeon. Such hydrological regimes could not be satisfied after the third phase of the TGR impoundment in 2005, which decreased the suitability of the flow (HSIF) and temperature (HSIT) regimes for the spawning of Chinese sturgeon. However, a significant complementary relationship between high HSIF and low HSIT was found in all years of successful spawning after 2005, but it did not occur in the nonspawning years of 2013, 2015 and 2017, which could be the reason for spawning failure in these years. In the years 2018 and 2019, although the complementary relationship existed, the spawning failure in these 2 years was probably due to few parent fish. Therefore, we suggest that future conservation measures should focus on both optimizing operations of the TGR to ensure suitable hydrological conditions and restocking with adult fish to ensure successful spawning.