Impact of dam construction on the spawning grounds of the four major Chinese carps in the Three Gorges Reservoir

The construction of dams has tremendously altered the water flow and environmental conditions of natural rivers, severely affecting the habitat and reproduction activities of aquatic organisms. In this study, we constructed a reproduction suitability model for the four major Chinese carps (FMCCs) in the upper reaches of the Yangtze River and related this to a one-dimensional hydrodynamic model of the Three Gorges Reservoir (TGR). Stochastic flood modelling was used to simulate a series of river floods before and after construction of the Three Gorges Dam (TGD). The simulated floods were used as the input boundaries of the hydrodynamic model to then simulate complex flow conditions in the upper reaches of the Yangtze River before and after dam construction, so as to simulate the spatial distribution and changes in the spawning grounds of the FMCCs. We found that the reproduction suitability for the FMCCs in the TGR decreased during the post-dam period, and most of the spawning grounds moved to the river reach above Zhongxian. The FMCCs were found to initially have a total of nine spawning grounds, but during the post-dam period there were six spawning grounds likely in the 60-km stretch of Zhutuo–Jiangjin section and in the 255-km stretch of Banan–Zhongxian section. By setting four different water levels in front of the dam (corresponding to no-dam, low-dam, middle-dam, and high-dam scenarios), we examined the effects of dam sizes on the spatial distribution of the FMCCs' spawning grounds. The results showed that the area of affected spawning grounds positively correlated with the dam's size, with high-dam scenario (corresponding to the construction of the TGD) being especially impactful through a net loss of spawning grounds extending 101 km upstream from the dam. The results of this study offer scientific guidance for the ecological regulation of the TGR.