Responses of spawning thermal suitability to climate change and hydropower operation for typical fishes below the Three Gorges Dam

Abstract River thermal regime is a critical factor affecting the initiation of spawning and breeding success in freshwater fish species. The four main domestic carps (FMDCs) and Chinese sturgeon are representative warm-water and cool-water fishes with different spawning thermal requirements in the Yangtze River Basin, China. This study focused on these two types of fish species below the Three Gorges Dam (TGD), and thermal response curves of fish spawning were constructed. The daily thermal suitability index (TSI) was simulated based on scenario analysis of climate-induced water warming and cooling. The variation in four TSI-based spawning indicators was then analyzed to evaluate the response of fish spawning to climate change and hydropower operation. The results showed that the spawning time is likely to be advanced by 1.3 days on average for the FMDCs and delayed by 2.1 days on average for the Chinese sturgeon with a 0.2 °C increase in the water temperature. For the FMDCs, climate warming would narrow the spawning window, although it may advance the spawning time that has been postponded by TGD operation, and climate cooling would widen the spawning window but further postpone spawning. For the Chinese sturgeon, climate warming would aggravate the negative impacts of TGD operation by further narrowing the spawning window and delaying spawning, while climate cooling would likely offset the negative impacts of TGD operation on fish spawning. We suggest that adjusting hydropower operation rules, e.g., elevating the water temperature in the early spawning period of the FMDCs to expand the spawning window and reducing the water temperature by at least 1.2 °C during the spawning period of the Chinese sturgeon, is essential for maintaining and restoring the natural spawning process and populations. Our study is also beneficial for inferring the cumulative impacts of cascading hydropower operations and provides insights for the management and conservation of fish species with different thermal tolerances in other hotspot regions.