Study on the change of spawning habitat suitability of four major Chinese carps in the lower reaches of the dam

The Yichang-Chenglingji reach is important spawning ground for the Four Major Chinese Carps (FMCC) in the Yangtze River, which once accounted for 42.7% of the total spawning quantity in the Yangtze River basin. The impoundment of Three Gorges Reservoirs as well as other reservoirs in the upper reach have changed the flow and sediment regimes in the study reach. On the one hand, the flow regimes in the spawning ground during the spawning period is changed. On the other hand, the reduction of sediment concentration has greatly increased the water unsaturation, causing the morphological response of the spawning ground like riverbed erosion and scouring. These changes have potential impacts on the habitat suitability of the spawning ground for domestic fish. In this paper, based on the measured cross section topographic data of the studied reach and the spawning amount data of FMCC in typical years, the habitat simulation model including one-dimensional hydrodynamic model and habitat model based on fuzzy logic is created. The factors of water level fluctuation and flow velocity which profoundly influence the spawning activity of the FMCC are considered, the Weighted Usable Area (WUA), which can reflect the suitability of the habitat, is calculated, and the impact of flow discharge, water level fluctuation (reflected by variation of discharge ΔQ), and channel erosion on the WUA of FMCC during spawning period is compared. The results show that both the discharge and the variation of discharge significantly affect the suitability of the habitat. In general, the WUA basically increases first and then decreases with increasing discharge, and there is an optimal ΔQ value under different discharges, and the optimal ΔQ increases with the increase of discharge. During the years from 2003 to 2018, the Yichang-Chenglingji reach witnessed significant erosion, and the volume of bankful channel increased by 17.9%. According to the calculation results of the habitat model, the channel erosion did not change the general relationship between discharge,ΔQ and WUA curve, but it would affect the value of WUA, mainly manifested in the reduction of WUA, of which the maximum impact was about 6%, and the impact mainly occurred when the flow submerges the floodplain. In general, discharge, variation of discharge and channel erosion will all affect WUA, so in the future, on the one hand, it is necessary to pay attention to the impact of reservoir operation on spawning of domestic fish, and create flow and flow process suitable for spawning; on the other hand, it is also important to seek measures to alleviate the reduced habitat suitability caused by channel erosion.