Disentangling the effects of hydro-morphological changes on the reduction in the spawning habitat of four major Chinese carps in the Middle Yangtze River

The Middle Yangtze River serves as a critical spawning habitat for the Four Major Chinese Carps (FMCC). The operation of the Three Gorges Dam (TGD) on the Yangtze River has significantly altered flow and water temperature regimes, reduced sediment concentration, and led to changes in river morphodynamics in the downstream reaches. These hydro-morphological changes have negatively impacted the spawning habitat suitability of FMCC, however, for the multiple hydro-morphological changes induced by the TGD, existing research primarily focuses on hydrological processes, and has not yet quantitatively distinguished the individual contributions of each factor to habitat loss. To disentangle the joint effects of hydro-morphological changes on habitat, we first constructed a one-dimensional physical habitat model that considered the spawning requirements of water level fluctuation, flow velocity, and water temperature for FMCC spawning. Then, we simulated and calculated habitat quality and quantity in pre-and post-TGD scenarios and examined the effects of hydro-morphological changes on the reduction in the spawning habitat of FMCC. Our results indicate that the Weighted Usable Area ( WUA ) for spawning decreased by approximately 12.1% during the spawning period from April to July after the TGD operation. Of this reduction, changes in water temperature accounted for 46.0%, alterations in the flow regime contributed 24.8%, and changes in river morphology were responsible for the remaining 29.2%. The most significant habitat loss occured in May, with the WUA decreasing by 19.6% relative to the pre-dam period. Decreased water temperatures due to TGD operation predominantly reduced WUA in April and May, while flow regime changes primarily reduced WUA in June and July. Additionally, channel erosion reduced WUA by decreasing wetted area and lowering the suitability of water level fluctuation, with varying spatial effects on habitat suitability changes. To mitigate WUA loss during spawning seasons, we propose an optimal combination of discharge quantity (Q) and daily discharge increment (ΔQ), which can inform dam operation schemes to reduce the impacts of altered flow regimes. This research provides insights into the comprehensive effects of hydro-morphological changes on fish habitat. It underscores the impact of river erosion due to dam operations and lays a foundation for the future ecological operation of hydropower projects and initiatives for micro-habitat restoration.