Cumulative ecological effects of cascade reservoir operation: A multi-life-stage fish habitat perspective

Cascade reservoir development has transformed natural rivers into river–reservoir systems, yet previous modelling approaches have often focused on single life stages or limited river reaches, leaving cumulative ecological effects across fish life cycles insufficiently quantified. Here, we develop a multi-life-stage habitat suitability modelling framework that explicitly links hydraulic and thermal alterations induced by cascade reservoir operation to key biological processes. The framework incorporates five key life stages of potamodromous fish—migration, spawning, incubation, juvenile growth, and overwintering—and enables comparison of habitat conditions under alternative regulation regimes. Applied to the upper Yangtze River between the Xiangjiaba Dam (XJBD) and the Three Gorges Dam (TGD), the model captures pronounced reorganization of habitat suitability driven by changes in flow regime, water temperature, and water-level variability along river–reservoir segments. Model results showed lower suitability for migration and spawning in reservoir reaches after dams operation, but higher suitability for incubation and juvenile stages in slow-flow backwater and reservoir reaches. Deeper water also provided more suitable overwintering conditions. Comparison of modeled comprehensive habitat suitability under different ecological-priority scenarios suggests that maintaining free-flowing reaches between cascade reservoirs is important for supporting suitable habitats across fish life stages. By resolving life-stage-specific responses within a unified modelling framework, this study provides a process-based tool for diagnosing cumulative ecological impacts of cascade regulation, and offers a transferable approach for assessing habitat dynamics in other large regulated river systems.