A New Method for Determining the Ecological Flow Regime to Support Sustainable Restoration of Target Fish Habitats in Impaired Rivers

Large-scale river degradation constitutes a global challenge, rendering the ecological restoration of impaired rivers ever more crucial. While ecological restoration projects have enhanced the quality of river habitats, given the dynamic nature and complexity of river and lake ecosystems, the achievement of sustainable restoration of fish habitats and the assurance of its effectiveness continue to face numerous challenges. Consequently, this study proposes an improved approach to determine the ecological flow requirements of fish habitats in impaired rivers. In relation to the screening of key species, a bespoke evaluation index system has been developed specifically for impaired rivers lacking rare and endemic fish species. Primary data were collected via field surveys, ecological monitoring, and a review of the literature, while the analytic hierarchy process (AHP) was utilized to quantitatively identify key species. In the development of the assessment framework, three core indicators were integrated: habitat-weighted usable area (WUA), habitat connectivity index (HCI), and microhabitat heterogeneity index (RMH). Incorporating the ecological requirements of key fish species across different life stages, a systematic analysis was undertaken to explore the ecological response effects of different indicator combinations under varying flow regimes. The results revealed that a flow rate of 160 m³/s gives rise to an inflection point in the RMH diversity index at 1.618, whereas a flow rate of 240 m³/s results in a significant inflection point in the HCI at 0.652. At a flow rate of 260 m³/s, the WUA attains 2,007,928 m². The optimal ecological flow range was determined to be 160–240 m³/s for the breeding period (March–June), 240–260 m³/s for the foraging period (July–October), and 120 m³/s for the winter period. These findings provide a theoretical framework for the restoration of target fish populations in similarly degraded rivers.