Engineered freshwater flows promote fish reproduction and stock recovery in the estuarine ecosystem

Freshwater inflow to estuaries is essential for regulating key biological processes and sustaining critical ecological functions. However, under combined pressures from climate change and anthropogenic activities, freshwater discharge frequently declines below the threshold required to maintain ecosystem health. Currently, the freshwater-flow level required to sustain key biological production processes in estuaries remains poorly constrained, limiting the development of quantitative environmental-flow targets for estuarine ecosystem management. This research examines the effects of varying spring freshwater discharge on fish spawning and fishery recovery based on long-term field observations in the Yellow River Estuary (YRE), China. The results revealed a significant positive relationship between spring streamflow and fish egg abundance, while fishery stocks showed a nonlinear increase across an intermediate range of spring discharge. Based on the joint responses of fish egg abundance and fishery stock, we identified an approximate spring discharge transition range of 3.0–4.0 billion cubic meters, with 3.5 billion cubic meters serving as a management-relevant reference value for the YRE. Because the study is based on observational evidence, this value should be interpreted as an empirical reference point rather than as a fixed ecological threshold. This study provides a framework for evaluating how managed spring freshwater discharge may support fish reproduction and fishery-resource recovery in regulated temperate estuaries, offering useful information for integrated water and ecosystem management.