International Journal of Aquaculture, 2025, Vol.15, No.4, 175-183 http://www.aquapublisher.com/index.php/ija 177 dams and other river channels are important reasons for the sharp decline in the number of migratory freshwater fish (Figure 1). In addition, river channel interception, channelization and flood control dams have destroyed the upstream and downstream connections of rivers and floodplain ecology, suppressed the recovery capacity of flood alluvial plains, and caused many fish that rely on seasonal floods to lose their key habitat (Arthington, 2018). In short, while meeting the needs of water supply, power generation and shipping, water conservancy projects have caused serious physical damage and isolation effects to fish habitats, greatly aggravating the degradation of aquatic habitats. Figure 1 The Jinsha (upper Yangtze) River in China shows the location of 10 dams above the Three Gorges Dam and Gezhouba Dam (Adopted from Dudgeon, 2024) 3.2 Pollutant enrichment and water quality deterioration Industrial and agricultural pollution is one of the main chemical causes of aquatic habitat degradation. Agricultural non-point source pollutants (fertilizers, pesticides, etc.) and domestic sewage enter the water through runoff, causing eutrophication and harmful algae blooms, consume a large amount of oxygen in the water, and produces bad environment nitrogen and phosphorus enrichment caused by fish death (Kumari, 2022). Industrial pollution (heavy metals, organic pollutants) and urban domestic sewage lead to increased toxicity of water, which can damage the immunity and reproductive system of fish, change the levels of sex hormones, and inhibit fish health even at low concentrations. Pollutants discharged on land have intensified the circulation of nutrients at the bottom of the water body, weakening the stability of the bottom-water habitat. Under the pressure of pollution, the number of aquatic plants and benthic invertebrates has decreased, the food chain structure has been disrupted (Brain and Prosser, 2022), and the resources on which fish rely on are reduced. At the same time, the increase in water temperature and changes in precipitation patterns caused by climate change are also changing the physical and chemical conditions of the water body, exacerbating the negative impact of water quality deterioration. 3.3 Overfishing and other human factors Overfishing puts indirect pressure on habitat by directly reducing the number of fish individuals. Continuous high-intensity fishing has led to a younger age structure and a smaller size of fish populations, destroying the replenishment and reproduction process of natural populations. According to a report by the World Fish Migration Foundation, overfishing accounts for about one-third of the threat to freshwater migratory fish. In addition, invasive alien species and biological invasions will also compete with local fish for habitat and food, changing ecological balance. Rapid changes in land use (such as urbanization, riparian development, wetland landfill) reduce the available habitat space for fish. Climate change makes water temperature and flow fluctuate more
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