International Journal of Aquaculture, 2025, Vol.15, No.4, 175-183 http://www.aquapublisher.com/index.php/ija 176 biological composition from the source to the estuary, forming a rich habitat structure (such as torrents, slow flows, drift belts and beach dam areas, etc.). This complex environmental structure provides fish with diverse living space and food resources (O’Mara et al., 2024). For example, free-flowing natural rivers have nurtured nearly 50% of the world's fish population, and many of them are migratory and require connected habitats in different sections to complete the spawning and foraging processes. River ecosystems usually have strong self-purification capabilities, but are also susceptible to upstream runoff, erosion, and recharge water quality (Wan et al., 2025). Fish often have ecological behaviors of river migration in rivers, and their population structure and distribution are highly sensitive to hydrological changes in the basin and habitat connectivity. Therefore, maintaining the natural flow and connectivity of the river is crucial to maintaining the diversity of river fish. 2.2 Lake and wetland ecosystems Lake and wetland ecosystems belong to static or semi-static environments, which are characterized by slow water flow, stable vertical structure of water body and large water volume, which are prone to water temperature stratification and nutrient deposition. Lakes are mostly inland water bodies with rich nutrients, with strong occlusion and high ecosystem productivity, but they are also prone to eutrophication and algae blooms. Wetlands (including swamps, estuary swamps, floodplain, etc.) are between land and water, with interlaced ecological characteristics, and are important places for fish reproduction, nurturing and overwintering (Cutler et al., 2024). Lakes and wetlands provide fish with a rich food chain basis (such as phytoplankton and invertebrates) and are key habitats for many freshwater fish. Compared with rivers, lake wetlands often have more complex species communities and ecological corridors, but are also more susceptible to runoff input and internal circulation (Bai et al., 2022). The study found that fish diversity in lakes and wetlands is often restricted by factors such as lake size, connectivity, and nutritional status. For example, large lakes such as Poyang Lake and Dongting Lake in the middle and lower reaches of the Yangtze River in China attract a large number of migratory and settled fish to reproduce every year due to their relatively stable water levels and abundant aquatic plants (Maileht et al., 2024). Protecting lake wetlands requires comprehensive consideration of the overall water quality improvement of lake basins and the restoration of tidal flats and wetland vegetation to support the needs of fish throughout their life cycle. 2.3 Marine and coastal ecosystems Marine ecosystems include offshore, far sea, coral reefs and mangroves and are the largest aquatic habitat system in the world. Compared with freshwater systems, the ocean has a wider spatial scale and physical environmental heterogeneity, such as constant salinity but drastic temperature changes with latitude, and the current transport of matter (Griffin et al., 2025). Coastal ecosystems such as coral reefs, algae reefs and mangroves provide complex reef structures and juvenile habitats for numerous marine fish, supporting rich species diversity (Ghimayen et al., 2024). Marine habitats are usually relatively productive, with marine fish populations having high mobility and large-scale migration behavior. Due to human development of marine resources, marine ecosystems are also facing threats such as overfishing, marine pollution and climate change. In general, the habitat characteristics of marine ecosystems are vast areas, large depths, and abundant species, but due to marine biology and physics laws (such as light depth and nutrient distribution), fish have clear niche stratification and seasonal distribution laws in different sea areas. 3 Main Causes of Habitat Degradation 3.1 Water conservancy project construction and habitat breaking Water conservancy projects such as dams, reservoirs, and waterway improvement built by humans have changed the natural hydrological dynamics of the original rivers, causing habitat fragmentation and significant changes in the physical environment. The dam storage has slowed down the river flow rate, a large amount of sediment and nutrients deposited in the upstream reservoir area, and the sand content and nutrient input of the water in the downstream river section have sharply decreased, destroying the basement conditions and floating food chain required for fish to lay eggs (Dudgeon, 2024). After the basin connectivity is cut off, migratory fish have difficulty returning to the spawning area through obstacles, and their life cycle is seriously disrupted. Statistics found that
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