International Journal of Aquaculture, 2025, Vol.15, No.4, 175-183 http://www.aquapublisher.com/index.php/ija 178 violently, leading to a mismatch of timing of migration and reproduction, further weakening the ability of fish population recovery (Girkar et al., 2017). Habitat degradation is the result of a combination of multiple factors. Human engineering activities, pollution emissions, overuse of resources and climate drive have jointly accelerated the deterioration of fish habitat in aquatic ecosystems. 4 Effects of Habitat Degradation on Fish Populations 4.1 Fish diversity and distribution range decline Habitat degradation has led to a significant decline in fish species diversity. A large number of studies and monitoring results show that global fish populations have shown a widespread decline due to the long-term deterioration of the aquatic environment (Jan et al., 2023). For example, a global assessment showed that between 1970 and 2020, the population of migratory freshwater fish fell by an average of 81%, and the number of many once prosperous species decreased dramatically or even endangered. River breakage and habitat isolation have reduced the distribution range of many fish species, not only the population has decreased, but also the number of cases of regional extinctions has increased (Grokhovska and Konontsev, 2020). Statistics found that due to dams and other river barriers, more than half of the assessed freshwater fish populations are at risk of extinction within certain geographical ranges, and on average, each species may lose about 3.3% of their habitat range. Degradation also leads to simplification of the food web structure and loss of ecological niches. The original rich fish ecosystem succession often degenerates into simple systems dominated by a few dominant species. 4.2 Reduced fish population and resources As habitat quality declines, the number of individual fish and overall resource volumes have significantly decreased. A large number of cases show that fish catches and biomass are declining in contaminated or modified water bodies. Habitat deterioration often leads to a decrease in fish survival rate, a decrease in the survival rate of juvenile fish, and difficulty in population renewal. In highly polluted or vegetative waters, the survival environment of fish eggs and juvenile fish is destroyed, weakening the fish's replenishment capacity; the cut-off of rivers blocks the migration route of migratory species, resulting in failed reproduction. The study pointed out that among the multiple cases reported by the World Fish Migration Foundation, river fish populations have dropped dramatically over the past fifty years, with few parent fish left in many basins to breed populations. The decline in fish resources will also affect local fishermen's livelihoods, and fishing catches continue to decline in many areas, further leading to additional human pressure on water resources and the destruction of ecosystem services (Crane et al., 2020). 4.3 Fish behavior and ecological functions are disturbed Changes in habitat structure interfere with the life history process and ecological behavior of fish. For example, the migration and spawning behavior of fish are highly dependent on river connectivity and specific hydrological conditions. Once the river channel is blocked by the dam, the fish cannot complete the migration back to the spawning area, thus destroying the entire life cycle. In addition, water quality changes can also affect fish migration timing, foraging and social behavior (Napit, 2024); high temperature and low oxygen conditions may force fish to adjust their distribution depth or migrate to areas with better water quality. Habitat degradation also alters the interactions of fish with other species, such as benthic fish in eutrophied lakes that may trigger swamp cycles, aggravate water turbidity, interfere with plant growth, and thus affect other fish species (Einarsson, 2016). Overall, habitat degradation has a comprehensive adverse impact on fish population structure, population dynamics and ecological functions by destroying the habitat conditions and life cycle environment on which fish rely on. 5 Aquatic Habitat Restoration Methods 5.1 Engineering ecological restoration In response to habitat damage caused by water conservancy projects, engineering ecological restoration methods focus on restoring water connectivity and physical structures, such as demolishing unnecessary dams, building fish paths or cascade aqueducts. Demolition of aging dams can quickly restore river continuity and improve the
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