International Journal of Aquaculture, 2025, Vol.15, No.3, 135-148 http://www.aquapublisher.com/index.php/ija 143 the original species of multiple lines and conduct hybridization experiments. In terms of reservoir cage farming, the government strictly controls the cage delivery density and advocates "multi-nutritional integrated aquaculture (IMTA), that is, planting aquatic plants or stocking filter-feeding fish around the cage to absorb breeding waste and reduce environmental impact. Some cases show that in the reservoirs in northeastern Brazil, farms combine tilapia with local herbivorous carp and shellfish, and the water quality indicators have been significantly improved and the comprehensive benefits of breeding have been improved (Camarago and Amorim, 2020). In addition, Brazil pays great attention to preventing the impact of foreign invasion of tilapia on local organisms. Although tilapia has established self-producing groups in Brazilian waters, the government has explicitly prohibited the introduction of tilapia in ecologically sensitive areas (such as the Amazon River Basin) to protect the unique local fish diversity. 7 Ecological Risks and Sustainable Breeding Challenges 7.1 Ecological invasion caused by the spread of alien species With its tenacious survival ability, tilapia has also become an alien fish with invasive potential while introducing it into breeding. Escaped tilapia populations have been found in the natural waters of many tropical and subtropical countries, which have had some impact on the local ecosystem. The risk of ecological invasion of tilapia is mainly reflected in the following aspects: After tilapia enters natural water bodies, due to lack of natural enemies and strong reproductive power, the population can grow rapidly, which may pose a threat to local fish through competition and predation. In the Shanmei Reservoir in South China, the invaded Qi's tilapia and hybrid tilapia have formed reproductive populations, occupying the main proportion of catches, causing significant squeeze on indigenous fish resources (Shuai and Li, 2022). Studies in Ethiopia, the Philippines and other places have also reported that the invasion of tilapia has led to a decline in some local cichlid populations. Secondly, tilapia can become a carrier of pathogens, bringing diseases in the breeding environment into the wild. The study found that Nile tilapia may introduce parasitic monoclonal flukes (such as Cichlidogyrus sclerosus) during foreign invasion, and local native fish have experienced healthy declines after infection with these parasites. The ecological engineering role of tilapia will also change the water environment. They feed on plankton and debris, and high-density tilapia populations may increase water turbidity and inhibit aquatic vegetation growth through feeding and agitation, thereby affecting ecosystem structure (Cassemiro et al., 2017). 7.2 The pressure of intensive aquaculture on water quality and soil systems Although large-scale intensive farming of tilapia increases output per unit area, it inevitably generates environmental pressure, which is mainly reflected in the pollution and degradation of water quality and bottom quality. In high-density farming ponds, large amounts of bait and fish excretion will lead to eutrophication in the water: the content of ammonia nitrogen, nitrite and phosphorus increases, and the over-propagation of planktonic algae, causing large fluctuations in dissolved oxygen in the water day and night and even hypoxia. When the water quality of the pond deteriorates, not only will the fish be damaged in health, but pollutants will also be discharged into the surrounding water. Research shows that every ton of tilapia produced produces about 3 050 kg of nitrogen and 57 kg of phosphorus. If improperly disposal is discharged into rivers and lakes, it will cause algae blooms and water quality in local waters. In addition, in the long-term mode of not replacing the pool water or recycling, toxic metabolites such as hydrogen sulfide may accumulate at the bottom of the pool, threatening fish survival. High-density farming can also lead to deterioration of the pond bottom mud environment. The residual bait and fish manure are deposited at the bottom, decomposed by microorganisms to consume oxygen, forming a highly reducing black and odorous sludge, which not only releases harmful gases such as ammonia nitrogen and methane, but also destroys the beneficial microbial community at the bottom of the pool. The organic matter content in the pond mud in some aging tilapia fish in southern China is much higher than that in normal soil, and it is in a sour and odorous state, and it needs to be regularly silted and changed to the bottom. In addition, intensive farming may induce heavy metal and drug residue problems. Heavy metal elements entrained in feed and water sources are enriched in fish bodies and sediments, which may ultimately affect human health through the food chain (Melo Júnior et al., 2023; Shafiujjaman et al., 2024).
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