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The self-pollution comes mainly from the discharge of solid and liquid wastes from the breeding process, such as residual baits, animal feces and excrement, and dissolved components of solid materials. In addition, fertilisers, disinfectants, antibiotics, and other drugs that are introduced during the farming process are also part of the source of self-pollution.
The self-pollution feeding culture for bait farming mainly includes cage culture and pond culture.
1, cage culture
The waste produced by cage culture is mainly residual fish, fish feces and excrement. The ultimate environmental impact of these wastes is mainly contained nutrients such as nitrogen, phosphorus and organic matter. During the aquaculture process, no matter whether the fish, fish, or compound feed that were crushed and processed by small trash fish can not be fully utilized after feeding, the non-feeding parts and fish feces enter the water body and deposit to the bottom layer. The amount of solid waste is related to cage culture methods, culture types, food types, and management methods. The rate of deposition of cage fish farms in foreign countries was reported to be 5 kg/m 2 /day, and the deposition rate of solid waste of rainbow trout in cage culture was 1496 g/m 2 /day. It is estimated that only 27% to 28% of the nitrogen in the artificially fed seawater cage fishery system in the form of bait and fry is recovered through fish harvest, and 23% is accumulated in sediments. One of the effects of enrichment of organic matter in sediments is the increase in oxygen consumption of heterotrophic organisms, the decomposition of sediments, the release of inorganic nutrients such as nitrogen and phosphorus, and stimulation of the growth of aquatic plants and algae, in the absence of oxygen. It also releases toxic ammonia and sulfides that impede the growth and health of fish. Since soluble inorganic nitrogen is a limiting nutrient for the growth of coastal phytoplankton, the artificial feeding activities in cage culture areas involve the amount of nitrogen, phosphorus, vitamins, iron, manganese, and other trace elements brought by nearby sea areas, especially nitrogenous nutrients. Salt provides the necessary material basis for the proliferation of phytoplankton and the occurrence of red tides. In addition, similar to the shellfish's own pollution, the cage culture will also affect the benthic community changes. Through research, it has been found that with the change of the oxygen saturation of the substrate, benthic species continuously undergo succession. Animal species and biomass decrease. This is mainly due to the enrichment of sediment organics and increased oxygen consumption. When the oxygen demand exceeds the supply, the bottoms will become anoxic, thus changing the chemistry and ecology of the sediment. At present, the foods used in cage culture in coastal Guangdong are basically chilled fish, and artificial feeds are rarely used. The waste of food and self-pollution caused by feeding the fish are more serious. According to the survey, the pollution indicators of water quality and sediment quality in cage aquaculture sea areas such as Yulin Bay, Cangqian Bay, and Dapeng Bay have improved to varying degrees compared to non-farming areas. With the development of cage aquaculture, the environmental impact of self-pollution will further increase.
2, pond fish
The intensive culture pond ecosystem is a semi-manually controlled ecosystem. Artificial stocking makes the fish and shrimp population cultivated an absolute dominant population in the system. Artificial feeding is the main source of energy. Like other artificially-cultivated ecosystems, there are also problems with continuous bait production in pond culture. Residue baits are ponds. The main source of breeding their own pollution. For example, in shrimp aquaculture, artificial input of shrimp into the shrimp pond accounts for about 90% of the total input nitrogen, of which only 19% is converted into nitrogen in the shrimp body, and most of the rest is about 62%-68% accumulated in the sludge at the bottom of the shrimp pond. In addition, 8% to 12% of the suspended particulate nitrogen, dissolved organic nitrogen, dissolved inorganic nitrogen and other forms exist in water. Even in the best managed shrimp farms, as many as 30% of the feed will never be eaten by shrimp. The nutrients and organic matter dissolved in the shrimp farms will affect the nutritional level of the cultivated water and cause pollution to the shrimp pond itself. Important factor. Organic residues such as residual baits and excrement in shrimp ponds can also produce a large amount of ammonia nitrogen after being decomposed by microorganisms in seawater. Ammonia nitrogen is a common toxic substance in shrimp ponds. Not only does it have a lethal effect on shrimps at high concentrations, even at safe concentrations. The range also has significant effects on the physiological function of shrimps, such as increasing the oxygen consumption of shrimp, impeding its nitrogen excretion, reducing its adenosine triphosphatase activity, and destroying its ability to regulate osmotic adjustment; more importantly, shrimp under the stress of ammonia nitrogen, The decline in disease resistance is more prone to disease. Similar to the shellfish culture, the decomposed bottom bait in shrimp ponds will also cause a drop in dissolved oxygen and pH in seawater. Studies have shown that excess shrimp bait breaks down in the bottom of the pool and the dissolved oxygen in seawater drops from 8 mg/l to zero within 24 hours, and the pH drops from 8 to 6. The self-pollution of the shrimp farming industry has also contributed to the occurrence of red tides to a large extent and may cause large-scale infectious shrimp disease. Nitrogen and phosphorus nutrients dissolved in the bait are not only aquaculture environment itself, but also a source of pollution in the adjacent sea area. There is a positive correlation between the aquaculture area of ​​the shrimp farm in the Inner Gulf, Thailand, and the average nitrogen concentration in the bay, suggesting that nitrate may be derived from nitrogen from shrimp farms. The nutrient load discharged from shrimp ponds also increases the primary productivity in the adjacent sea areas, causing eutrophication of the water bodies. In some areas, the pollutants discharged from shrimp farms have exceeded the receiving capacity of the nearby water bodies.
The development of marine aquaculture and marine environmental protection are the unity of opposites. On the one hand, marine aquaculture requires an environment with less pollution and fresh water. Seawater pollution is an important factor constraining the development of marine aquaculture. However, mariculture must be accompanied by self-pollution. The self-pollution not only affects the aquaculture industry itself, but also poses an increasingly serious threat to the offshore marine environment, and ultimately jeopardizes its further development. Therefore, we must do our best to control and reduce the occurrence of self-pollution, and seek a way to develop the sustainable development of mariculture and environmental protection.
Taken together, prevention and reduction of self-pollution in mariculture should be considered from the following aspects.
1. To formulate and improve various related laws and regulations, and to strengthen macroeconomic regulation and control, we should learn from the experience of advanced countries in management, and formulate and improve relevant laws and regulations in light of China's actual conditions, and strengthen law enforcement to achieve the purpose of macro-control. It can be started from two aspects. First, the overall planning of the breeding area, the implementation of environmental impact assessment, determine the environmental capacity or aquaculture capacity. The second is to introduce the management methods of industrial sewage into the aquaculture industry, and to collect seawater aquaculture sewage charges or aquaculture sewage discharge taxes, which will play a role in controlling and reducing the impact of marine aquaculture on the environment. The environmental capacity of aquaculture in a bay or a coastal area can be understood as the maximum amount of aquaculture wastewater that can be borne by a water body under water quality indicators and hydrodynamic conditions determined by its functional planning and use. Stimulated by the pursuit of economic benefits, many cultivation and self-pollution are the result of excessive breeding capacity. Therefore, through the research of aquaculture capacity, the culture density can be controlled within the water body carrying capacity so that the cultured pollutants will not exceed the self-purification capability of the water body, such as the material circulation flux that the water exchange can provide, and the excess nutrients of other organisms in the water body. Absorptive capacity and so on. Therefore, the determination of aquaculture capacity is a need for sustainable development of the aquaculture industry.
2. Adjusting and optimizing the culture structure The use of the physiological characteristics of different cultured organisms to carry out multi-species polyculture will not only be beneficial to the ecological balance of the cultured organisms themselves and the aquaculture waters, but also make full use of and bring into play the productive potential of the aquaculture waters. According to the characteristics of higher nutritional level of intensive aquaculture water, the use of large seaweeds to absorb excess nutrients and intercropping the farmed animals and algae can reduce the organic matter and nutrient load of the water body along with the harvest of algae, and at the same time improve the economic benefits of breeding. Ecological Benefits. At present, there are more common shellfish polyculture, mixed polytrophy of shrimp and algae, and polyculture of fish and algae. For example, between kelp and mussels, the income is 47% higher than that of a single seaweed in the same sea area, more than twice that of a single mussel; the seaweed and scallop are bred, and its income is 35 times higher than that of a single seaweed. Scallops increased by 29%. In addition, shrimp shellfish polyculture, polyculture of shrimp and fish, etc., can use filter-feeding shellfish to remove particulate organic matter and microalgae in shrimp ponds, and use omnivorous fish to remove shrimp bait and organic debris. Can reduce the incidence of shrimp. Develop seabed growth, use and reduce the underlying sedimentary nutrients. The residual baits in the intensive culture process and the metabolites of the cultured animals mostly settle to the bottom of the culture zone and the bottom sediment, making the organic detritus and biological communities at the bottom abundant. Through the introduction of artificial reefs and seeded seeding methods, it is possible to develop the increased culture of benthic economic species such as abalone, sea urchins, and sea cucumbers. It is a new breeding route with low investment, high profitability, and high potential. Reduce the self-pollution of the bottom layer of water.
3. Improve feeding techniques, improve feed quality, and the production of scientifically reared residual foods is an important factor in the formation of aquaculture's own pollution. Therefore, improving feed quality, such as feed particle binding, conversion, etc., will certainly reduce the water body. Pollution is of great benefit. For example, in Finland, by increasing the conversion rate of bait and reducing the phosphorus content, the aquaculture production increased three times in 1987-1991, and the phosphorus emission from fish farming only increased by 30%. In shrimp farming, adding 50% of photosynthetic bacteria to the feed and regularly putting sorbents into the pool can absorb and adsorb hydrogen sulfide, amines and eliminate pathogenic viruses and other harmful substances and microorganisms in the water. Regular application of beneficial microbial preparations in aquaculture ponds can serve to degrade organic matter and improve the quality of cultured water and sediment, thereby reducing the degree of self-pollution. In addition, by increasing the feeding technology, such as timing, fixed-point feeding, determining the appropriate amount of feeding, setting the feeding station, etc., it can also reduce the waste of bait and the amount of residual bait generated. 4. Treatment of aquaculture wastewater After aquaculture wastewater is discharged after purification, it can effectively reduce the nutrient load in the aquaculture sea area and adjacent waters. The simplest method is precipitation treatment, that is, to allow the waste water to be put into the sedimentation tank for a few days before discharge to allow the suspended matter to settle at the bottom of the tank. The more complete wastewater treatment plant consists of a particulate separation device, a bioreactor (removal of ammonia nitrogen and organic matter), and an oxygenation device. Treated wastewater can significantly reduce particulate suspended matter, nitrogen, phosphorus and chemical oxygen demand. Recently, the author has also achieved good results in the treatment of shrimp aquaculture wastewater with large-scale algae, filter-feeding shellfish and microbial preparations.
The harm and countermeasures of self-pollution in feeding culture
The aquaculture self-pollution refers to the state of the water body affected by the ecological function of the water body due to the fact that the aquaculture activity itself has caused the concentration of pollutants in the aquaculture water body and its adjacent water areas to exceed the normal level.