The harm of heavy metals to fish and its pollution control

Heavy metal pollution of aquatic animals is a common food hygiene problem all over the world. Such as Canada. America. In Japan, the mercury content of a large number of fish and shellfish exceeds the prescribed limit due to river pollution. Such problems have brought serious harm to mankind. The pollution of heavy metals in freshwater fish is very complicated. there are many kinds of toxic chemicals involved in a wide range, such as mercury, arsenic, lead, chromium, pot and so on, in which arsenic is non-metallic, but it is often considered as heavy metals.

1 main sources of toxic metals

The main sources of toxic metals polluted by aquatic animals can be divided into exogenous and endogenous (biological enrichment). Exogenous pollution sources include feed, water quality, sediment, air, drugs and so on.

1.1 pollution of toxic chemicals in feed in modern large-scale intensive farming, feeds used in intensive farming systems are generally added with pharmaceutical additives that promote animal growth and disease prevention, drug residues that are toxic and side effects to farmed aquatic animals due to overuse. In addition, plant feed materials are bound to apply a certain amount of pesticides in the planting process, resulting in pesticide residues in crops, cereals and their skins and shells as fish feed must contain a certain amount of pesticide residues.

1.2 pollution of toxic chemicals in water body there is not only biological pollution, but also serious chemical pollution. The main source of heavy metal pollution in water body is untreated industrial wastewater and domestic sewage. Oil leakage and pesticides washed in rivers with Rain Water can make heavy metals such as mercury and radiation deposit in the bottom of water body. The pollution of heavy metals in water quality is extremely complex. the common inorganic substances are mercury, lead, copper, zinc, arsenic, alum, barium and other heavy metals as well as cyanide, fluoride and so on. Fish live in water all their lives, and some of the heavy metals in the water inhaled during breathing enter the systemic circulation through the mouth, and the surface of the fish is also easy to absorb heavy metals and enter the fish body.

1.3 toxic chemicals in the contaminated soil are mainly heavy metal elements such as mercury, lead, pot, copper, manganese, zinc, arsenic and so on. The toxic chemicals in the soil mainly come from industrial "three wastes", pesticides, chemical fertilizers, garbage and sewage, as well as unharmless faeces. Due to the erosion of Rain Water, a large number of heavy metal pollutants enter the soil, which indirectly causes heavy metal pollution of aquatic products.

1.4 pollution of toxic chemicals in the air is produced by human beings. Life activities, fuel combustion emissions, factory production contains heavy metals such as mercury, lead, pot, zinc, arsenic. Industrial waste gases such as alum and barium are discharged into the air and gradually spread around, causing pollution by natural settlement or falling with raindrops in soil and water. The pollution of feed, air, water and soil is not isolated, but interrelated and influenced each other. Pollutants transform and migrate each other to form circular pollution of the environment, resulting in direct or indirect pollution of aquatic products by toxic chemicals.

2 enrichment of heavy metals by fish

2.l Mercury polluting the environment can enter the human body through the food chain through biological enrichment and has a strong ability to accumulate mercury. Fish can accumulate 1000 times more mercury than the surrounding water environment. The results showed that when the content of mercury in water reached 0.001~0.01mg/L, through the transfer and concentration of Chlorella → water flea → goldfish, the content of mercury in fish was 800 times higher than that in water after 35 days. Poisoning can also be caused by long-term consumption of fish and shellfish with high levels of methyl mercury. Japan's "water disease" is caused by the fact that residents around Shuijiewan often eat this aquatic organism seriously polluted by mercury. The longer the fish is in mercury-containing water, the more mercury the fish contains. At the same time, the microorganisms in the surface mucus of fish have a strong methylation, which can convert inorganic mercury into methylmercury. Therefore, almost all mercury in fish exists in the form of methylmercury, which increases the harm to human beings.

2.2 Aquatic organisms have a strong concentration of arsenic. General fish contain 1.1 mg/kg of arsenic. The content of arsenic in shellfish in the culture area with high arsenic concentration is as high as 100 mg/kg. The arsenic content of sea shrimp can reach 174 mg/kg, and that of small sea shrimp can reach 15~40mg/kg.

2.3The enrichment coefficients of chromium in marine organisms are as follows: seaweed 60, 120, 000, invertebrates 2, 000, fish 2000. Through enrichment and the role of food chain, the content of chromium in animal food is higher than that in plant food, such as fish, meat and eggs are 0.l~0.5mg/kg, vegetables and fruits are all below 0.1mg/kg.

2.4. the concentration of aquatic organisms in pot fish and shellfish is 4500 times higher than that of water. For example, the seawater of Tokyo Bay in Japan contains 0.1 ~ 0.3 μ g / L of cadmium, while the content of copper in the sediment is as high as 1.0~1.7mg/kg. The copper content in the fish and shrimp caught in this area is 0.l~0.3 mg/kg.

Lead entering the water body can be entered into aquatic products through the enrichment of aquatic organisms. According to research, the enrichment coefficient of some aquatic organisms to lead in seawater can be as high as 1400 times.

(3) hygienic and safety standards for aquatic products

Canada and Japan set the maximum limit of mercury in fish as 0.1~0.15mg/kg and in shellfish as 0.5mg/kg. At present, China's food hygiene standards stipulate that the content of mercury in fish and shellfish should not exceed 0.3mg/kg, and methyl mercury should not exceed 0.2mg/kg. In 1973, WHO (World Health Organization) recommended that adults' weekly intake of mercury should not exceed 0.3mg and methylmercury should not exceed 0.2mg, based on an analysis of mercury levels that can cause human poisoning.

The standard of arsenic content in food varies from country to country. The former Soviet Union stipulated that it should not be detected, Switzerland stipulated that it should not be detected, and the United States stipulated that meat and eggs were 0.5mg/kg. WHO and FAO proposed that the allowable limit of arsenic in food is 0.1mg / kg. Who stipulates that the allowable residue of arsenic in food is 0.l~lmg/kg, while meat and eggs in the United States should not exceed 0.5mg/kg according to arsenic acid.

In 1973, WHO and FAO proposed that the weekly allowable intake of chromium per person was tentatively set at 0.0067~0.0083mg/kg.

In 1972, the WHO stipulated that the daily intake of cadmium in various foods per person was "none", and the weekly intake of adults was temporarily allowed to be 0.4-0.5 mg. In 1988, the FAO/WHO temporarily stipulated that the weekly acceptable intake of silver was 7 ~ 8.3 μ g / kg body weight. The 16th Joint Committee of WHO and FA0 recommended that the amount of steel allowed from food per person per month be 4OO~500 pg.

WHO tentatively determined that the weekly tolerance of lead was 3 mg/kg body weight for adults and 0.025mg/kg body weight for children. In 1975, the British government stipulated that the content of lead in aquatic food should not exceed 1.0 mg/kg. Our country stipulates that the lead content in general food should not exceed 1 mg/kg or 1 mg/L.

4 methods of prevention and control of heavy metal pollution

As the use of pesticides, antibiotics, sulfonamides and other pesticides is still one of the important measures to control animal and plant diseases and insect pests and promote the growth of livestock, poultry and crops months ago, the continued use of these chemicals in agriculture, animal husbandry and fishery is inevitable, and the variety and quantity used are constantly increasing. The environmental pollution caused by industrial "three wastes" is still a serious problem. Therefore, in order to protect the environment and prevent the pollution of animal food, prevention and control measures need to be taken, and the main measures can be summarized as follows:

4.l actively control the industrial "three wastes" to eliminate their toxic chemical pollution to animal food, and the relevant industrial and mining enterprises should actively reform the process. Eliminate the industrial "three wastes" in the process of production. At the same time, it is necessary to actively carry out environmental analysis and food hygiene testing.

4.2 strengthen the management of the production and use of highly toxic pesticides to stop the use of pesticides prepared with mercury and arsenic. Actively develop new varieties of pesticides with high efficiency, low toxicity and low residues in order to reduce drug residues in aquatic products.

4.3 strengthen the use and management of fish drugs to make necessary regulations on the types, targets, methods, doses and conditions of use of drugs used, so as to prevent abuse. The use of antibiotics that can easily lead to drug resistance should be prohibited. The drug withdrawal period is stipulated, and there should be a certain drug withdrawal period before the capture of aquatic products.

4.4 prohibition of abuse of food additives when additives must be used in food production, non-toxic, harmless, nutrition and hygiene should be put in the first place. Try not to use or use less food additives. When using additives, the hygienic standards for food additives set by the state shall be strictly followed, and excessive abuse shall be strictly prohibited.

4.5 actively carry out the detection of contamination and residual toxins in animal food and conscientiously implement the provisions on the allowable dose of toxic chemical substances in China's food hygiene standards, which shall not exceed. Strengthen the detection of heavy metal content of fish and shellfish. If it is found that the pollution of fish and shellfish in a certain water area is serious, fishing, sale and consumption should be prohibited.

With the improvement of people's living standards and China's accession to the WTO, the demand for safe and non-toxic aquatic products will significantly increase, which requires sufficient supply of green aquatic products to the market. In order to obtain green aquatic products, there must be a supply of green feed, which also puts forward higher requirements for the research and development of aquatic feed. Sustainable fishery requires not only to ensure or increase aquaculture production, but also to protect the ecological environment of water areas, but the existing fishery production technology in China is difficult to meet this requirement. It is necessary to develop a new type of aquatic feed that has little pollution to the water environment and is harmless to fish and human body, that is, "green and environment-friendly new aquatic feed", whose core includes: no drugs are added; feed preparation technology based on digestible nutrients by applying nutrition balance theory; scientific feed processing technology such as expanded feed processing technology; implementation of feed quota feeding technology according to the law of fish growth and development.