The Harm of Heavy Metal Pollution to Fish and Prevention and Control Measures
The Harm of Heavy Metals to Fish and Pollution Prevention and Control
The problem of heavy metal pollution in aquatic animals is a relatively common food hygiene issue worldwide. For example, in Canada, the United States, and Japan, due to river pollution, the mercury content in large quantities of fish and shellfish exceeded the specified limits. Problems such as these have brought serious harm to humans. The situation of heavy metal pollution in freshwater fish is very complex, with many types of toxic chemical pollutants involved and a very wide scope, commonly including mercury, arsenic, lead, chromium, cadmium, etc. Among these, arsenic is a non-metal, but it is often considered together with heavy metals.
1 Main Sources of Toxic Metals
The main sources of toxic metal pollution in aquatic animals can be divided into exogenous and endogenous (biological enrichment). Exogenous pollution sources include feed, water quality, sediment, air, drugs, etc.
1.1 Pollution of Toxic Chemical Substances in Feed In modern large-scale intensive aquaculture, feed used in intensive culture systems generally contains medicated additives that promote animal growth and prevent disease. Due to excessive use, drug residue problems arise that are toxic and have side effects on cultured aquatic animals. In addition, plant-based feed ingredients inevitably have certain amounts of pesticides applied during cultivation, resulting in pesticide residues in crops. Cereals and their outer skins, husks, etc. used as fish feed inevitably contain certain amounts of residual pesticides.
1.2 Pollution of Toxic Chemical Substances in Water Bodies Water not only has biological pollution but also serious chemical pollution problems. The main sources of heavy metal pollution in water bodies are untreated industrial wastewater and domestic sewage; oil tanker leaks, pesticides washed by rainwater into rivers, all can cause heavy metals such as mercury and cadmium to deposit in water body sediment. Heavy metal pollution of water quality is extremely complex. Commonly seen inorganic substances generally include heavy metals such as mercury, lead, copper, zinc, arsenic, vanadium, barium, as well as cyanides, fluorides, etc. Fish live in water throughout their lives. During respiration, the water they inhale containing some heavy metals enters the systemic circulation through the oral cavity, and the fish body surface also easily adsorbs heavy metals that enter the fish body.
1.3 Pollution of Toxic Chemical Substances in Soil The toxic chemical substances in soil are mainly heavy metal elements such as mercury, lead, cadmium, copper, manganese, zinc, arsenic, etc. The toxic chemical substances polluting soil mainly come from industrial "three wastes," pesticides, chemical fertilizers, garbage and sewage, as well as untreated manure. Due to rainwater washing, large amounts of heavy metal pollutants enter the soil, thereby indirectly causing heavy metal pollution of aquatic products.
1.4 Pollution of Toxic Chemical Substances in Air In human production and living activities, exhaust gas discharged from fuel combustion, and industrial waste gas containing heavy metals such as mercury, lead, cadmium, zinc, arsenic, vanadium, barium, etc. from factory production, are discharged into the air, gradually spreading to the surroundings, and through natural sedimentation or falling with raindrops onto soil and water bodies, causing pollution. The pollution of feed, air, water, and soil is not isolated but interconnected and mutually influencing. Pollutants transform and migrate among the four, forming cyclic environmental pollution, thereby causing direct or indirect pollution of aquatic products by toxic chemical substances.
2 Enrichment of Heavy Metals by Fish
2.1 Mercury in the polluted environment can all enter the human body through the food chain via biological enrichment Aquatic organisms have a very strong ability to accumulate mercury. Fish can accumulate mercury at concentrations 1000 times higher than the surrounding water environment. Experiments have shown that when the mercury content in water reaches 0.001-0.01 mg/L, through the transfer and concentration of Chlorella → Daphnia → goldfish, after 35 d the mercury content in the fish body can be 800 times that of the water. Humans who consume fish and shellfish with high methylmercury content over a long period can also suffer poisoning. Japan's "Minamata disease" was poisoning that occurred because residents around Minamata Bay regularly ate aquatic products severely polluted by mercury. The longer fish remain in mercury-containing water, the more mercury accumulates in their bodies. At the same time, microorganisms in the mucus on the fish body surface have a strong methylation effect and can convert inorganic mercury into methylmercury. Therefore, mercury in fish bodies exists almost entirely in the form of methylmercury, thereby increasing the danger to humans.
2.2 Aquatic organisms have a very strong concentrating effect on arsenic Generally, fish contain 1.1 mg/kg of arsenic. Shellfish in aquaculture areas with higher arsenic concentrations contain as much as 100 mg/kg of arsenic. Large sea shrimp can contain up to 174 mg/kg of arsenic, and small sea shrimp can contain 15-40 mg/kg.
2.3 Marine organisms have an enrichment effect on chromium, with concentration factors of: seaweed 60-120000, invertebrates 2-9000, fish 2000. Through enrichment and the food chain, the chromium content in animal foods is higher than in plant foods. For example, fish, meat, and eggs contain 0.1-0.5 mg/kg, while vegetables and fruits are all below 0.1 mg/kg.
2.4 Aquatic organisms can enrich cadmium. The concentration in fish and shellfish bodies is 4500 times higher than the water content. For example: seawater in Tokyo Bay, Japan contains 0.1-0.3 μg/L of cadmium, while the sediment contains as much as 1.0-1.7 mg/kg of copper. The copper content in fish and shrimp caught in that area is 0.1-0.3 mg/kg.
2.5 Lead entering water bodies can enter aquatic products through the enrichment effect of aquatic organisms. According to research, the enrichment coefficient of certain aquatic organisms for lead in seawater can be as high as over 1400 times.
3 Hygiene and Safety Standards for Aquatic Products
Canada and Japan set the maximum limit of mercury in fish at 0.1-0.15 mg/kg, and for shellfish at 0.5 mg/kg. Currently, China's food hygiene standards stipulate that the mercury content in fish and shellfish shall not exceed 0.3 mg/kg, of which methylmercury shall not exceed 0.2 mg/kg. In 1973, WHO (World Health Organization) based on analysis of mercury levels that can cause human poisoning, recommended that the allowable weekly mercury intake for adults should not exceed 0.3 mg, and methylmercury should not exceed 0.2 mg.
The standards for arsenic content in food vary among countries. The former Soviet Union stipulated that it must not be detected, Switzerland stipulated 1 mg/kg, the United States stipulated 0.5 mg/kg for meat and eggs, WHO and FAO proposed an allowable limit standard for arsenic in food of 0.1-1 mg/kg, WHO stipulated the allowable residue of arsenic in food as 0.1-1 mg/kg, and the United States calculated based on arsenic acid that meat and eggs shall not exceed 0.5 mg/kg.
In 1973, WHO and FAO proposed that the allowable weekly chromium intake per person be tentatively set at 0.0067-0.0083 mg/kg.
In 1972, WHO stipulated that the daily cadmium intake from various foods for each person was "none," and tentatively allowed an adult weekly intake of 0.4-0.5 mg. In 1988, FAO/WHO tentatively stipulated the weekly acceptable intake of silver as 7-8.3 μg/kg body weight. The 16th Joint Committee of WHO and FAO recommended that the allowable monthly intake of cadmium from food for each person be 400-500 pg.
WHO tentatively set the adult weekly tolerance for lead at 3 mg/kg body weight, and for children at 0.025 mg/kg body weight. The British government stipulated in 1975 that the lead content in aquatic food shall not exceed 1.0 mg/kg. China stipulates that the lead content in general food shall not exceed 1 mg/kg or 1 mg/L.
4 Prevention and Control Methods for Heavy Metal Pollution
Since the use of pesticides, antibiotics, sulfonamides and other agents is still one of the important measures for preventing and controlling animal and plant diseases and pests and promoting the growth of livestock, poultry and crops, the continued use of these chemical agents in agriculture, animal husbandry and fisheries is unavoidable, and the varieties and quantities used are still continuously increasing. The pollution of the environment by industrial "three wastes" remains a serious problem. Therefore, in order to protect the environment and prevent pollution of animal-derived foods, prevention and control measures need to be taken. The main measures can be summarized in the following aspects:
4.1 Actively treat industrial "three wastes" to eliminate toxic chemical substance pollution of animal-derived foods. Relevant industrial and mining enterprises should actively reform processes, eliminate industrial "three wastes" in the production process. At the same time, environmental analysis and food hygiene testing work should be actively carried out.
4.2 Strengthen management of the production and use of highly toxic pesticides. Stop the use of mercury- and arsenic-based pesticides. Actively develop new pesticide varieties with high efficiency, low toxicity, and low residues to reduce drug residues in aquatic products.
4.3 Strengthen the use and management of fish medications. Make necessary provisions for the types of drugs used, targets, methods of use, dosages, and conditions of use to prevent abuse. Antibiotics that easily lead to drug resistance should be prohibited. Withdrawal periods should be stipulated; there should be a certain withdrawal period before aquatic products are harvested.
4.4 Prohibit abuse of food additives. When food production must use additives, non-toxic, harmless, nutritious and hygienic considerations should be given top priority. Food additives should be used as little as possible or not at all. When using additives, the national food additive hygiene standards should be strictly implemented, and excessive abuse is strictly prohibited.
4.5 Actively carry out testing of pollution and residual toxins in animal-derived foods. Earnestly implement the provisions on allowable doses of toxic chemical substances in China's food hygiene standards, and they must not be exceeded. Strengthen testing of heavy metal content in fish and shellfish. If serious pollution of fish and shellfish in a certain water area is found, 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 aquatic products without toxic side effects will increase significantly, which requires sufficient green aquatic products to supply the market's needs. To obtain green aquatic products, there must be a supply of green feed, which also places higher demands on the research and development of aquatic feed. Sustainable fisheries require both ensuring or increasing aquaculture production and protecting the water ecological environment, and China's existing fishery production technology is difficult to meet this requirement. It is necessary to develop new-type aquatic feed with low pollution to water environments and harmless to both fish bodies and human bodies, namely "green, environmentally friendly new-type aquatic feed." Its core includes: not adding any drugs; feed formulation technology that applies the nutritional balance theory to formulate feed according to digestible nutrients; scientific feed processing technology such as extruded feed processing technology; and feeding technology that implements feed quota feeding according to the growth and development patterns of fish.
1 Main Sources of Toxic Metals
The main sources of toxic metal pollution in aquatic animals can be divided into exogenous and endogenous (biological enrichment). Exogenous pollution sources include feed, water quality, sediment, air, drugs, etc.
1.1 Pollution of Toxic Chemical Substances in Feed In modern large-scale intensive aquaculture, feed used in intensive culture systems generally contains medicated additives that promote animal growth and prevent disease. Due to excessive use, drug residue problems arise that are toxic and have side effects on cultured aquatic animals. In addition, plant-based feed ingredients inevitably have certain amounts of pesticides applied during cultivation, resulting in pesticide residues in crops. Cereals and their outer skins, husks, etc. used as fish feed inevitably contain certain amounts of residual pesticides.
1.2 Pollution of Toxic Chemical Substances in Water Bodies Water not only has biological pollution but also serious chemical pollution problems. The main sources of heavy metal pollution in water bodies are untreated industrial wastewater and domestic sewage; oil tanker leaks, pesticides washed by rainwater into rivers, all can cause heavy metals such as mercury and cadmium to deposit in water body sediment. Heavy metal pollution of water quality is extremely complex. Commonly seen inorganic substances generally include heavy metals such as mercury, lead, copper, zinc, arsenic, vanadium, barium, as well as cyanides, fluorides, etc. Fish live in water throughout their lives. During respiration, the water they inhale containing some heavy metals enters the systemic circulation through the oral cavity, and the fish body surface also easily adsorbs heavy metals that enter the fish body.
1.3 Pollution of Toxic Chemical Substances in Soil The toxic chemical substances in soil are mainly heavy metal elements such as mercury, lead, cadmium, copper, manganese, zinc, arsenic, etc. The toxic chemical substances polluting soil mainly come from industrial "three wastes," pesticides, chemical fertilizers, garbage and sewage, as well as untreated manure. Due to rainwater washing, large amounts of heavy metal pollutants enter the soil, thereby indirectly causing heavy metal pollution of aquatic products.
1.4 Pollution of Toxic Chemical Substances in Air In human production and living activities, exhaust gas discharged from fuel combustion, and industrial waste gas containing heavy metals such as mercury, lead, cadmium, zinc, arsenic, vanadium, barium, etc. from factory production, are discharged into the air, gradually spreading to the surroundings, and through natural sedimentation or falling with raindrops onto soil and water bodies, causing pollution. The pollution of feed, air, water, and soil is not isolated but interconnected and mutually influencing. Pollutants transform and migrate among the four, forming cyclic environmental pollution, thereby causing direct or indirect pollution of aquatic products by toxic chemical substances.
2 Enrichment of Heavy Metals by Fish
2.1 Mercury in the polluted environment can all enter the human body through the food chain via biological enrichment Aquatic organisms have a very strong ability to accumulate mercury. Fish can accumulate mercury at concentrations 1000 times higher than the surrounding water environment. Experiments have shown that when the mercury content in water reaches 0.001-0.01 mg/L, through the transfer and concentration of Chlorella → Daphnia → goldfish, after 35 d the mercury content in the fish body can be 800 times that of the water. Humans who consume fish and shellfish with high methylmercury content over a long period can also suffer poisoning. Japan's "Minamata disease" was poisoning that occurred because residents around Minamata Bay regularly ate aquatic products severely polluted by mercury. The longer fish remain in mercury-containing water, the more mercury accumulates in their bodies. At the same time, microorganisms in the mucus on the fish body surface have a strong methylation effect and can convert inorganic mercury into methylmercury. Therefore, mercury in fish bodies exists almost entirely in the form of methylmercury, thereby increasing the danger to humans.
2.2 Aquatic organisms have a very strong concentrating effect on arsenic Generally, fish contain 1.1 mg/kg of arsenic. Shellfish in aquaculture areas with higher arsenic concentrations contain as much as 100 mg/kg of arsenic. Large sea shrimp can contain up to 174 mg/kg of arsenic, and small sea shrimp can contain 15-40 mg/kg.
2.3 Marine organisms have an enrichment effect on chromium, with concentration factors of: seaweed 60-120000, invertebrates 2-9000, fish 2000. Through enrichment and the food chain, the chromium content in animal foods is higher than in plant foods. For example, fish, meat, and eggs contain 0.1-0.5 mg/kg, while vegetables and fruits are all below 0.1 mg/kg.
2.4 Aquatic organisms can enrich cadmium. The concentration in fish and shellfish bodies is 4500 times higher than the water content. For example: seawater in Tokyo Bay, Japan contains 0.1-0.3 μg/L of cadmium, while the sediment contains as much as 1.0-1.7 mg/kg of copper. The copper content in fish and shrimp caught in that area is 0.1-0.3 mg/kg.
2.5 Lead entering water bodies can enter aquatic products through the enrichment effect of aquatic organisms. According to research, the enrichment coefficient of certain aquatic organisms for lead in seawater can be as high as over 1400 times.
3 Hygiene and Safety Standards for Aquatic Products
Canada and Japan set the maximum limit of mercury in fish at 0.1-0.15 mg/kg, and for shellfish at 0.5 mg/kg. Currently, China's food hygiene standards stipulate that the mercury content in fish and shellfish shall not exceed 0.3 mg/kg, of which methylmercury shall not exceed 0.2 mg/kg. In 1973, WHO (World Health Organization) based on analysis of mercury levels that can cause human poisoning, recommended that the allowable weekly mercury intake for adults should not exceed 0.3 mg, and methylmercury should not exceed 0.2 mg.
The standards for arsenic content in food vary among countries. The former Soviet Union stipulated that it must not be detected, Switzerland stipulated 1 mg/kg, the United States stipulated 0.5 mg/kg for meat and eggs, WHO and FAO proposed an allowable limit standard for arsenic in food of 0.1-1 mg/kg, WHO stipulated the allowable residue of arsenic in food as 0.1-1 mg/kg, and the United States calculated based on arsenic acid that meat and eggs shall not exceed 0.5 mg/kg.
In 1973, WHO and FAO proposed that the allowable weekly chromium intake per person be tentatively set at 0.0067-0.0083 mg/kg.
In 1972, WHO stipulated that the daily cadmium intake from various foods for each person was "none," and tentatively allowed an adult weekly intake of 0.4-0.5 mg. In 1988, FAO/WHO tentatively stipulated the weekly acceptable intake of silver as 7-8.3 μg/kg body weight. The 16th Joint Committee of WHO and FAO recommended that the allowable monthly intake of cadmium from food for each person be 400-500 pg.
WHO tentatively set the adult weekly tolerance for lead at 3 mg/kg body weight, and for children at 0.025 mg/kg body weight. The British government stipulated in 1975 that the lead content in aquatic food shall not exceed 1.0 mg/kg. China stipulates that the lead content in general food shall not exceed 1 mg/kg or 1 mg/L.
4 Prevention and Control Methods for Heavy Metal Pollution
Since the use of pesticides, antibiotics, sulfonamides and other agents is still one of the important measures for preventing and controlling animal and plant diseases and pests and promoting the growth of livestock, poultry and crops, the continued use of these chemical agents in agriculture, animal husbandry and fisheries is unavoidable, and the varieties and quantities used are still continuously increasing. The pollution of the environment by industrial "three wastes" remains a serious problem. Therefore, in order to protect the environment and prevent pollution of animal-derived foods, prevention and control measures need to be taken. The main measures can be summarized in the following aspects:
4.1 Actively treat industrial "three wastes" to eliminate toxic chemical substance pollution of animal-derived foods. Relevant industrial and mining enterprises should actively reform processes, eliminate industrial "three wastes" in the production process. At the same time, environmental analysis and food hygiene testing work should be actively carried out.
4.2 Strengthen management of the production and use of highly toxic pesticides. Stop the use of mercury- and arsenic-based pesticides. Actively develop new pesticide varieties with high efficiency, low toxicity, and low residues to reduce drug residues in aquatic products.
4.3 Strengthen the use and management of fish medications. Make necessary provisions for the types of drugs used, targets, methods of use, dosages, and conditions of use to prevent abuse. Antibiotics that easily lead to drug resistance should be prohibited. Withdrawal periods should be stipulated; there should be a certain withdrawal period before aquatic products are harvested.
4.4 Prohibit abuse of food additives. When food production must use additives, non-toxic, harmless, nutritious and hygienic considerations should be given top priority. Food additives should be used as little as possible or not at all. When using additives, the national food additive hygiene standards should be strictly implemented, and excessive abuse is strictly prohibited.
4.5 Actively carry out testing of pollution and residual toxins in animal-derived foods. Earnestly implement the provisions on allowable doses of toxic chemical substances in China's food hygiene standards, and they must not be exceeded. Strengthen testing of heavy metal content in fish and shellfish. If serious pollution of fish and shellfish in a certain water area is found, 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 aquatic products without toxic side effects will increase significantly, which requires sufficient green aquatic products to supply the market's needs. To obtain green aquatic products, there must be a supply of green feed, which also places higher demands on the research and development of aquatic feed. Sustainable fisheries require both ensuring or increasing aquaculture production and protecting the water ecological environment, and China's existing fishery production technology is difficult to meet this requirement. It is necessary to develop new-type aquatic feed with low pollution to water environments and harmless to both fish bodies and human bodies, namely "green, environmentally friendly new-type aquatic feed." Its core includes: not adding any drugs; feed formulation technology that applies the nutritional balance theory to formulate feed according to digestible nutrients; scientific feed processing technology such as extruded feed processing technology; and feeding technology that implements feed quota feeding according to the growth and development patterns of fish.