Biomagnification of mercury and other toxins in fish


BACKGROUND - In the 1950s, inhabitants of the seaside town of Minamata, on Kyushu island in Japan, noticed strange behavior in animals. Cats would exhibit nervous tremors, and dance and scream. Within a few years this was observed in other animals; birds would drop out of the sky. Symptoms were also observed in fish, an important component of the diet, especially for the poor. When human symptoms started to be noticed around 1956 an investigation began. Fishing was officially banned in 1957. It was found that the Chisso Corporation, a petrochemical company and maker of plastics such as vinyl chloride, had been discharging heavy metal waste into the sea for decades. They used mercury compounds as catalysts in their syntheses. It is believed that about 5,000 people were killed and perhaps 50,000 have been to some extent poisoned by mercury. Mercury poisoning in Minamata, Japan, is now known as Minamata disease.



It is an unfortunate fact that fish and shellfish concentrate mercury in their bodies by a process called biomagnification. This is often in the form of methylmercury, a highly toxic organic compound of mercury. Fish products have been shown to contain varying amounts of heavy metals, particularly mercury and fat-soluble pollutants from water pollution. Species of fish that are long-lived and high on the food chain, such as marlin, tuna, shark, swordfish, king mackerel and tilefish (Gulf of Mexico) contain higher concentrations of mercury than others.

Mercury is known to bioaccumulate in humans, so bioaccumulation in seafood carries over into human populations, where it can result in mercury poisoning. Mercury is dangerous to both natural ecosystems and humans because it is a metal known to be highly toxic, especially due to its ability to damage the central nervous system. In human-controlled ecosystems of fish, usually done for market production of wanted seafood species, mercury clearly rises through the food chain via fish consuming small plankton, as well as through non-food sources such as underwater sediment.

The presence of mercury in fish can be a particular health concern for women who are or may become pregnant, nursing mothers, and young children.


Legislation on a global scale is believed by some to be needed for this issue because mercury pollution is estimated to be so far-reaching. Pollution from one country does not stay localized to that country. Despite the need by some, international regulation has been slow to take off. The first forms of international legislation appeared in the 1970s, beginning as agreements about shared bodies of water. The next step was the Stockholm Declaration, which urged countries to avoid polluting the oceans by dumping. The 1972 Oslo Convention and the 1974 Paris Convention were adopted by parts of Europe. Both lessened polluting the ocean with mercury, the former by banning the dumping of ships and aircraft into the ocean and the latter by obligating participants to reduce land-based pollution on coastlines. The first real global legislation regarding mercury pollution was the Basel Convention of 1989. This convention attempts to reduce the movement of mercury across borders and primarily regulates the import and export of toxic chemicals, including mercury. In 1998 the Convention on Long-Range Transboundary Air Pollution was adopted by most of the European Union, the United States, and Canada. Its primary objective is to cut emissions of heavy metals. The convention is the largest international agreement on mercury established to date. In the early 21st century, the focus of mercury regulation has been on voluntary programs. The next phase in legislation is a global effort, and this appears to be what the Minamata Convention hopes to accomplish. The Minamata Convention, named after the Japanese city that suffered horribly from mercury pollution, has taken four years of negotiation but was finally adopted by delegates from over 140 countries. The convention will come into power after 50 countries have signed it. The Minamata Convention will require all participants to eliminate, where possible, the release of mercury from small-scale gold mining. It will also require a sharp reduction in emission from coal burning.


There are three types of mercury emission: anthropogenic, re-emission, and natural, including volcanoes and geothermal vents. Anthropogenic sources are responsible for 30% of all emissions, while natural sources are responsible for 10%, and re-emission accounts for the other 60%. While re-emission accounts for the largest proportion of emissions, it is likely that the mercury emitted from these sources originally came from anthropogenic sources.

Anthropogenic sources include coal burning, cement production, oil refining, artisan and small-scale gold mining, wastes from consumer products, dental amalgam, the chlor-alkali industry, production of vinyl chloride, and the mining, smelting, and production of iron and other metals. The total amount of mercury released by mankind in 2010 was estimated to be 1,960 metric tons. The majority of this comes from coal burning and gold mining, accounting for 24% and 37% of total anthropogenic output respectively.

Re-emission, the largest emitter, occurs in a variety of ways. It is possible for mercury that has been deposited in soil to be re-emitted into the mercury cycle via floods. A second example of re-emission is a forest fire; mercury that has been absorbed into plant life is re-released into the atmosphere. While it is difficult to estimate the exact extent of mercury re-emission, it is an important field of study. Knowing how easily and how often previously emitted mercury can be released helps us learn how long it will take for a reduction in anthropogenic sources to be reflected in the environment. Mercury that has been released can find its way into the oceans. A 2008 model estimated the total amount of deposition into the oceans that year to be 3,700 metric tons. It is estimated that rivers carry as much as 2,420 metric tons. Much of the mercury deposited in the oceans is re-emitted, however; as much as 300 metric tons is converted into methyl mercury. While only 13% of this finds its way into the food chain, that is still 40 metric tons a year.

Much (an estimated 40%) of the mercury that eventually finds its way into fish originates with coal-burning power plants and chlorine production plants. The largest source of mercury contamination in the United States is coal-fueled power plant emissions. Chlorine chemical plants use mercury to extract chlorine from salt, which in many parts of the world is discharged as mercury compounds in waste water, though this process has been largely replaced by the more economically viable membrane cell process, which does not use mercury. Coal contains mercury as a natural contaminant. When it is fired for electricity generation, the mercury is released as smoke into the atmosphere. Most of this mercury pollution can be eliminated if pollution-control devices are installed.

Mercury in the United States frequently comes from power plants, which release about 50% of the nation's mercury emissions. In other countries, such as Ghana, gold mining requires mercury compounds, leading to workers receiving significant quantities of mercury while performing their jobs. Such mercury from gold mines is specifically known to contribute to biomagnification in aquatic food chains.

The farming of aquatic organisms, known as aquaculture, often involves fish feed that contains mercury. A study by Jardine has found no reliable connection between mercury in fish food affecting aquaculture organisms or aquatic organisms in the wild. Even so, mercury from other sources may still affect organisms grown through aquaculture. In China, farmed fish species, such as bighead carp, mud carp, and Siniperca chuatsi, carried 90% of total mercury content in all of the measured fish in a study by Cheng. This study also concluded that mercury bioaccumulates through food chains even in controlled aquaculture environments. Both total mercury and methyl mercury absorption was found to be derived from sediments containing mercury, not mainly from fish feed.

The Hawaii Institute of Marine Biology has noted that fish feed used in aquaculture often contains heavy metals such as mercury, lead, and arsenic, and has dispatched these concerns to organizations such as the Food and Agriculture Organization of the United Nations.

Elemental mercury often comes from coal power plants, and oxidized mercury often comes from incinerators. Oil-fired power plants also contribute mercury to the environment. The energy industry therefore is a key player in the introduction of mercury into the environment. When addressing the issue of reducing seafood mercury bioaccumulation on a global scale, it is important to pinpoint major energy producers and consumers whose exchange of energy may be the root of the problem. 


Mercury content in fish does not affect all populations equally. Certain ethnic groups, as well as young children, are more likely to suffer the effects of methyl mercury poisoning. In the United States, Wallace gathered data that indicated 16.9% of women who self-identify as Native American, Asian, Pacific Islander, or multiracial exceed the recommended reference dose of mercury. A study done on children of the Faroe Islands near Great Britain showed neurological problems stemming from mothers consuming pilot whale meat during pregnancy.

Regulation and health

While various studies have shown high concentrations of mercury accumulated in fish, medical cases often go unreported and pose a difficulty in correlating mercury in fish with human poisoning. Environmental issues cover a broad range of areas, but medical cases that are associated with pollutants released into the environment by factories or construction areas cause public health issues that affect not only the environment but also human well-being. Substances poisonous to the human body in a particular amount or dose may not cause any symptoms over time. While there are limits to how much of anything the body can have, mercury is a particular poison that produces immediate physical symptoms when the body has been accumulating it over a period of time.

In the United States, the Environmental Protection Agency measures the amount of mercury concentrated in human blood that does not pose fatal health outcomes. The agency is in charge of enforcing regulations and policies that cover a range of environmental topics. Analysis of blood mercury concentrations in childbearing women has proved that exposure to methyl mercury (MeHg) occurs primarily through the consumption of fish. The U.S. FDA highly recommends against pregnant woman and young children consuming raw fish. Pregnant women and young children often lack strong immune systems and are more at risk for foodborne illnesses.

Medical cases and exposure to mercury

In the United States, the EPA serves as an advisory organ to set the levels of mercury that are non-fatal in humans. Symptoms of exposure to high levels of methyl mercury include disturbed vision, hearing, and speech, lack of coordination, and muscle weakness. Medical studies have examined the correlation of fish consumption and health issues. American studies have presented evidence of fish consumption and its effects on child development. Longitudinal studies agree that human activities are what release and accumulate mercury in marine life. Addressing the issues of fish consumption forces health officials to recognize the sources of mercury in the human body. Specific Native American tribes are vulnerable to a high exposure of mercury. Studies have determined that these native peoples in the United States suffer more from mercury poisoning and illness than any other cohort group in the country. This is due to the fact that fish is a main source of protein. Exposure risk was assessed through a medical study, thus raising judicial issues of whether the public health of these groups is a priority in the United States.


The American College of Obstetricians and Gynecologists note that, considering all the dangers and benefits, the overall result of eating fish in the United States is likely to improve personal health rather than damage it. The college argues that the omega-3 polyunsaturated fatty acids found in fish have a health benefit that outweighs the harm from mercury or polychlorinated biphenyls. Even so, the College also suggests limiting fish consumption for pregnant women. A risk-benefit study weighing the risks of mercury consumption against the benefits derived from fish in Alaska showed that the benefits outweigh the risks when consuming salmon for both cardiovascular health and infant neurological development, and that MeHg data for non-oily fish needs to be of high quality before relative risk can be reliably identified. The Seychelles Child Development Study traced more than seven hundred mother-child pairs for nine years, and found no neurological problems in the children resulting from both prenatal and postnatal methylmercury exposure. A study done with marketed fish in Oman concluded that, except in a few rare cases, the fish available for consumption had lower levels of mercury than limits defined by various health organizations. Clearly, these studies call into question whether or not normal everyday consumption of fish is dangerous in any way, and at very least justify the creation of place-based and culturally relevant consumption advisories. They do not take into account cases of severe mercury poisoning, such as that found in Minamata disease.

Selenium is an element that is known to counteract some of the dangers of ingesting mercury. Multiple studies have been done, such as those in New Jersey and Sweden, that take into account selenium as well as mercury levels. Fish often do contain selenium in conjunction with bioaccumulated mercury, which may offset some of the dangers associated with the mercury ingested.

Species with characteristically low levels of mercury include shrimp, tilapia, salmon, pollock, and catfish (FDA March 2004). The FDA characterizes shrimp, catfish, pollock, salmon, sardines, and canned light tuna as low-mercury seafood, although recent tests have indicated that up to 6 percent of canned light tuna may contain high levels. A study published in 2008 found that mercury distribution in tuna meat is inversely related to the lipid content, suggesting that the lipid concentration within edible tuna tissues has a diluting effect on mercury content. These findings suggest that choosing to consume a type of tuna that has a higher natural fat content may help reduce the amount of mercury intake, compared to consuming tuna with a low fat content. Also, many of the fish chosen for sushi contain high levels of mercury.

According to the US Food and Drug Administration (FDA), the risk from mercury by eating fish and shellfish is not a health concern for most people.[55] However, certain seafood might contain levels of mercury that may cause harm to an unborn baby (and especially its brain development and nervous system). In a young child, high levels of mercury can interfere with the development of the nervous system. The FDA provides three recommendations for young children, pregnant women, and women of child-bearing age:

Do not eat shark, swordfish, king mackerel, or tilefish (Gulf of Mexico) because they might contain high levels of mercury.
Eat up to 12 ounces (2 average meals) a week of a variety of fish and shellfish that are lower in mercury. Five of the most commonly eaten fish and shellfish that are low in mercury are: shrimp, canned light tuna, salmon, pollock, and catfish. Another commonly eaten fish, albacore or ("white") tuna depending on its origin might have more mercury than canned light tuna. So, when choosing your two meals of fish and shellfish, it is recommended that you should not eat more than up to 6 ounces (one average meal) of albacore tuna per week.
Check local advisories about the safety of fish caught by family and friends in your local lakes, rivers, and coastal areas. If no advice is available, eat up to 6 ounces (one average meal) per week of fish you catch from local waters, but consume no other fish during that week.

Research suggests that selenium content in fish is protective against the toxic effects of methylmercury content. Fish with higher ratios of selenium to methylmercury (Se:Hg) are better to eat since the selenium binds to the methylmercury allowing it to pass through the body un-absorbed.

In 2012 the European Food Safety Authority (EFSA) reported on chemical contaminants they found in the food of over 20 European countries. They established that fish meat and fish products were primarily responsible for methylmercury in the diet of all age classes. Particularly implicated were swordfish, tuna, cod, pike, whiting and hake. The EFSA recommend a tolerable weekly intake for methylmercury of 1.3 μg/kg body weight.











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