Fish and shellfish concentrate mercury in their bodies, 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, tilefish, northern pike, and lake trout contain higher concentrations of mercury than others.
The presence of mercury in fish can be a health issue, particularly for women who are or may become pregnant, nursing mothers, and young children.
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Mercury and methylmercury are not soluble, so they primarily accumulate in the viscera, although they are also found in the muscle tissue. Within the food chain when a fish is consumed by a predator, the mercury level is accumulated. Since fish are less efficient at depurating than accumulating methylmercury, fish-tissue concentrations might in very large fish increase over time. Thus species that are high on the food chain amass body burdens of mercury that can be ten times higher than the species they consume.[citation needed] This process is called biomagnification.
In 1950s, inhabitants of the seaside town of Minamata, on Kyushu island in Japan, noticed strange behavior in animals. Cats would exhibit nervous tremors, 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 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. 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.
The complexities associated with mercury transport and environmental fate are described by USEPA in their 1997 Mercury Study Report to Congress.[1] Because methylmercury and high levels of elemental mercury can be particularly toxic to a fetus or young children, organizations such as the U.S. EPA and FDA recommend that women who are pregnant or plan to become pregnant within the next one or two years, as well as young children avoid eating more than 6 ounces (one average meal) of fish per week.[2]
A book by Jane Hightower, Diagnosis Mercury: Money, Politics and Poison, published in 2008, is largely about humans exposed to mercury through eating large predatory fish such as swordfish, shark, king mackerel, large tuna, etc.[3][4][5]
Much of the mercury which eventually finds it way into fish originates with coal burning power plants and chlorine chemical plants. 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. 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.[6]
In the United States, the FDA has an action level for methylmercury in commercial marine and freshwater fish that is 1.0 parts per million (ppm). In Canada, the limit for the total of mercury content is 0.5 ppm. The Got Mercury? website includes a calculator for determining mercury levels in fish.[7]
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.[8] Many of the fish chosen for sushi contain high levels of mercury.[9]
According to the US Food and Drug Administration (FDA), the risk from mercury by eating fish and shellfish shall not be a health concern for most people.[10] However, certain seafood might contain levels of mercury to harm an unborn baby (and especially its brain development and nervous system) or in cases of young child's interfere with the development of the nervous system. The FDA makes the following tree recommendations for child-bearing age and pregnant women and young children:
These recommendations should be are considered when feeding fish and shellfish to young children, but in proportionally smaller and controlled quantities.[10]
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  | NRDC: Mercury Contamination in Fish - Guide to Mercury in Sushi - NRDC: Mercury Contamination in Fish - Guide to Mercury in Sushi |
| | Mercury Study Report to Congress - Mercury Study Report to Congress | Mercury | US EPA |
| | FDA tests show risk in tuna - Topic Galleries -- chicagotribune.com |
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