Why Does Fish Contain Mercury?

Fish are low in saturated fat and high in protein and omega-3 oils, which makes them good for your health. However, all fish contain some degree of mercury, and exposure to high concentrations of particular types of mercury can put your health at risk. While most of us are aware of concerns about the mercury content of fish, most people don't know exactly what the risks are, or how they develop.

Mercury and Methylmercury

• Mercury is a naturally occurring metal that can be found in oil, soil, rocks and water. It is also distributed by human activities such as metal mining and coal-fired power generation. Mercury can take one of three forms: It is "elemental" (metal) in its primitive state; "inorganic" when it is combined with elements like sulfur, chlorine and oxygen; and "organic" when it is combined with hydrogen or carbon. Of these three forms, organic mercury is the most dangerous to humans because it is toxic; methylmercury is organic mercury.

Mercury in Oceans and Fish

• In elemental form, mercury in oceans is sediment. Micro-organisms transform ocean sediment into organic methylmercury; plants then absorb it through osmosis, and fish absorb it through their gills and tissues. Mercury binds to the proteins in a fish's muscles, which makes it impossible to remove through cooking. Fish develop higher concentrations of mercury through the process of bioaccumulation, meaning they absorb more than they excrete. Fish also increase their mercury levels when they feed on plants or other fish with high mercury levels.

Fish Mercury Levels

• Fish will contain more or less mercury based on their size, species, diet and location. Generally speaking, larger, predatory fish at the top of the food chain have higher levels of mercury. Top on the list of many health advisories, including those published by Health Canada, are risks associated with eating excessive amounts of shark, rays, bluefin tuna, swordfish, barramundi, gemfish, orange roughy and ling. By contrast, smaller fish such as herring and salmon, as well as crustaceans such as prawns, lobsters and oysters, tend to contain little mercury. To put concentrations into context, consider that an average herring will have a mercury content of 0.01 parts per million, whereas a shark can have concentrations of 1 ppm or more.

Concerns

• The human bloodstream readily absorbs and transmits methylmercury through the body; it reaches the brain and, in pregnant women, developing fetuses. According to Health Canada, methylmercury has been "associated with impaired functions of the central and peripheral nervous systems." Developing fetuses are especially vulnerable to high levels of mercury exposure; such exposure can lead to developmental damage such as decreased I.Q., blindness, learning disabilities, coordination challenges and seizures. Excessive methylmercury exposure in adults can lead to tremors, vision and hearing problems, loss of muscle coordination, memory loss, and death in extreme cases.

Considerations

• The United States Food and Drug Administration, the Canadian Food and Inspection Agency, and similar government agencies around the world monitor fish mercury levels. These agencies advise pregnant women and young children to be selective about the types and amounts of fish they eat; the EPA recommends that they don't eat shark, swordfish, king mackerel or tilefish. Similarly, Health Canada advises that women who are pregnant or breastfeeding should limit themselves to 150 grams of these types of fish per month; other Canadians are advised to limit themselves to 150 grams per week.