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How Does Mercury Get into Fish and Seafood?

Mercury in Fish

Many types of fish are rich in omega-3 fatty acids that have been linked to a number of health benefits, but some popular species, such as swordfish, king mackerel, shark and some tuna, contain significant levels of mercury. At high levels, mercury exposure can harm the heart, brain, lungs, kidneys and immune system of people of all ages, and even at relatively low levels, it can harm fetuses, infants and young children.

But how does this toxic compound get into fish in the first place?

The high levels of mercury found in fish like swordfish, king mackerel and certain types of tuna are primarily caused by industrial mercury emissions around the world, with coal-burning power plants being the largest human-caused source of mercury emissions to the air in the United States, according to the US Environmental Protection Agency (EPA). Mercury in the air eventually settles into oceans, lakes or rives, or onto land where it can be washed into water. Once in water, mercury can be transformed into methylmercury by bacteria. Methylmercury is a highly toxic form of mercury that is absorbed by certain aquatic microorganisms, and that can build up in fish and other marine animals when they eat those microorganisms.

Coal-burning power plants aside, there are also a number of other sources that release mercury into to the environment. These include gold mines, waste from dental amalgams, broken industrial thermometers, and manufacturing of certain metals and cement.

There are also some consumer products that contain mercury, including certain antique clocks, barometers and mirrors; batteries; CFLs and other fluorescent light bulbs; necklaces and other jewelry; certain interior and exterior paints; skin-lightening creams; thermometers and thermostats; and certain medicines.

Here are some ways you can help reduce the amount of mercury that gets into water and ultimately into fish and humans who eat fish:

  • Recycle old or unused mercury-containing products whenever possible
  • Look for non-mercury alternatives when buying new products
  • Correctly dispose of mercury-containing products at the end of their useful lives

Why do some fish contain more mercury than others?

Larger, long-lived deep-water fish species and predatory species typically contain the highest levels of mercury. Mercury levels can also fluctuate within a single species depending on the age of the fish, fat concentrations, food source, season and region. This means that by making more informed decisions you can reduce the amount of mercury from fish and seafood while still enjoying their wonderful nutritional benefits. Good choices (i.e. high omega-3, low mercury fish) include anchovies, salmon, sardines and herring, but also Pacific oysters have been singled out as an excellent low-mercury seafood source of healthy omega-3 fatty acids. At the other end of the spectrum are fish that contain so much mercury that you might want to limit their consumption even though some of them contain significant amounts of omega-3s. Such high-mercury fish include shark, bluefin and albacore tuna, marlin, tilefish from the Gulf of Mexico, swordfish, king mackerel, skipjack and yellowfin tuna, and orange roughy (deep sea perch). Especially pregnant and breastfeeding women and young children should avoid eating these fish.

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