Environmental Protection Agency
As a rule, the bigger the fish, the higher the mercury content.
What is mercury?
Mercury is an element that occurs naturally in the environment. It can also be released into the atmosphere through industrial pollution. Because mercury dissolves easily in water, it accumulates at the bottom of bodies of water.
What is methylmercury? How does it enter the food chain?
In environments without oxygen, mercury combines with carbon to become methylmercury, a highly toxic compound. Bottom-dwelling fish consume methylmercury particles on the ocean floor and are then eaten by larger fish. Large fish such as sharks, swordfish and king mackerel contain the largest amounts of methylmercury because they're higher up in the food chain.
Why is methylmercury dangerous?
Methylmercury is toxic. It can harm the immune system, the gastrointestinal system, and even the genetic code. It attacks the central nervous system. High levels of exposure will result in brain damage and eventual death.
Most people already have some levels of methylmercury in their system from eating fish, but effects of mercury exposure depend on factors like age and amount of exposure. Trace amounts of methylmercury are excreted over time.
Should I stop eating fish?
No. The health benefits of fish outweigh the side effects of methylmercury exposure. The FDA recommends the following:
1. Do not eat shark, swordfish, king mackerel or tilefish. These fish typically contain high levels of mercury.
2. It's not risky to eat up to 12 ounces a week of fish and shellfish that are lower in mercury, such as shrimp, canned light tuna, salmon, pollock, and catfish.
3. You may also eat up to 6 ounces of albacore tuna, but the level of mercury is higher in albacore tuna than in light tuna, so substitute other types of fish for your other 6 ounces.
Why should pregnant women avoid methylmercury?
Developing embryos are particularly sensitive to methylmercury, so pregnant women and women trying to become pregnant are advised to avoid all large fish. They should also eat no more than one six-ounce serving of tuna a week, and should limit overall consumption of fish to no more than 12 ounces a week, according to the FDA.
What if I want to catch and eat my own fish?
Contact your local health department about mercury advisories in the waters where you plan on fishing. Fish that is caught recreationally may be OK, depending on whether your local waters have higher or lower mercury levels. If no advice is available, eat up to one meal a week of fish from local waters, but don't consume other fish that week.
Sources: Food and Drug Administration; Environmental Protection Agency; U.S. Geological Survey; and other NPR reports