Mercury is an element that is usually not harmful. But in the environment, mercury changes to methylmercury. This organic form is easily absorbed by living things. It builds up in fish and animals. The more mercury in a fish or animal, the more harm it can cause.

Tuna is a large predatory fish at the top of the food chain. It has the most methylmercury. Plankton is tiny food for almost all small and some large marine animals. Smaller marine animals are food for bigger marine animals. As mercury moves up the chain, the larger fish and the longest-lived fish end up with the mercury that has been absorbed by the smaller plants and animals.

Because tuna is at the top of the food chain, it has the most mercury.

The fetus is very sensitive to the harmful effects of mercury. The harm done cannot be reversed even when the fetus is no longer exposed to the mercury. Mercury is always harmful. But it causes less harm as you get older. An adult seems to be less harmed by higher amounts of mercury, compared with a fetus, child, or adolescent.

State and tribal health programs regularly post information about mercury in area fish. They offer guides on how much fish from certain lakes, rivers, and coastal waters a person can safely eat.

Mercury harms the brain. It can cause a wide range of brain and nerve problems.

Shark, swordfish, king mackerel, tilefish, orange roughy, and big-eye tuna have such high levels of mercury that pregnant people, people who are breastfeeding, and young children should not have even a single serving of these types of fish. All of these fish are higher up the food chain.

People who are pregnant or breastfeeding should consume between 8 and 12 ounces per week of a variety of seafood from choices that are lower in mercury. Seafood low in mercury include anchovy, cod, herring, oyster, salmon, freshwater trout, and canned light tuna.