Mercury is a naturally occurring heavy metal that can build up in the bodies of fish in the form of methylmercury, which is highly toxic. Nearly all fish and shellfish contain traces of mercury, and some seafood contains other contaminants known as persistent organic pollutants (POPs) . As small fish are eaten by larger fish up the food chain, concentrations of mercury and POPs increase, so that large, predatory deep-ocean fish tend to contain the highest levels. Therefore, it is best to avoid eating large fish, such as shark, swordfish, tilefish, and king mackerel.
Fish that contain high levels of mercury include shark, ray, swordfish, barramundi, gemfish, orange roughy, ling, and southern bluefin tuna. Mercury levels differ from one species of fish to the next, due to factors such as the type of fish, size, location, habitat, diet, and age. Fish that are predatory (eat other fish) are large and at the top of the food chain, and so tend to contain more mercury.
The risks from mercury in fish and shellfish depend on the amount of fish and shellfish eaten and the levels of mercury in the fish and shellfish. For most people, the risk from mercury by eating fish and shellfish is not a health concern. However, some fish and shellfish contain higher levels of mercury that may harm an unborn baby or young childs developing nervous system. If exposed to high amounts of mercury from food or other sources, severe or even fatal effects in the kidney, lungs, digestive tract, or cardiovascular system could develop.
The FDA recommends that adults should eat up to 12 ounces (two average meals) per week of a variety of cooked seafood as long as they avoid the large predatory ocean fish mentioned above and pay attention to local seafood advisories. For women who are pregnant or may become pregnant, nursing mothers, and children ages 12 and younger, the following recommendations are suggested:
- Do not eat shark, swordfish, king mackerel, or tilefish.
- Eat up to 12 ounces (two average meals) per week of a variety of fish and shellfish that are lower in mercury. Five of the most commonly eaten fish that are low in mercury are shrimp, canned light tuna, salmon, pollock, and catfish.
- Another commonly eaten fish, albacore ("white") tuna, has more mercury than canned light tuna. So, when choosing your two meals of fish and shellfish, you may eat up to 6 ounces (one average meal) of albacore tuna per week.
