Methylmercury in Seafood

Whole Foods Market® consistently offers the highest quality seafood caught and farmed from the best sources around the world. We also pride ourselves on keeping our customers informed of the latest health and environmental issues. In reference to advice concerning mercury in fish and shellfish, as provided by the U.S. Food and Drug Administration (FDA) and the U.S. Environmental Protection Agency, (EPA), here are some frequently asked questions about methylmercury in seafood.

What is methylmercury and what is the concern?

Methylmercury is the dominant form of mercury found in aquatic animals, including fish. At elevated levels, it can be harmful to the developing nervous systems of babies in utero and young children, affecting cognitive, motor, and sensory functions. The more methylmercury that accumulates in a person’s bloodstream, the longer the exposure time, and the younger in age of the person, the more severe the effects may be.

For pregnant women, women of childbearing age who may become pregnant, nursing mothers, or children, the FDA and EPA advise against eating fish that might contain high levels of mercury. Since effects from too much mercury can also occur in adults outside of childbearing age, consumption of fish that may contain high levels of mercury should be limited.

How does methylmercury accumulate in fish?

Although mercury occurs naturally in the environment, the primary source of mercury to the oceans is from the burning of coal, in addition to some industrial and mining activities. Through rain, snow and runoff, mercury can enter streams, oceans, rivers, and lakes where, through the influence of certain bacteria, it undergoes a chemical transformation into methylmercury, which can be toxic. Methylmercury is greatly concentrated by planktonic microorganisms from seawater and then assimilated into animals that eat these microorganisms. Fish absorb methylmercury from their food as they feed on aquatic organisms but lose almost none of the methylmercury that they consume. Thus, as larger, longer living species feed on prey lower on the food chain, they continue to accumulate mercury over time. Cooking preparation and heat do not reduce the mercury levels.

As reported in their study Toxicological Effects of Methylmercury (2000), the National Academy of Sciences stated:

“Because of the beneficial effects of fish consumption, the long term goal needs to be a reduction in the concentrations of mercury in fish rather than the replacement of fish in the diet by other foods. In the interim, the best method of maintaining fish consumption and minimizing mercury exposure is the consumption of fish known to have lower methylmercury concentrations.”

Which commercially available fish might contain the highest levels of methylmercury?

King Mackerel, Shark, Swordfish, Tilefish, Tuna

What about tuna?

Since canned “light” tuna is processed from smaller species of tuna, it typically has lower concentrations of mercury than either canned albacore (“white”) tuna or tuna steaks/fillets. Accordingly, the FDA and EPA advise limiting intake of both albacore tuna and tuna steaks/fillets to up to 6 ounces (one average meal) of tuna steak eaten per week.

How can I reduce my exposure to mercury?

You can reduce your exposure to mercury by eating a variety of fish known to have low mercury levels. While individuals outside of the more vulnerable, sensitive population groups may enjoy low mercury fish more frequently, the FDA and EPA recommend that women who are or may become pregnant and nursing mothers eat up to 12 ounces (2 average meals) per week of a variety of fish and shellfish that are lower in mercury. Young children should be served smaller portions.

Which fish are considered low in mercury?

In general, smaller, short-lived fish have less mercury than larger, long-lived fish. Typically the older and larger the fish, the greater the potential for higher concentrations of mercury. Commercially available fish lower in mercury include:

  • Catfish
  • Clams
  • Cod
  • Crab
  • Farmed Salmon
  • Flounder/Sole
  • Haddock
  • Herring
  • Ocean perch
  • Oysters
  • Rainbow trout
  • Sardines
  • Scallops
  • Shrimp
  • Spiny lobster
  • Tilapia
  • Trout (farmed)
  • Wild Salmon

What other foods provide the omega-3 essential fatty acids that are found in significant quantities in fish?

As an alternative to eating fish, purified (often called “molecularly distilled”) fish oil supplements offer omega-3 fatty acids with lower levels of contaminants. Omega-3-enriched eggs offer another alternative source of essential fatty acids, and micro algae-based omega-3 supplements, available in our Wellness & Beauty department, are a vegetarian alternative.

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