Mercury and Selenium in Seafood

Healthy, nutritious, sustainably caught Mackerel on ice at Monterey Fish Market

All living organisms that evolved with the planet Earth contain trace quantities of mercury. Elemental mercury is washed into streams and the sea, where bacteria convert it into organic methylmercury, which can enter the food web. At the same time, plants and other organisms are busy converting methylmercury back to inorganic compounds. This constant cycling of mercury from elemental to organic and back again has occurred for eons. Only recently, through x-ray spectroscopy, has the type of mercury found in seafood (methylmercury cysteine) been identified.

Initial studies have found that the mercury found in seafood may be less toxic than the form of mercury on which fish consumption advisories have been based. In addition, a number of studies have found that the essential element selenium, high amounts of which are found in ocean fish, sequesters mercury, thus neutralizing its toxic effects. This may be the reason why studies have never shown an epidemic of child developmental problems in coastal populations whose diets have been comprised in large part of seafood.

A U.S. National Academy of Sciences report in 2000 examined the dangers of mercury and concluded that the risk of harm from the intake of mercury from eating fish for the majority of people was low. It is unlikely that mercury is a threat to a healthy adult who eats a normal and varied diet.

In many cultures around the world, fish are a much greater part of the diet than in the United States. In Japan, for example, seafood consumption is four to five times higher than in the United States, and for centuries the very largest tunas, those most likely to bio-accumulate large amounts of methylmercury, have been a favorite Japanese food without causing apparent problems.

Omega-3 bearing seafood is an important part of a healthy diet for all people, especially pregnant women and young children. Scientific evidence has shown that omega-3 fatty acids, found exclusively in seafood, are essential to the complete development of the brain, nervous system, immune system, and retinal system during pregnancy and the first two years of life. However, the risk of mercury exposure to pregnant women, infants, and children should not be minimized. Fetal growth and infancy are the periods in human development when exposure to methylmercury poses the greatest threat to the nervous system. Methylmercury easily crosses the placenta, and the mercury concentration may rise to 30 percent higher in fetal red blood cells than in those of the mother. More studies are needed to understand the effects of mercury on human development, but common sense tells us that the developing fetus and infants are especially vulnerable.

Prudence dictates that pregnant or nursing mothers and those trying to become pregnant should follow the FDA guidelines that recommend eating omega-3-rich seafood while avoiding fish testing high for mercury.

Pregnant and nursing mothers and those trying to become pregnant should:

  • Avoid the largest long-lived predatory ocean fish, such as swordfish, marlin, large tuna, and sharks. Also to be avoided are King mackerel, barracuda, conger eel, groupers, jacks, large bluefish and Gulf of Mexico tilefish*. Avoid wild freshwater fish such as whitefish, pike, pickerel, walleye, and lake trout.
  • Choose the following fish, which are very high in omega-3 fatty acids yet contain nondetectable to very low levels of mercury; sardines, mackerel, sablefish, herring, anchovies bonito and sockeye salmon. Other very good choices are other species of wild salmon, Pacific coast albacore and skipjack tuna.
  •  Consider that Aquacultured fish with higher levels of omega-3s and nondetectable to very low levels of mercury: Trout, char, sturgeon, barramundi, hamachi, and kampachi are high in omega-3s, while catfish and tilapia have moderate levels of omega-3s.
  •  Consider that seafood that contains low to moderate levels of omega-3s and nondetectable to very low levels of mercury: cod, pollock, haddock, hake, flounder, sole, croaker, spiny lobster, crab, squid, scallops, shrimp, clams, oysters, mussels, and farmed crawfish.
  •  Consider that canned light-meat (skipjack) tuna or low-mercury Pacific troll-caught albacore The FDA fish consumption advisory for commercial fish can be found at the FDA's center for food safety and nutrition at Fish-consumption advisories for recreational fisheries can be found at

Mercury Studies

The most well known California study was conducted by San Francisco Physician Dr. Jane Hightower, which ostensibly linked seafood consumption to mercury toxicity. Although Dr. Hightower warned that hers was a highly questionable study in terms of its scientific methodology NGO's and the California Attorney general used this study as a basis to require restaurant chains and supermarkets to label some seafood as being harmful under Proposition 65 (California's consumer right-to-know law that requires businesses selling products known to cause cancer, birth defects, or other reproductive harm to notify customers).

There are also two ongoing science-based studies: The first, conducted by a research team from the University of Rochester led by pediatric neurologist Gary Myers, is an ongoing study, which follows a subject base of 779 pregnant women and their children in the Seychelles Islands. The subjects typically ate large ocean going fish on average twelve to fourteen times per week. The study, published in the journals Neuro-Toxicology and Lancet, determined that after comparing the fifteen hundred children born during the study with a control group in the United States, "There is no evidence of neurodevelopmental risk from prenatal methylmercury exposure resulting solely from the consumption of ocean fish."

The second, referred to as the Faroe Islands study, was reported in the February 2004 issue of The Journal of Pediatrics. Conducted by a research team from the Harvard School of Public Health led by Philippe Grandjean, the study found "slight neurological anomalies" in some of the children in the study. However, the study followed Faroe Islanders who consume pilot whales, marine mammals that contain high levels of both PCBs and mercury, while containing very little selenium, an element that is thought to reduce the detrimental effects of mercury. The entire whale is consumed soon after capture, creating a pulse effect of exposure to a witch's brew of PCBs and mercury.

* Some of the most important factors that affect mercury levels in seafood are not reflected in available data. It is well known that larger, older fish and fish captured near notable mercury "hot spots," such as coal-burning power plants, chlorine production facilities, and offshore oil-drilling platforms accumulate higher levels of mercury. Seldom has this pertinent information been compiled or correlated in survey data. More extensive surveys and improved sampling methods would go a long way toward helping us understand the issues of mercury in seafood.

Back to top