Mercury and Selenium in Seafood

Putting Mercury Risk in Context

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.

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.

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.

Seafood Guidance for Pregnancy & Nursing

seafood and pregnancy

Omega-3–rich seafood is an important part of a healthy diet, especially during pregnancy and early childhood. These nutrients—found primarily in seafood—support the development of the brain, nervous system, immune system, and vision during pregnancy and the first two years of life.


At the same time, it’s important to be mindful of mercury exposure. Methylmercury can cross the placenta, and developing fetuses and infants are more vulnerable during this stage. While research continues, a balanced, common-sense approach is recommended: continue eating seafood while being selective about the types you choose.


  • Avoid large, long-lived predatory fish such as swordfish, marlin, sharks, and large tuna, along with species like king mackerel, barracuda, grouper, and tilefish. It’s also best to avoid certain wild freshwater fish like pike, walleye, and lake trout.
  • Instead, focus on omega-3–rich, low-mercury options like sardines, mackerel, sablefish, herring, anchovies, bonito, and sockeye salmon, along with other wild salmon and Pacific albacore or skipjack tuna.
  • Aquacultured fish such as trout, char, sturgeon, barramundi, hamachi, and kampachi are also strong options, while catfish and tilapia offer moderate omega-3 levels.
  • Many everyday seafood choices—like cod, pollock, haddock, flounder, crab, shrimp, scallops, clams, oysters, and mussels—are naturally low in mercury and can be enjoyed as part of a balanced diet.

Mercury Studies

One of the most well-known California studies was conducted by San Francisco physician Dr. Jane Hightower, which suggested a link between seafood consumption and mercury toxicity. Although Dr. Hightower herself noted limitations in the study’s scientific methodology, it was later used by NGOs and the California Attorney General as a basis for requiring certain seafood warnings under Proposition 65—California’s consumer right-to-know law regarding substances that may cause cancer, birth defects, or other reproductive harm.

There are also two widely referenced, science-based studies. The first, conducted by a research team from the University of Rochester led by pediatric neurologist Gary Myers, has followed 779 pregnant women and their children in the Seychelles Islands. These subjects regularly consumed ocean fish—often twelve to fourteen times per week. Findings published in journals such as Neuro-Toxicology and The Lancet reported no evidence of neurodevelopmental risk from prenatal methylmercury exposure associated with typical ocean fish consumption.

The second, known as the Faroe Islands study and reported in The Journal of Pediatrics, found slight neurological differences in some children. However, this population consumed pilot whale, a marine mammal known to contain high levels of both mercury and PCBs, while being low in selenium. The whale is typically consumed shortly after harvest, resulting in concentrated exposure to these compounds—conditions that differ significantly from typical seafood consumption.

It is also important to note that key factors influencing mercury levels in seafood are not always reflected in available data. Larger, older fish and those caught near known mercury “hot spots”—such as coal-burning power plants, industrial facilities, and offshore drilling areas—tend to accumulate higher levels. This type of information is not consistently captured in survey data, and more comprehensive studies would help improve our understanding of mercury in seafood.