Levels of mercury in tuna are on the rise

Protect your health by choosing the right low-mercury fish

Published: February 13, 2015 11:15 AM

Concentrations of mercury in tuna caught in the Pacific near Hawaii increased at a rate of around 4 percent annually from 1998 through 2008, according to a new study led by a University of Michigan researcher. The study suggests that rising levels of mercury emissions coming from sources such as coal-fired power plants that pollute open ocean waters are to blame for mounting levels of the toxin in Hawaiian yellowfin tuna, also known as “ahi,” which is often used in sushi or for grilled tuna steaks.

As we noted in our special report "Can Eating the Wrong Fish Put You at Higher Risk for Mercury Exposure?" almost all seafood contains mercury in varying amounts, but higher levels tend to accumulate in swordfish, shark, and larger, longer-living predatory fish. Certain types of tuna, including yellowfin and big eye, are also high in mercury. In fact, Food and Drug Administration data show that many samples have levels comparable to shark and swordfish. 

When you eat seafood containing methylmercury—the form found in seafood—more than 95 percent is absorbed into your bloodstream. Getting too much of the toxin can damage the brain and nervous system. Risks are highest for fetuses, but children and adults also can suffer harmful health effects, including problems with speech, walking, and fine motor coordination.

For the University of Michigan study on mercury in tuna, researchers reanalyzed data from four other studies that measured total mercury levels in the muscle tissue of yellowfin tuna that were caught in the North Pacific near Hawaii: 111 fish in 1971, 104 fish in 1998 and 14 fish in 2008. They found that concentrations of the toxin in Hawaiian yellowfins did not change between 1971 and 1998, but over the next 10 years, mercury levels shot up at a rate of 3.8 percent or more per year.

“While the sample size from 2008 is much smaller than the others and we would like to see more data from recent years to confirm this finding, this trend is highly concerning,” says Jean Halloran, director of food-policy initiatives at Consumers Union, the policy and advocacy arm of Consumer Reports.

A 2009 study led by researchers at the U.S. Geological Survey and Harvard University found that mercury levels measured in the waters of the northern Pacific Ocean had risen about 30 percent over 20 years. But the Hawaiian yellowfin tuna study is the first to document a consequent increase in mercury levels of open-water fish, according to the study’s authors. Calling for more stringent reductions in mercury emissions, they warned that if mercury continues to be deposited into the ocean at current rates, mercury levels in the North Pacific will double by 2050. “Mercury contamination of ocean fish is a serious global health issue," they concluded.

The evidence from these studies provides yet another reason to heed Consumer Reports' advice on ways to protect your health by reducing your dietary intake of mercury. To get the benefits of seafood while minimizing your exposure to the toxin, see our recommendations of low-mercury seafood choices, such as wild and Alaskan salmon (either fresh or canned) as well as our advice on higher-mercury fish to watch out for.

Also check these resources and tools for making safer seafood choices, including the Got Mercury? calculator, which shows how much of a given type of fish or shellfish you can eat per week based on your body weight without exceeding the maximum acceptable dietary limit for mercury set by the Environmental Protection Agency.

—Andrea Rock

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