Mercury in Seafood: Is It Still Safe to Eat Fish?

Scary news for seafood lovers: Not only do 84 percent of the world’s fish contain potentially unsafe levels of mercury, but eating certain types of seafood frequently may be particularly risky, troubling new research suggests.

Scientists have also solved a longstanding mystery: why deep-water fish, such as swordfish and bigeye tuna, contain more mercury than marine life that feeds near the ocean’s surface, such as flying fish and yellowfin tuna.

In research published earlier this year in Nature Geoscience, scientists from University of Michigan and University of Hawaii report that up to 80 percent of methylmercury—the highly toxic form of this heavy metal—is found deep in the ocean, which is why deep-feeding fish have the highest levels.

Particularly Hazardous to Kids

Methylmercury poses the greatest threat to children, since it can harm their developing brains and nervous system, the EPA reports. When babies are exposed to this toxin in the womb (through their mother’s consumption of contaminated seafood), it can impair memory, attention, language, fine motor, and other physical and mental skills, the agency adds.

The main source of exposure is eating fish and shellfish laced with this toxin—and the danger is likely to increase, since the new study predicts that levels of this industrial pollutant in the Pacific Ocean may double by 2050, unless action is taken to combat this growing threat to our seafood supply—and human health.

Here’s a closer look at the new research—and how to get the benefits of fish (such as heart-protective Omega-3 fatty acids), while minimizing exposure to mercury.

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How Does Mercury Get Into Fish?

In an earlier study, the same researchers found that, “predatory fish that feed at deeper depths in the open ocean, like opah and swordfish, have higher mercury concentrations than those that feed in waters near the surface, like mahi-mahi and yellowfin tuna,” study coauthor Brian Popp told Hawaii 24/7.

“We knew this was true, but we didn’t know why,” adds Popp, professor of geology and geophysics at University of Hawaii at Manoa. In the new study, the team used a mass spectrometer to analyze mercury in six predatory species and three species of prey fish, caught in the Pacific Ocean near Hawaii.

By analyzing the chemical signatures of methylmercury in the fish, the researchers discovered that the toxin travel thousands of miles through the air from Asian countries, where coal-burning power plants spew out mercury pollution.

Sunlight helps break down mercury pollution near the ocean surface, explaining why fish that feed there have lower levels. But the heavy metal also appears to bind to bacteria in organic matter that sinks to the lower depths where deep-water fish eat it.

Those fish, in turn, are eaten by large predatory deep-water fish at the top of the food chain. The nine species studied, ranked from deepest- to shallowest-feeding, were 2 species of lantern fish, swordfish, bigeye tuna, moonfish (opah), skipjack tuna, yellowfin tuna, mahi-mahi, and flying fish.

Facts About Eating Fish

Which Fish Contain the Most Mercury?

In a paper released earlier this year, the Biodiversity Research Institute (BRI) found that 84 percent of fish samples from around the world contain mercury concentrations exceeding the Environmental Protection Agency’s (EPA) health advisory guidelines. That means the amount wasn’t safe for human consumption more than once a month.

BRI reports that the greatest concentrations were found in marlin, Pacific bluefin tuna, swordfish, and king mackerel. Several species of shark were also high in mercury. The new study suggests that other deep-water fish, such as lantern fish, moonfish, and bigeye or skipjack tuna, could also be risky.

What Is the Healthiest Seafood?

BRI rates anchovy, sardines, flounder, salmon, cod and haddock as healthier choices that can be safely eaten on a weekly basis, while the new study found that of the 9 fish species studied, flying fish and mahi-mahi were among the lowest in mercury.

The EPA also recommends shrimp, canned light tuna, catfish, salmon, and pollack as commonly consumed seafood options that are low in the heavy metal toxin. Fish sticks and fast-food fish sandwiches are typically made from fish that are low in mercury.

3 Tips for Safe, Healthy Seafood Consumption

The EPA suggests that women and young children follow these 3 guidelines to get the health benefits of seafood, while reducing risk for harmful effects of mercury. New studies show that a Mediterranean diet that includes fish, fresh fruits and vegetables, oil olive, and nuts helps ward off dangerous diseases, including heart attacks and cancer,

  1. Avoid shark, swordfish, king mackerel, or tilefish because they’re high in mercury.
  2. Eat up to 12 ounces of seafood (2 average meals) weekly. Select from a variety of fish and shellfish that are lower in mercury. Since albacore ("white") tuna has more mercury than canned light tuna, limit yourself to a max of 6 ounces of albacore as one of your two weekly meals of fish.
  3. Check local advisories about the safety of fish caught in your local lakes, rivers, and coastal areas. If no advice is available, eat up to 6 ounces (one average meal) per week of fish you or friends catch, but don't eat any other fish during that week.

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