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U-M researchers unravel mysteries about mercury in fish

There are a lot of health benefits from eating fish. But some kinds of fish contain high levels of mercury. A form of mercury called methylmercury is toxic to people, and the main way that gets into our bodies is from eating fish. It can cause damage to the nervous system, the heart, kidneys, lungs and immune system.

From the EPA's websiteon mercury:

For fetuses, infants, and children, the primary health effect of methylmercury is impaired neurological development. Methylmercury exposure in the womb, which can result from a mother's consumption of fish and shellfish that contain methylmercury, can adversely affect a baby's growing brain and nervous system. Impacts on cognitive thinking, memory, attention, language, and fine motor and visual spatial skills have been seen in children exposed to methylmercury in the womb. Recent human biological monitoring by the Centers for Disease Control and Prevention in 1999 and 2000 (PDF) (3 pp., 42 KB, About PDF) shows that most people have blood mercury levels below a level associated with possible health effects. More recent data from the CDC support this general finding.

Mercury occurs naturally and it’s also produced by power plants that burn coal.

Mercury fingerprinting

Researchers from the University of Michigan and the University of Hawaii have done some detective work on mercury in fish in the Pacific Ocean.

Joel Blum is a professor of Earth and Environmental Sciences at the University of Michigan. He's the lead author of the study published in Nature Geoscience.

He and his team studied nine species of Pacific fish in order to better understand how mercury gets into those fish.

...about one-third of the mercury in the atmosphere comes from natural sources and about two-thirds comes from human sources.

“Previous researchers have shown through a variety of different means that one of the main sources of mercury to the atmosphere is from combustion of coal, and we know that that mercury is then deposited on the surface of the ocean with rainfall. But the big mystery [lay] in how that mercury was transformed into a toxic and bio-available form, and also where within the ocean that mercury was entering into the marine food webs.”

Blum says many previous studies have shown that Asia is the largest source of mercury to the atmosphere from burning coal – partly because so much coal is burned there, and also because there are fewer emission controls on the power plants.

He says most of the mercury in the ocean comes from the atmosphere. He says other studies have shown that about one-third of the mercury in the atmosphere comes from natural sources and about two-thirds comes from human sources.

So, why do some fish have more mercury than others?

Mercury builds up in the food chain, so some large predator fish contain high levels of methylmercury partly because they eat a lot of smaller fish that have mercury in their systems. But the researchers also made a big discovery: they found out why the level of mercury in fish varies depending on where they feed in the ocean.

"The depth at which the fish eat is also a very important and sometimes the most important factor."

“So what we knew previously was that the trophic level that fish feed at, meaning how high up they are in the food chain, was definitely a contributor to high levels of mercury, and that’s still the case. We also know that the age of the fish, that older fish tend to accumulate more mercury. But those two factors didn’t give us the full story. There were still some missing trends and puzzles that didn’t make sense and what we’ve found now is that in addition to those two factors, the depth at which the fish eat is also a very important and sometimes the most important factor,” says Blum.

So, the fish that eat farther down in the ocean will have more mercury in their systems than fish that feed at shallower levels.

“What we found out is that methylmercury was being partially broken down by sunlight. So on the surface of the ocean, there’s very little methylmercury, because once it gets formed by the microbes, it almost immediately gets broken down by sunlight. As you go deeper and deeper into the ocean, where you have less and less penetration of sunlight, the methylmercury that’s formed at depth does not get broken down by sunlight and therefore can build up to high levels,” he says.

What's a consumer to do?

Blum says the message should not be that people should stop eating fish.

“Fish is a very, very healthy food, it has a lot of really important nutrients in it. The key is that you have to be an intelligent consumer and you have to think about what species of fish you’re eating. Particularly for young women (of childbearing age, and pregnant women) and children, they need to eat those species of fish that have low levels of mercury and there are many, many wonderful fish out there that are very healthy and good for you, and by just avoiding a half dozen or so of these fish that have the highest levels, one can have a very healthy diet.”

He says the species highest in mercury are swordfish, tilefish, shark and several types of tuna.  In particular, Blum says swordfish steaks are a major source of mercury in the diets of Americans.

"People love swordfish because it tastes so meaty, and in fact it was the only fish I'd eat as a kid," says Blum.

Here's what the EPA says about fish that are high in mercury:

By following these three recommendations for selecting and eating fish or shellfish, women and young children will receive the benefits of eating fish and shellfish and be confident that they have reduced their exposure to the harmful effects of mercury. Do not eat Shark, Swordfish, King Mackerel, or Tilefish because they contain high levels of mercury. Eat up to 12 ounces (2 average meals) a week of a variety of fish and shellfish that are lower in mercury. Five of the most commonly eaten fish that are low in mercury are shrimp, canned light tuna, salmon, pollock, and catfish. Another commonly eaten fish, albacore ("white") tuna has more mercury than canned light tuna. So, when choosing your two meals of fish and shellfish, you may eat up to 6 ounces (one average meal) of albacore tuna per week. Check local advisories about the safety of fish caught by family and friends 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 catch from local waters, but don't consume any other fish during that week. Follow these same recommendations when feeding fish and shellfish to your young child, but serve smaller portions.

The Monterey Bay Aquarium's Seafood Watch recommends these fish as both low in mercury and sustainably-caught choices:

The Best of the Best: July 2013*

  • Atlantic Mackerel (purse seine from Canada and the U.S.)
  • Freshwater Coho Salmon (farmed in tank systems, from the U.S.)
  • Pacific Sardines (wild-caught)
  • Salmon (wild-caught, from Alaska)
  • Salmon, Canned (wild-caught, from Alaska)

*The Super Green list is based on dietary requirements for an average woman of childbearing age (18-45, 144 pounds) eating eight ounces of fish per week. The list also applies to men and children; children should eat age-appropriate portions to maximize their health benefits while minimizing risk. The recommendation of 250 mg of omega-3s refers to the combined level of two omega-3s of primary importance to human health: eicosapentanoic acid (EPA) and docosahexanoic acid (DHA).

You can learn more about choosing fish that are low in mercury from these sites:

The EPA's fish consumption advisory site

The EPA/FDA's "What you need to know about mercury in fish and shellfish"

Environmental Defense Fund's Seafood Selector

Rebecca Williams is senior editor in the newsroom, where she edits stories and helps guide news coverage.