Advice – Mercury in Seafood

Bull kelp on still waters.

MERCURY: Sources in the Environment, Health Effects,
and Politics

by Carl Safina

Think of it this way: fish is good for you; mercury is bad for you.

Fish carry mercury, but some fish have a lot and some have very little. So choose fish low in mercury. We’ll tell you how.

For most of us, most of the time, mercury is something we need to think about, but not worry about. (We pay our agencies—like the Environmental Protection Agency (EPA) and the Food and Drug Administration (FDA)—to do the worrying for us; but they don’t always do the best job possible.)

If you know what you’re doing, you can avoid seafood-related health problems involving mercury. Here are the questions to think about: are we eating fish high in mercury, how much are we eating, and will we be exposing a developing child to mercury?

Our report, Mercury: Sources in the Environment, Health Effects, and Politics summarizes a lot of what scientists and medical researchers have learned about mercury in the environment, in food, and in people. And, if you want to know, “Where did these people get this bit of information from?” this report tells you that, too, with extensive footnotes. If you want to know, Which fish are low in mercury?, there’s a helpful graphic on page 28, and a list showing low-, medium-, and high-mercury fish on page 33.


Mercury is tricky. It’s the only metal that is liquid at room temperature. It’s an element. It exists in several forms and various compounds. All are toxic. Some forms are lethal in very small quantities. Mercury effortlessly penetrates cell membranes and gets deep into living tissue, including the brain, and crosses the placenta of mammals where it can interfere with fetal development.

Fortunately, most people never come into contact with the most dangerous forms. But mercury is everywhere. In 2009 the U.S. Environmental Protection Agency tested game fish in 500 lakes nationwide; every single fish from every lake tested positive for mercury.

Mercury gets into the air from natural sources such as volcanoes, and human sources such as burning coal. Nowadays, human sources put about ten times as much mercury into the air as do volcanoes. Some mercury coming from the land and sea is natural; but today some of it is mercury that was first introduced through past human activities and is now recycling through living systems. About a third of the mercury getting into the living environment these days is being brought up by people from deep, locked-away sources such as coal, in which mercury is a common impurity.

People use mercury in various industrial processes, such as manufacture of chlorine, lye, and plastics, and to separate gold from stream sediments (in certain river systems, gold mining is a significant cause of mercury pollution). But the largest single human generated source of mercury comes from burning coal to create electricity. And that is greatly increasing.

In the U.S., 40 percent of human-generated mercury comes from coal-burning power plants. Existing technology could remove about 95 percent of the mercury from coal plant smokestacks, but few plants currently have this technology.

It’s a truly global issue. The amount of mercury emitted in the U.S. is a small part of world emissions. Two-thirds of human-generated mercury originates in Asia; China is now the world’s largest mercury polluter. Air currents carry mercury around the planet. More than half of the mercury that falls in America originates in Asia and elsewhere outside the country. Similarly, about two-thirds of the mercury generated in the United States falls outside the U.S.

After mercury falls to Earth, much of it either settles on water or gets washed into water. In water, bacteria convert mercury into a compound called methylmercury. That’s where most of the trouble starts. Methylmercury is the form most commonly absorbed by living things. In streams, lakes, rivers, and the ocean, methylmercury easily enters the food-web, getting into plankton and then plankton eating fish, and whatever eats them.

Mercury from natural sources has always entered the living environment. But what’s different now is that human industrial activities are greatly raising the amount of mercury getting into soils, water, wildlife, and people.

Today, the quantity of mercury in soil is about three times greater than before the Industrial Revolution. Teeth, fur and feather samples dating back 800 years show that preindustrial mercury concentrations in Arctic marine animals were only about one-tenth of present-day levels. North Pacific Ocean water samples taken in 2006 had mercury concentrations 30 percent higher than just 10 years earlier. Much of that is from increasing coal-burning as developing countries industrialize, and human populations grow.

Like people who eat too much fish, fish-eating wildlife—including birds such as herons, and aquatic mammals—can suffer neurological and developmental effects. Mercury has recently been implicated in egret declines in south Florida, with the birds suffering liver and kidney damage. Even insect-eating songbirds such as thrushes and tree swallows are experiencing mercury-induced reproductive problems and lower survival. Otters in Maine and Vermont and mink in Massachusetts and Connecticut carry mercury concentrations at potentially fatal levels.

Is this just a scare, or can mercury really make you sick? Answer: It can make you sick. Mercury is a particular concern for child-bearing-aged women and growing children.


Mercury that we send into the air and water comes back to us in seafood. And because mercury gets into living things easier than it leaves, it accumulates more with each rung of the food chain.

So just imagine a simple food chain: Mercury gets into water. Bacteria convert it to methylmercury, which gets into single-celled algae, which are eaten by tiny drifting animals called copepods. Soon one million copepods each carry an infinitesimally small amount of mercury. They’re eaten by, say, a thousand herring. Now those thousand herring have the mercury of a million copepods. Over a summer, a tuna eats the thousand herring; getting all the mercury that was in all those herring, mercury that’s been gathered by millions of smaller living things from a vast quantity of seawater. That’s how all the mercury from many creatures funnels into one.

The longer the food chain, the more toxified the top predators. Animals in each link in the food chain can have between two and seven times the mercury of their prey in the previous link. Levels in these top predators can be more than a million-fold higher than in the little creatures at the bottom of the same food chain.

When humans become the last link in the ocean food chain by consuming top predators such as sharks, swordfish, or tunas, they get a high mercury dose. Arctic peoples such as North American Inuit and the Faroe Islanders, who still eat a lot of seals, dolphins, and whale-meat, carry some of the highest mercury loads. But so do some people who eat a small piece of a big fish many times a week.


How much does it take to cause health problems? The thing is: experts disagree. Even so, there’s an easy, sensible path through this maze.

Fish-eating fishes like bass, snappers, or groupers have higher mercury concentrations than do low-on-the-food-chain shell bearing mollusks (clams, oysters, scallops), crustaceans (shrimp, lobsters, crabs), or mainly vegetarian fishes like tilapia. A grilled shark steak has a relatively high concentration of mercury; a baked clam has very little. (It doesn’t matter how you cook it or whether it’s raw; the mercury stays.)

So the first rule of thumb about which seafoods are high or low in mercury is: the more predatory the fish, the higher on the food chain—the more mercury. And that’s also pretty much the only rule of thumb. Big fish eat little fish, so the bigger, the more mercury. The relevant question becomes: Was it a big fish or a little one? The solution: eat small.

But how dangerous is any given meal to your health? That depends on how big a piece you eat relative to your body weight, whether you’re still growing, and whether you’re pregnant or likely to become pregnant—and whether you want to eat swordfish, shark, tuna, or other big fish once a year or several times a week.

If exposure stops, blood mercury levels drop to half within two months of exposure as mercury passes out of the body. If we habitually eat foods with mercury, mercury accumulates, and with age our mercury level increases.


High and chronic exposures have been responsible for some horrible cases of mass poisonings. In the 18th and 19th centuries, factory workers making felt hats using mercury-soaked fabric slowly developed twitches, tremors, and dementia, hence the term: “mad as a hatter.” For nearly 40 years in the mid-20th Century, a factory dumped mercury compounds into Minamata Bay, Japan. By the mid-1950s, thousands of residents of nearby fishing communities were stricken with neurological problems and with trouble seeing, speaking, and walking. Worse, many babies were born paralyzed and with gross deformities, and mental retardation. Well over 10,000 people were poisoned.

High doses of mercury can be fatal. But chronic, low-level exposure may cause permanent nervous system damage, affecting the brain and impairing sight, hearing, speech, learning abilities and IQ, attention span, and muscle coordination, fatigue, difficulty concentrating, thinning hair, digestive problems, muscle and joint pain, sleep disorders, immune system problems, and heart and vascular trouble. (Of course, those problems can arise from other causes too, making it difficult to determine when mercury is the cause.) Serious methylmercury poisoning can cause tingling, difficulty walking, skin rashes, mood swings, memory loss, liver and hormonal problems, and, in extreme cases, coma, convulsions, and death. A cost analysis by researchers at New York City’s Mount Sinai Hospital calculated yearly U.S. health costs from mercury pollution at $5.1 billion.

A U.S. government study of over 6,000 women between 1999 and 2006 found that 8 percent of women of childbearing age had blood mercury levels exceeding the Environmental Protection Agency’s guidelines for potential injury to an unborn child.

Mercury exposure in the womb or during infancy, can disrupt complex, fragile development, causing irreversible damage to a child’s brain—at much lower exposures than those affecting adults. Over 600,000 children annually are born with mercury levels that put them at risk for learning problems and impaired intelligence, according to an Environmental Protection Agency analysis of in-the-womb mercury exposure.

And yet—seafood is the best source of the omega-3 fatty acid DHA, which is critical for a baby’s brain and eye development, both before and after birth.

This has sparked unhelpful debating of yes-or-no questions: “Should I eat fish because it’s healthy?” Should I avoid seafood because it’s dangerous?” “Do the benefits of eating fish outweigh the risks?” Those are the wrong questions.

Because seafood is both nutritious and our main source of mercury, the answer isn’t, “avoid seafood;” the answer is, “avoid mercury.”


Here’s how to do it: First, change the questions to, “What fish should I eat? How much? And how often?”

But remember our rule of thumb: the bigger the fish, the more mercury. A large, old 500-pound mako shark that has been eating tunas and swordfish is going to contain all the mercury that has concentrated up the line, from swarms of plankton to many thousands of smaller fish like mackerel, then into the tuna and swordfish and on to the shark. Similarly, a 5-pound cod has eaten less food than has a 50-pound cod, so it has accumulated less mercury. Our rule of thumb holds: eat small.

If a fish cannot fit on a platter whole, it’s probably high in mercury. For instance, some of the lowest-mercury seafoods include shrimps, salmon, catfish, and pollock. You could fit them on a platter whole. But tuna? It would take a mighty large platter to fit a whole tuna, since yellowfin, bigeye, and bluefin tuna often exceed 100 pounds (bluefin can reach well over 1,000). That’s why fresh tuna comes in steaks. Canned tuna is usually yellowfin, skipjack, or albacore tuna. Most Americans eat so much canned tuna, in fact, that over one-third of the U.S. population’s mercury comes from canned tuna. And beware of fish sold as steaks; those are big fish, so limit your eating.

But what kind of advice is “beware,” or “limit?” In 2010 the Environmental Protection Agency and the Food and Drug Administration issued a joint consumer advisory. The guidance, directed at women who may become pregnant, pregnant women, nursing mothers, and young children, boils down to this: “Eat up to 12 ounces (2 average meals) a week of a variety of fish and shellfish that are lower in mercury… If you eat a lot of fish one week, you can cut back for the next week or two. Just make sure you average the recommended amount per week.” Because tuna steaks have more mercury than canned tuna, they recommend not exceeding 6 ounces per week of tuna steaks (

So it’s pretty easy to stay within government-recommended guidelines and to keep your mercury blood-level low. However, the guidelines don’t speak to men, or to women who aren’t potentially child-bearing. And the agencies’ list of fish low in mercury is poor and partially inaccurate. To clarify this, we include a very informative graphic on page 28 of our report and a table on page 33. The good news: some of the most popular and best-tasting seafood is also low in mercury.


The Safina Center wants to give you the full information picture so you don’t have to worry. But fishing industry lobbyists and publicists want you to believe there’s nothing at all to worry about. Their Orwellian-named Center for Consumer Freedom’s tells consumers that they can eat fish containing ten times the EPA recommended maximum dose. contends that mercury concerns are a “scare.” A U.S. based seafood industry lobbying group, National Fisheries Institute, actually wrote, “What published science shows is that higher blood mercury among moms may actually be a marker of optimal brain development in babies, because it indicates regular seafood consumption.”

Corporate profits in the energy industry have also affected our health by influencing mercury regulation. Companies operating the nation’s worst polluting coal-power plants raised millions of dollars to create pro-industry mercury rules while the George W. Bush administration ignored professional and scientific staff and advice from a federal advisory panel.


To keep fish safe, we need to reduce the release of mercury. That’s quite doable. Existing pollution control technology could lower global mercury discharges by up to 60 percent. The European Mercury Strategy, launched in 2005, includes a 2011 ban on mercury exports, a phase-out of mercury in goods and industrial applications, new rules for safe storage, and reductions in mercury emissions from fossil-fuel power plants and industrial facilities.

In the U.S., there’s been a sharp drop in mercury use in batteries, fungicides, and paints. Mercury emissions from U.S. municipal and medical waste incinerators have gone down by 90 percent.

But what about coal? New regulations enacted by the Obama Administration in December, 2011 will, for the first time, regulate emissions from coal-fired power plants under the Clean Air Act. These new rules will remove over 90 percent of the mercury when power plants burn coal. More than half of all coal-fired power plants already use the proposed technologies. Compliance will cost about $11 billion yearly, but the EPA estimates benefits of up to $140 billion each year. In other words, every dollar spent reducing power plant pollution can yield up to $13 in health and economic benefits.

That’s the way forward, and cause for optimism.


Further Reading

MERCURY: Sources in the Environment, Health Effects, and Politics, The Safina Center (formerly Blue Ocean Institute) Full Report
Tuna Surprise: Mercury in School Lunches, Mercury Policy Project