Mercury Contamination in Seafood

Mercury contamination in fish can have serious implications for human health. (Photo credit: Ramon Cami/Fotolia)

Mercury (Hg) is a heavy metal that is found in the Earth’s crust. When fossil fuels are burned, it is released into the atmosphere. Manufacturers also use mercury as a catalyst to make industrial products such as fungicides, vinyl chloride, and even paper. It also is used in paints to keep them from going bad. Precipitation can transfer mercury from the atmosphere to lakes, rivers, and streams. Industrial plants sometimes discharge it directly into aquatic environments in their wastewater. It can get trapped into the sediment and be stored for a long time. Microorganisms that can survive without oxygen turn elemental mercury into methyl mercury (sometimes referred to as MeHg). Although elemental mercury can cause health effects such as kidney disease, methyl mercury is even more toxic, and lasts in the environment for a longer time. It can be absorbed by the gills of fish, or may be consumed by fish that eat plankton that has absorbed mercury. Methyl mercury can build up in the tissues of fish and shellfish, so that the concentrations within the animals’ bodies is much greater than within the water in which they live.

When fetuses are exposed to methyl mercury from the seafood their mother eats, the development of their nervous system, especially the brain, can be affected. Studies have found that when pregnant women eat a lot of fish, they have relatively high levels of methyl mercury in their system. Their children have lower IQs and score lower in tests of cognition. However, fish is high in protein and long-chain omega-3 fatty acids, which are essential for brain health. Pregnant women who eat only small amounts or no fish at all also have children with lower cognition and IQ scores due to the lack of omega-3 fatty acids in their diets. The best practice for all people, and especially pregnant or nursing women, is to eat fish about twice a week but to choose fish that are not likely to contain high amounts of mercury.

Mercury levels depend on the size, age, and geographical location of the seafood. Usually, whitefish such as cod and haddock do not have high mercury levels, while dark fish have higher amounts. Unfortunately, high mercury levels and long-chain omega-3 fatty acids tend to appear in the same fish species, so cod and haddock have lower long-chain fatty acids as well as lower mercury levels.  But small fatty fish that are often canned, such as sardines and chunk light tuna, have higher long-chain omega-3 fatty acids and relatively lower levels of mercury than other types of fish.

Earlier studies linking canned tuna to high mercury levels have unfortunately scared off some people from eating it. The average American now eats less than 50 mg/day of omega-3s, though the recommendation is 250 mg/day. It’s important to learn the whole story. Chunk or solid albacore, or white tuna, has higher levels of mercury than chunk light tuna. Albacore is a larger species of fish and can contain mercury levels that are three times the levels found in yellowfin or skipjack tuna. Canned chunk light tuna is usually made from yellowfin or skipjack. Albacore that are caught in the United States or Canada have lower mercury levels than those that are imported. Like albacore, bluefin tuna is a large fish species that is high in mercury. Thankfully, it is not normally found in canned tuna. It is, however, a very common component of sushi. A study in New York sampled several restaurants and found that if people ate six pieces of tuna-containing sushi per week they would consume more than the EPA-accepted level of mercury. At expensive restaurants, the levels were even higher than the cheaper sushi places. Chefs at expensive restaurants are more likely to choose the larger, more expensive fish without realizing that mercury levels in these fish are higher.

Much of what we know about the human health effects of mercury on both adults and children comes from a single disaster. In the 1950s, methyl mercury was discharged from an industrial plant into Minamata Bay, Japan. Around 3,000 people that ate the contaminated fish and shellfish were killed, disabled, or otherwise affected. Many experienced neurological symptoms such as numbness, tremors, vision problems, confusion, and depression. Some effects were immediate, and others took years to surface.  Some women that were seemingly unaffected gave birth to severely disabled children. At the time, many people in the rest of the world were not that concerned about what is now known as “Minamata Disease,” because methyl mercury was not being dumped directly into the water elsewhere in the world. Not long afterwards, however, it was discovered that microorganisms in the water were converting organic mercury into methyl mercury. Since that discovery, there has been a decrease in the direct release of any mercury by industrial plants. However, there are still large amounts of mercury released through fossil fuel burning.

Besides neurological symptoms, adults who consume high levels of mercury are also more at risk for cardiovascular disease, negative hormonal changes, effects on their reproductive systems, and lowered motor skill functioning. Guidelines for acceptable mercury levels in the United States are set by the Environmental Protection Agency (EPA) and the Food and Drug Administration (FDA). These standards are also used by the World Health Organization (WHO). The guidelines are based on a person’s weight, and are set using the standard of a 154-pound adult. People who weigh less or more should adjust the recommended amounts accordingly. Mercury can be measured by taking a hair sample at a doctor’s office and sending it to a laboratory for testing. Hair samples have been found to correlate with blood levels and are an easy and accurate way of measuring the amount of mercury in a person’s system.

It doesn’t take much mercury to have a great effect on the environment. One estimate suggests that it would take about one tablespoon of mercury released drop by drop into a 20-acre lake over a year to poison the entire lake. Even if the amount of mercury in the water is small, it can be magnified in the tissues of animals that live in that lake. Mercury levels accumulate as you go up the food chain. Bigger fish that prey on smaller fish are more likely to have high mercury levels. Large predatory fish such as king mackerel, tilefish, swordfish, and shark are consistently high in mercury. One way of avoiding high mercury levels is to eat smaller species or species that eat plants rather than other fish. Mercury poisoning is not restricted to ocean-dwelling fish, either. Freshwater fish in the United States can also have high levels of mercury. Walleye and Northern Pike are two species that are normally long-lived in the wild. Mercury levels in these fish are high enough that it is recommended that they not be consumed at all. Before fishing, check with local fishery authorities to learn what contaminants may be a concern in the lakes you use. One way to reduce mercury levels in the fish we purchase is to eat farmed fish. Although they often come from the same waters as wild fish, they are usually harvested when they are younger and at a smaller size, so they have had less time to accumulate mercury in their tissues.

 The choices that we make as consumers have a big impact on our environment. The United States is the third largest consumer of seafood worldwide, behind Japan and China. As people become educated about mercury levels and other factors impacting our environment, we are increasingly willing to pay more money for better-quality food that supports, rather than depletes, our environment. The Monterey Bay Aquarium has developed the Monterey Bay Seafood Watch Guide. This guide supplies information to people about which fish are most likely to have low contaminants and to be caught in a way that does not diminish sea life populations. The Seafood Watch Guide is small enough to be kept in a wallet so it is available to reference whenever it might be needed, such as at the grocery store or at a restaurant. Spending money on the species recommended by the Seafood Watch Guide sends a message to the fish buyers, restaurant owners, and fishing industry itself that we don’t want to eat seafood that causes risk to ourselves or that has not been caught or farmed in an environmentally friendly way.

More to Explore

Monterey Bay Seafood Watch Guide
Environmental Defense’s Seafood Selector
FDA Data on Mercury Levels in Seafood
EPA Limits on Mercury in the Environment
Minamata Bay Mercury Contamination

Society and the Environment: Changing Seas

catching fish

Overfishing from higher trophic levels means commercial fishers must harvest from lower trophic levels to meet demand.

Most of the food we eat comes from agriculture and farming, but we also rely on the fishing industry. About 15% of the animal protein consumed in the world comes from fish and other marine and aquatic organisms. But many fish species have been overharvested. The swordfish and cod fisheries of the North Atlantic and the salmon fishery off the northwestern coast of the United States are examples of depleted fisheries. In many parts of the world, sharks are disappearing rapidly because of the demand for shark fin soup. Some fisheries now contain so few fish that harvesting them is not economical. And the size of some of the harvested fish that remain are now smaller because they don’t survive long enough to grow.

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