Questions and Answers About Mercury in Fish

What is mercury? How does it get into fish? Mercury (Hg) occurs in the environment naturally and as a result of human activity. It's released into the air when rocks erode, volcanoes erupt and soils decompose. It's also released when power plants burn coal, incinerators burn mercury-containing waste, and when mercury thermometers break.

Mercury enters lakes and other waters in rain, snow and runoff. Certain type of bacteria convert it to a form that's easily absorbed by insects and other small organisms. Small fish eat the insects and small organisms; big fish eat the small fish. In this way, mercury levels accumulate up the food chain so that large predator fish such as walleye contain the highest amounts. When people eat these fish, they accumulate mercury as well.

People who frequently eat fish high in mercury can accumulate levels that may harm their health. That's why the Wisconsin Department of Natural Resources and Division of Public Health advise people about the number of meals and species they can safely eat. By following these recommendations, known as the fish consumption advisories, Wisconsin residents and visitors can continue to enjoy the fish they catch and benefit from this low-fat, low-cost source of nutrition, while reducing their exposure to mercury.

Why should we be concerned about mercury in fish?

Fish is the main source of mercury in human diets. Ingesting too much mercury can harm your health. At low doses, mercury can harm the developing nervous system in a fetus or may affect cardiovascular and immune systems in adults. At high doses, mercury can affect your central nervous system, triggering such health problems as memory loss, slurred speech, hearing loss, lack of coordination, loss of sensation in fingers and toes, reproductive problems, coma, and possibly death.

Who is most at risk?

Fetuses and babies are most at risk because their developing nervous systems are much more sensitive to mercury than adults' nervous systems. An estimated 60,000 children born each year in the United States are at risk of suffering neurological and learning problems because their mothers eat large amounts of mercury-contaminated fish and seafood during their pregnancy. Fish is good food and can give your child a healthy start if you eat a variety of fish that are low in mercury and other contaminants.

Do we have to stop eating fish?

No. Continue to enjoy eating fish. The nutritional benefits - fish are low in saturated fat and high in protein - outweigh the potential risk of mercury as long you follow advice for the number of meals and species you can safely eat. Most people have a very low risk of suffering mercury-related health problems. If you eat a lot of fish, you can lower your exposure to mercury by eating fish known to have low mercury levels. Small fish such as perch and bluegill have less mercury than large predator fish, such as walleye, northern pike or bass. There is no need to stop eating those shore lunches! It's safe to catch and eat fish from any of the lakes or rivers on the advisory as long as you follow the safe eating guidelines.

I've eaten fish from a lake that has just been put on the advisory. What is going to happen to me or my children?

In the future, use the fish advisory as a guide for how much fish to consume from lakes and rivers carrying advisories. Most people do not eat fish frequently enough to cause a problem. The risks to you and your children from eating a few meals of contaminated fish are still relatively small. We recommend you should avoid eating fish more frequently than the advisory recommends.

So what you're saying is you've got to eat a ton of fish from these places to have a problem, right?

Not exactly. It depends on where you catch the fish, the mercury levels found in fish from that water, and the fish size and species. Carefully read the advisory. You may not have to change your eating habits at all, or you may choose to keep the sizes and species of fish that best fit your diet.

Are the fish I buy in a store contaminated too? How about fish in restaurants?

Most ocean fish you'll find at the store - species such as pollock, shrimp, and salmon - have very low levels of mercury. However, the U.S. Food and Drug Administration (FDA) [exit DNR] recommends that pregnant women and those planning to get pregnant not eat swordfish, shark, king mackerel and tilefish because these species contain high levels of mercury. FDA has found that women of childbearing age can safely consume up to 12 ounces of fish per week (2 average meals per week). Wisconsin fish fry typically is cod, haddock, or perch, which are among the safest species of fish to eat.

You have little cause for concern if you eat or buy a variety of commercial fish or eat less than one meal per week of fish. Regardless of whether the fish you eat is store-bought or caught from Wisconsin waters, monitor the amount and species of fish you are eating and follow the state's fish consumption advisory to decide which are safest.

Can I tell whether a fish is high in contaminants just by looking at it?

No. Sometimes people see tumors or lesions on fish and think the fish must be contaminated. That's usually not the case. The tumors and lesions are more likely caused by a parasite in the fish's skin. Also, you cannot tell by the clarity of the water whether or not a water body is contaminated. Cloudy or muddy water may not have contaminated fish, but the opposite can also be true. Consult the advisory for more information.

How is mercury different from polychlorinated biphenyls (PCBs), which are also found in some fish?

Like mercury, PCBs can pose a health threat to developing fetuses and young children. PCBs also have been linked to immune dysfunction and cancer in adults. Because they build up in your body over time, it's important to reduce your lifetime exposure to PCBs, starting at an early age. The state's health advisory lists waters contaminated by PCBs as well as mercury.

However, there are many important differences:

What fish have been tested?

Wisconsin has more than 15,000 lakes and 40,000 miles of rivers. The DNR has been able to test fish from over 1,500 sites over the past 35 years, focusing efforts on popular lakes and rivers and those near industrial centers. Because of the large amount of data we've gathered, we know how much mercury is generally found in different fish species. As a result, most - but not all - inland lakes carry the same statewide advice for how many meals of certain species people can safely eat while reducing their risk of exposure to mercury. Some waters have more restrictive advice due to higher concentration of mercury and PCBs. There are also separate recommendations for sensitive populations including women of childbearing years, nursing mothers and all children under the age of 15.

Can I swim in waters with an advisory?

The amount of contaminants you would inadvertently ingest as a result of swimming in waters contaminated with environmental toxins is insignificant. However, ingesting surface waters should be avoided unless properly treated.

What is being done to reduce mercury contamination?

Wisconsin has taken many actions to reduce mercury in the environment, including banning the sale of mercury in batteries and working with communities and citizens to remove mercury from thermometers and other equipment in homes, schools, and at work. The department also is working to develop rules to require major sources to reduce their mercury emissions. DNR also actively supports a mercury reduction initiative led by Minnesota and is working with other Great Lakes states and EPA to develop a broader mercury reduction effort.

For more information on mercury in fish, please contact:

Candy Schrank, Aquatic Toxicologist
(608) 267-7614