Contaminants and Minnesota Fish - EH: Minnesota Department of Health

Contaminants and Minnesota Fish

How do Minnesota's fish compare with other states?

Minnesota has one of the most extensive fish monitoring programs in the United States. It is not because Minnesota has some of the most contaminated fish; rather, Minnesota has more lakes and river miles than other states. All of the Great Lake states, and Ontario, face many of the same problems with mercury and PCB contamination, and all issue fish consumption advisories. However, Lake Superior is the least contaminated of the Great Lakes. Mercury contamination in Wisconsin and Ontario's inland lakes is comparable to that in Minnesota. Guidelines that states issue for inter-state border waters may differ because of differences in how health risks are interpreted.

What contaminants are found in Minnesota fish?

In Minnesota, mercury is the contaminant in fish that causes the most concern. Air pollution is the major source of mercury that contaminates the fish in Minnesota’s lakes and rivers - see Reducing Mercury Contamination in Fish. Over time, fish can accumulate relatively high mercury concentrations. That’s why it’s important to make choices about the fish you eat and how often you eat it.

Fish in Lake Superior and major rivers such as the Mississippi River may contain PCBs (PDF). These synthetic oils had many uses and are found in electrical transformers, cutting oils, and carbonless paper. Although they were banned in 1976, they do not easily break down and remain in the water and lake sediments for years. PCB levels in Minnesota waters are slowly decreasing.

Dioxins are inadvertently produced through a number of human activities as well as by natural processes. Results from an US EPA study of contaminants in fish from lakes across the US indicate that dioxins are found in every fish tested. The levels of dioxins in the fish tested from Minnesota as part of this study are low overall and low in comparison to other areas of the country. Dioxins accumulate in animal fat and are therefore also present in meat and dairy products. At this time MDH does not provide advice to limit fish consumption based on dioxins in fish.

Per- and Polyfluoroalkyl Substances (PFAS) are a family of manmade chemicals that have been used for decades to make products that resist heat, oil, stains, grease and water. Common uses include nonstick cookware, stain-resistant carpets and fabrics, as components of fire-fighting foam, and other industrial applications. Perfluorooctane sulfonate (PFOS) is the PFAS that can build up to levels of concern in fish. The Minnesota Pollution Control Agency is investigating the sources of perfluoroalkyl substances in fish.

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Health risks of eating contaminated fish

It may take months or years of regularly eating fish to accumulate levels of PCBs, methylmercury, and PFAS to build up in your body that are a health concern. The consumption advice given by MDH is intended to keep  contaminants below levels that may cause health effects.

As you follow the fish advisory, the amount of methylmercury you take into your body is safely eliminated between meals. Large amounts of mercury may harm the nervous system. Young children, developing fetuses and breast-fed babies are at most risk, because small amounts of mercury can damage a brain that is just starting to form or grow. Too much mercury may affect a child’s behavior and lead to learning problems later in life. The first symptoms of adult mercury poisoning include incoordination and burning or tingling sensation in the fingers and toes. As mercury levels increase, your ability to walk, talk, see, and hear may all be affected in subtle ways.

Exposure to PCBs is linked to infant development problems in children whose mothers were exposed to PCBs before becoming pregnant. The consumption advice for PCBs is intended to protect children from developmental problems. PCBs also cause changes in human blood, liver, and immune functions of adults. In addition, PCBs cause cancer in laboratory animals and may cause cancer in humans.

Currently, cancer will affect about one in every two people in Minnesota, primarily due to smoking, diet, and hereditary risk factors. If you follow the advisory over your lifetime, the PCBs in the fish you eat may not increase your cancer risk at all. At worst, using Environmental Protection Agency methods to calculate risk from a lifetime of eating contaminated fish, it is estimated that approximately one additional cancer case may develop in 10,000 people eating contaminated fish, according to this advisory. Eating fewer meals of contaminated fish will further decrease your cancer risk.

PFAS and Health

Minnesota’s fish consumption guidelines were developed following the Great Lakes Consortium for Fish Consumption Advisories Best Practice for Perfluorooctane Sulfonate (PFOS) Guidelines (PDF).

MDH continually reviews ongoing research  to ensure that our guidelines reduce exposures and protect public health. As new studies and science become available, our advice may be revised to reflect additional information.

How do contaminants get into fish?

Contaminants can reach rivers and lakes from local sources such as improperly stored wastes and abandoned dumps. If a local source is identified, it may be possible to clean it up and decrease the contamination of the lake or river. However, contaminants also reach remote and pristine lakes from the atmosphere.

Once in a lake, mercury is converted to methylmercury by bacteria. Fish absorb methylmercury from their food. Mercury is tightly bound to proteins in all fish tissue, including muscle. There is no method of cooking or cleaning fish that will reduce the amount of mercury in a meal. Larger, older fish and fish which eat other fish accumulate more contaminants than smaller, younger fish which eat less contaminated prey.

Fish absorb fat-soluble chemicals like PCBs and dioxins from water, suspended sediments, and food. PCBs and Dioxins concentrate in the fat of fish, and in fatty fish such as carp and catfish. Cleaning and cooking a fish to remove fat will lower the amount of PCBs and dioxins in a fish meal.

PFAS are very stable chemicals that do not change or break down in the environment. As a result, they may build up in fish. The Interagency Fish Contaminant Monitoring Program (PDF) works with MPCA to monitor fish for PFAS. As with mercury, removing the fat when cleaning or cooking does not reduce the amount of PFOS in the edible parts of the fish.

Harmful Algal Blooms and Fish Consumption

Use caution when fishing and consuming fish caught from waters where a bloom may be occurring

Toxins from algae can accumulate in the entrails (guts) of fish and occasionally in the muscle (fillet) of fish. Levels in fish depend upon the severity of the bloom in the area where the fish are caught.

In general, fish that are caught in areas of a waterbody where major blue-green algae blooms occur may be safe to eat, as long as the guts of the fish are discarded. However, there is some uncertainty about the levels of algal toxins that can accumulate in filets, so anglers may want to wait a week or two after algal blooms are over before fishing and eating fish from waters where a bloom is occurring.

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Updated Wednesday, 18-May-2022 12:19:52 CDT