A body of water is listed as impaired when more than 10% of a fish species in a lake or river have a mercury concentration in fillets that exceeds 0.2 parts per million. If the mercury level is below 0.57 ppm, the impaired waters are included under the Statewide Mercury Total Maximum Daily Load. Lakes and rivers with mercury levels in fish above 0.57 ppm require additional reductions.
- Fish consumption advice (Minnesota Department of Health)
- Minnesota's Fish Contaminant Monitoring Program
Mercury accumulates in fish after it has been converted to the chemical compound methylmercury; other forms of mercury do not magnify in concentration up the food chain. Methylmercury is created by bacteria in highly organic portions of aquatic systems, such as the sediment of lakes and wetlands. Zooplankton pick up the methylmercury as they filter the water and feed on algae. When small fish eat zooplankton, the methylmercury builds up in their bodies as the fish grow bigger and older. Small fish are eaten by larger fish, and the concentration of methylmercury increases at each step in the aquatic food chain. It is highest in large walleye, northern pike, and other predatory fish.
For larger predatory fish to be safer to eat, MPCA scientists say that we must reduce mercury emissions to 789 pounds per year, a 76% reduction from 2005 levels. Working with stakeholders, the MPCA has developed a plan to meet this goal by 2025. Eventually, the level of mercury in Minnesota waters should be low enough that the fish in them can be eaten once a week. But a third of the mercury comes from natural sources, such as minerals in rocks and volcanoes, and people will need to continue monitoring their fish consumption because of it.
More research is needed
Frequently asked questions
How can you tell that a lake is contaminated with mercury?
It isn't possible to tell without conducting scientific analyses of the water, fish, or sediments. Nothing in a lake's outward appearance is likely to indicate whether the lake or its fish are contaminated.
Where can I find fish consumption guidelines for Minnesota waters?
MDH provides guidelines to help people make choices about which fish to eat and how often. Statewide Safe-Eating guidelines are based on mercury and other contaminants measured in fish throughout the state.
Certain characteristics of lakes, such as the run-off area, acidity or alkalinity, adjacent wetlands, and biological activity in the lake all affect the level of methylmercury contamination in the lakes' fish. Check the MDH Waterbody specific guidelines to see if the waters you fish have more restrictive guidelines.
You can find the characteristics of particular lakes, including information on fish consumption guidelines, on the Minnesota Department of Natural Resources' Lake Finder.
Would it be better if I just didn't eat any fish?
Fish are low in fat and a good source of protein and other nutrients. Benefits of eating fish outweigh the risks if you follow the guidelines from the Minnesota Department of Health.
Is there mercury in ocean fish too?
Ocean fish can be contaminated with mercury, although levels tend to be lower. Fish that eat other fish (such as tuna, shark, and king mackerel) will have the highest mercury levels.