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How does mercury get into fish?

To understand how mercury contaminates fish, consider the mercury cycle. It begins with mercury being emitted to the atmosphere by sources such as coal-burning power plants. The mercury washes out of the air with precipitation and comes down on land and water.

In the mud of lakes, rivers and wetlands live bacteria then convert around 10 percent of this mercury into methylmercury, an organic compound readily absorbed by the digestive systems of animals and people (other forms of mercury are not as easily absorbed by the digestive system).

Microscopic animals in the water ingest the methylmercury as they feed on algae and bacteria. These tiny creatures are eaten by small fish, which are eaten in turn by progressively larger fish, up the aquatic food chain. At each level, the concentration of methylmercury in the flesh of fish increases threefold or more, a process called "biomagnification."

Big fish not “keepers” if you want to avoid mercury

As a result, the higher concentrations of methylmercury are found in large predator fish, such as walleye, northern pike, and swordfish. Eating these fish gives a much higher exposure to mercury than eating small, young predators or pan fish such as bluegills or perch.

Consequently, you can eat small fish more often than large predator fish. Since methylmercury is bound to the proteins in the flesh, there's no method of cleaning or cooking fish that can reduce methylmercury.

Methylmercury concentrations in fish vary widely because of physical, chemical and biological factors. For example, lakes with a high proportion of wetlands in their watersheds are more likely to have high mercury concentrations in predator fish, because more mercury merhylation rakes place in wetlands.

Mercury levels in water and sediments are three to four rimes higher today than a century ago. One might expect fish contamination to be the same. There is evidence that sulfate-contaminated rain over the last century increased methylmercury, so that fish in some lakes were up to 10 rimes more contaminated than they were prior to industrialization.

Part of the reason that mercury contamination in fish is slowly declining in Minnesota (about 1 percent a year) is that the federal Clean Air Act Amendments of 1990 succeeded in reducing sulfur emissions by 50 percent nationwide. Mercury emissions have also declined.
If these trends continue as expected, so should the decline of mercury concentrations in fish.

Find out more about about the mercury cycle from the government’s U.S. Geological Survey Web site.

-- Sam Brungardt
(Contact the author at 651-282-6410 or by email at sam.brungardt@pca.state.mn.us)