Mercury is found in the environment because of natural and human activities. When moving through the environment, mercury goes through a series of complex changes. Through these changes in lake and river sediments, an organic form of mercury – methylmercury – is created. Methylmercury is very persistent in the environment and moves up the food chain to predator species. It can accumulate in people who eat these predator species. Predator species of fish include all species of black bass (largemouth, smallmouth and spotted), striped bass, white bass, hybrid striped bass, walleye, sauger, saugeye, flathead catfish, muskellunge and northern pike.

In order to protect the most sensitive populations, pregnant or nursing women, women of childbearing age and children younger than 15 years of age are advised to eat no more than one meal per week of predator fish. Since women beyond childbearing age and males older than 15 years of age are at less risk for the effects of methylmercury, these groups may continue to enjoy as many meals of predator fish as they please. For more information, see Facts About Illinois' Methylmercury Advisory.

More restrictive meal advice applies to fish taken from the following bodies of water.