Fish consumption advisories for methylmercury now account for more than three-quarters of all fish consumption advisories in the United States. Forty States have issued advisories for methylmercury on selected water-bodies and 13 states have statewide advisories for some or all sportfish from rivers or lakes. Coastal areas along the Gulf of Mexico, Maine, and the Atlantic Ocean from Florida through North Carolina are under advisories for methylmercury for certain fish.
You can read more about it here, "Mercury in the Environment" from the US Geological Survey http://www.usgs.gov/themes/factsheet/146-00/
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