Because fish eliminate mercury at a very slow rate, it accumulates in their tissues and organs where it cannot be removed by filleting or cooking, unlike organic contaminants that concentrate in the skin and fat. South Dakota samples at least 10 lakes each year for a panel of 25 contaminants including mercury. Prior to that, testing found only one other body of water with mercury levels above the FDA's action level of 1ppm. That advisory was issued in 1974 for the Cheyenne River and the contamination was traced to mining activity in the Black Hills. The testing is a joint effort of the SD Department of Game, Fish and Parks, the SD Department of Environment and Natural Resources, and the SD Department of Health.
Fish is an important source of high-quality protein that provides many health benefits.
At the 1ppm mercury level, the Department of Health recommends that people space their meals of such fish to limit consumption to safe levels. The department's advisory follows the FDA's action level and recommends that healthy adults eat no more than 7 ounces of fish per week with mercury levels close to or slightly above 1ppm. Women who plan to become pregnant, are pregnant or are breast-feeding, and children under age seven should eat no more than one 7 ounce meal of such fish per month. The FDA action level of 1ppm is 10 times lower than the lowest levels associated with negative neurological effects observed in mercury poisoning incidents. There have been no incidents of mercury poisoning related to fish consumption in humans reported to the department in South Dakota.