What is mercury?
Mercury is a hazardous chemical that occurs naturally in the environment and is also used in industrial applications.
How does it get into freshwater fish?
In lakes and other bodies of freshwater, small organisms convert naturally occurring inorganic mercury into its organic form, methyl mercury. Methyl mercury binds with particles and sediments eaten by smaller fish. Larger game fish prey on these smaller, mercury contaminated fish. 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.
How do we know what the extent of the problem is in South Dakota?
South Dakota samples at least 10 lakes each year for a panel of 25 contaminants including mercury. Prior to the current state testing program, only one other body of water was found 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.
How concerned should I be about exposure to mercury from eating freshwater fish?
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.
NOTE: Regarding fish from freshwater lakes, ponds, and streams where mercury levels are not currently known, the Environmental Protection Agency makes the following recommendation:
If you are pregnant or could become pregnant, are nursing a baby, or if you are feeding a young child, limit consumption of freshwater fish caught by family and friends to one meal per week. For adults one meal is six ounces of cooked fish or eight ounces uncooked fish; for a young child one meal is two ounces cooked fish or three ounces uncooked fish.
In addition, the Food and Drug Administration (FDA) has issued advice on mercury in fish bought from stores and restaurants, which includes ocean and coastal fish as well as other types of commercial fish. FDA advises that women who are pregnant or could become pregnant, nursing mothers and young children not eat shark, swordfish, king mackerel, or tilefish. FDA also advises that women of childbearing age and pregnant women may eat an average of 12 ounces of fish purchased in stores and restaurants each week. Therefore, if in a given week you eat 12 ounces of cooked fish from a store or restaurant, then do not eat fish caught by your family or friends that week. This is important to keep the total level of methylmercury contributed by all fish at a low level in your body.
Is mercury a risk with swimming or other recreational activities in freshwater?
No. There is no known health risk related to mercury from swimming, boating, or catch and release fishing. While mercury levels may be high in fish, the chemical's concentration is typically as much as 100,000 times lower. Handling fish is highly unlikely to expose an individual to elevated levels of mercury.
Do other states test for mercury contamination and issue advisories?
Many states test freshwater fish for mercury contamination and according to the EPA more than 30 have issued fish consumption advisories. South Dakota's neighboring states of Minnesota, North Dakota, Nebraska and Montana have all issued mercury-related consumption advisories in the past.