The list of South Dakota lakes with worrisome levels of mercury in fish grew again in 2012, but state environmental specialists say that doesn't mean people should stop eating fish.
Based on testing last year, three lakes in eastern South Dakota were added to the existing list of 12 lakes — five of them west of the Missouri River — where the state recommends limiting consumption of certain fish species. And an existing mercury advisory at another East River lake was expanded to include another fish species.
Patrick Synder of the state Department of Environment and Natural Resources in Pierre said, however, that people should not be afraid to eat fish in South Dakota. That includes fish from lakes covered by the advisories, he said.
"I think the big message is that fish is still an important source of protein with its omega 3 oil," Snyder said. "We're not recommending not eating fish but just limiting consumption of these fish that have advisories."
Elm Lake in Brown County near Aberdeen in northeastern South Dakota was added to the list in 2012 for walleyes 22 inches and longer. Also added were Lake Minnewasta and Middle Lynn in Day County of the northeast, both for walleyes 18 inches and longer.
That brought the advisory total to 15 lakes. In addition, the previous mercury advisory for walleyes 18 inches and longer at Island Lake in Minnehaha and McCook counties near Sioux Falls was extended to smallmouth bass 18 inches and longer.
The 15 lakes on the list came from a total of 160 bodies of water in the state that have been tested since 1994. State Game, Fish & Parks Department fish crews do the sampling, and the fish are then submitted for laboratory analysis.
DENR, GF&P and the state Department of Health work together on the mercury program. The state samples at least 10 lake a year for 25 potential contaminants, including mercury.
PCBs and pesticides are also on the check list, although it's rare to have them detected in South Dakota fish samples. Mercury has been the ongoing problem in South Dakota, as in many other states.
None of the three South Dakota lakes added to the list in 2012 were west of the Missouri River, where there are already five bodies of water on the list. The closest to Rapid City is Newell Lake, north of Newell in Butte County, where advisories are in place for walleyes and northern pike 18 inches and longer.
The state issues an advisory when mercury levels in fish samples reach 1 part per million. The recommendation is then for healthy adults to eat no more than 7 ounces of fish under advisory per week. Children under age 7 and women who are breast feeding, pregnant or plan to become pregnant should eat no more than 7 ounces of such fish per month, according to the advisory.
Other West River lakes on the list are: Pudwell Dam in Corson County, for walleyes 18 inches and longer; Lake Isabel in Dewey County, northern pike 25 inches and longer, largemouth bass 17 inches and longer; Coal Springs Reservoir in Perkins County, northern pike over 25 inches; Lake Roosevelt in Tripp County, largemouth bass 18 inches and longer, northern pike over 24 inches.
In a few instances, environmental officials believe they can pinpoint the source of the mercury. In Coal Springs Reservoir, for example, it is likely because the reservoir was built on a coal seam that had been mined. Mercury is naturally associated with coal.
But usually it is difficult to determine the exact source of mercury in specific lakes, said Shannon Minerich, an environmental scientist with DENR in Pierre.
"In our advisory lakes, there can be a handful of different possible reasons," Minerich said. "There are a lot of variables with mercury."
The culprit in fish advisories is an organic form of mercury called methylmercury, which is formed by natural actions when mercury enters a water body. That organic form can be easily absorbed by plants in lakes, which are then eaten by small fish, which are eaten by bigger fish.
Mercury levels magnify up the food chain, so larger predator fish are typically the most likely to make the advisory list. Fish species on the current advisory list are walleyes, northern pike, largemouth bass and smallmouth bass, although other species are tested.
Mercury is a naturally occurring element but also finds its way into the environment by industrial activities, such as coal-fired power plants and improper disposal of products containing mercury.
It is difficult, however, to tie mercury levels in a lake to a specific power plant.
"Overall, power plants contribute to global mercury in the air," Snyder said. "But the mercury in these lakes isn't necessarily from power plants around here."