More fish and another lake in western South Dakota have been added to the list of state waters with mercury problems.
Recent analysis of fish sampled in Newell Lake in Butte County north of Newell detected levels of mercury in northern pike more than 18 inches long above the 1 part per million threshold in fish tissue recommended by the Food and Drug Administration.
Newell already had been added to the state list in January with a fish advisory on walleyes 18 inches and longer, based on previous testing.
More recent sampling also led state officials to extend the existing mercury advisory on Roosevelt Lake near Colome in Tripp County, which had applied only to largemouth bass 18 inches and longer, to also include northern pike more than 24 inches. And Coal Springs Reservoir near Meadow in Perkins County has been added to the advisory list, for northern pike over 25 inches long.
Recent sampling results also extended the advisory in East River waters to Reid Lake in Clark County for walleyes more than 23 inches long and to Opitz Lake in Day County for northern pike more than 26 inches long.
There are now 12 lakes on the state mercury advisory list, five of them west of the Missouri River.
John Lott, fisheries section chief for the state Game, Fish & Parks Department in Pierre, said the mercury advisories do not mean listed fish are unsafe to eat. But they do, as the name suggests, advise people to consider the mercury content when they decide how much of the listed fish to eat, he said.
"The whole idea of the advisory is to have an informed public," Lott said. "It's not to discourage them from eating fish, but just to use wise judgment in the size and type of fish they eat, and how often."
Mercury can cause serious neurological problems when ingested at certain levels. Lott said those levels have been shown to be much higher than the 1 ppm threshold used by the state.
Nonetheless, the state advisory recommends that healthy adults not exceed 7 ounces of fish under advisory each week. Seven ounces is roughly the size of two decks of playing cards.
Children younger than 7 years old should have no more than one 4-ounce meal of listed fish per month. And women who are breast feeding, pregnant or about to become pregnant should limit their consumption of listed fish to 7 ounces per month.
It is virtually impossible to pinpoint the exact source of mercury in fish tissue. The potentially harmful element exists naturally in the environment but also is produced by human activities. Those include power generation at coal plants, internal-combustion engines and improper disposal of materials containing mercury.
Whatever the source of mercury in the environment, it tends to invade the aquatic cycle when vegetation is flooded and decay of those materials occurs. Then it builds in the food chain and tends to concentrate in larger predator fish, which are the ones most often listed in advisories.
Higher water levels after years of drought might have been a factor in the recent increase of advisory listening for state waters, Lott said.
"Our water levels have come up a lot since 2008, and one of the contributing factors with mercury is the decomposition of plant matter in a lake and the bacteria that take the atmospheric mercury and make it into methyl mercury, which can be taken into fish tissue," he said.
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In some lakes, the advisory has potential for daily impacts. Bitter Lake in Day County is a hot walleye lake, a popular place for anglers and a listed lake with an advisory on walleyes of all sizes, as well as larger northern pike.
Yet there is no indication that the advisory has affected fishing pressure at the lake, Lott said.
"We've had 150 boats a day a lot of days at the one boat ramp we have in service on Bitter," he said.
It isn't clear how much the advisory on Bitter Lake has affected actual consumption of walleyes caught in the lake, he said.
"I don't know if it discourages any anglers from harvesting fish there, but at least on our part, they have been informed," he said.
On some other advisory lakes, the impact on anglers is less widespread. Newell Lake, for example, doesn't have a large population of either larger walleyes or larger northern pike.
"We pick up a few in our sampling, but we don't normally see a lot," said Gene Galinat, GF&P regional fisheries manager in Rapid City. "We do see quite a few smaller pike in there."
Coal Springs Reservoir in Perkins County is also not a big-pike hot spot, he said.
"There aren't a lot of pike in there," Galinat said. "It had pretty much dried up during the drought. We did see that some of the pike made it through."
The state advisory list could grow larger next year if the state decides to lower its threshold level for mercury in fish tissue from the FDA standard of 1 ppm to an Environmental Protection Agency level of .3 ppm. The EPA is encouraging that lower standard.
Working with the state Department of Environment and Natural Resources and the South Dakota Department of Health, GF&P fisheries crews expanded their sampling this year to get a broader base of data on state fishing waters and fish species. That additional information will help in determining next year whether to go to the lower threshold and give a baseline for further testing.
Contact Kevin Woster at 394-8413 or email@example.com