• Contaminant levels in walleye warrant caution; state issues consumption advisory
Walleye is one of South Dakota's best known and most popular fish – for both catching and eating.
But a recent news release by the state Department of Health put out a "consumption advisory" for Newell Lake in Butte County.
There could also be a slight concern for walleye caught in eastern South Dakota, says a fisheries expert at South Dakota State University.
Recent testing of walleye larger than 18 inches caught in Newell Lake found them to have mercury levels approaching 1 part per million (ppm). The Food and Drug Administration has set a mercury level of 1 ppm as the point at which consumption should be spaced to prevent potentially dangerous accumulations of mercury.
The South Dakota 2011 Fishing Handbook, which is available online, cites eight lakes that presently have elevated mercury levels. None are in Brookings County. Close to home, however, are the West Highway 81/Twin Lakes in Kingsbury County, which have shown elevated mercury levels in walleye greater than 18 inches in length and northern pike longer than 19 inches.
The number of South Dakota lakes that have demonstrated high mercury levels in their fish is a moving target every year.
One of South Dakota's leading experts on the causes of high mercury levels in fish is Steven R. Chipps, a professor of Wildlife and Fisheries working out of the Northern Plains Biostress Lab on the South Dakota State University campus.
He explained, "I say (the number and location of lakes) changes every year in the sense that we're adding one or two lakes every year; because we haven't tested every single lake in South Dakota for mercury. Every year we add about 10 to 12 new lakes to sample and monitor."
Sloughs or lakes?
Chipps spoke with the Register last week and explained in layman's language the complicated chain of events that lead to mercury contamination in fish.
Pinpointing the location of the Kingsbury County bodies of water, he explained, "If you're going toward Madison, there's a couple big sloughs east and west sides. We refer to it as the 'West-81 sloughs.'"
Are they really lakes or sloughs? Actually, a bit of both.
He added, "They used to be sloughs; now they're large sloughs, I guess. The water certainly expanded in those. They used to be semi-permanent wetlands; now, of course, theyre large lakes. They're flooded right to the road."
These large-slough turned large-lake bodies of fresh water are more suseptible to having walleye with mercury levels near the one ppm that drive a "consumption advisory" from the state's department's of Environment and Natural Resources; Game, Fish and Parks; and Health.
Chipps looks back "to the late '90s when we had that first slug of high water and all the lakes started to expand in eastern South Dakota." Many of the smaller bodies of water and wetlands "doubled, tripled, quadrupled in size, flooding new vegetation. Previously unflooded areas were now flooded."
Tied to that is "the most significant source of mercury in the environment"; it comes via what Chipps called "atmospheric deposition."
He explained that "dry particles and even precipitation – rain and snowfall – brings the ionic form of mercury that's in the atmosphere down to the landscape where it's ultimately deposited on vegetation."
He added that the predominant source of the ionic form of mercury is "coal-fired power plants, sulfuir dioxide emissions and such."
Then when flooding with new water occurs, it kills the grass and vegetation. The mercury that "is bound to the plants and things gets reincorporated to the 'aquatic food web.'
"There's a certain series of bacteria that convert that ionic form of mercury into what we call 'methyl mercury.'"
From that bacterial level, each step-up of consumption in the food chain leads to higher concentrations of the methyl mercury; Chipps called the process "bio-magnification."
The end result was that "lakes that expanded more tended to have walleye, pike and some of those sport fishes with higher mercury levels than lakes that didn't (expand)."
Lower mercury levels
Larger lakes, however, are not by virtue of their size going to yield fish with higher mercury levels. As an example, Chipps used Lake Poinsett, about 35 miles north of Arlington on U.S. 81 and one of South Dakota's largest natural lakes.
"In fact," he said, "the mercury concentrations in fish in Lake Poinsett are quite low, relative to the other lakes. The reason for that is Poinsett has an outlet; the water goes into the Big Sioux.
"It's continuing being flushed, if you want to think of it that way."
Fish that ingest mercury tend to keep it; so it accumulates, meaning that in most instances larger and older fish will tend to have more mercury than younger, smaller fish.
"Their elimination rates are very slow," Chipps explained. "In humans, we do eliminate mercury; but it's still a neurotoxin to us in the methyl mercury form.
"So when we eat something like fish that may have high mercury concentrations, it can be dangerous for us." And for some population groups it poses more hazards.
EPA numbers conservative
The South Dakota Department of Health points out that some simple precautions should be taken when eating fish caught in waters where advisories are in effect.
Adults should eat no more than seven ounces per week and children younger than 7 years old should eat no more than one 4-ounce portion of the specified fish per month. Women who are pregnant, planning to become pregnant or breast-feeding should eat no more than one 7-ounce meal per month. Seven ounces of fish is about the size of two decks of playing cards.
Contact John Kubal at firstname.lastname@example.org. The South Dakota Department contributed to this report.