Mercury levels in certain fish in 17 South Dakota waters are high enough to prompt warnings about eating them.
The state maintains a list of those advisories, which encourage people -- particularly children, pregnant women, women planning to get pregnant or those who are breast feeding -- to limit the amount of listed fish they eat.
And an ongoing lake sampling program tries to keep track of mercury levels in waters throughout the state to see if more advisories are needed.
A state Game, Fish & Parks Department fisheries crew was on Whitewood Creek Thursday to take small tissue samples from brown trout for testing. The crew used electrofishing gear to temporarily stun the trout so they could be netted and checked in two sections of the creek in and near Deadwood.
The crew used a mix of baking soda and acetic acid in create a carbon-dioxide mixture that reduced oxygen levels enough to make the normally rambunctious trout easier to handle. Then the fish were measured and weighed before crew members took a small tissue sample for testing.
"The reason we're looking for mercury, it's one of those heavy metals we know is toxic for humans and mercury has the ability to bio-accumulate, or bio-magnify, and what that means is it's difficult to get out of your body," Gene Galinat the regional fisheries manager for the state Game, Fish & Parks Department in Rapid City said. "And fish find the same thing."
The work here on Whitewood Creek is part of a larger effort to keep track of mercury levels in fish populations across the state. GF&P crews sample fish in lakes, streams and rivers every few years for signs of mercury. Those with a problem are put on the advisory list.
Most fish on the advisory list are larger predator fish, such as northern pike. The larger predators feed on smaller fish, which have themselves fed on lower life forms in the food chain. Mercury concentrates as it moves up the chain.
"What happens is that it gets picked up in the algae and insects eat it. It gets into the panfish and other smaller fish and then the predator fish, the northern pike and others, eat those fish and they get more mercury," Galinat said. "Typically we're looking at the bigger predator fish being on the list."
Pinpointing the source of mercury problems in lakes is difficult. The element is naturally in the environment but typically become s problem because of man-made mercury sources, including coal-fired power plants.
Despite pollution a century of gold mining, an extensive and expensive clean-up effort beginning decades ago has done much to restore Whitewood Creek. It sustains a wild brown trout population and is not listed on the mercury advisory list. Becuase of it's history and continued mining in the area, Whitewood Creek is intensively monitored by mining companies and other government agencies.
But Galinat says the state includes Whitewood Creek in its mercury listing program to make sure.
"We sample different waters every year," Galinat said. "The last time we were on Whitewood Creek was probably 10 or 12 years ago. So we're just on our cycle to come back to Whitewood Creek."
Today's fish samples will be analyzed at a state Health Department lab. Galinat hopes for good news. So do those who fish the creek with more traditional gear.