Whatever the source, under the right conditions in the environment, mercury can change into methyl mercury, which binds with particles and sediments to be incorporated into the food chain and accumulate in living tissues.
In lakes and rivers, that can mean trouble for fish and for the people who eat them.
“It ends up in the insects, the insect-eating fish and the predator fish, which is where it really starts to accumulate,” state Game, Fish & Parks Department regional fisheries manager Gene Galinat of Rapid City said.
Large predator fish such as walleyes, northern pike and largemouth bass are most likely to show the higher levels of mercury. And on Newell Lake, the advisory recommends limited consumption of walleyes 18 inches and longer after tissue testing revealed a .96 level, close enough to 1 part per million to merit inclusion on the list.
Galinat said additional testing will be done in coming years to see if other fish, particularly the popular largemouth bass, need to be added to the advisory. High, murky water conditions at the lake have complicated sampling there, leaving uncertainty about the largemouth bass population, Galinat said.
“We didn’t get a real good sampling on the bass in there,” he said. “Now that we’ve done this screening and got that level on walleyes, we’ll be back more often to sample those top-line predator fish.”