This commentary was written by Brad Johnson of South Dakota Searchlight, an online news organization.

Anglers catching fish in zebra mussel-infested lakes may be reeling in adult walleyes with mercury content 72% higher and perch with 157% higher levels than those being caught on non-infested lakes.

In a recently-released report, researchers studied 21 Minnesota lakes, 12 containing zebra mussels and nine without. The study reports that zebra mussels alter a lake’s makeup in several ways. The razor-sharp mussels filter the water, eating algae, zoo plankton and phytoplankton, resources used by small fish.

They can make lakes crystal-clear, enhancing native and invasive plant growth and reducing the number of small bait fish near shore. The excrement from zebra mussels contains concentrated amounts of mercury, and that excrement forms mats on the soil near shore.

Walleye and perch, the study said, lacking normal food sources, begin foraging for scarce food in the lake bottom, ingesting the mercury-laden material.

A 2020 Minnesota study also shows that zebra mussels stunt the growth of walleye and perch.

Given the threats, one wonders why South Dakota is so lackadaisical in its approach to containing the spread of not just zebra mussels, but all invasive aquatic species.

Fishing alone is extremely important to South Dakota’s economy, with approximately 225,000 anglers spending nearly $531 million annually, according to a state Department of Game, Fish & Parks 2022 economic impact study.

A 2019 study, “The Economic and Social Values Associated with Small South Dakota Lakes,” reports “that for every dollar spent in South Dakota by anglers fishing a lake in this study there was an economic return of $1.70.”

The study, by South Dakota State University Ph.D. student Aaron Sundmark, focused on the impact of the state’s approximately 400 small lakes.

South Dakota has 577 lakes and reservoirs under state management, and lakes are prominently featured in state tourism campaigns. Yet the state invests little in protecting these vital resources.

The South Dakota Lakes and Streams Association has implored South Dakota political leaders and Game, Fish & Parks to take the zebra mussel spread more seriously. The group is frustrated by the state’s limited effort.

The invasive mussels entered South Dakota in 2014 in Lewis and Clark Lake in the Missouri River. They have steadily marched across the state. They now infest 22 lakes as well as the Missouri River, James River and Big Sioux River systems including Sand Lake Refuge, according to GF&P’s aquatic invasive species website, SD Least Wanted.

The high mercury concentrations especially can have serious health effects, particularly on unborn children.

“Mercury contamination of aquatic ecosystems is a substantial public health issue,” the study said, “and mercury is responsible for about 80% of all fish consumption advisories globally.”

Because South Dakota’s government has had a limited response to the spread of zebra mussels, organizations like SDSLA have been forced to take the lead.

SDLSA has sponsored a $100,000-plus study of the economic impact of zebra mussels on South Dakota.

The study is being conducted at the University of South Dakota in partnership with South Dakota State University. Assisting in the study is Nanette Nelson, a research economist with the University of Montana. Her 2019 Montana study predicted that if zebra mussels were to colonize all the water bodies in that state, they would cause up to $122 million in mitigation expenses, up to $112 million in lost revenue and up to $497 million in lost property value.

Results of SDLSA’s study are about a year away. In addition to higher mercury levels in fish, zebra mussels attach themselves to structures such as power systems at the hydroelectric dams on the Missouri River.

Back in Minnesota, researchers found that mercury concentrations increased as the fish grew larger.

The study said, “Median mercury concentration in adult walleye of average size (about 16.5 inches) from lakes with zebra mussels was 0.30” parts per million (ppm) or 1.4 times greater than the 0.22 ppm level that is the level where people should begin limiting fish consumption.

“Mercury is in every lake anyway because of atmospheric conditions,” SDLSA President Steve Charron noted, “and is most often found laying around in the shallow parts. When fish start scavenging around in things they don’t usually eat, mercury increases in fish.

“It is just another warning shot to pay attention as zebra mussels spread. The water may be clear but the weeds and the mercury that come with it show that it is not a good thing to have zebra mussels in your lake.”

The Minnesota study was conducted by researchers from the University of Minnesota College of Food, Agricultural and Natural Resource Sciences, Minnesota Aquatic Invasive Species Research Center, and the U.S. Geological Survey.