The Lake Where You Live

Lake residents and lake lovers fear zebra mussels perhaps more than any other aquatic invasive species. They encrust pier supports and boat hulls. They cover almost any hard object in the water. Their broken shells can cut waders’ feet. They multiply by the millions.

All that, and they alter a lake’s ecosystem, filtering out algae and disrupting the food web. And now a research team in Minnesota has found that levels of toxic mercury in walleyes and perch are higher in that state’s lakes infested with zebra mussels than in lakes without them.

Mercury of course is a subject of health advisories against eating more than certain amounts of fish from some northern Wisconsin lakes, including a favorite bluegill lake of mine. So the research is concerning. 

The principal investigator for the research project at the University of Minnesota is Gretchen Hansen. In one of my books I cited another of her studies involving invasives’ impact on fish populations. It showed that young walleyes were smaller than normal after their first year in lakes with spiny water fleas.

The Minnesota Star Tribune reported, “Average-sized walleyes sampled from mussel-impaired lakes such as Gull, Pelican and Tenmile were more than twice as likely to exceed mercury thresholds tied to human health … They contained 72 percent higher mercury concentrations compared to walleyes in uninvaded lakes.”

The study found mercury levels even higher in perch from 12 mussel-infested lakes that were surveyed. Hansen, s fisheries, wildlife and conservation biology professor, told the newspaper, “It was quite a large difference. I was surprised to see this level of mercury. It’s another way that zebra mussels are impacting our lakes, our food.”

The logical question: Why would this happen? The researchers hypothesize that the mussels change water chemistry in such a way that inert mercury already in the lake can take on a different form that small aquatic organisms can absorb. Fish feeding on those creatures then take in the mercury, which over time can accumulate (biomagnify) in their flesh. 

Like Minnesota, Wisconsin has advisories for eating fish with mercury. For example, statewide, the DNR recommends that women under age 50 and children under 15 eat no more than one serving of walleye per month; more protective advisories are listed for waters where mercury, a central nervous system toxin, is known to exceed a certain threshold. 

In the current study, walleyes on average 16.5 inches long were more than twice as likely to contain at least 0.22 parts per million of mercury (Minnesota’s threshold level) in lakes with zebra mussels versus those without. Average-sized perch of average size were 50 times more likely to exceed the threshold. 

The study was published in Science of the Total Environment, a peer-reviewed journal. Besides the university, contributors included the Minnesota DNR and the Mercury Research Lab at U.S. Geological Survey.

This research underscores the importance of preventing the spread of zebra mussels, which can be carried in boat ballast, bait buckets, live wells and other containers in the form of tiny larvae called veligers. It appears that Clean Boats Clean Waters and Clean-Drain-Dry initiatives are becoming more important than ever.

Ted Rulseh resides on Birch Lake in Harshaw and is an advocate for lake protection and improvement. His Lakeland Times and Northwoods River News columns are the basis for a book, “A Lakeside Companion,” published by The University of Wisconsin Press. Ted may be reached at [email protected].