Walleyes and perch taken from Minnesota lakes infested with zebra mussels contain starkly higher levels of mercury than those taken from uninvaded lakes, according to new University of Minnesota research published this week.
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, the study found. They contained 72% higher mercury concentrations compared to walleyes in uninvaded lakes. The study found even higher concentrations of the neurotoxin in perch taken from the 12 study lakes impaired by zebra mussels, an invasive species.
“It was quite a large difference,” said Gretchen Hansen, fisheries, wildlife and conservation biology professor, the principal investigator. “I was surprised to see this level of mercury. It’s another way that zebra mussels are impacting our lakes, our food.’’
Angela Preimesberger, a fish consumption guidance scientist at the Minnesota Department of Health, said she’s very interested in the findings and will consider the implications as a member of the state’s Interagency Fish Contaminant Monitoring Program. Any changes to local consumption guidelines issued by the state will be considered after collaborative discussions, she said.
A central hypothesis in the study is that the invasive mussels change lake chemistry in a way that alters pre-existing, inert mercury into a form that can be absorbed by bugs and other organisms in the food web. For comparison, researchers also sampled perch and walleyes in nine uninvaded lakes, including Koronis, Big Sandy and Shamineau.
Minnesota’s broad fish consumption guidelines get stringent when fish tissues are tainted with mercury in concentrations over 0.22 parts per million. At that mark, and up to 0.95 parts per million, the consumption guideline is for people in sensitive populations not to eat more than one meal a month. That applies to people who are or may become pregnant, people who are breastfeeding or plan to breastfeed and children under age 15.
In study lakes containing zebra mussels, walleye mercury concentrations exceeded 0.22 parts per million in average-sized walleyes at a rate of 77%, compared to 35% in uninvaded lakes.
Put another way, adult walleyes of average size (16.5 inches long) were more than twice as likely to exceed the 0.22 parts per million threshold in lakes with zebra mussels compared to uninvaded lakes. Adult yellow perch of average size were 50 times more likely to exceed 0.22 parts mercury per million in lakes with zebra mussels.