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.

Hansen, in an interview, stressed that people should be concerned about the study's findings and be vigilant of how invasive species are disrupting Minnesota's lakes. But the new research doesn't carry the message that it's not safe to eat wild-caught fish or that fish from lakes invaded by zebra mussels are toxic and shouldn't be consumed, she said. Smaller fish generally have lower concentrations of mercury, for instance.

"We need to pay attention to what size fish we eat,'' Hansen said. "Fish is normally a sustainable and healthy food source and still can be. We should be aware of risks and benefits of all of our food sources, and perhaps pay special attention to monitoring mercury in lakes with zebra mussels.''

Mercury is a well-known contaminant in fish and it causes the most concern among public health officials. It's responsible for about 80% of all fish consumption advisories globally.

The state health department says large amounts of mercury may harm the nervous system. The department says young children, developing fetuses and breast-fed babies are at most risk, because small amounts can damage a brain that is just starting to form or grow. Too much mercury may affect a child's behavior and lead to learning problems later in life. Adults, too, can have health effects, including heart and kidney damage, health officials say. Consumption advisories recognize that the amount of mercury you take into your body is eliminated, to a degree, between meals.

The new study, published Tuesday in the peer-reviewed scientific journal Science of the Total Environment, started in 2019 and involved the state Department of Natural Resources and the Mercury Research Lab at U.S. Geological Survey (USGS). Funding came from USGS and Minnesota's Environment and Natural Resources Trust Fund.

Previous research in the Great Lakes suggested a correlation between zebra mussels and higher mercury levels in fish, but Hansen's team is the first to publish a study relating to mercury in fish sampled from mussel-contaminated inland lakes. The study lakes — all actively managed for walleye fishing by the DNR — were chosen for their similarities. The study did not include Minnesota's 10 largest walleye lakes.

"Mercury concentrations were higher in fish of all life stages from lakes containing zebra mussels compared to uninvaded lakes," the researchers wrote in their journal article. The article concluded: "Zebra mussel invasion may result in a greater potential mercury consumption risk to humans."

In Minnesota, 16% of lakes managed for walleyes by the DNR contain the sharped-shelled mussels — often in such massive concentrations that they blanket large portions of lake bottoms.

Researchers hypothesized that these mats — including the mussels' waste products — snuff out a layer of oxygen and give rise to certain bacteria that transform or "methylate" mercury that would otherwise be inert. Once methylated, the mercury is taken up by bugs, snails and other organisms that get eaten by perch, walleyes and other fish. When lakes are altered in this way, it concentrates food consumption in near-shore areas where the mercury methylation is occurring, according to researchers.

Perch are common table fare, but they also act as a primary food source for walleyes — a species so coveted by anglers that it was designated by the Legislature in 1965 as Minnesota's state fish.

Hansen said she's interested in conducting additional, related research. Meanwhile, she will be collaborating with state health officials over the study's findings, she said.