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 has found another negative effect of invasive zebra mussels in lakes: higher levels of mercury in fish.

Researchers from the University of Minnesota, the U.S. Geological Survey and other agencies studied 21 Minnesota lakes and compared mercury concentrations in the tissue of two species, walleye and yellow perch.

The results were unexpected: They found adult walleye in lakes infested with zebra mussels had 72 percent higher mercury concentrations than in lakes without the invaders. In yellow perch — a common prey for walleye — the mercury concentrations were 157 percent higher. 

Smaller and average-sized walleye and perch from lakes with zebra mussels also were much more likely to exceed 0.22 parts per million of mercury, a threshold that triggers more restrictive advisories on how much fish humans can safely eat.

Gretchen Hansen, an associate ecology professor at the U of M and one of the lead authors, said there were other surprising results.

“We saw this effect in multiple species and across multiple life stages,” she said. “It wasn’t just the adult walleye. We saw similar responses of yellow perch, and also young-of-year, or juvenile or baby fish.”

Those juvenile fish are not as likely to be eaten by humans. But Hansen said the findings point to larger trends of what’s happening with mercury in infested lakes, and the pathways by which fish are affected.

“It’s like the whole system is being changed in a way that leads to higher mercury in the prey of walleye and in the walleye themselves,” she said. 

Mercury is released into the air, often by coal-fired power plants and other industries. It mainly enters lakes through atmospheric deposition, carried by rain or snow. 

When mercury enters a lake, it’s in pure elemental form. Bacteria in the water convert it to a more toxic form called methyl mercury, which can be absorbed by fish or humans.

How much mercury gets into fish depends on several factors, Hansen said, including the lake’s food web and where fish are eating.

When zebra mussels infest a lake, they can form dense mats on the bottom. That creates sediment without oxygen, an ideal environment for the bacteria needed to convert mercury, Hansen said.

At the same time, filter-feeding zebra mussels are sucking nutrients out of the water and depositing them on the bottom as waste, she said. As a result, walleye and perch may shift their feeding habits nearer to shore, where more mercury is available.

“I think of it as sort of like a one-two punch,” Hansen said. “Neither one of these processes would affect mercury in fish necessarily by themselves.”

Hansen said researchers focused on 21 medium-sized lakes considered good for walleye, but not among Minnesota’s 10 largest. Most are clustered in the middle of the state. Twelve have zebra mussels and nine do not. Otherwise, they share similar characteristics.

Popular fishing lakes are more likely to be infested with zebra mussels, because they tend to have more boaters, Hansen said.

“It just points to the magnitude of potential effects on species like walleye (that) are going to be felt in those places, because that’s where zebra mussels are showing up,” Hansen said. She said the team is interested in further study to learn whether the same effect is happening in other lakes.

The study was published in the journal Science of the Total Environment. 

Hansen said the findings don’t mean it’s unsafe to eat fish. She said people might want to be more cautious eating fish from lakes with higher densities of zebra mussels, possibly by eating smaller fish or those lower on the food chain.

The Minnesota Department of Health issues fish consumption 

 for many Minnesota lakes based on mercury levels. The guidelines are lower for children under age 15 and people who are now or may become pregnant or breastfeed.

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Kirsti Marohn

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Researchers found adult walleye in lakes infested with zebra mussels had 72 percent higher mercury concentrations than in lakes without the invaders. Smaller walleye also were more likely to exceed mercury thresholds meant to protect human health.

Dan Gunderson | MPR News

Matthew Hintz for MPR News

Reminders about invasive species are posted at a public boat launch.

Study: Walleye in lakes with zebra mussels have higher mercury levels

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