Zebra mussel larvae discovered in Vermilion, Kabetogama lakes

For years, Lake Vermilion and Lake Kabetogama seemed special.

As Minnesota’s other large and heavily fished lakes fell one-by-one to zebra mussel infestations, the two sprawling wooded giants in northern St. Louis County remained clean. Many hoped that the unusually low amount of calcium in the two waters made it impossible for the devastating invasive mussels to grow their shells.

Those hopes were dashed recently when the Minnesota Department of Natural Resources said it found zebra mussel larvae in the two lakes, as well as in nearby Crane and Namakan lakes.

The discovery could upend ecosystems and change the way people fish, boat and swim in some of the largest and most widely used lakes in northern Minnesota. Local residents and guides are hopeful, though, that the lakes’ nutrient makeup will stave off devastation.

“It’s very sad,” said Jeff Forester, executive director of Minnesota Lakes and Rivers Advocates. “And it’s preventable.”

Zebra mussels are about fingernail-sized and typically spread when boaters unknowingly pick them up in infested waters and carry them to new lakes. They choke out other aquatic life and damage fish habitat.

With the addition of Vermilion and Kabetogama, the mussels have now spread to all 10 of Minnesota’s largest and most popular fishing destinations, including Lake of the Woods, Mille Lacs, Upper Red, Leech, Winnebigoshish, Cass, Pepin and Rainy lakes.

There is still hope that the mussels will not be able to overrun and forever alter the water and aquatic life inside Vermilion and Kabetogama, as they have at other Minnesota lakes.

The calcium levels are low enough that the mussels may not be able to explode in number, said Richard Rezanka, an invasive species specialist for the DNR.

Rezanka spent Wednesday diving in Vermilion, searching small islands and outcrops to try to pinpoint where the mussels are breeding.

“We still don’t know, is it a really small isolated population?” he said. “Will they expand like they’re so famous for doing? It’s a big question mark. No crystal ball.”

Justin Chromy, a fishing guide on Lake Vermilion, points to other lakes in the Canadian Shield, where the waters are surrounded by exposed bedrock and little soil, such as Lake of the Woods and Rainy Lake.

“You can see in those lakes that they’ve cleared a bit, but not to the extreme extent where you see that gin-clear water in central Minnesota,” he said. “I don’t anticipate it’ll change the fishery too much in Vermilion.”

The Vermilion Lake Association has been one of the most active organizations in the state, working for decades to keep out mussels and other invaders, inspecting boats at launches and leading public education campaigns.

But with more than 15,000 boats launched into the lake each year from dozens of sites, it’s impossible to check them all.

“We tried awfully hard to keep them out,” said Terry Grossman, the association’s fisheries director.

Now the attention will turn to keeping more of the zebras from coming in.

They cluster together in beds by the tens of thousands, attaching themselves to any hard surface they can find, including rocks, wood, the shells of larger mussels, pipes, docks and swimming rafts. Their hard, sharp shells cut the feet of swimmers. They also cause expensive damage to water intake pipes and power plants.

Zebra mussels have caused extensive damage to lakes and rivers throughout the Midwest since they hitchhiked their way into the Great Lakes through the ballast water of ocean freighters in the 1980s.

They reached Minnesota by 1989, when they were discovered in Duluth Harbor. They’ve since been found in about 400 of the state’s lakes and wetlands as well as in hundreds of stretches of river.

University of Minnesota researchers found in 2024 that walleye and perch pulled from infested waters had 72% higher mercury concentrations as fish from lakes without zebra mussels. The researchers believe the invaders 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.

Mercury is one of the big concerns for Forester, whose family has a cabin on Vermilion.

The DNR did not find adult mussels in any of the four lakes, but the presence of the larvae shows that the invasive species has established itself and is reproducing.