Zebra mussel larvae have spread to one of Minnesota's biggest, best-known walleye lakes.

The nearly million-acre Lake of the Woods, which straddles the Canadian border, had a "substantial" amount of the aquatic invasive species larvae in one of three sites where water was sampled, the state Department of Natural Resources announced Friday. The DNR has added Minnesota's side of the lake to its list of waters infested by the mussels.

The news is a setback to anglers and others who regularly visit the giant lake on Minnesota's Northwest Angle, but officials emphasized that no juvenile or adult mussels were found. They are hopeful that the lake's natural features, including its low calcium levels and robust sturgeon population, will prevent the mussels from taking hold. "With so much traffic coming up to Lake of the Woods … certainly there was a chance those things could be spread," said Joe Henry, executive director of the Lake of the Woods Tourism Bureau. "We are hoping that ... [they] don't make it to maturity."

Zebra mussels have been spreading throughout the state and continent.

They can change a lake's food supply as the tiny, sharp-shelled mollusks multiply, filter out nutrients and make the water more clear and less habitable to aquatic plants. They can cut swimmers' feet, attach to boat motors and damage water pipes.

Listing Lake of the Woods as an infested body of water will mean tighter regulations on some activities such as harvesting bait and using lake water for other purposes.

Minnesota law requires boaters and anglers to clean and drain all water from boats and trailers and dump unwanted bait in the trash.

Since invasive species can be small and difficult to see, boaters are urged to spray their vessels with high-pressure water and rinse with very hot water or dry them for at least five days before moving to another waterway.

It's not known how the mussel larvae got to the 70-mile-long Lake of the Woods, the sixth largest freshwater lake in — or partially in — the United States. In addition to being transported by people, invasive species can sometimes be introduced from a connected water or tributary.

At this point, Lake of the Woods' population of walleye and sauger is "extremely healthy," Henry said, and he and others say that walleye populations haven't suffered greatly from zebra mussels in some other large lakes, including Lake Erie.

DNR officials said they will monitor the lake closely, hoping that its water chemistry and existing fish will help fight off a mussel invasion.

"There is information out there that says lakes that have low calcium, for example, may not have as severe or may not have as bad an infestation," said Phil Talmage, the DNR's Area Fisheries Supervisor in Baudette.

"Lake of the Woods has a very robust lake sturgeon population. There's a chance they may prey on the zebra mussels. That's been documented in other systems as well."

Nick Painovich, longtime co-owner of the Zippel Bay Resort on the lake's south shore, said he's been talking to biologists, including his son, about the potential for zebra mussels in the lake that has been his livelihood for 40 years.

He and others hope that the mussels will find it difficult to attach themselves on the sediment-bottomed south shore.

If the lake's brown water becomes more clear, he said, it may change how anglers approach fishing.

"I don't think it'll destroy our walleye fishery," Painovich said. "We'll see if it changes where we find fish and how we fish them, I guess."

Pam Louwagie • 612-673-7102