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Zebra mussel larvae confirmed in Rainy Lake in St. Louis County (published September 1, 2021)

The Minnesota Department of Natural Resources has confirmed zebra mussel larvae in Rainy Lake, near International Falls in St. Louis County. Rainy Lake straddles the Minnesota and Canada border. Around one-third of Rainy Lake is located within Voyageurs National Park. The zebra mussel larvae were found in waters outside the park boundary.

Four of five DNR water samples taken in July of 2021 contained zebra mussel larvae, suggesting a reproducing zebra mussel population in Rainy Lake. These samples followed up on a July 2020 report of a single adult zebra mussel.

At that time, the DNR did not find zebra mussels in Rainy Lake and none of the microscopic zebra mussel larvae were found in water samples.

The DNR has been in contact with the Canadian government, the National Park Service (NPS) and the Bois Forte Band of Chippewa about this zebra mussel confirmation. The NPS collects water samples from Rainy Lake and connected lakes as part of a cooperative long-term monitoring program. 

While recent NPS samples did not show veligers, the larger monitoring effort will be important to understanding how extensively zebra mussels may be spread in this connected system of lakes.

Whether or not a lake has any invasive species, Minnesota law requires boaters and anglers to:

  • Clean watercraft and trailers of aquatic plants and prohibited invasive species,
  • Drain all water by removing drain plugs and keeping them out during transport, and
  • Dispose of unwanted bait in the trash.

Some invasive species are small and difficult to see at the access. To remove or kill them, take one or more of the following precautions before moving to another waterbody:     

  • Spray with high-pressure water.
  • Rinse with very hot water (120 degrees for at least two minutes or 140 degrees for at least 10 seconds).
  • Dry for at least five days.

Zebra mussels can compete with native species for food and habitat, cut the feet of swimmers, reduce the performance of boat motors and cause expensive damage to water intake pipes.

Anyone who suspects a new infestation of an aquatic invasive plant or animal should note the exact location, take a photo or keep the specimen, and contact a Minnesota DNR aquatic invasive species specialist.

More information is available at