The Minnesota Department of Natural Resources confirmed on July 23 reports of zebra mussels in Lake Oscar in Douglas County.

A lakeshore property owner contacted the lake association after finding a three-quarter-inch adult zebra mussel. The lake association reported it to Douglas County aquatic invasive species staff who, in turn, contacted the DNR. Divers from the DNR found a total of 78 adult zebra mussels in four locations on the lake.

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

The DNR had earlier confirmed small patches of flowering rush, an invasive aquatic plant, in Grants Lake in Douglas County. According to a July 18 release, it’s the first time the species has been found in the county.

The DNR staff has begun removing the plants by hand, which has proven effective for small populations. They will monitor the site for any regrowth and take additional control measures if required.

Flowering rush is a weed-like plant with pink flowers that can overtake habitat, crowd out native vegetation and make it difficult for boats to access open water. It’s a perennial that grows 1 to 4 feet high in shallow water. It flowers in early summer through mid-fall.

In deeper water, it can grow in a submerged form that does not produce flowers. Flowering rush can be difficult to identify when not in bloom, as it closely resembles many beneficial native plants such as the common bulrush.

People can spread flowering rush primarily by moving watercraft or illegally releasing aquatic garden plants into public waters. It reproduces by spreading small onion-like buds, which can be hidden in mud and debris and can stick to boots, waders and other fishing and hunting gear.

Flowering rush is a prohibited invasive species in Minnesota, which means it is illegal to possess, import, purchase, transport or introduce this species except under a permit for disposal, control, research or education.

Lake Alexander in Morrison County

The DNR also confirmed zebra mussels in Lake Alexander in Morrison County on July 18 in another news release.

DNR divers found two adult zebra mussels, about one-half inch in length, near Soldiers Island on the west side of the lake. Two more invasive mussels of the same size were found at the boat access on the east side of the lake.

Reports of zebra mussels in 2018 in another part of the lake could not be substantiated, despite numerous searches in 2018 and 2019.

Preventing the spread of invasive species

Whether or not a lake is confirmed to harbor invasive species, Minnesota law requires boaters and anglers to:

  • Clean watercraft and trailers of aquatic plants and invasive species.

  • Drain all water by removing drain plugs and keeping them out during transport.

  • 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 launching watercraft in 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.

People should contact an area DNR aquatic invasive species specialist if they think they have found zebra mussels or any other invasive species that has not already been confirmed in a lake.

More information is available at mndnr.gov/ais.