Bob Gilsdorf holds a rock with a zebra mussel on the left and a native snail on the right on the dock of his Pelican Lake home in 2012. DNR officials said Friday that Cass Lake and many of the lakes near it are now considered zebra mussel infested. Ann Arbor Miller / MPR News 2012

Zebra mussels have been found in Cass Lake, the first time the invasive species have been confirmed in the Bemidji area, the Minnesota Department of Natural Resources said Friday.

A citizen discovered the mussels earlier in the week on the southeast corner of Cedar Island, a popular beach and swimming area. The DNR followed up with its own research and found zebra mussels "in a variety of sizes," the department said in statement.

Because of the find, Cass Lake, one of Minnesota's largest, will be designated as zebra mussel infested, as will the stretch of the Mississippi River between Cass Lake and Lake Winnibigoshish, the DNR said. Connecting rivers and lakes, including Buck, Andrusia, Wolf, Pike Bay, Pug Hole, Kitchi, Little Rice and Big Rice lakes, will also be designated as zebra mussel infested.

• Map: See where zebra mussels have spread in Minnesota

"While no zebra mussels have been found in these lakes, they are heavily used by boaters traveling from Cass Lake," the DNR said.

The agency this summer confirmed zebra mussel discoveries in other northwest Minnesota Lakes, including popular Lake Melissa, near Detroit Lakes.

More than 175 Minnesota lakes and rivers are now infested with zebra mussels, an invasive species that can push out native mussels. They can also cause more algae and weed growth — they filter the lake water, allowing sunlight to penetrate more deeply.

They attach to boats, making it easy for them to travel and contaminate other waters.

Boaters are required to clean their boats after leaving a lake to keep from transporting mussels that might attach to the boat hulls.

The DNR and county officials across Minnesota have been working this summer to beef up boat inspection and enforcement. Gov. Mark Dayton earlier this year signed legislation to distribute $10 million a year to local governments to fight the mussels and other aquatic invasive species.