Now the Mississippi and 12 other waters are also considered tainted in the species' march north.
Lake Winnibigoshish, one of Minnesota's largest and most popular walleye waters, has become the latest premier lake to become infested with zebra mussels.
The discovery in the state's fourth-largest lake is an ominous sign that the efforts to prevent the invasive creatures from expanding steadily northward have failed.
Worse, Winnibigoshish's waters flow into 13 other lakes or rivers, including the Mississippi River, all of which will now be considered infested. First discovered in Minnesota in the Duluth harbor in 1989, zebra mussels have spread to about 90 lakes and rivers, including Mille Lacs, Pelican, Gull, Minnetonka and Pepin.
Their presence in Winnibigoshish marks the first time they have been found in the Grand Rapids area, home to many smaller lakes that could become the next to be affected.
The mussels alter ecosystems, but no one knows yet how they might change fish populations, fishing or other water recreation -- the backbone of Minnesota's $11 billion tourism industry.
"It's disappointing when any lake is infested. But this is a large lake, and a lot of people use it,'' said Ann Pierce, Department of Natural Resources invasive species unit supervisor.
Though the DNR has imposed restrictions and beefed up enforcement to slow the spread of aquatic invasive species -- including random roadside stops last summer -- some say the discovery at Winnie proves the state is losing the battle.
"I'm disappointed, but I'm not surprised,'' said Tom Neustrom, a well-known Grand Rapids fishing guide. "It was inevitable with the way people move from lake to lake. Eventually I believe all the major walleye lakes will be infested. It's just something we're going to have to live with.''
But Pierce said the DNR isn't willing to concede.
"I wouldn't say it's us losing the battle,'' she said. "We have an opportunity to stop the spread; we all have to play a part.''
The state's 2.3 million boaters already are required by law to empty bilges, live wells and minnow buckets and remove drain plugs from their boats when they leave the water. But last year about 14 percent of boaters were caught violating the law.
Larvae in middle of lake
The latest bad news came when DNR officials announced they found two microscopic zebra mussel larvae, called veligers, in water samples collected from the middle of Winnie last July. They recently examined those samples. Though they haven't yet found adult zebra mussels -- and ice cover will prevent them from doing that now -- the presence of veligers leads them to believe that adult zebra mussels have established a breeding population in Winnie.
"We can't guarantee that, but it's far more likely than the slight possibility we stumbled on some released accidentally,'' said Gary Montz, a DNR research scientist.
Officials don't know how zebra mussels got into the lake, except that they likely arrived via boats, minnow buckets or other human activity. And they don't know what impact the invaders will have on Winnie's famed walleye fishery.
Zebra mussels filter water and consume tiny plankton that provide food for fish. Lakes often become clearer, prompting vegetation growth. And their razor-sharp shells also affect swimmers.
Veligers float for weeks, meaning that downstream waters could become infected.
They eventually attach to something and grow into a dime-size mussel. Once they are established they reproduce rapidly, with one female producing up to 1 million eggs a season.
Winnie will be officially designated as infested on Feb. 12.
"The size of the lake may delay locating an adult population, but the presence of veligers suggests there is likely a reproducing population," said Rich Rezanka, a DNR invasive species specialist. "This listing will allow [visitors] to take additional precautions to prevent inadvertent spread to other lakes."
With the infested designation come restrictions on anglers and boaters harvesting bait and transporting water.
Doug Smith • 612-673-7667