The number of zebra mussels on Lake Mille Lacs has dropped this year for the first time since they were discovered eight years ago.
But officials say that doesn't mean the explosion of the invasive species is over, or that their numbers will continue to fall.
There are still plenty of them in the state's most popular walleye lake.
A Department of Natural Resources survey completed this week found an average of about 1,070 zebra mussels per square foot in surveyed areas. That's down from nearly 1,300 per square foot last year.
"We don't know if this leveling off is a natural phenomenon, when they reach a peak of abundance, or whether it's simply weather-related,'' said Rick Bruesewitz, DNR area fisheries supervisor. "We wonder if our late ice-out last spring had anything to do with it. It's too early to say.''
Officials say it's typical for populations of invasive species to grow rapidly, then fall back.
But this year's slight decline doesn't diminish the concern they have for the potential impacts.
"At the levels they are at, they can siphon an enormous amount of water,'' Bruesewitz said.
Zebra mussels filter up to a quart of water daily and consume algae, which is food for zooplankton, essential food for small fish. The result can be much clearer water. And clearer water wouldn't be beneficial to walleyes and could spark vegetation growth.
The population explosion has been remarkable: In 2009, divers counted just four zebra mussels per square foot; in 2010 they counted 14 per square foot. Last year, that number increased nearly 100 times.