Zebra mussels decline on Lake Mille Lacs

After growing exponentially for several years, the zebra mussel population on Lake Mille Lacs has fallen for the second consecutive year — indicating the population might be leveling off.

"It seems to have stopped increasing and maybe is declining a bit,'' said Tom Jones, Department of Natural Resources regional fisheries treaty coordinator.

That's welcome news for the beleaguered lake, one of the state's most popular fishing destinations.

Zebra mussels were discovered in the 200-square-mile lake in 2005, and their population exploded. The average density of infested areas peaked at 1,269 zebra mussels per square foot in 2012. The per-square-foot density fell to 1,072 last year and 875 this year.

"We expected a rapid increase, and then a leveling off, and we're seeing that,'' Jones said.

He doesn't know where the population will settle, but its numbers remain impressive.

About 35 percent of the lake bottom is infested with the invaders, Jones said. The remaining 65 percent of the lake bottom is too soft to support them.

Based on densities he's documented, Jones calculates there are about 2.2 trillion zebra mussels weighing about 2 billion pounds, dwarfing the lake's walleye biomass.

"That was eye-opening,'' Jones said.

Historically, there's been about 2 million pounds of walleyes in the lake, though officials believe there's only one-third that amount today due to a puzzling decline in the walleye population.

Officials still aren't sure what impact, if any, zebra mussels have had on Mille Lacs. The mussels filter vast amounts of water, but water clarity hasn't changed since they showed up, Jones said. Officials will continue to monitor water chemistry in the lake, to watch for changes.

Jones presided over a public meeting last week in Isle on walleye reproduction in Mille Lacs. About 65 people attended. Another session will be 6:30-8 p.m. Thursday in the New Brighton Community Center in New Brighton.

An ongoing study is examining stomach contents of walleyes, northerns and smallmouth bass to determine feeding patterns.

Jones said there are many variables that could be affecting the walleye population, including changing fish populations, climate change, zebra mussels and another invasive, spiny waterfleas, which eat zooplankton — key food for small fish.

Doug Smith • doug.smith@startribune.com