Despite "aggressive treatment," zebra mussels continue to reproduce and establish at Hennepin County's Christmas Lake, the Department of Natural Resources conceded Friday.
The recent discovery of more zebra mussels there is a blow to Minnesota's efforts to defeat the invasive pest. Officials had focused much of their experimental work on Christmas Lake, including the use of treatment that began a year ago that promised to kill zebra mussels without harming the rest of the ecosystem.
In April, the DNR felt like it had won a skirmish against the Christmas Lake zebra mussels when it announced that a three-step treatment to kill a pocket of mussels in the lake had been effective on a "small, isolated infestation" that was detected early" and that divers had found no evidence of zebra mussels.
The news on Friday, however, was downbeat after the discovery of 16 mussels.
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"None of these newly discovered zebra mussels were found in the area of Christmas Lake that was treated earlier this year, but most of those we found were juveniles," Keegan Lund, DNR invasive species specialist, said in a statement.
"That tells us reproduction has occurred and this population is established," the agency acknowledged. "Because zebra mussels are scattered across the lake and reproducing, current management options are not feasible."
More than 200 Minnesota lakes and rivers are now confirmed with zebra mussels, an invasive species that can push out native mussels and cause other problems for lakes and boaters.
They have sharp edges that can cut if stepped on. 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.
DNR officials continue to plead with the public to protect against zebra mussel contamination by cleaning boats, draining all water and keeping plugs out while transporting boats between lakes and disposing of unwanted bait in the trash.
"There is a common misconception that zebra mussels 'are everywhere' and that their spread is inevitable. The reality is, zebra mussels have been confirmed in less than 2 percent of Minnesota lakes, and more Minnesotans than ever before know and follow invasive species laws," Lund said.
The DNR said it will continue to work with the local homeowners on assessing Christmas Lake. While the agencies involved in the treatment project are disappointed, the agency says the information gained from the effort was "worthwhile and will be used for future treatments in other bodies of water."
The DNR last week announced plans for an experimental treatment of Ruth Lake in Crow Wing County, saying the success at Christmas Lake had informed plans for Ruth.
It's not clear now if treatment plans for Ruth will change given the defeat at Christmas Lake.