Zebra mussels found near Orwell Reservoir [UPDATED]

Published 12:27pm Tuesday, May 7, 2013 Updated 12:31pm Tuesday, May 7, 2013

There’s bad news on two fronts when it comes to zebra mussels in Otter Tail County.

On Tuesday, the Minnesota Department of Natural Resources said a new treatment failed to kill zebra mussels in Rose Lake near Vergas. At the same time, the DNR has also confirmed that zebra mussels have been found in the Otter Tail River below Orwell Reservoir.

Low water levels in the Otter Tail River below Orwell Reservoir revealed zebra mussels of various sizes attached to rocks and debris.

“The presence of zebra mussels below Orwell Reservoir is not surprising as these zebra mussels were found downstream of where the Pelican River enters the Otter Tail River,” said Nathan Olson, DNR invasive species specialist in Fergus Falls. “Zebra mussels were discovered in the Pelican River and Pelican Lake near Pelican Rapids in September 2009.”

The DNR has since seen zebra mussels move downstream into Lizzie and Prairie lakes, and it now appears they have found their way into the lower Otter Tail River.

As a result of these zebra mussel discoveries, Lake Miltona and the lower reach of Otter Tail River, from the confluence of the Pelican River downstream to the Bois De Sioux River confluence, will be designated as infested waters.

Lake Ida will also be designated for precautionary reasons because it is the next lake downstream of Miltona. The Pelican River is already designated as infested waters from Bucks Mill Drive downstream to the confluence with Otter Tail River.

To treat Lake Irene in Douglas County and Rose Lake in Otter Tail County, DNR used copper sulfate, a chemical commonly used to treat lakes for swimmers’ itch.

Olson says the agency did not find any zebra mussel larvae or large zebra mussels during scuba searches this summer. But Olson says during fall searches the DNR did find adult zebra mussels in both lakes.

Olson says the DNR cannot confirm that zebra mussels have reproduced in either lake. But the recent find means efforts to kill the zebra mussels were unsuccessful.

In both cases, boat lifts used in infested lakes were moved to non-infested waters.

DNR biologists collected water samples to look for larval zebra mussels (veligers) and conducted scuba and lake equipment searches to look for juvenile and adult zebra mussels throughout the 2012 open water season. Field work to evaluate the eradication efforts was completed last fall.

“We did not find any veligers in our water samples or large zebra mussels during our scuba searches last summer,” Olson said. “However, during our fall searches, we did find adult zebra mussels in both lakes.”

Olson added that because monitoring efforts did not produce any veligers or juvenile zebra mussels, the DNR cannot confirm that zebra mussels have reproduced in either lake. The recent find of adult mussels means that despite early detection and a rapid DNR response, efforts to kill the zebra mussels in these lakes were not successful.

DNR staff will continue to monitor these lakes next year to evaluate the zebra mussel populations.

The DNR has also confirmed that water samples taken late this summer from Kerbs, Paul and Rusch lakes in Otter Tail County contained zebra mussel larvae (veligers).

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