October 26, 2006

On Tuesday and Wednesday of this week (Oct. 24-25), the Department of Natural Resources (DNR) coordinated a helicopter assault on the carp population in Swan Lake, which is located in Nicollet County.

Two helicopters owned and operated by Teri-John Aviation out of St. Peter delivered some 3,880 gallons of liquid rotenone to nearly 3,000 acres of water remaining in the lake.

"We're extremely hopeful that the rotenone will kill the vast majority, if not all, of the carp that have been reproducing in Swan Lake," said Ken Varland, DNR southern region wildlife manager at New Ulm. "If any do survive, however, we're banking on Mother Nature to produce winterkill conditions that will eliminate them."

Varland said "tens of thousands" of dead carp were already observed by Wednesday afternoon.

Carp, a species not native to North America, root up aquatic vegetation, causing turbidity and other water quality problems. Eventually, if left unchecked, carp-infested waters become intolerable for other fish species and water-dependent wildlife such as waterfowl.

Rotenone is a naturally occurring extract derived from the roots of various tropical plants. While the product is safe to humans and most species of wildlife and domestic animals when applied according to label instructions, it is deadly on gill-breathing animals such as fish.

Varland said rotenone has been used effectively for years to eliminate carp from wetlands and shallow lakes. It is believed, however, that the Swan Lake project is the largest of its kind in Minnesota, if not the nation. At 10,000 acres (15 square miles), Swan Lake is considered the largest prairie pothole marsh in the nation.

"We were alarmed when the presence of carp was verified in Swan Lake in November of 2005 and knew immediately that we had to do everything in our means to get rid of them," Varland said. "And when we explained the dire situation to Swan Lake duck hunters and others, they immediately got behind us, even though they knew that would mean very limited duck hunting access this fall."

The DNR plan called for immediately lowering water levels in the lake as much as possible and then treating any remaining water with rotenone just prior to freeze-up this year. Minimizing the amount of water needing to be treated serves several purposes: it lessens the cost, increases the probability of a complete kill, creates conditions more conducive to a winterkill and promotes the growth of beneficial aquatic vegetation.

Varland said helicopters began spraying rotenone on the lake about noon on Tuesday and continued until dark. They then resumed again at mid-morning on Wednesday and finished up early in the afternoon. An ice cover both mornings delayed the start of the application, forcing a continuation into the second day.

"This was an extremely complicated project to carry out, especially since the rotenone application came during the duck season," Varland said.

With some 130 duck hunting cabins and numerous other access sites to the lake, Varland said extra measures were taken to keep people off the water during the actual application. Public notices were sent out, signs posted and DNR staff stationed at all potential access sites.

More than 50 DNR staff from various divisions participated in the project. "The logistics involved in something like this is amazing," Varland said. "Staff from the Nicollet area wildlife office, particularly Kristy Zajac who coordinated the project and ran the command center, did an outstanding job pulling this all together. And the support we got from other DNR divisions was tremendous."

And now it's Mother Nature's turn, Varland said. "We've done everything within our means to protect the fabulous treasure that is Swan Lake. Now, if we get the winterkill conditions we're hoping for, our expectation is that next spring we'll find all the carp are gone and the lake being replenished by snow melt and the spring rains."