As the crow flies, Swan Lake lies about six miles due north of the Minnesota River.
But carp can’t fly.
So it’s anyone’s guess about just how they managed to infest the sprawling, 10,000-acre waterfowl lake just west of Nicollet.
Stein Innvaer, a Minnesota Department of Natural Resources assistant area wildlife manager stationed at Nicollet, said it was believed that the waterfalls located a few hundred yards from where Nicollet Creek, the lake’s outlet, empties into the Minnesota River would serve as a natural barrier to prevent any upstream migration.
He speculated that perhaps the persistent fish found an avenue of sheet water or even traveled up tile lines to reach the lake.
Another possibility, he said, was that the fish came from nearby Middle Lake. Even though it is in a completely different watershed, a ditch flowing from that lake eventually joins with one flowing from Swan Lake.
However they got there, by 2005, Swan Lake, rich in heritage and history as a waterfowl haven, had fallen on tough times as as huge schools of the roughfish roamed, uprooting aquatic plants and clouding the water.
Duck hunters scanned mostly empty skies as the lake, lacking good food sources, became a fly-over zone for many migrating waterfowl.
It was time for some action.
The lake was partially drained that summer with the hope that winter conditions would facilitate a winter kill of existing fish populations. A mild winter nixed that so the draw-down continued into 2006 until only 3,000 acres of water remained.
That fall, an application of rotenone, a chemical that causes fish to suffocate, was then applied to ensure a complete die-off of surviving carp.
And things have been getting nothing but better on the popular hunting lake.
“With the fish gone, it has been a textbook recovery,” Innvaer said. “Opening day bag checks of hunters the following year were the highest ever recorded — about four ducks per hunter.”
Noting that muskrat populations are a good indicator of a healthy system, he said that trappers took an estimated 50,000 of the furbearers from the lake this past season.
“It’s just ideal out there right now,” he said.
To ensure it stays that way, work is underway to replace the previous water control structure that dated back to the late 1980s.
Key to the new structure, in addition to light-weight aluminum stop logs connected to a cranking system to facilitate easier removal for water level management, will be a fish barrier, something the previous structure lacked.
Innvaer said several designs including a velocity tube, a smooth-walled, inclined culvert that carp are unable to navigate through, and an electronic fish barrier were considered.
“It just wasn’t the right place for a velocity tube and you can imagine the expense of operating an electronic barrier,” he said.
Instead, the barrier will be of more conventional — closely spaced metal bars and a device called a “head knocker” to prevent fish from leaping over it — and designed to be easily cleaned of vegetation and debris.
Design and construction is being done by Ducks Unlimited staff and will be completed in the near future, well before any fish runs might begin.
The $125,000 project is being paid for with money raised from the 3/8 of 1 percent sales tax increase approved by voters in 2008 as the Clean Water, Land and Legacy Amendment.
John Cross is a Free Press staff writer. Contact him at (507) 344-6376 or by e-mail at email@example.com.