By John Cross
Free Press Staff Writer
MANKATO — The deadline for moving fish houses from lakes in the southern two-thirds of the state is midnight Monday but Ted Witt of Rapidan already pulled his from Loon Lake near the community of Lake Crystal last weekend.
But before he packed up, he was seeing things that didn’t bode well for the lake and its fish in these waning winter days.
“The crappies were coming into our holes after we drilled them,” he said. “They didn’t bite, they just kind of sat there.”
He suspects their strange behavior was due to falling oxygen levels in spite of an aeration system that is operating on the lake to prevent such events.
“It would be a shame to lose the lake ... if it winterkills, it will be another five years before the fishing is any good there again,” he said.
It has been a long, hard winter and some area lakes prone to suffer winterkills indeed are at a tipping point, says Hugh Valiant, supervisor at the Department of Natural Resources Fish Hatchery in Waterville.
And given the extended period of wintery conditions — area lakes have been virtually snow-covered since mid-December — aeration systems installed on many lakes to prevent such occurrences are being severely tested.
Valiant said that the effectiveness of an aeration system depends largely on the amount of open water it creates. “The amount of open water is proportional to the gas exchange that occurs,” he said, adding that variables like the wind can effect the amount of oxygen infused into a lake.
And while smaller, less efficient systems may have pulled lakes through in the past, they may not be able to this year.
Predicting or preventing winterkill is hardly an exact science, something that DNR crews are discovering as they regularly have monitored oxygen levels in area lakes during the past several weeks.
Some lakes where crews expected to find dramatically falling oxygen levels have been holding up remarkably well.
Among them include Lake Henry, Lake Elysian, and Albert Lea Lake.
Lake Henry, a shallow lake utilized as a walleye rearing pond, has no aeration system, but curiously has been holding its own this winter.
Lake Elysian and Albert Lea Lake, both sprawling shallow water bodies, have suffered winterkills during past severe winters, even with aeration systems in operation.
This year, in spite of the prolonged winter conditions, oxygen levels are holding up well in those and some other shallow lakes, and Valiant said he suspects the wet conditions the area experienced way back in October may be the reason.
“After all of that wet weather in October, we had a pretty good flow in the streams,” he said. “Those lakes that have good stream flow are holding up — we don’t expect them to winterkill.”
And curiously, some of the lakes have higher oxygen levels at the bottom than they do right beneath the ice, exactly the opposite of what is usually found, prompting Valiant to speculate that curly-leafed pondweed still may be growing in spite of the lack of light penetrating the snow and ice.
Another important variable in the winterkill picture, Valiant said, is how quickly levels drop. When oxygen levels decline gradually, some fish can adapt to tolerate levels as low as .5 parts per million.
Notably, northern pike, perch, black bullheads — even walleye — can tolerate oxygen levels well below 1 part per million. Species that don’t adapt well to low oxygen levels include bass, bluegills, crappies, and surprisingly enough, the ubiquitous carp.
When winterkill is imminent on a lake, DNR Fisheries frequently opens the lake up to liberalized fishing regulations to allow the harvest of the fish without regard to limits with the use of nets, etc.
So far this season, Dora and Pepin Lakes in Le Sueur County are the only area lakes that have been opened to liberalized fishing, and those regulations expire today.
A complete list of state lakes open to liberalized fishing can be found at the DNR Web site at www.dnr.state.mn.us, clicking on Fishing and then going to Angler Alerts.
In the meantime, other lakes indeed are poised to suffer winterkill after all of these weeks of wintery conditions.
What those lakes need, Valiant said, is warmer weather that will infuse lakes balanced on the edge with oxygen-rich run-off.
Unfortunately, above-freezing temperatures are tough to find in the near forecast.
John Cross is a Free Press staff writer. Contact him at 344-6376 or by e-mail at jcross@mankatofreepress.com.
