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DNR warns of possible winterkill on certain area lakes

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outdoors Park Rapids, 56470
Park Rapids Minnesota PO Box 111 56470

The lakes in our region have common similarities and diverse differences. Influenced by numerous variables, each body of water has its own set of challenges that affect native fish and wildlife.

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Minnesota's Department of Natural Resources (DNR) monitors our lakes regularly to ensure the water, its inhabitants (fish) and visitors (birds, beavers and reptiles) are all in good health.

One area of observation that becomes critical throughout the winter months is dissolved oxygen. Measured in parts per million (ppm), dissolved oxygen is the free oxygen tied between water molecules. Eutrophic lakes use oxygen rapidly due to their higher nutrient levels, most commonly phosphorus introduced by land use.

The phosphorus and other nutrients contribute to more vegetative growth and when those weeds decompose, oxygen use increases.

Oxygen consumption is more noticeable during the winter months versus the open water period. Light-blocking snow and ice prevent sunlight from penetrating deep enough to make plants grow. Throughout late spring, summer and early fall, lakes naturally aerate when the wind blows, but a frozen surface prevents the water from stirring, even when wind speeds are high.

If oxygen levels decline slowly, fish may gradually acclimate or simply move to areas with higher oxygen content. But if oxygen levels drop quickly, then there's greater risk for fish mortality.

Doug Kingsley, Park Rapids Area Fisheries supervisor, says that quite often winterkill is partial and not all fish species die. The most susceptible fish are bass and bluegill, then crappie and walleye which are moderately tolerant, northern pike are fairly tolerant, while bullhead and carp are the most tolerant.

However, if one or more of the less hearty species are lost during a partial winterkill, a gap remains in the lake's balance, creating a change in predation and reproduction.

"If a lake loses a couple weaker species, the more tolerant fish like carp and bullhead really begin to multiply and that creates some major issues," says Kingsley.

Dissolved oxygen readings are gathered at the lake's deepest point; in one-foot increments on shallower bodies of water and in three foot increments for deeper lakes. Although there is no "magic number" in terms of oxygen levels that trigger winterkill, Kingsley notes that as a rule of thumb, oxygen levels less than 3 ppm are considered low, while 1ppm or less indicates a good chance for fish kill.

Over the past few months, the DNR has closely monitored winterkill conditions on a handful of area lakes. Portage, Peysenske, Moran, Ojibway and Stocking lakes in Wadena County are all on the DNR's radar.

Moran Lake's oxygen levels at the surface were only 0.3 ppm during the survey in mid-March, indicating a high chance for at least a partial winterkill. Peysenske Lake was questionable earlier, but Kingsley says it's recovering and winterkill will likely be minimal as is the case with Portage Lake.

Stocking Lake in Wadena County is at greater risk. "The DNR started aerating the lake because of low oxygen levels, but it hasn't seemed to help," states Kingsley.

Despite aeration, Stocking Lake's dissolved oxygen is 1.6 ppm at the surface and .3 near the bottom.

"Upper and Lower Ojibway was an interesting study," says Kingsley. "The lower basin was at .8 ppm, but the upper basin was at 10 ppm, meaning there was plenty of oxygen on one half of the lake, but not on the other."

Kingsley adds that a partial winterkill can be helpful in certain situations.

"If a lake is overpopulated with stunted panfish, a partial winterkill curbs those numbers, allowing the remaining panfish to grow much larger."

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