No air down there: Dissolved oxygen levels could lead to fish kills
WORTHINGTON — A harsh winter with early ice and snow cover may lead to winter fish kills on several southwest Minnesota lakes, according to the state’s Department of Natural Resources.
Ryan Doorenbos, Area Fisheries Supervisor for the DNR’s Windom office, said dissolved oxygen levels on a dozen area lakes are low enough that game fish and possibly some rough fish may not survive.
“Anything below 2 (parts per million of dissolved oxygen) is highlighted in red,” Doorenbos said of a report posted on the DNR’s website. “In our minds, it’s something to be a little bit concerned about.”
The lakes with dangerously low levels of dissolved oxygen include Oaks and Talcot in Cottonwood County; Loon and Pearl in Jackson County; Lime, Louisa and North Wilson in Murray County; East Graham and Clear Lake (near Dundee) in Nobles County and three lakes in Martin County. The Windom Fisheries office monitors lakes in eight counties, from the South Dakota state line east to Watonwan County and north of the Iowa line to portions of Lincoln and Lyon counties.
“Typically in a normal winter, when we get heavy amounts of snow … you get a lot of snow that sits on top of the ice, and that’s when we get concerned,” Doorenbos said. “This year is different. We really didn’t have a huge system that dumped lots of snow, but it’s added up over time.”
That snow on area lakes is the equivalent of “shutting the sun off,” he said, adding that light can’t penetrate through the snow and ice to promote photosynthesis and oxygen production.
“Basically all that’s happening is oxygen consumption,” Doorenbos said.
In a typical winter, oxygen levels decline through early- to mid-February before some thawing begins to occur and light is able to penetrate through the ice. Yet with thaw potential still weeks away, Doorenbos said the fisheries office is monitoring the situation.
On some lakes, the DNR has been notified that carp are coming to the surface of open water in areas where aeration systems are operating, and that is an indicator of stress on fish species.
If carp are showing signs of stress, game fish like walleye and panfish will as well. Doorenbos said northern pike and yellow perch are hardier species and can survive low levels of dissolved oxygen.
The DNR takes readings of dissolved oxygen on area lakes on a regular basis, and the Okabena-Ocheda Watershed District began tabulating its own readings before Christmas.
Logan Ahlers, a college student home on winter break, has taken readings on lakes Okabena, Ocheda and Bella for the past few weeks. He has found variable oxygen levels between the lakes, and even in different areas on the same lake.
His lowest reading was 1.5 ppm dissolved oxygen on Lake Bella earlier this month, while his highest reading, also on Lake Bella, was over 11 ppm this week. Testing on both the east and west basins of Lake Ocheda has yielded dissolved oxygen levels of 2-4 ppm, while Lake Okabena has shown the best levels of dissolved oxygen at 7-8 ppm.
“An oxygen level of 8 parts per million is good,” said OOWD Administrator Dan Livdahl. “In cold water conditions, you could get up to 12 to 14 parts per million before the water is saturated with oxygen.”
The watershed district began tracking dissolved oxygen levels in the three lakes last summer after investing in a meter and probe for testing. Livdahl said there are several reasons for taking measurements, from monitoring aquatic life to revealing what is happening with the chemistry of bottom sediments. Measurements may also show how dissolved oxygen is impacted by algae.
“We wanted to know if oxygen levels were getting so low that harmful nutrients were being released into the lake and used by algae,” Livdahl said.
This winter, however, monitoring is focused on fish health.
“We are really interested in what’s happening in Lake Ocheda and Lake Bella,” Livdahl said. “We’ve had really high numbers of carp and buffalo fish in both lakes. We’ve also seen higher levels of game fish in those lakes, and the dissolved oxygen level shouldn’t be high enough to support game fish.”
If there is a winter kill in the watershed district, Livdahl expects to see it in pockets on Lake Ocheda and Lake Bella. The watershed board has talked in recent years about doing a draw-down, at least on Lake Ocheda, in hopes of creating a fish kill to reduce the extremely high numbers of rough fish.
Perhaps this year, Mother Nature will provide the means for a winter kill.
Meanwhile, Doorenbos said lakes that do experience winter kill, specifically of game fish like walleye, will be restocked by the DNR.
“We’ll do some follow-up netting as soon as the ice comes off,” Doorenbos said. “If we suspect a pocket kill or winter kill, we’ll assess what species are present or absent … and of we do experience a significant fish kill, we’ll come in with walleye fry.”
Daily Globe Reporter Julie Buntjer may be reached at 376-7330.