Many popular ice fishing lakes, especially those shallower prairie waters holding perch, are at risk of losing some or all of their populations to low dissolved oxygen levels. Simonson Photo.

By Nick Simonson

Following an early April snowstorm and continued cold conditions across much of central and eastern North Dakota, concerns for extensive winterkill events on many lakes, particularly those in the southeastern portion of the state, continue to grow.  As of March 24, the North Dakota Game & Fish Department (NDG&F) listed 85 waters on its website that were likely experiencing low dissolved oxygen levels this year, with a large portion of them falling into the region managed by Fisheries District Supervisor BJ Kratz.

“At this point I expect at least 20 percent of the lakes in my district will have some potential for winterkill. Of course we won’t know that for sure until we get out and survey those and see what things look like after the ice goes off,” Kratz details.

While most smaller prairie lakes are likely the ones facing winterkill issues and sustained low oxygen levels which are detrimental to fish, Kratz suggests that many factors beyond depth and water quality impact resiliency to winterkill events.

“No lake is the same.  We’ve seen lakes this winter where we’ve had a maximum depth of 10-to-12 feet be really good all winter long. Various algae forms require different densities of light, so if you have certain species, they might thrive in a low light condition.  If that light is fairly stable through the winter they’ll continue to thrive,” Kratz suggests, adding, “It’s really difficult to predict.  Depth doesn’t have a lot to do with it, in general, but the bigger the lakes typically and the deeper the lakes, they just have a better oxygen reserve, that is the bank to hold the oxygen through the winter for those fish when we freeze up, until the ice goes off.”

While many anglers think inflows of water from springs, aquifers and other sources beneath the surface of a lake are beneficial in preventing winterkill, Kratz explains that only those seeps which impact the surface likely have an effect.

“It depends on what level the spring comes into the lake.  If you’ve got a big hillside and you’ve got springs that are around the lake itself and you’ve got an area of open water, that can help oxygenate the water around it, just through the atmospheric component of it.  Typically, groundwater has very little dissolved oxygen in it. When you have water that flows into a lake at some depth, you’re not going to typically bring any more oxygen into that lake.

In addition to the extension of solid ice layer on most waters and a hefty snowpack covering the state’s lakes into April, NDG&F agents are finding themselves against a biological brick wall as well for their restocking efforts.  With the delayed end to the ice season, it is likely that spawning fish to be transported into the winterkilled waters will be harder to intercept before laying their eggs – a process determined by both photoperiod and water temperature – and that will slow down the recruitment of a new population in those lakes impacted by winterkill this season.

“Most everyone has already started to plan accordingly based on the worst case scenario.  We’re going to certainly want to try to stock every lake that had a winterkill simply because we’re going to be staring over in these places basically with a full bank of water and maybe in some situations, no fish,” Kratz says, continuing, “the problem that we’re probably going to run into this year – let’s talk perch for example.  We try to trap-and-transport or move our perch in the spring before they spawn so that we can get potentially that first spawn off that year, in which case you have an age class.  Well, this year, we’re probably looking at ice off in a lot of these places at the end of April, certainly into May, and those perch are probably going to want to drop their eggs pretty quickly, so our fish aren’t going to be bearing eggs probably as much as they typically would in a given year,” he concludes.

A list of North Dakota’s lakes facing potential winterkill issues and those exhibiting low dissolved oxygen can be found online at: gf.nd.gov/fishing/dissolved-oxygen-monitoring.  Anglers are encouraged to report fish kills observed after ice out to the NDG&F via their website, or of other clues prior to ice out, such as smelling foul odors when punching holes through the ice in their late-season hardwater outings.