January 23, 2024
From the pages of In-Fisherman's annual 2024 Tactical Ice Fishing Guide
During winter, factors such as structure, forage, cover, and spawning habitat often are less important than dissolved oxygen in determining fish location. In some lakes it also plays a key role in fish survival. The dynamic relationship among light levels, aquatic plants, fish, and oxygen content becomes a balancing act that can teeter and collapse.
I visited with former Minnesota Department of Natural Resources Fisheries Supervisor Tim Brastrup and Fisheries Specialist Mike Knapp to ask questions about oxygen and its effects on our winter fishing. Brastrup began by relating a field experience.
“One winter, we were checking oxygen levels in a potential winterkill lake covered with deep snow,” he said. “We filled a couple small jars with lake water and placed them on the snow. A couple minutes passed while we calibrated the D.O. (dissolved oxygen) meter. The first reading was 14 ppm (parts per million). ‘Can’t be,’ I thought. So, we dropped the probe into the hole again and the meter read zero—no oxygen.
“Looking into the jars, we could just make out a trace of algae. In just a few minutes of light exposure, the algae had photosynthesized enough oxygen to saturate the water. Merely that short exposure let plants restore oxygen levels.”
Most anglers probably ignore the oxygen factor because we can’t see it, feel it, smell it, or touch it. Lacking a D.O. meter, we can’t accurately measure it, either. But if you’ve visited a lake in spring that has had a winterkill, you certainly sense its effects as decaying fish line the shore.
Knapp offered an explanation. “Lakes with dense vegetation, abundant fish populations, high fertility, and moderate to shallow depths are more prone to winterkill. In winter, most plants die back and respire because of the reduction in light penetration. The decomposition process uses a lot of oxygen. Combine that consumption with the oxygen used by fish, zooplankton, and invertebrates and you can go into deficit. All it takes is about 4 inches of wet snow to shut off light from above.
“The greatest oxygen debt occurs because of the ice itself, as it seals the lake from its main oxygen source—wind-driven surface mixing. Often the only thing adding oxygen is groundwater from springs. Indeed, groundwater springs probably are the main reason we rarely see a complete winterkill. Some fish usually find pockets of oxygenated water. It doesn’t take many adult fish to repopulate a lake, often within two to three years, especially in the absence of predators. In some cases, a winterkill is a boon to a lake with stunted panfish or pike because surviving fish have access to more food.
“The fish that survive, particularly panfish, suddenly have access to all the food resources in that lake and begin growing rapidly in spring,” Knapp said. “The lake’s total zooplankton hatch is available to a limited number of fish. We’ve seen lakes two years after winterkill go from malnourished bluegills to healthy 1-pound fish.”
For anglers interested in trophy-class panfish, pike, and other species, keeping track of local winterkill events, then being among the first to tap these fast-regenerating fisheries, is a solid plan. The window of opportunity may not last long, however, as another winterkill can occur in any given year.
Oxygen and Fish Movement
In deeper, less-fertile waters, and in winters with thinner ice and less snow cover, oxygen may remain plentiful. In such cases, fish movement isn’t affected by oxygen level. At times, fertile to moderately fertile lakes also don’t have large-scale winterkills. In these waters, though, oxygen declines in the same general manner and can affect fish location.
In many panfish waters, oxygen can become a critical element by late ice.
“At first ice—say December—oxygen is likely to be 8 to 12 ppm from top to bottom,” Knapp said. “In January, on the other hand, you might have 10 ppm just below the ice, but 6 feet down only 5 ppm, and near the bottom only 2 ppm. Finally, late in winter, oxygen at the surface might be at 1 to 2 ppm, and 5 feet down drop to near 0.”
According to Knapp, because of the decomposition that occurs in the deep basin, lakes lose their oxygen from the bottom up.
“You see the effect with species like crappies,” he saiod. “Anglers often find them at early ice suspended, say 30 feet down over a 40-foot basin. In March the fish are more likely 5 to 10 feet down, holding in the most oxygen-rich water. In extreme cases, the only oxygen can be just beneath the ice. These upward shifts also are related to the vertical movements of phytoplankton and zooplankton that seek light, oxygen, or each other. Each of these factors are linked.”
Fish also often move from deeper water onto shallow flats as winter wears on. In some cases, bluegills and crappies are on these flats even though no live plants are available. Again, water closer to the surface holds more oxygen. In many lakes, water deeper than 6 to 8 feet doesn’t have any oxygen.
As a side note, one of the most exceptional walleye bites unfolds late in winter on shallow flats more typically associated with panfish. These shallow movements usually begin when ice thickness is near its winter maximum. This isn’t just a last-ice deal, but rather one that develops up to a month prior to ice-out. Fewer anglers remain on the ice at this stage, and in some states the walleye season has closed. Good fish can be found during the day in 3 to 8 feet of water at this time and they’re usually hungry. It’s likely oxygen plays a role in this pattern.
In the future we hope to learn more about the role oxygen plays in winter fish movements. The first step is persuading manufacturers to produce affordable, angler friendly D.O. meters. The technology exists. Inquiring angling minds want to know.