The Wild Side: Late season ice and winterkill on area water ways
DNR Fisheries, Biologist John Kubisiak, has been dealing a lot with questions and concerns about late winter ice and the potential for winterkill in area lakes and flowages provide a guest column for this week’s Wildside.
As the DNR Fisheries Biologist managing waters in Oneida County, one of the questions I’ve been hearing lately is “Will we see a lot of winterkill this year?” Winterkill is when fish die under the ice due to a lack of oxygen. A lake that has not winterkilled in the past is unlikely to experience winterkill this year.
However, there are a number of area lakes that are prone to winterkill and experience it every few years. It is very likely that these “usual suspects” will experience a more severe winterkill this year.
Lake water becomes saturated with oxygen during fall mixing, so lakes normally go into winter with as much dissolved oxygen as they can hold. Even after lake water is sealed off from the atmosphere by a layer of ice, photosynthesis by plants under the ice continues to produce oxygen. However, as the winter goes on, light levels and photosynthesis are reduced by short day-length, thick ice and a heavy snow load. The largest demand for oxygen usually comes from decomposing plant and animal material on the lake bottom, so the oxygen is lowest close to the lake bed. In lakes suffering from low oxygen, the fish position themselves right under the ice, to take advantage of the best conditions available.
Lakes that are prone to winterkill tend to be shallow, with little oxygen-holding water volume. They have a lot of vegetation and thick sediments that use up oxygen. Winters like we are experiencing this year, with prolonged ice cover and heavy snow, are big years for winterkill. Winterkill rarely kills all the fish in the lake, and we often see increased fish growth for few years after a winterkill because the surviving fish have less competition for food. Lakes where all the fish are lost provide frog and amphibian habitat.
Some lakes that are prone to winterkill have aeration systems. Aeration works by pumping air through hoses into the lake. The air bubbles diffuse some oxygen into the water, but more importantly they cause the water to mix and melt the ice. The open water is able to exchange oxygen with the atmosphere. Even large aeration systems are not designed to completely eliminate winterkill because it is impractical to aerate the entire lake. However, aeration can greatly reduce the severity of winterkill by providing a refuge where much of the fish population can find enough oxygen to pull through.
Winter isn’t over yet, and the weather we get between now and ice-out will determine how severe the winterkills are. The long, sunny days of late winter are good for plant growth, and spring rains can bring a flush of oxygen-rich water percolating through cracks and holes in the ice, helping reduce tough conditions.
However, if ice cover hangs on late into spring and we continue to get snow, then conditions will continue to deteriorate under the ice. Sooner or later, spring will come and our lakes will become liquid and well-oxygenated once again.
If you suspect a lake may be approaching winterkill conditions, there is not a lot that can be done at this late date, but sometimes it helps to be prepared for what may be coming. Dissolved oxygen meters are available for loan around the region.
In the Rhinelander area, contact Oneida County Aquatic Invasive Species Coordinator Michele Sadauskas at 715-369-7836 or email firstname.lastname@example.org.
The first sign of winterkill is often eagles congregated at open water areas, where they can feast on dead fish. If winterkill is observed, you should report the lake, species affected and relative number to the local DNR Fisheries Biologist.
For Oneida County Lakes, contact John Kubisiak in Rhinelander at 715-365-8919 or e-mail JohnF1.Kubisiak@Wisconsin.gov.