All You Need to Know About Winterkill

Mar 29, 2018

As our weather turns to spring, some of our thoughts might turn to winterkill. In this episode of Wildlife Matters, the masked biologist looks at lakes under ice and why winterkill happens.

Every year when spring rolls around, I can expect fisherman and lakeshore owners to show concern about the potential for winterkill, a term for fish dying under the ice due to a lack of oxygen.  A lake that has not winterkilled in the past is probably not likely to experience winterkill this year.  However, there are a number of area lakes that are prone to winterkill and experience it every few years.  Lakes like these stand the risk of a winterkill depending on how it goes between now and ice out.

Since we tend to breathe oxygen from the air with our lungs, we may not spend a lot of time thinking about dissolved oxygen. For fish to survive under water though, they need to be able to pull dissolved oxygen out of the water by pulling it across their gill plates. In the spring and summer, oxygen can be mixed into lakes naturally by wave action and rainfall. Cold water is heavy, and sinks. Water at the surface is warmed by the sun, and sits above the colder water, forming a noticeable thermocline. As fall arrives, the days get shorter and the sun is lower in the sky, so the lake surface begins to cool. Eventually, in late fall, the surface water cools more than the water below summer’s thermocline and it sinks to the bottom, forcing that water to rise up, causing the lake to turn. Lake water mixes significantly during this time and becomes saturated with oxygen. Consequently, 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 with prolonged ice cover and heavy snow cover 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 snow continues to cover the ice and deflect the sun’s rays, then conditions will continue to deteriorate under the ice.  However, no matter the case spring will come and our lakes will become liquid and well-oxygenated once again.

Striving to make new things familiar and familiar things new, this is the Masked Biologist coming to you from the heart of Wisconsin’s great Northwoods.