The ice abides: When winter kills

When winter starts this way – snowy and very cold from early December – we know we’re in for a long slog until spring. We may worry about the birds, about deer starving in their yards, and about the fish in our lakes, especially if those lakes are shallow. We’ve all heard of winterkill. Can a lake really freeze clear to the bottom?

Well, not likely. But that doesn’t mean a long, cold winter can’t kill fish. It can, and it does. Only it’s not the ice that kills them – not directly anyway. They die from lack of oxygen, which is to say, from suffocation. The ice seals the lake, cutting off the supply of fresh air. The water’s oxygen level then has only one way to go, and that’s down. The fish themselves, and the decay of organic matter, use up oxygen. If the ice cover remains for too long, there’s so little oxygen left that the fish can’t breathe.

Fish need a certain level of dissolved oxygen in the water – at least 2 parts per million. An oxygen concentration below 1 part per million is lethal to many if it persists. Fish that are the most vulnerable are those in shallow lakes with lots of vegetation and mucky bottoms rich in organic matter.

“Winterkill begins with distressed fish gasping for air at holes in the ice and ends with large numbers of dead fish, which bloat as the water warms in early spring,” says an article on the Michigan Department of Natural Resources website. Of course, some fish tolerate low oxygen better than others. Bass, walleyes and bluegills are fairly tolerant; perch and northern pike are more so. Bullheads can withstand severe oxygen depletion.

“February is usually a critical period and is the best time to check the oxygen content of lakes prone to winterkill,” says the Michigan DNR article. “A good mid-winter thaw about then often recharges the lake’s oxygen supply by means of photosynthesis and melt water. Conversely, a prolonged winter, with continuous snow cover and late ice-out increases the chance of winterkill.”

People on many small lakes protect them against winterkill by pumping in air with motor-driven aerators. It’s effective, though not a permanent solution. The real answer is to reduce the amount of nutrients entering the lake – nutrients that feed an abundance of plants that ultimately die, decompose, and deplete the winter oxygen supply.

Property owners can help by avoiding lawn fertilizers and making sure their septic systems are functioning properly, instead of seeping nutrient-rich water into the lake. Of course, some lakes are just naturally rich in nutrients, and in such cases there isn’t much lake residents can do, short of an extremely costly process of dredging out sediment. Winterkill in such cases is just part of the natural cycle. 

What happens to lakes that go through winterkill? Well, seldom do all the fish die. Enough usually make it through to reproduce. If the lake has an inlet stream, fish may come in that way and repopulate it faster. Nature takes its course; things heal. Some even maintain that a minor or moderate fish kill can be good for a lake: The fish populations are thinned out so there is more food for the survivors, which then prosper and grow.

Still, a winter die-off is not something to look forward to. If you live on a shallow lake, or if you have such a lake as a favorite fishing spot, a long and harsh winter can be a legitimate cause for worry.