Winterkill and Other Fish Kills

What is Winterkill?

Floating dead fish as a result of winterkillWinterkill is a term used to describe the loss of fish over the winter because dissolved oxygen was lacking in a waterbody. Submerged vegetation and algae create oxygen through the process of photosynthesis. During the winter, oxygen production is often reduced because growing ice cover and accumulating snow on the lake limit the amount of sunlight reaching vegetation. In small, shallow lakes the available oxygen can quickly be used up by fish and by bacteria that feed on dead and decaying vegetation during the process of decompositions. When the oxygen level declines, less tolerant fish species, and fish in poor condition overall, can begin to suffocate and die.

Winterkill is a natural process and not all results are detrimental. In lakes with high numbers of carp for example, periodic winterkill can thin out their numbers. In lakes that support gamefish, the fish population can rebound quite dramatically in years following winterkill. Intraspecific competition can be reduced following a winterkill, allowing fish to grow faster and to larger sizes. While some area lakes have installed aeration systems to reduce the possibility of winterkill, others are managed as "boom or bust" fisheries, at times providing some of the hottest fishing action in our area. In our walleye production ponds, stocking fry following winterkill generally results in better fingerling production and can occasionally produce great fishing. Winterkill in walleye production ponds eliminates disruptive benthivores, other competitors, and carry-over walleyes that can hamper young of year walleye fingerling production.

Some species of fish are more vulnerable to winterkill than others. Trout require the most oxygen and start to stress at oxygen levels below 5 ppm. Bluegill and largemouth bass also are moderately sensitive to lowered oxygen levels. Walleye, yellow perch, northern pike, common carp and crappie species have intermediate tolerances down to about 2 ppm, while bullheads and fathead minnows are the most tolerant of low oxygen. Winterkills seldom result in the death of all fish in a lake. Lakes with regular winterkill events are usually dominated by bullhead species.

Dissolved oxygen monitoring during the winter and trap net assessments after ice-out help us identify which lakes experienced winterkill, but often the only way to see if a lake has winterkilled is to observe dead fish at ice out and to follow up with nets for fish community assessment.


Dissolved Oxygen Monitoring

The Waterville Area Fisheries office monitors dissolved oxygen levels in area lakes that have aeration systems or have a history of winterkill. Dissolved oxygen is measured from the surface where the hole is cut and down to the substrate or wherever the dissolved oxygen probe cord ends (usually 10 feet). Typically one site is tested on each lake although more may be conducted if winterkill seems imminent or readings are below 2.0 milligrams/liter or parts per million. Readings on aerated lakes are usually taken near the aerator to see if an oxygen refuge is being maintained.


Lakes are monitored periodically as the winter progresses and the results are posted as soon as possible. If you do not see a lake listed, it likely does not have a history of winterkill, is not aerated, or was deemed unnecessary for testing.


Note: These lists are informational only and should not be considered complete or "official." Winterkill can be difficult to predict and may or may not occur regardless of the following results. Unit of measure for numbers listed below is milligrams/liter or parts per million dissolved oxygen.


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