As the ice finally breaks up on Michigan's lakes and canals, residents may get a rude awakening along the shoreline: stretches of dead fish and other aquatic life. State wildlife managers say the scene can be jarring, but in many cases it is a natural phenomenon called winterkill, not a sign that a lake has been poisoned or is collapsing for good.
In yesterday's press release, the Michigan Department of Natural Resources (DNR) said winterkill is the most common type of fish mortality event and is especially likely in shallow lakes, ponds, streams and canals. “Winterkill is the most common type of fish mortality event,” said Aaron Switzer, the DNR’s fish production program manager. The agency noted that these die-offs are usually localized and “do not affect the overall health of fish populations or fishing quality,” according to the Michigan Department of Natural Resources.
How winterkill suffocates lakes
When thick ice and deep snow keep sunlight from reaching underwater plants, those plants stop producing oxygen and start to die off. Bacteria that break down the decaying plant material then burn through what dissolved oxygen is left, creating low-oxygen zones that can suffocate fish. Carcasses trapped under the ice can stay preserved until thaw, and they may appear fuzzy from secondary fungal infections even though the fungus did not cause the deaths, as reported by CBS Detroit. A rapid warmup after a long cold stretch can add extra stress and make the die-offs more noticeable all at once.
Where you're most likely to see it
Shallow, soft-bottomed lakes packed with aquatic vegetation are prime territory for winterkill, and urban canals that collect nutrient runoff and pollutants from roads, lawns and failing septic systems are especially vulnerable, according to WILX. The DNR adds that turtles, frogs, toads and crayfish can also be affected, with most sightings turning up in protected bays and other pockets where oxygen levels sink the lowest.
What to do if you find dead fish
The DNR is asking the public to report fish kills through its Eyes in the Field system and to contact the nearest DNR office or Michigan’s Pollution Emergency Alert System at 800-292-4706 if they suspect the deaths are not from natural causes. Those reports help wildlife managers track and investigate unusual events, although routine winterkill generally calls for no response beyond documentation, per the Michigan Department of Natural Resources.
