GRAND RAPIDS, Mich. — In a pond off the beaten path in Grand Rapids, hundreds of dead fish — carp, bluegill and bass — are piled up against a beaver dam. Bloated and belly side up, they rot in the spring sun.
It's a smelly, likely natural sight known as a winterkill.
The suffocating effects of Michigan's coldest season cause this type of event, according to the Michigan DNR, leading to large-scale die-offs of plants and animals.
"When you see that, you think, 'What's wrong?'" Aaron Switzer, fish production program manager for the DNR's fisheries division, said. "But it's a natural phenomenon."
When bodies of water — typically small, shallow ponds — freeze in the winter, the ice cover limits the amount of oxygen and sunlight that can enter the water. Unable to photosynthesize, aquatic plants then die and begin to decompose — a process that sucks up even more oxygen — leaving fish and other underwater species struggling to breathe.
Before long, they die, too.
"It'd be like throwing a fish in a bucket and leaving it there," Switzer said. "There's just no reproduction of oxygen."
A "normal" winter in Michigan with near-average snowfall has led to a relative increase in winterkills across the state, Switzer said.
FROM THE DNR: Natural fish kills may be common during spring thaw
At the retention pond in Grand Rapids, hundreds of fish, if not thousands, died due to the natural phenomenon. Given its relatively urban location, the body of water likely also deals with a high amount of nutrient-rich runoff, which increases the likelihood of a winterkill.
"There were a couple of things going against the fish in this pond," said Joel Betts, an aquatic ecologist.
Betts says timing and appearance are also important in determining the causes of these die-offs. While winterkills reveal themselves when the ice melts around springtime, fish kills in the warmer months can be cause for concern.
"If you see it in the summer, then it could be a symptom of a really unhealthy lake," Betts said.
In addition, dead fish with lesions, hemorrhaging or bulging eyes can be signs of disease or a pathogen, while bloating and a fuzzy-looking appearance caused by fungal growth are indicators of a winterkill.
"It's good to report fish kills, because there could be a number of reasons why they could happen," Betts said.
Though smelly and unsightly, the loss of life brought on by a winterkill doesn't typically last. Nature takes its restorative course, the DNR says.
"Most of the time, the impact on the ecosystem is minimal," Switzer said. "These smaller ecosystems rebuild quite rapidly."
To report a fish kill, click here.
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