A dog walker came upon an unsettling sight Saturday at Glacial Park near Ringwood – what appeared to be hundreds of dead carp in a pond with as many crows snacking on the carcasses.

The fish were most definitely carp, said Zac Cannizzo, conservation ecology Manager at the McHenry County Conservation District. Glacial Park is one of the district’s wildlife areas.

What occurred is a “winter kill,” said Caitlynn Martinez-McWhorter, district marketing and education director. It is “a very common occurrence, especially in small ponds.”

It is even more common with carp, Cannizzo said.

“These fish will move into an area, a depressed area, and get trapped” by ice, he explained. “Their outlet gets frozen over.”

Because carp are large fish, they consume oxygen at a higher rate than other, smaller native fish, Cannizzo said. The fish managed to trap themselves in a low-oxygen area, where bacteria is eating the rotting plants and algae. Unable to get enough oxygen, they die.

The process reads like something out of a true-crime podcast.

“The bodies are trapped in the ice and when the ice melts, on a warm day like we had on Friday, the bodies float to the surface,” Cannizzo said.

Large native fish – like bass and blue gills – tend to overwinter in deeper water and are not often subject to these kinds of kills, he added.

Andy Plauck, a fisheries biologist with the Illinois Department of Natural Resources, agreed winter kills are “quite common when we have harsh winters. ... We haven’t had this harsh of a winter since 2013-14 in northern IL, so many ponds haven’t had bad kills in recent years.”

While it’s concerning to see so many dead fish, the birds, coyotes, raccoons and the pond itself benefit.

“It’s beneficial to the marsh, that there is less carp,” Cannizzo said.

Carp are a nonnative species – they were brought to North America by early European settlers as a food fish but are not considered a sport or game fish.

“There are people and some cultures that eat them ... but we do not tend to eat them anymore,” he said.

That doesn’t stop small predators from feasting on the dead fish trapped in the shallow water.

“Coyotes, raccoons, birds ... will clean it up in relatively short order,” Cannizzo said.

If the winter kill had been near one of the more-used walking trails, conservation district staff would have gone out to collect fish so users wouldn’t have to see it.

The conservation district prevents these kinds of kills in their more popular – and stocked – fishing spots by aerating the water through the winter, Martinez-McWhorter said.

While winter kills are common, conservation district officials want users to tell them when they see issues no matter the time of year.

Fish kills midsummer can mean there is an algae bloom or a chemical spill, Cannizzo said. While neither is common in their streams and ponds, it can happen, he said.

“Even if the cause is something natural, we can go out and check it to make sure it is natural – better safe than sorry," Cannizzo said.

Conservation district users should call 815-338-6223 or email MCCD@MCCDistrict.org to report issues they may see. Cannizzo also encourages them to take photos to include in reports.