Dead fish floating to the surface of ponds would seem to be a sign of pollution. At this time of the year, however, their deaths were almost certainly by natural causes, according to myjournalcourier.com.
In response to dead fish sightings across the state of Illinois, scientists assure the public it happens both naturally and annually. Basically, snow and ice during the winter months block the sun from providing life to algae and underwater plants, which provide oxygen to fish. Without enough air, the fish die. When the snow melts and the ice thaws, their carcasses float to the top.
Because 2014 experienced so much snow and ice, this year's casualties were greater than usual. Some ponds yielded hundreds of dead fish.
"If you've had an oxygen depletion kill like this, odds are you're going to have it again, especially if your pond is relatively old, relatively shallow, infertile," IDNR fisheries biologist Dan Stephenson told ABC. "And if it happens once, odds are it's going to happen again."
Since shallow ponds are more likely to freeze over and be covered by snow, they're more susceptible to losing their fish over the winter. Deeper bodies of water also have a greater store of oxygen -- why lakes that freeze over are able to support their fish during the colder months, according to myjournalcourier.
The effect has some implications for pond owners and overseers in charge of stocking their body of water. Though general guidelines are typically determined by size, a more specific limit can be imposed through a biologist's inspection. Thus, smaller ponds might gain special exemptions during particularly icy winters like 2014. Complicating matters, however, is that not all dead fish float to the surface, making it difficult to get an accurate count.
