The ones that got away

It’s been a rough winter all right. Just ask the fish.

Area fishing holes could be at low population levels this spring, because of the long, cold winter just past and the unusually abundant snowfall that came with it.

Carl Gorra, a park operations manager with the Naperville Park District, said winter kill was much more severe this year than usual.

“We’re seeing a lot more dead fish than we’ve ever seen before,” he said.

Because fish depend on dissolved oxygen for their survival, the enduring thickness of the ice on local ponds and lakes, and the snow that sat on the surface for many weeks, are to blame.

The effect was particularly severe in shallower ponds, Gorra said. That’s because ponds with little depth lack places fish can go to take refuge from the thickening ice.

The Will County Forest Preserve District hasn’t received reports from anglers or employees of unusual numbers of dead fish, but Andrew Hawkins expects that will change in the coming weeks.

“Ice and snow cover in harsh winters does not allow for sunlight to reach aquatic plants, which in turn reduces the amount of oxygen the plants will produce,” Hawkins explained in an email. “If the plants begin to die and start to decompose, even more dissolved oxygen will be used up in the water. When the dissolved oxygen falls too low, then fish kills occur.”

The district’s 80-acre Whalon Lake in the southeast corner of Naperville, a former quarry, is home today to bluegill, redear sunfish, channel catfish, bass and walleye.

Most of the fatalities in the Naperville park ponds have been gizzard shad, Gorra said, although some ponds have turned up significant numbers of bluegill that couldn’t survive the winter.

In the DuPage County Forest Preserve District, rangers and ecologists have spotted fish die-off at some of the 30 fishing holes, including Herrick Lake in Wheaton, Cricket Creek in Addison and East Branch in Glendale Heights, but agency experts aren’t deeply alarmed by it.

“Localized die-offs like these can occur naturally any winter, particularly in smaller and shallower lakes and ponds, and are not a sign that aquatic habitats are in danger,” fisheries ecologist Dan Grigas said in a news release. “They’re usually a result of seasonally low levels of dissolved oxygen, which start to return to normal as the ice starts to melt.”

Although the mortality levels aren’t unprecedented, district staff will be keeping an eye on the water bodies throughout the spring, and adjusting their normal stocking activities if necessary, Grigas said.

Gorra said Park District staff have seen hundreds of dead fish, but not thousands, as has been the experience for other agencies in the region. He thinks the grave status of the fisheries is temporary.

“Populations will probably be back to normal before too long,” he said.

The problem of fish kills isn’t limited to the Naperville area. The phenomenon is being seen around the state.

The Illinois Department of Natural Resources Division of Fisheries is receiving reports of fish kills on lakes and ponds around Illinois.

Dead fish have become apparent as ice cover thaws, department officials said.

Biologists with the department said that occasional fish kills are natural and occur when light cannot penetrate ice, slowing the growth of algae and plants that produce oxygen.

It is often difficult to determine the full extent of a winter fish kill since not all fish killed may be visible. As a result, many pond owners are seeking additional information on the cause, and advice on restocking after a winter fish kill, officials said.