MADISON, Wis. -- Winterkill, the most common natural cause of fish die-offs, has the possibility of being severe in some areas of Wisconsin this year because of the harsh winter and record-breaking snowfalls.

Winterkill, or "freeze-out," is caused by low oxygen levels during the ice-covered period. Thick ice and heavy snow cover prevent light from reaching underwater plants, reducing the amount of oxygen that plants produce.

"We expect to see partial winterkills every year, but we haven't had severe winterkills in almost 20 years," said Paul Cunningham, Department of Natural Resources fisheries ecologist. "This really is a unique situation in terms of lower than average temperatures and higher snowfall amounts."

Fish most likely to be affected by winterkill include walleyes, bass and bluegills. The fish normally die in late winter, but may not be noticed until a month after the ice leaves the lake because the dead fish are temporarily preserved by cold water. Once the water warms in early spring, large numbers of dead fish may float ashore.