UNDATED (AP) -- The Wisconsin Department of Natural Resources says the winter's cold temperatures and high amounts of snow may lead to a large amount of fish deaths.

The agency says what's known as winterkill is the result of low oxygen in the water caused by ice and snow blocking light from reaching underwater plants.

Fish ecologist Paul Cunningham says it may be the worst winter-related fish die-off in 20 years.

He says the state had been in a period of milder winters, but this past winter was a more extreme event.

Cunningham says the DNR will have a better idea of how many fish have died after the ice melts.

He says some winterkill is beneficial to the lakes because surviving sport fish may grow larger.