As Iowa continues its spring thaw, the Department of Natural Resources has received many reports of dead fish in lakes and ponds across the state.

Though some past fish kills in Iowa have been linked to polluted waters, these ones occur naturally, according to the DNR, as the ice and snow of winter blocks sunlight from reaching aquatic plants, which in turn, stop producing oxygen. The longer the snow and ice cover lasts, the less oxygen is in the water.

Most winter kills do not wipeout entire fish populations.

“We get a lot of calls from farm pond owners who think they lost all of their fish in their pond to winter kill. Our advice to them is to fish the pond in the spring, note the species, number and size of what you catch and talk to their local fisheries biologist about the health of the pond,” said Joe Larscheid, chief of the fisheries bureau for the Iowa Department of Natural Resources.

“What is important to understand it that this is a natural phenomenon and has been occurring in lakes, ponds and river backwaters throughout our history,” Larscheid said. “On the positive side, winter kills create a surplus of food that allows the remaining fish to experience rapid growth over the following year or two.”

Winter kills are visible shortly after ice out when fish that died during the winter float and are blown to shore. In certain lakes, like Rathbun, Black Hawk, Storm and Coralville, these dead fish are often a source of food for channel catfish that will go on a feeding spree. Many anglers see this as an early season fishing opportunity for trophy sized channel catfish.

Though many fish kills are natural, the DNR said it will continue to monitor waters in case kills can be linked to man-made factors.

“If in doubt, give your local fisheries biologist a call so we can discuss your situation,” Larscheid said.

These DNR has received reports of fish kills at the following lakes:
Swan Lake (Carroll), Badger Creek Lake (Madison), Clark Lake (Cerro Gordo), Kuhn Wildlife Pond (Cerro Gordo), Pilot Knob Pond (Winnebago), Alice Wyth Lake (Black Hawk), Middle Sabula and Green Island lakes (Jackson), Credit Island Lagoon (Scott), and a storm water retention pond in Guttenberg.