REGIONAL—Hiking by the water may be something to avoid for a few weeks this spring as some N’West Iowa ponds and shallow lakes experience winter kill.
Winter kill is a seasonal event where fish wash up on shorelines and beaches in the spring after ice clears off lakes.
Mike Hawkins, a biologist for the Spirit Lake District Fisheries of the Iowa Department of Natural Resources, explained that winter kill happens when oxygen levels get too low.
“It’s like a bank, you’ve only got so much oxygen there and as you use it up or take oxygen out of the system through biological oxygen demand or decay, you can run out of it before the ice goes off in the spring,” he said.
A contributing factor to winter kill is thick ice and snow cover, which prevent aquatic plant life from photosynthesizing and generating more oxygen. Hawkins said the relatively mild winter helped prevent much winter kill, but drought conditions created issues in smaller ponds and shallow lakes.
“We went into winter with a lot of water bodies at low water condition or as low as folks have seen for a while, and that puts you at a higher risk of winter kill,” Hawkins said.
That’s been the case at a small pond at Vande Weerd Pit south of Hospers along the Highway 60 expressway. Sioux County Conservation director Robert Klocke said the pond may have been completely winter killed, with 200-300 fish dead.
“Between the low water levels and then thick ice and snow on those ponds, it’s likely a complete kill out,” Klocke said.
The pond will be restocked later this spring with bass, bluegill and catfish.
A larger pond at Alton Roadside Park winter killed several hundred fish, big and small, which Klocke said was largely due to low water levels reducing oxygen available. Across Highway 10 at the Dunlop Wildlife Area, an aerator in the water prevented the pond from freezing over and causing winter kill.
The much larger Dog Creek Lake southeast of Sutherland also experienced winter kill, mostly of small fish. For seasonal fish kills like this, park rangers and county conservation staff generally leave the fish to decay naturally or be picked over by scavenger birds.
Lyon County Conservation director Craig Van Otterloo said winter kills in ponds and lakes in Lyon County were limited. The only county-managed property where winter kill was reported was a backwater pond formed after flooding three years ago.
“None of our deep lakes or ponds had any winter kill in it,” Van Otterloo said. “We’re in really good shape.”
Although hikes by the water may be less pleasant at the moment, Hawkins said this year’s winter kill is unlikely to be detrimental to fishing.
“Some fish will make it through and those that do, they’ve got almost unlimited growth potential and they will reproduce very quickly to fill that void up,” Hawkins said.
Although the DNR restocks certain lakes to keep populations of native fish from being overwhelmed by invasive species like carp, Hawkins said there is less need for that this year.
Private landowners who have experienced winter kill may reach out to the fisheries office for phone consulting and guidance on what to do. Hawkins said in most cases, restocking will not be necessary unless a winter kill pond already was unhealthy.
“Small pond owners should keep track of what fish they have left in the spring, do a little sampling with their fishing pole in the spring to see if they have any bluegill or bass,” Hawkins said. “If they do, those systems should recover very quickly.”