The DNR is investigating dead suckers, chubs, minnows and sunfish found along a three-mile-long stretch of Mud Creek in O'Brien County on Tuesday.
The kill was discovered about three miles west of Paullina and reported to the DNR Tuesday. It appears to be caused by silage pile runoff from Roorda Dairy.
Runoff from silage, which is agricultural material fed to animals, is high in organic matter. Oxygen in the creek is used up when silage runoff is consumed by bacteria. This lack of oxygen can be lethal to fish.
Heavy rains on Saturday night likely caused runoff from a concrete area and silage pile at the dairy to flow into a stormwater ditch leading to the creek. Dilution from Mill Creek seems to have prevented a more extensive kill.
"Producers need to take measures to properly contain feed stockpiles and silage piles to prevent runoff from entering our creeks, lakes, or rivers, even after heavy rainfall," said Ken Hessenius, head of the DNR Spencer field office.
The DNR estimates that hundreds of fish have been killed in this incident and is currently conducting fish counts. The DNR will continue to investigate and may consider possible enforcement action.