The Iowa Department of Natural Resources is trying to figure out what caused large amounts of fish to die along a 20 mile stretch of water near Spencer, Iowa.

The DNR is investigating the fish kill, which runs along Stoney Creek northwest of Spencer.

A call came in Monday reporting the dead fish, but residents in the area said they had been seeing dead fish over the weekend.

“We may or may not be able to track down the source of this fish kill, because it likely occurred several days ago,” said Ken Hessenius, supervisor of the DNR Spencer field office. “If people would remember to call the field offices or the 24-hour spill line as soon as they see something that would increase our chance of finding the pollutant source.”

DNR staff believe it is from fertilizer or animal manure because of elevated ammonia levels found just west of Spencer.

The DNR spill number is 515-725-8694.