About 40,000 fish died in Crane Creek last week, but it's unclear what killed them. (Photo courtesy of Iowa DNR)For the second time in a little more than a year, something has killed thousands of fish in a northeast Iowa creek without leaving obvious clues to identify it, according to state conservation officers.Early estimates indicate about 40,000 fish died last week in Crane Creek, where their carcasses were found in a 20-mile stretch of the stream from near Readlyn to Dunkerton.They included sought-after game fish such as bass and northern pike, along with crawfish and snails, said Brett Meyers, an Iowa Department of Natural Resources environmental specialist. A fish kill investigation on Friday and Saturday found few living fish in the creek. Those that were alive appeared to be “in distress,” Meyers said.A construction crew that was working on a bridge over the creek reported it Friday after seeing dead fish. Initial sampling of the creek water did not reveal elevated levels of common contaminants such as ammonia — an indicator of manure or fertilizer pollution. There were no abnormal, chemical smells.“These things are tough to figure out, especially when there’s nothing obvious with the field tests when you get out there as soon as you can,” Meyers said.A similar incident happened in June 2023, according to DNR records, when an estimated 1,500 fish died in the same area of the creek.Both fish kills had roughly the same upstream starting point — near the creek’s intersection with 250th Street in Bremer County — but last year’s went for about 5 miles.“Stream flows were low and water was clear, with no unusual odors,” the DNR noted last year.The department concluded that an unidentified pollutant that was heavier than water likely killed the fish, because the dead ones were larger or otherwise more likely to be near the creek bed. Smaller fish near the surface didn’t die at the same rate.This year’s fish kill investigation is still pending, and Meyers hopes information from the 2023 investigation will aid the current one. There are numerous potential contamination routes into the creek in that area — including underground drain tiling from farm fields and a nearby city.Lab tests for a greater variety of pollutants are pending and might not be done for weeks, Meyers said. The DNR has also enlisted the help of the Iowa Department of Agriculture and Land Stewardship to determine whether pesticides might be to blame. Meyers said area farmers have been spraying their fields in recent days and weeks, which is often done with planes.The DNR is not aware of any recent heavy rainfall that might have carried contaminants into the creek.The post Northeast Iowa creek has second mystery fish kill appeared first on Iowa Capital Dispatch.First published by Iowa Capital Dispatch.