Investigators with the Iowa Department of Natural Resources are trying to determine what caused a fish kill this week in Fayette County.

According to a press release, someone reported the fish kill Wednesday morning. Dead brown trout were discovered below a storm water drain that flows into Otter Creek near West Union.

DNR staff spent the day at the site conducting field tests. So far, officials have not been able to definitively pin-point the cause of the fish kill.

Residents reported they first saw dead fish Tuesday around 3 PM, according to the press release. DNR officials said it appears the kill affected only the naturally reproducing trout, not other fish species that were swimming in the creek.

The dead trout ranged in size from about two to 16 inches, DNR officials said.

According to DNR fisheries staff, trout are more sensitive to temperature changes and chemicals. While at Otter Creek, they conducted a fish count.

Since the cause of the fish kill hasn't been determined, the investigation is ongoing. DNR officials said they plan to focus on identifying the source of pollutants that could have entered the creek through a storm drain.

Iowa Department of Resources investigators encourage people to report fish kills as early as possible to the agency's 24-hour spill line at 515-725-8694. Officials said early reporting helps DNR staff determine the pollutant source before it washes downstream.

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