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DNR: Warm water believed to be to blame for catfish kill on Rock River

JANESVILLE — The Wisconsin Department of Natural Resources is examining a fish kill earlier this month that fisherman claim left a large number of catfish dead along the Rock River.

According to emails obtained from the state DNR, a Janesville bait shop owner reported that fisherman had seen “thousands” of dead catfish above the Centerway Dam at Janesville around July 30-31.

DNR fisheries officials could not verify that estimate. However, DNR emails indicate that around that time fish and game wardens found 50 or 60 dead catfish on the river above the dam in Beloit, and that others were found further north near Jefferson. Most were young fish, according to the emails.

Kurt Welke, a fisheries manager for Rock, Dane, and Green counties, said it’s not clear exactly what killed the fish or why the kill just involved catfish. However, officials believe the kill could have been linked to rising water temperatures.

Welke said a spate of extremely hot weather in July heated the river, creating algae conditions that could have robbed the fish of oxygen or led to an increased concentration of bacteria or viruses.

Catfish are spawning now, and spawning fish tend to be more sensitive to changing water conditions, Welke said.

Welke said most of the fish recovered were badly decomposed, making it harder to pinpoint an exact cause. He said wardens checked the fish for signs they could have been killed during commercial net fishing, but the fish showed no apparent signs of net injuries.

Welke added there was no evidence of a chemical spill or contamination.

The DNR has had no further reports of dead catfish in the river, Welke said, but the agency reported a recent fish kill at Lake Kegonsa that involved white bass, perch and drum.