Dan Sallee, a DNR spokesman, said Monday it is the first time the species has been found in the waterway before it feeds into the Mississippi River. The 285-mile Rock River begins in several lakes in the southeastern Wisconsin and flows south into the Mississippi River below Rock Island.

"I don't want to see new water contaminated with this bad, bad fish," Sallee told the Rockford Register Star.

A fisherman caught the carp on Sunday below the Fordam Dam in Rockford. It weighed between seven and eight pounds and was 22 inches long.

The DNR later determined the fish was an Asian carp, Sallee said.

He said he hopes the carp is an isolated case and that the river doesn't have a breeding population.

The fish could have swum upstream from the Mississippi. It also could have been dumped from an angler's bait bucket when it was young or it could have been released into the river as an adult, Sallee said.

The Mississippi and Illinois rivers have established bighead populations, which entered those waterways after escaping fish farms during flooding.