Five Asian bighead carp have been caught recently on the St. Croix River near Stillwater, the farthest north the invasive species has been found on the river, the Minnesota Department of Natural Resources reported Tuesday.

The fish were caught about seven miles upstream of the previous point where they had been confirmed.

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The catch more than doubles the four bighead carp that had previously been caught in the river. One carp showed up in 1996 and three others were found in 2011.

The carp have been slowly migrating up the Mississippi River system after escaping from fish farms in southern states where they were used to keep pond water clean.

They are voracious filter-feeders, can grow to 60 pounds and remove the tiny organisms that make up the base of the aquatic food chain. They are similar to Asian silver carp which also are moving north into Minnesota waters.

"The DNR thanks the anglers who have reported the capture of bighead carp and have sent photos so they could be immediately verified," said Nick Frohnauer, DNR invasive fish coordinator, in a statement. "Knowing these details allowed our invasive carp crew to do follow up sampling in a timely manner."

Anglers reported catching the carp and the DNR then responded with intensive sampling in an effort to determine if more invasive carp were in the area. DNR staff set additional gill nets and conducted electrofishing. The agency also has hired a commercial fishing operator to seine a large bay to see if additional carp are present.

Silver and bighead carp are two of four species of invasive Asian carp threatening the Mississippi River and other Minnesota ecosystems, along with black and grass carp. Populations of bighead and silver carp are established in the Mississippi River and its tributaries as far north as Iowa. They also are threatening to enter the Great Lakes in Chicago as they move up the Illinois River system.

The Minnesota DNR is regularly monitoring for invasive carp by using targeted surveying and contracted commercial fishing. The agency also is working with the University of Minnesota's Aquatic Invasive Species Research Center, federal agencies, and other universities researching ways to prevent the spread and manage populations of invasive carp.

Anyone who catches a bighead, silver, black or grass carp is asked to immediately contact the DNR at (888) 646-6367 or Do not release the fish. Take a photo, and store the fish until it can be delivered to the DNR.