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grass carp caught in wisconsin river (dnr photo)

By Matt Hrodey

A state Department of Natural Resources boat recently caught a kind of Asian carp – the hefty “grass carp” – in the Milwaukee River. Capture of the fish, classified by the DNR as an invasive species, is evidence it’s still being stocked illegally in private ponds. Another grass carp was recently caught, for the first time, in the Wisconsin River. It likely migrated from the Mississippi River, where the large fish have become more common in recent years.

Grass carp are so called because they’re ravenous eaters that feed on aquatic plants. Golf course owners and other caretakers of private ponds sometimes stock the fish to clear the ponds of plant life. Grass carp are illegal in this state – but it’s legal in Illinois to stock a genetically engineered variety that’s incapable of breeding. Other states, including Iowa – believed to be the source of grass carp found in the Mississippi River bordering Wisconsin – place no restrictions on the fish.

The DNR has never found evidence of the fish breeding in Wisconsin. They are Asian carp by definition but are not one of the three species federal and state officials fear could migrate from Chicago canals to Lake Michigan. But grass carp are despised nonetheless: Their eating habits can starve out other fish and encourage destructive algae blooms.

one caught in milwaukee river

Despite the lack of breeding, the carp have turned up from time to time in the Milwaukee River. Those found are typically the genetically-engineered sterile variety, suggesting they migrated or escaped from some pond owner’s private stock. Flooding helps the fish escape from ponds and make headway into larger waters. The massive floods that hit Iowa in 2008 were blamed for a surge in grass carp found later in fish harvests taken from the Mississippi River.

Heavy spring rains such as those that have fallen across Wisconsin can also help carp escape from ponds or encourage them to venture into unexplored tributaries. John Lyons, a fisheries biologist for the DNR, says the 40-inch carp caught in the Wisconsin River on April 27 likely swam up into the Lower Wisconsin River from the Mississippi, which is flooded.

As for the Milwaukee River fish, a 32-incher caught on April 21, the source is unknown. This carp, along with the Mississippi specimen, will be genetically tested to determine if they’re the sterile variety usually found in Wisconsin waters.

Will Wawrzyn, another DNR fisheries biologist, says the fish may have migrated from a private pond somewhere. He netted the fish on a DNR boat conducted fish population surveys on the river. The carp was caught where the river passes through Downtown Milwaukee. There’s little evidence of a grass carp outbreak – but neither is there much evidence to the contrary. “Hopefully we don’t find too many more,” he says, “but you never know.”

In 1990, an Arkansas company was charged in federal court with illegally selling some 2,000 grass carp to country clubs in Wisconsin, including ones in Wauwatosa, Franklin, Big Bend, West Bend, Brookfield and Oconomowoc. Some of the owners said they didn’t know the fish were illegal. The fish were also sold, according to court documents, to a handful of private pond owners in Waukesha, Ozaukee and Milwaukee counties. The company claimed the fish sold were of the sterile variety.

Life expectancy for a grass carp is about ten years, but since 2000, the DNR has caught several of the fish in the Milwaukee River, suggesting they are somehow migrating from Illinois or continue to be stocked in Southeastern Wisconsin ponds.

“We’d just as soon not have them at all,” says Ron Benjamin, fisheries supervisor for the Mississippi River.

Grass carp are numerous in the Missouri and Illinois rivers but seem to have struggled to establish themselves in the upper reaches of the Mississippi.

One reason, Benjamin suggests, may be Wisconsin’s rugged “game fish” populations – fish such as northern pike that offer tough competition for the carp.

“These fish are going to have to shoulder their way into our ecosystem,” he says.

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