Nathan Wallick is a firefighter in the American city of Peoria, Illinois. He also likes to use bows and arrows for sport. Now, that sport is helping him make extra money and improve the local environment. People pay Wallick to lead hunts on the Illinois River to kill Asian carp, an invasive fish.

　　Nathan Wallick is one of several private boat operators helping to stop Asian carp from entering Lake Michigan, one of the largest bodies of water in the United States.

　　The hunt takes place in an area of the river about 160 km from Lake Michigan. Wallick's customers shoot arrows at the carp as the silvery fish jump out of the water behind the boat.

　　Wallick told Reuters, "The name of the game is try and knock them down as much as possible and keep them from getting into the Great Lakes."

　　On a recent hunting trip, Wallick and his customers caught about 100 carp. That is not very many compared to the number of fish caught by commercial fishing boats working for the Illinois Department of Natural Resources.

　　Kevin Irons is a program manager with the department. He told Reuters they have removed 3.9 million kilograms of Asian carp from the upper Illinois River since 2010.

　　This June 22, 2017, file photo provided by the Illinois Department of Natural Resources shows a silver carp that was caught in the Illinois Waterway, approximately nine miles away from Lake Michigan.

　　This June 22, 2017, file photo provided by the Illinois Department of Natural Resources shows a silver carp that was caught in the Illinois Waterway, approximately nine miles away from Lake Michigan.

　　Irons said his department gives the fish caught in removal operations to tradespeople. The sellers process them into non-food products such as fish oil, fertilizer and pet food.

　　Irons added that his agency welcomes the relatively small aid from carp fishermen like Wallick.

　　So far, Lake Michigan and three of the five other Great Lakes - Superior, Huron and Ontario - remain free of Asian carp, Irons said. Just two carp have ever been seen in the Illinois Waterway - a shipping path that links Lake Michigan to the Illinois River. Both were caught.

　　Four Asian carp species now live in much of the Mississippi River. One species, grass carp, has made it into one of the Great Lakes - Lake Erie.

　　The Asian carp can weigh up to 45 kilograms. The fish feed entirely on small plants and animals that float freely in oceans and other bodies of water. The carp threaten to upset food chains of the waterways they live in while crowding out native fish. In the lower Illinois River, for example, they now make up 70% of water life, Irons said.

　　Asian carp come from China. They were brought into the United States in the 1970s to help control growth of plant life in commercial catfish ponds in the South. The carp found their way into the Mississippi River through flooding and accidental releases. Since then, they have invaded many of the nation's rivers and streams.

　　I'm Jonathan Evans.

　　Nick Pfosi reported this story for the Reuters news service. Jonathan Evans adapted it for VOA Learning English. Caty Weaver was the editor.