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This week, biologists dumped poison into a major Chicago waterway: The Sanitary and Ship Canal. That's one way they are trying to stop a fish invasion of Asian carp. They've also put up an underwater electric barrier but they are about to turn it off for maintenance. Scientists fear the carp could decimate native species if they get into the Great Lakes.

NPR's David Schaper reports.

DAVID SCHAPER: Winter in Chicago: the wind, the cold, the snow. So, who would want to stand out on Navy Pier every morning to catch little eight to ten inch perch? People like John Calderon, that's who.

Mr. JOHN CALDERON (Fisherman): Well, I'm out here all winter. I can take the cold, you know, I'm out here when there's ice here, we have to break through it just to get them, you know.

SCHAPER: Why? Calderon says Lake Michigan perch taste great. But he and other fisherman worry their delicious catch may disappear if the Asian carp get into Lake Michigan.

Mr. CALDERON: Oh, they'll kill all our fish because they've go to eat. They eat a lot. They're the biggest eaters of all, and I don't want them around.

SCHAPER: Asian carp are voracious eaters. They can grow four feet long and weigh up to 100 pounds. They are filter feeders at the bottom of the food chain and eat constantly, consuming up to 40 percent of their body weight in plankton each day. They are prolific at reproducing, some call them the rabbits of the water. So, Asian carp can quickly starve and crowd out native species of fish.

In the two decades since floodwaters washed them out of southern fish farms where they were imported to control algae, Asian carp have taken over some parts of the Mississippi River system. In the Illinois River, for example, Asian carp now make up more than 90 percent of the fish in some locations. The silver Asian carp can jump 10 feet up into the air when startled and with so many of them in Illinois, several boaters and jet-skiers have been injured by the flying fish. And they are swimming north, with Asian carp DNA recently found in water samples just eight miles from Lake Michigan.

Mr. JOEL BRAMMEIER (President, Alliance for the Great Lakes): There's never been a more important time for the federal and state governments to stand up and do the right thing.

SCHAPER: Joel Brammeier is president of the Chicago-based Alliance for the Great Lakes.

Mr. BRAMMEIER: If we don't take the right steps today, we are consigning the Great Lakes to a future as carp ponds.

SCHAPER: The most dramatic step yet in the fight to keep Asian carp from entering Lake Michigan is the biggest fish kill in Illinois history. Wednesday night, biologists dumped 2,300 gallons of a fish-killing chemical into a six-mile-long portion of the Chicago Sanitary and Ship canal. It killed tens of thousands of native fish, in hopes of killing any Asian carp that might be in the waterway. So, the U.S. Army Corps of Engineers can turn off an underwater electric barrier on the canal for maintenance. The poisoning is controversial. Though Asian carp DNA had been detected in the shipping canal, no actual fish have been seen this close to Lake Michigan until biologists found a dead one late yesterday. John Rogner is with the Illinois Department of Natural Resources.

Mr. JOHN ROGNER (Assistant Director, Illinois Department of Natural Resources): We believe this will reduce or eliminate any remaining doubt of the need for this operation. We've now confirmed with a body what the e-DNA evidence has suggested that Asian carp are indeed knocking on the door of the Great Lakes.

SCHAPER: Crews will continue netting dead fish this weekend and Rogner says he won't be surprised if more Asian carp are found. Last month, scientists discovered Asian carp DNA upstream of the kill zone and well beyond the $10 million electric barrier designed to zap and stun the fish into turning back.

Mr. HENRY HENDERSON (Natural Resources Defense Council): It hasn't worked.

SCHAPER: Henry Henderson of the Natural Resources Defense Council is one of several environmentalists calling for the Army Corps of Engineers to temporarily close three Chicago navigational locks into Lake Michigan. Michigan Governor Jennifer Granholm is threatening legal action to protect a $7 billion a year Great Lakes fishery and force the Corps to close the locks. Granholm's Great Lakes director, Ken DeBeaussaert.

Mr. KEN DEBEAUSSAERT (Director, Great Lake): In the short term, we're concerned that the emergency response plan that was developed isn't sufficient. And that closing the locks would be an appropriate additional protection for the lakes that should be pursued.

SCHAPER: Federal officials say they are considering closing the locks. In the meantime, commercial fishing and DNA water sampling in canals will try to pinpoint exactly how close Asian carp are to getting into the Great Lakes.

David Schaper, NPR News, Chicago.

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