It's Asian Carp Against the Current

"They test the barrier a couple of times, and they usually go far away and don't come back up here again," Shea said. "We've had only one fish get through."

That fish -- not an Asian carp -- passed through the barrier in the churning wash of a tug pushing a barge, Shea said. It happened in 2002, and the transmitter soon fell to the bottom of the 25-foot-deep canal, where it still emits a signal. Researchers concluded that the fish did not survive the trip.

The closest an Asian carp has been spotted to the fence is about 20 miles, also in 2002, said Shea, who reported that the nearest concentration is believed to be 60 miles from the fence, which is about 25 miles southwest of Chicago and the entrance to Lake Michigan. Monitors conduct monthly surveys, just to be sure.

The Chicago Sanitary and Ship Canal is a valuable choke point because it is the major liquid link between the Mississippi River Basin and Chicago, which sits along Lake Michigan. The trench, built in the 1890s in response to deadly cholera and typhoid epidemics, was engineered to carry sewage away from Chicago and its drinking-water supply. Most famously, it reversed the flow of the Chicago River. Fifty-five years after the canal opened, the American Society of Civil Engineers called it one of the seven wonders of American engineering.

Keeping Asian carp out of the Great Lakes is considered essential at a time when countless invasive species, often traveling in the ballast water of large ships, are changing the ecology of the world's largest surface supply of fresh water. The lakes cover 94,000 square miles, and Michigan alone has 3,288 miles of coastline.

Two of the most troublesome recent arrivals are the round goby and the zebra mussel, a fingernail-size shellfish native to the Caspian Sea region. Wags call the Asian carp a hundred-pound zebra mussel.

"There have been massive changes in the Great Lakes," said David Jude, a University of Michigan professor who sits on the advisory committee that oversees the electrified fence. "We sure as heck don't need a big fish that's coming here, feeding on this zooplankton and competing with other fish in the lakes."

Illinois has invested $2 million in the new fish-shocking project, with at least half a dozen other states making smaller contributions. The federal government committed about $7 million. For now, the $500,000 annual expense for electricity, upkeep and a manager is to come from Illinois coffers, but the state is hoping to make it a federal issue -- and expense.

"We think it's an issue of federal concern. It's interstate and international waters, like from the Great Lakes into the national shipping conduit," said Scott Stuewe, acting chief of the fisheries division at the Illinois Department of Natural Resources. He has no doubts about the importance of the project.

"If you've seen what tarpon look like on the end of a line, like a sport fish on TV, that's kind of what these things look like," Stuewe said. "They've been proliferating up through the Illinois river system. We tend to think they may proliferate in the Great Lakes, but we just don't want to find out."