The fish that could eat the Great Lakes is only 6 miles away from Lake Michigan now — maybe.

The big, ugly and unbelievably hungry Asian carp has been making its way up the Mississippi for two decades and now appears to be closer than ever to migrating en masse to the world's largest body of fresh water.

If it starts reproducing there, scientists say, it's likely to eventually consume much of the plankton that forms the basis of the food chain that supports what's estimated to be a $7 billion sports fishery.

"These fish are extraordinarily prolific, and if they establish themselves in the Great Lakes, the Great Lakes are done," said Joel Brammeier, president of the Alliance for the Great Lakes.

To stop the carp, some environmentalists say we should consider spending an untold sum to redo the engineering feat that reversed the flow of the Chicago River and linked the

Mississippi basin with the Great Lakes a century ago. That would stop the invading fish — maybe.

Such are the uncertainties confronting federal officials as they consider how to cope with the greatest threat to face the Great Lakes fishery in modern times.

The trouble is, the defining words in the Asian carp story seem to be: maybe and if.

The lakes already may be in big trouble — if the lone blackhead carp that was found last month in Lake Calumet, six miles from Lake Michigan, was one of many.

And radical changes in the Chicago waterways may be able to save the lakes from the invaders from afar — if they can be done on time, and if the monster fish doesn't find some other way into the lakes, which is something that may already be happening.

Big eaters

One thing's for sure: this is no zebra mussel, no nuisance stuck to the bottom of a boat or a pipe drain.

One Asian carp species, the silver carp, gets so startled by boat motors that they leap 10 feet out of the water and come flying at unsuspecting boaters.

Not all Asian carp do that, but many are gigantic fish that can weigh up to 100 pounds and eat up to 20 percent of their body weight every day.

In doing so, they hog the plankton at the bottom of the food chain and crowd out other species. In some parts of the Illinois River, for example, Asian carp now make up 95 percent of the biomass.

"It's essentially wiped everything else out," said Tom Marks, New York director of the Great Lakes Sport Fishing Council.

Brought to the United States to swallow up algae at southern catfish farms decades ago, the carp escaped from those farms during flooding in the early 1990s and over the years made a home throughout the Mississippi basin. If the fish establish themselves in the Great Lakes, look out.

"There would be a huge potential for a very considerable, perhaps catastrophic damage to the fisheries," said David M. Lodge, a professor of biological sciences at the University of Notre Dame whose environmental DNA research has shown evidence that the fish are in the Chicago waterways near Lake Michigan.

"Lake Erie is at the highest level of risk," Lodge said.

That is because of its shallow depth, warmer waters and plentiful food supply.

The risk appeared to grow much more imminent when a single bighead carp was caught in Lake Calumet, near Chicago, last month. It's only six miles from Lake Calumet to Lake Michigan, and there's nothing in between that could stop the fish from spreading.

While a $30 million system of electronic barriers is being built south of the Chicago waterways, environmentalists believe the fish caught in Lake Calumet — and perhaps many more — have evaded those barriers.

So for the Great Lakes, the existential question may be: What does one fish mean?

To Brammeier and other environmentalists, the discovery backs up Lodge's research showing that the fish are on the doorstep of the Great Lakes.

"One fish usually doesn't mean one fish; it typically means a population," Brammeier said.

But industrial interests — who are fighting to stop efforts to close Chicago waterways to fight off the fish — aren't so sure.

"This is the first fish they've found in 10 months of searching," said Jim Farrell, executive director of the Illinois Chamber of Commerce's Infrastructure Council. "Someone could have put it there."

Sales ban That's no conspiracy theory. Single carp have been found for years in inland lagoons, and five were found in western Lake Erie between 1995 and 2003.

How did they get there?

Scientists believe those fish may have been unwilling participants in an ancient Asian tradition.

While Asian carp are a dangerous nuisance here, they are a food staple in China, and have been available for purchase live in Asian groceries for years.

The Asian tradition: "If you're going to eat one, you buy two and release one," Lodge noted.

Not surprisingly, the sale of the live fish is now banned in the Chicago area, but business interests still focus on this cultural practice as a way of minimizing the Asian carp threat.

Yet one immediate and partial solution would be comparatively inexpensive in government dollars: closing the locks in the Chicago waterway system that were built when the Mississippi and the Great Lakes were connected by man a century ago.

That's something Sen. Kirsten E. Gillibrand, D-N.Y., has been calling for since last December.

"The most important thing would be shutting down the locks," just because it would be fast and immediately effective, Gillibrand said.

But a wide range of Chicago-area business interests, ranging from industries that do waterway shipping to the operators of the city's famous boat tours, fear that lock closure would disrupt their business.

"Yes, there's lots of talk about invasive species, but let's also talk about people with jobs, who work on these waterways," said Lisa Frede, director of regulatory affairs for the Chemical Industry Council of Illinois.

And some environmentalists worry that lock closure would be a temporary Band-aid that many might mistake for a cure for a grievous injury.

"Lock closure is no good solution," said Jennifer Nalbone, director of navigation and invasive species at Buffalo-based Great Lakes United. "It's arguably easier for the fish to get through an open lock rather than a closed lock, but it's not fail-safe."

Moreover, "lock closure stops people from working together" on a more comprehensive solution, Nalbone said.

Race against time

A more comprehensive solution could range anywhere from ecological separation — somehow making the Chicago waterways uninhabitable to fish, perhaps by warming the water— to hydrological separation: that is, undoing the water link between the Mississippi system and the lakes.

The Chicago District of the Army Corps of Engineers would do any of that work, but environmentalists complain that the Corps has been slow to react to the growing carp threat.

"There is a failure to take this seriously," said Henry Henderson of the Natural Resources Defense Council.

Contacted Thursday, a spokesman at the Corps office in Chicago said a day later that it would be difficult to get anyone from the Corps to comment on the Friday of a holiday weekend.

To push the Corps into action, a bipartisan, bicameral group of lawmakers, including

Gillibrand and Sen. Charles E. Schumer, last week introduced a bill that would force the Corps  to complete a study within 18 months on the feasibility of permanently severing the Mississippi River and Great Lakes basins.

The Corps had said such a study could take years, but the lawmakers — which include Illinois senators — said that's far too long.

"We are clearly in a race against the clock," said Schumer, D-N.Y.

The trouble is, the clock — and the carp — may already be winning.

"I think the effort to stop the carp from reaching Lake Michigan failed several years ago," said Marks, of Derby, who runs a fishing charter business out of Sturgeon Point.

Not only is there DNA evidence that the fish are in the Chicago waterways, there's also the fact that the carp are popping up in other areas close to the lakes. Just last week, an Asian carp was found in the Wabash River in Indiana, which shares a flood plain with the Maumee River, which enters Lake Erie at Toledo, Ohio. In other words, the fish could establish themselves in Lake Erie someday in the same way they took hold in the Mississippi.

Lodge said there's no way of knowing how long it would take for the carp to start breeding in the lakes, or how long it would take before other species are crowded out of their habitat.

And Nalbone, of Great Lakes United, agreed.

"It's a mystery, and we've got to get comfortable with not knowing," she said. "This could be a real-life experiment, we don't want to see."