The 20-foot flat-bottomed johnboat roared up the Illinois River at 30 miles per hour, leaving a roiling brown wake. Suddenly, the water exploded.

Twenty pounds of fat, silvery carp shot into the air, twisting 5 feet above the river's surface before slamming back into the water.

"Watch out - south of here, they wham right into the boat or jump in," said Nate Caswell, a U.S. Fish and Wildlife Service biologist who was driving the boat.

Neither happened on this trip, but for the next 15 minutes, dozens of the ugly, hulking Asian carp performed their odd airborne ritual, flipping, leaping and skipping across the water.

"They freak out at the slightest disturbance, but nobody knows why," Caswell said. "We want to keep them out of the Great Lakes if we can, but the fact is they're on the way."

It's not just fish that are freaking out these days. The carp have worked scientists, resource managers and politicians into a frenzy.

Throughout the Great Lakes basin and along the Mississippi River, officials and researchers are desperately trying to fashion a way to stop the species' advance.

The beasts have made their way upriver for more than a decade and are now less than 50 miles from Chicago's Lake Michigan lakefront, held back by an electrified "dam."

The carps' voracious eating habits (one fish eats half its body weight per day) and breeding habits (they're also known as "river rabbits") could wipe out the base of the Great Lakes' food chain. Beyond the ecological disaster that they would wreak, the carp could devastate the lakes' $4.5 billion commercial and sport fisheries.

Add to that the grotesque spectacle that has been captured on videotape several times: Flying carp slamming into boaters, crashing into boats and flopping wildly in hulls.

"I've been hit by them - everyone in my crew has been," said Caswell, who has been monitoring the carp for a year. "One of these days, some boater going 50-60 miles an hour is going to take a 10-pound carp to the noggin, get knocked out of the boat and drown."

Pam Thiel, who supervises the carp project on the Illinois River, said such spectacles "seem like slapstick humor, but it's really black humor when you realize what these carp could do to the Great Lakes."

The carp are the most recent poster child in a widespread battle against invasive, non-native species, which federal officials estimate cause $137 billion in economic losses nationwide every year. More than 160 of these species have invaded the Great Lakes; a similar number have moved into the Mississippi River. They include such varying species as sea lampreys and zebra mussels.

But most Minnesotans haven't heard of the Asian carp, much less been alarmed by their approach.

"I've been talking about these things for several years, but not a whole lot of people are paying a lot of attention," said Jay Rendall, exotic species program coordinator for the Minnesota Department of Natural Resources. "When they do start showing up here, they will."

Four species of Asian carp - grass, silver, black and bighead - were imported into the United States in the 1970s by southern aquaculture operators who were using them to clean up fish farm waste and aquatic plants from the bottoms of their ponds. Then, they got out "through the accidental and intentional, legal and illegal release," said a Fish and Wildlife Service bulletin.

They began moving north, up the Mississippi and its tributaries, piling up below dams, crowding out other fish species and filling commercial fishing nets so full that the nets couldn't be lifted.

They have become so ubiquitous in the river's lower reaches that when a fish kill of undetermined origin occurred four years ago on the Mississippi in Illinois, biologists discovered that 97 percent of the fish were Asian carp.