HAVANA, Ill. (AP) — As scientists aboard a research boat activate an electric current, the calm Illinois River transforms into a roiling, silvery mass. Asian carp by the dozen hurtle from the water as if shot from a gun, soaring in graceful arcs before plunging beneath the surface with splashes resembling tiny geysers.

Water quality specialist Thad Cook grunts as a whopper belts him in the gut. His colleagues duck and dodge to avoid the missile-like fish that plop onto the deck, writhing madly until someone can grasp the slimy, slithering critters and heave them over the side.

It's like a scene from a Hitchcock movie — and indeed the flying carp have played villainous roles in many a YouTube video. Biologists, however, fear a different kind of horror story may be taking shape underwater: a war for survival between the aggressive Asian carp newcomers and native species important to people who catch fish for a living or fun.

"We suspect at some point there will be a real crash in the populations of some of these native fishes," said John Chick, an aquatic ecologist with the National Great Rivers Research and Education Center on the Mississippi River near St. Louis.

While years of study have turned up ominous signs that the carp are capable of crowding out other species and changing ecosystems, the worst-case scenario scientists expect to unfold hasn't yet been realized. Some scientists say that dire predictions about the damage carp can do may be premature. That makes the research Chick and his colleagues are conducting critical: It likely will influence how the debate over managing waterways made vulnerable by carp plays out in Congress and the courts.

U.S. research so far has focused on rivers, where Asian carp are most plentiful. A common method for determining which fish are in a given location is to shoot an electric charge into the water that temporarily stuns them so crews can scoop them up in nets. Silver carp, an Asian type known for springing from the water when startled, manage to jump first.

Imported decades ago to cleanse algae-choked aquaculture ponds and sewage treatment lagoons, they escaped during floods and have marched up the Mississippi watershed in more than two dozen states, ranging as far as Kansas and the Dakotas. They're taking dead aim at the Great Lakes, with the leading edge on the Illinois River some 55 miles south of Lake Michigan, although their DNA has been found in Chicago a mere 6 miles from the lake.

"All you've got to do is look at tributaries of the Mississippi or Ohio rivers and you'll find them," said biologist Ron Brooks of the Kentucky Department of Fish and Wildlife Resources.

Those environments are hotbeds for research, and could foretell what will happen if carp invade other waterways. What's missing, though, is smoking-gun evidence that Asian carp will devastate other fish. Even in places such as the Illinois and Mississippi, where carp are rampant, changes have been incremental.

For example, while indigenous bigmouth buffalo and gizzard shad both have gotten skinnier since the carp arrived, the buffalo's population has declined only moderately while the shad have fluctuated. Commercially harvested buffalo are found on grocery shelves from Alabama to Minnesota. The shad are crucial prey for bass and other sport fish.

"When you get a species invasion ... typically you see some native species decline or go extinct locally," said biologist Jim Garvey of Southern Illinois University at Carbondale. "We haven't seen that yet. We're kind of wondering what the heck's going on."

Chick and others have found that silver and bighead carp, the most menacing of several Asian varieties in the U.S., eat the same food as the bigmouth buffalo and gizzard shad. A separate study detected weight declines among buffalo and shad in the Illinois River, believed to have the largest concentrations of Asian carp.

Researchers are looking for proof that Asian carp are at least partially to blame for the drop-offs. "It's just a correlation at this point," Chick said.

Other research supports the potential that the newcomers could take over the neighborhood. Garvey and associates reported this year that Asian carp account for more than 60 percent of the biomass — the combined weight — of all fish species along the lower 150-mile stretch of the Illinois River. They also make up virtually all fish longer than 16 inches.

Such findings stir unease in the Great Lakes region, which could become the Asian carp's next frontier. Ravenous and prolific, the carp typically weigh 30 to 40 pounds but can exceed a hefty 100 pounds. They gorge up to one-fifth of their body weight daily on plankton — tiny plants and animals that nearly all fish eat.

Many fear the carp would unravel food webs supporting a $7 billion Great Lakes fishing industry. Silver carp, the ones that leap from the water with enough force to break boaters' noses, could give the region's tourism a black eye.

Knowing how damaging carp can be is important because the fight against them is costing big bucks — and could get lots pricier.

Government agencies have spent more than $150 million on technology to repel the invaders, including an electric barrier in a Chicago-area canal linking Lake Michigan with the carp-infested Illinois River. Five states are suing the federal government to blockade the canal, which would take years and cost billions. Shipping and tour boat groups say that step would be as ruinous to them as Asian carp would be to the fishing industry.

"This kind of research gives an early warning and justification to do everything possible to keep them out," said Marc Gaden, spokesman for the Great Lakes Fishery Commission. "The more understanding you have of what makes these fish tick and what's happening in the ecosystem where they've already invaded, the closer you get to maybe discovering ways to get them under control."

A U.S.-Canadian team Thursday released a risk analysis for the Great Lakes. Some experts have questioned whether the lakes have enough warmth and food to support Asian carp. But the report found that the hardy fish would find hospitable conditions in bays, nearshore areas and tributary rivers feeding all five of the lakes — even chilly Superior. Warm, shallow Lake Erie, with the most abundant fish numbers, is an especially ripe target.

It could take as few as 10 pairs of males and females to establish a successful population if they can find good spawning areas, and more than 70 rivers around the Great Lakes region appear suitable, the report said. If they slip into Lake Michigan and establish a foothold, they could journey northward to Lake Huron and southward into Lake Erie within a decade, it said.

Scientists are also digging through online databases for clues about how Asian carp have affected lake ecosystems in other countries. Duane Chapman, a U.S. Geological Survey biologist, says silver carp have driven down populations of native species in Europe similar to the Great Lakes' prized walleye and yellow perch.

Chick offered one possible explanation for why the carp's impact hasn't been more dramatic so far: There may be still enough food — for now — to ward off starvation in the Illinois and Mississippi rivers, which are richer with algae and zooplankton than most of the Great Lakes. So the expected die-off of other fish could take years to develop, until a tipping point is reached.

Calculating damage from Asian carp is slow and often frustrating work, thanks in part to the ever-changing nature of rivers. Fluctuating water levels, nutrient runoff and temperatures also affect fish numbers. But researchers are working to solve the mystery before the fish proliferate in the Great Lakes, determined to beat the clock and prevent the feared disaster damage.

"No one knows for sure what would happen," Garvey said. "But we don't want to get to that point. We're looking at some really scary scenarios."