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."