When scientists discovered six years ago that aggressive Asian carp had made their way up the Mississippi River's tributaries toward the Chicago area, the Obama administration and alarmed state officials pledged swift action to head off an invasion they feared could devastate fishing and boating on the vital Great Lakes.

Since then, U.S. federal agencies have spent more than $300 million on stopgap measures, including placing electric barriers on one likely route, a shipping canal that leads to Lake Michigan. But as the carp get closer, the quest for a surefire deterrent seems to be coming up empty.

An advisory panel that has debated solutions for several years is scheduled to hold what may be its final meeting Thursday, with no sign of a consensus plan, several members said in interviews.

Even if talks continue, chances are growing that the carp will arrive before anything conclusive is done to stop them. At their recent pace, the first young carp could reach Lake Michigan within two years, although a number of obstacles could slow them considerably.

"It's one of the things that keep me up at night," said U.S. Sen. Debbie Stabenow, a Michigan Democrat whose state borders four of the five Great Lakes. "Asian carp could devastate our Great Lakes and the hundreds of thousands of jobs that depend on them."

Blocking waterways an option

The most effective measure proposed — blocking waterways that connect the Mississippi River watershed with Lake Michigan — is favoured by a majority of the eight Great Lakes states but widely unpopular in two of them, Illinois and Indiana. The U.S. Army Corps of Engineers says it could cost up to $18 billion, a figure supporters contend is exaggerated.

Separating the watersheds would disrupt shipping on rivers and canals in the Chicago area, where barges annually haul an estimated $29 billion worth of coal, chemicals and other freight. The added delays would shift more cargo to already-packed highways and railroads, said Benjamin Brockschmidt of the Illinois Chamber of Commerce.

Also, scientists acknowledge that even if the waterways are blocked, Asian carp eventually might reach the Great Lakes anyway— for example, from careless anglers dumping bait buckets — and that their effect on other fish is still speculative.

"Severing a critical part of the nation's water transportation network is too high a price to pay for a solution that is not guaranteed to stop the spread of invasive species," said Tom Allegretti, president of American Waterways Operators, which represents barge and tugboat companies.

2 types of greatest concern

Environmental groups and the region's fishing and boating industries, which generate $23 billion annually on the lakes, are most worried about two varieties of Asian carp: bighead and silver, which weigh dozens of pounds and gorge on the same tiny plant and animal life that feeds the lakes' other fish.

Scientists are still measuring their impact in rivers, but under worst-case scenarios, the large carp could leave popular sport fish to go hungry and suffer population drop-offs. Asian carp are edible but bony, and most Great Lakes fish connoisseurs regard them as a poor substitute for the walleye and whitefish.

Additionally, silver carp are notorious for springing from the water when startled, sometimes ramming boaters with bone-cracking force — a hazard that some fear could damage the Great Lakes' tourism industry.

Several carp species were imported from Asia in the early 1970s to cleanse algae from fish farms and sewage treatment ponds in the South. They escaped during flooding and have migrated up the Mississippi and its tributaries, including the Illinois River, which leads to a network of Chicago-area rivers and canals and Lake Michigan.

'We'd not seen that kind of movement'

Carp DNA was detected in those waters in 2009, spurring calls for urgent action.

This fall, crews discovered two small silver carp farther up the Illinois River than ever before. It meant the leading edge of the juvenile population had advanced 106 kilometres since January — to within 124 kilometres of Lake Michigan.

"We'd not seen that kind of movement in the last four or five years," said Charles Wooley, deputy Midwestern regional director of the U.S. Fish and Wildlife Service. "And all of a sudden, boom."

The juveniles have a better chance of slipping past the electric barriers alive than the larger adults.

Even if the carp get through, scientists say, it could take years to establish breeding populations, and it's doubtful they would spread completely across the Great Lakes, although they could overrun shore areas and tributary rivers where popular species like perch and trout breed and people enjoy water sports.

'No one has done it before'

One proposal with broad support is opening another line of defence at a lock-and-dam near Joliet, Ill., about 16 kilometres south of the electric barriers, which could buy more time. Already, a 3 kilometre-long earthen dam has been built near Fort Wayne, Ind., to cut off another possible invasion route through the Maumee River into Lake Erie.

Yet the Corps of Engineers says a study of what technologies to deploy near Joliet — underwater noisemakers, carbon dioxide bubble screens, hot-water chambers — won't be completed until early 2019.

"It's not like building a levee or dredging a river," said Jeff Heath, the study manager. "No one has done it before."

The delays are stoking frustration among those fearing the worst from a carp invasion.

"We've been taking this slow march," Stabenow said, "when it needs to be much more of a sprint."