The U.S. Supreme Court refused Monday to order emergency measures that might prevent Asian carp from reaching the Great Lakes, despite a warning that the exotic fish pose a "dire threat" to the region's environment and economy.

Michigan and four neighboring states wanted the U.S. Army Corps of Engineers to install nets in two Chicago-area rivers and to expedite a study of permanent steps to head off an invasion by bighead and silver carp, which have advanced up the Mississippi River and its tributaries to within 88.5 kilometres of Lake Michigan.

Scientists say if the large, prolific carp spread widely in the lakes, they could starve out native species and devastate the $7 billion US fishing industry.

The justices' ruling, which was issued without comment, was their fourth rejection of pleas by the states for interim steps — including closure of navigational locks in the Chicago waterways — while their lawsuit against the corps is pending in a federal district court.

"It is our duty to aggressively fight to protect the Great Lakes from invasive species," Michigan attorney Bill Schuette said. "So while I'm disappointed, we will continue forward on all fronts."

Faster action demanded

Minnesota, Wisconsin, Ohio and Pennsylvania have joined Michigan in demanding faster action against the carp, which eat vast amounts of tiny plants and animals at the base of the food chain.

The Corps of Engineers says it will complete in 2015 a study of ways to prevent migration of fish and other species between the Great Lakes and Mississippi watersheds. The five states and environmental groups say that's not quick enough, because it could take many additional years to carry out the study's recommendations.

They advocate placing barriers in Chicago-area waterways to cut a link between the watersheds created more than a century ago when engineers reversed the flow of the Chicago River to flush the city's sewage toward the Mississippi. A recent report by groups representing Great Lakes states and cities proposed three methods for doing so, with estimated costs as high as $9.5 billion US.

Because permanent separation could take nearly two decades to complete, the states are suing for short-term actions. Their petition to the high court requested installation of block nets in the Little Calumet and Grand Calumet rivers along with quicker completion of the corps study.

Federal officials say the nets would do more harm than good and the study timetable is necessary because of its complexity.

The Obama administration has devoted more than $100 million US to shielding the lakes from the carp and recently announced plans to spend $51.5 million US this year. Plans include operating and monitoring an electric fish barrier near Chicago, stepped-up commercial fishing in the area, and field testing new strategies such as high-pressure underwater guns and pheromones that could lure carp into lethal traps.

Chicago business interests oppose separating the watersheds and closing the locks, which they say would disrupt commercial barge and pleasure boat traffic.

"Thankfully, the Supreme Court came down on the right side of this issue," said Mark Biel, spokesman for a group called UnLock Our Jobs. "Suitable and effective steps are being taken to ensure we protect the lakes. Unfortunately, those who have used this issue for their own political agendas will continue to push for devastating actions that prevent the Great Lakes region from working together on a comprehensive solution."

Joel Brammeier, president of the Chicago-based Alliance for the Great Lakes, said the court ruling was "not so much a setback as a way station on the road to separation. We've got to keep our eye on a long-term outcome."