CHICAGO (AP) -- A federal judge Monday threw out a lawsuit filed by five states that want barriers placed in Chicago-area waterways to prevent Asian carp from invading the Great Lakes, but said he would consider new arguments if the case were filed again.
Michigan, Wisconsin, Minnesota, Ohio and Pennsylvania claimed that the U.S. Army Corps of Engineers and Chicago's Metropolitan Water Reclamation District are causing a public nuisance by failing to physically separate a network of rivers and canals from Lake Michigan.
Scientists have detected DNA from two types of Asian carp in the waterways, bighead and silver carp. They say if the voracious carp gain a foothold in the Great Lakes, they eventually could out-compete native species and severely damage the region's $7 billion fishing industry.
U.S. District Judge John Tharp said he couldn't order the agencies to do what the states want because federal law requires the corps to keep shipping channels open between Lake Michigan and one of the Chicago waterways -- the Des Plaines River -- and prohibits constructing dams in any navigable waterway without Congress' consent.
In a written ruling, Tharp said he was "mindful of, and alarmed by, the potentially devastating ecological, environmental, and economic consequences that may result from the establishment of an Asian carp population in the Great Lakes." But he said the proper way for the states to win approval of separating the waterways is through Congress.
Tharp left the door open for further court action, however, saying the states might be able to find other grounds for a nuisance claim that wouldn't have the effect of asking the corps and city agency to violate federal law.
"There may be room in which the (states) can still maneuver," he wrote.
Joy Yearout, spokeswoman for Michigan Attorney General Bill Schuette, said his office had not decided on its next step.
"We're certainly disappointed," Yearout said. "We're reviewing the ruling and we will consult with the other states on how to move forward."
The U.S. Department of Justice, which represents the Army corps, had no immediate comment, spokesman Dean Boyd said.
Tharp's decision is the latest legal setback for the five states, which were joined in their suit by the Grand Traverse Band of Ottawa and Chippewa Indians. The U.S. Supreme Court has refused their request -- four times -- to order temporary measures such as closing Chicago shipping locks and installing block nets in the waterways.
Asian carp gobble huge amounts of plankton, which are tiny plants and animals crucial to aquatic food webs. Silver carp are notorious for springing from the water like missiles, sometimes colliding with boaters.
They were imported in the 1970s to cleanse Deep South aquaculture and sewage treatment ponds. Some escaped during floods and have migrated northward in the Mississippi River and its tributaries. They've advanced to within 55 miles of Lake Michigan in the Illinois River, which connects with other waters that reach Lake Michigan.
An electric barrier in the Chicago Sanitary and Ship Canal is designed to stop them. But scientists have detected DNA from Asian carp beyond the barrier just a few miles from the lake. The five states contend the only sure way to block the carp's path is to place dams or other structures in the waters, which Illinois officials say would damage the local economy and cause flooding.
Joel Brammeier, president of the Chicago-based Alliance for the Great Lakes, said the judge's ruling would not deter advocates of separating Lake Michigan from the Mississippi River watershed, which he said was "still the clear solution."
Dismissing the states' lawsuit was "the right thing to do," said Lynn Muench, a vice president of the American Waterway Operators, which represents tugboat and barge operators. "Hopefully this closes the ongoing threat to navigation, but we'll see."