A federal judge has blocked a third and perhaps final attempt to close Chicago-area shipping locks, saying Asian carp do not appear to be an imminent threat and closing the locks might not keep them from reaching Lake Michigan anyway.

In a long-awaited ruling released Thursday, U.S. District Judge Robert M. Dow said that "the bottom line is that even giving every benefit of doubt ... plaintiffs cannot establish a showing of irreparable harm."

Michigan Attorney General Mike Cox, who spearheaded the unsuccessful lock-closing effort in federal court and twice last year in the U.S. Supreme Court, said he will fight on.

"Our legal fight against Asian carp will continue, but President (Barack) Obama could stop the spread of Asian carp with the flick of a switch," Cox said. "Obama's persistent failure to stop Asian carp is a slap in the face to Great Lakes citizens genuinely concerned about preserving their livelihood."

Attorneys for the defense speculated that another legal challenge was unlikely.

"I have a hard time thinking that Michigan will want to put the resources into prosecuting this case again," said David Rieser, an attorney for the defense. The two sides are due back court Jan. 7 to determine how the case will proceed.

Locking gates were built into the Calumet-Sag Channel and the Chicago River to limit the amount of water releasing out of Lake Michigan when engineers reversed the flow of the Chicago River at the turn of the last century. The U.S. Army Corps of Engineers and the Metropolitan Water Reclamation District control the locks to limit flooding during heavy rains and to allow for the passage of millions of tons of cargo aboard ships and boats.

In a move initiated by Cox while he was campaigning to become governor, five Great Lakes states sued the federal government in July to force temporary closing of locks until other carp-control methods could be put in place.

Critics, though, called the lock-closing effort a "politically motivated" attack and said it could devastate the regional shipping industry and put residents who live in flood-prone areas at risk.

A native of China with no known predators in the U.S., Asian carp have overwhelmed native fish populations by out-competing them for food, jeopardizing the Great Lakes' estimated $7 billion annual commercial and recreational fishing industry.

jhood@tribune.com