The Seventh Circuit Court of Appeals has issued a decision in the ongoing legal saga regarding the invasive Asian carp that is threatening the Great Lakes ecosystem. On August 24, 2011, a three-judge panel of the Seventh Circuit denied the request of five Great Lakes states for a preliminary injunction requiring the Metropolitan Water Reclamation District of Greater Chicago ("District") and the Army Corps of Engineers ("Corps") to take certain measures to prevent the migration of Asian carp into Lake Michigan.

The plaintiffs (Michigan, Minnesota, Ohio, Pennsylvania, and Wisconsin) had appealed a district court denial of their request for a preliminary injunction. In the district court proceedings, the plaintiffs alleged that the Corps' and the District's operation of the Chicago Area Waterway System ("CAWS") would allow the invasive Asian carp to migrate into the Great Lakes. The CAWS is a system of canals, channels, locks, and dams located from the mouth of the Chicago River and other points of Lake Michigan to various tributaries of the Mississippi River. The plaintiffs argued that, if the Asian carp invade Lake Michigan, the $7 billion fishing industry that is dependent on the Great Lakes ecosystem will collapse.

The Seventh Circuit affirmed the district court's decision, but on different grounds. Disagreeing with the district court, the Seventh Circuit held that the plaintiffs had established "a non-trivial chance that the carp will invade Lake Michigan in numbers great enough to constitute a public nuisance." The court stated that, if such an invasion occurred, the plaintiffs would suffer irreparable harm. However, the court held that measures planned by the defendants and other state and federal agencies to prevent Asian carp from reaching the Great Lakes negated the need for a preliminary injunction. The court did warn that if these measures were not taken, or if new information was revealed later on, an injunction might be warranted.

The Seventh Circuit's decision does not dispose of the plaintiffs' underlying public nuisance and statutory claims, which remain pending in the district court. Ultimately, the plaintiffs are seeking a permanent injunction requiring the Corps to separate the waters of the Illinois River basin, which are infested with the carp, and the CAWS, which is estimated to contain low numbers of the carp, from Lake Michigan. The plaintiffs' request for a permanent injunction will be decided using a more stringent legal standard than their request for the preliminary injunction. Since the evidence on the exact location and threat posed by the Asian carp is continually evolving, however, it remains to be seen whether the plaintiffs will ultimately be successful.