BILLINGS – A federal appeals court ruled Thursday that a pesticide used intentionally by a state agency to get rid of unwanted fish is not a pollutant under the Clean Water Act.

The ruling by a three-judge panel of the 9th U.S. Circuit Court of Appeals came in a lawsuit over a state Department of Fish, Wildlife and Parks plan to kill nonnative fish in Cherry Creek and Cherry Lake, southwest of Bozeman, to reintroduce threatened westslope cutthroat trout.

Bill Fairhurst of Three Forks sued, arguing the department was required by the federal Clean Water Act to secure a special permit before applying antimycin, a commonly used fish poison.

The project went forward using antimycin, killing rainbow and Yellowstone cutthroat trout, the lawsuit said.

The appellate panel noted that there was no claim the antimycin had any unintended effects, or that any residue remained after the poisoning.

The panel said a chemical pesticide intentionally applied according to instructions approved by the Environmental Protection Agency “with no residue or unintended effect is not ‘waste’ and thus not a ‘pollutant’ for the purposes of the Clean Water Act.”

Bruce Rich, a regional fisheries manager for the department, said the project – in its third year – potentially could go on for 10 years. He said he could not immediately comment on the decision.

Cherry Lake and part of the stream are on national forest land, but most of the stream and its tributaries are on Ted Turner’s Flying D Ranch. Turner is financing most of the $500,000 project.