BOISE - The U.S. Fish and Wildlife Service said Tuesday it won't classify the mountain whitefish in the Big Lost River as an endangered species, ending an environmental group's attempt to protect the fish.

After examining the fish's DNA and evolution, the agency said the river's mountain whitefish doesn't merit a review because it isn't that different from similar fish across the country.

The decision means the agency doesn't have to investigate environmentalists' concerns that farming and ranching in eastern Idaho are hurting the environment and decimating mountain whitefish in that part of the state.

Fish and Wildlife Program Manager Steve Duke said the Western Watersheds Project, which was looking to protect the fish, didn't submit enough scientific evidence to prove the fish living in the isolated Big Lost River has evolved differently from mountain whitefish in other states, including Montana, Nevada and Wyoming.

"If we had information that it lived under conditions that were unique compared to other populations, that would have been different," he said. "We just didn't have that information."

He pointed to the white sturgeon in the Kootenai River as an example of a fish that met a standard of uniqueness because it has adapted to the cold, fast-flowing waters of the North Idaho river by spawning at low temperatures. The species was listed as endangered in 1994.

Western Watersheds Executive Director Jon Marvel said the federal agency didn't thoroughly examine the fish and that his group plans to scour the decision for faulty logic that could open the agency to a lawsuit.

"We are disappointed," he said. "The mountain whitefish's habitat has been declining for years."

The group, which petitioned the agency to protect the mountain whitefish in 2006, has succeeded in defending Idaho animals and plants in the past. Last year, it got the slickspot peppergrass plant listed as an endangered species, one of only two species that received federal protection that year.

The mountain whitefish lives in basins across the state and the West. The Big Lost River is unique because it is completely cut off from other rivers by mountains and ancient lava flows. As the river flows past Arco, it eventually sinks and disappears into the Snake River Plain.

Western Watersheds said Big Lost's isolation made the mountain whitefish that live and breed there unique in shape and color. The group said the fish needs federal protection because it is facing threats from livestock grazing and farmers taking water out of the river to irrigate their fields.

The nonprofit added that changing regulation of livestock grazing in the watershed would protect the species.