(Courthouse News) – The Ninth Circuit ruled Friday the U.S. Fish and Wildlife Service incorrectly denied endangered listing to a cold-water fish living in the Upper Missouri River Basin in Montana.

A three-judge panel found Fish and Wildlife failed to supply evidence of increased population of the Arctic grayling in Montana rivers, and didn’t properly account for climate change when it declined to list the species as endangered in 2014.

“FWS clearly stated in the 2014 finding that the number of breeding arctic grayling increased in the Big Hole River, and omitted the DeHaan study’s evidence to the contrary,” U.S. Circuit Judge Richard Paez wrote for the unanimous panel. “We conclude that in ignoring available data FWS acted in an arbitrary and capricious manner.”

The panel ordered the agency to reconsider the listing with a fuller account of the scientific record on hand.

Plaintiffs Center for Biological Diversity, Western Watersheds Project, Butte resident Pat Munday and former Montana fishing guide George Wuerthner called the ruling a major victory.

“Montana’s dwindling Arctic grayling populations will need all of the help we can give them to survive in the face of a warming climate,” said Earthjustice attorney Jenny Harbine, who argued the case. “The court’s decision offers hope for these magnificent fish, and now it’s up to the Fish and Wildlife Service to translate that hope into action to protect the grayling under the Endangered Species Act.”

The Arctic grayling is a salmonid fish that prefers cold-water rivers – some subspecies live and spawn in lakes – and historically roamed throughout the Upper Missouri River watershed, Montana, Wyoming and as far afield as Michigan.

Due to a myriad of threats to its habitat, the current population of Arctic graylings occupy less than 10 percent of its historical range, relegated mostly to the Big Hole River watershed in Montana.

Climate change is a daunting specter for the fish as any water temperature above 77 degrees Fahrenheit is hostile to its survival.

With all this in mind, Fish and Wildlife first considered listing the species in 1982. In 1994, the service concluded that listing was warranted, but higher priorities took precedence.

By 2007, the agency determined that Endangered Species Act protections were unnecessary because the population segment in question was not distinct. A lawsuit ensued, along with a settlement that stipulated the agency would determine whether to list the graylings in 2010.

Finally, Fish and Wildlife – citing a study that showed Arctic graylings were on the increase in the Big Hole River watershed – decided against listing the fish as endangered. But the plaintiffs claimed the agency relied too heavily on one study to the exclusion of several others that demonstrated a declining population in the river.

The panel agreed.

“FWS cannot rely on its briefing in this case to explain why the 2014 finding relied on the Leary study rather than the DeHaan study,” Paez wrote. “The explanation must be evidenced from the listing decision itself.”

The panel also criticized the agency for failing to fully consider the likelihood of rising stream temperatures on the species.

“Although there have been improvements in stream flow and water temperature since 2010, the water temperatures are still above those that are ideal for the Arctic grayling both in the main stem of the Big Hole River and its tributaries,” Paez wrote. “In sum, the 2014 finding that thermal refugia in the Big Hole River would aid survival of the arctic grayling was arbitrary and capricious.”

Fish and Wildlife will review the decision and formulate its next steps.

“This decision just came out, so we have no other immediate response to offer at this point,” said agency spokesman Ivan Vicente.

While the plaintiffs said the ruling was a positive step forward, they maintain an actual listing by Fish and Wildlife is the only way to reverse the steep habitat loss caused by extensive water diversions from the Big Hole River by farmers.

“I fish the Big Hole River often and, like many anglers, I treasure our grayling as a beautiful native fish,” said Pat Munday, a college professor who wrote a popular book about the river. “It is incredibly sad that we must sue the Fish and Wildlife Service to follow the law and protect grayling. I sincerely hope this time the agency will live up to its mission to put politics aside and finally protect the grayling.”