DENVER, Colo.— A federal judge has found that the U.S. Fish and Wildlife Service’s denial of Endangered Species Act protection for the Rio Grande cutthroat trout was arbitrary and unlawful. The finding, filed late Thursday, came in response to a lawsuit by the Center for Biological Diversity.

Today’s decision means the agency must reconsider listing the trout under the Act. The fish still survives in southwestern Colorado, along with southwestern, north and east-central New Mexico.

“We’ve been fighting to save Rio Grande cutthroat trout for more than 20 years. It’s a relief to have it one step closer to getting the help it so badly needs,” said Michael Robinson, a conservation advocate with the Center. “The trout is barely hanging on in a small number of tiny, isolated headwater streams. The fish is severely threatened by habitat degradation, non-native trout and now climate change.”

In 2008 the Fish and Wildlife Service found the trout warranted protection but then failed to actually grant it, instead putting it on a waiting list. Although the status of the trout had not improved, the agency reversed course in 2014 and determined the trout didn’t warrant protection.

To justify this reversal, the agency changed the criteria for what it considered a healthy, secure population from one with 2,500 fish to one with only 500 fish. The Service failed to provide any justification for this shift.

“The Rio Grande cutthroat trout was endangered in 2008 and in 2014 and remains endangered to this day,” said Robinson. “The Fish and Wildlife Service moved the goal posts in order to get to a politically driven decision that the trout doesn’t warrant protection. It’s well past time this beautiful fish gets the safeguards it needs.”

The Center first petitioned for the trout in 1998. After two lawsuits the Service finally agreed it needed protection in 2008. It took another court-ordered settlement for the agency to consider the trout’s status in 2014, and then it was again denied protection.

“The Endangered Species Act has saved 99 percent of species under its protection and put hundreds on the road to recovery. It can save the Rio Grande cutthroat trout too,” said Robinson. “The livestock industry and states like Colorado and New Mexico oppose trout protections. But those protections would benefit fish and people alike by protecting clean water and provide yet another reason to reduce greenhouse gas emissions.”

Characterized by deep crimson slash marks on its throat, the fish once swam throughout the Rio Grande, Pecos and Canadian river basins from Colorado to southern New Mexico. It is now limited to a small number of tiny headwater streams in only about 10 percent of its historic range.