SALT LAKE CITY - Federal endangered species protections won't be extended to Utah's state fish, the Bonneville cutthroat trout.

The U.S. Fish and Wildlife Service says populations of the fish have increased in the last three decades. The agency says the fish is widely distributed across parts of Utah, Idaho, Nevada and Wyoming.

The Bonneville trout - one of 14 cutthroat subspecies in the West - is found in 35 percent of its historic habitat and occupies more than 2,300 miles of stream in those four states, according to the agency.

"Based on our analysis, it's doing well," said Paul Abate, a Fish and Wildlife Service biologist in Salt Lake City who helped lead the latest review of the species.

That review of the cutthroat examined threats from land-use practices, hybridization, nonnative predators and climate change.

The agency found that none of the threats warranted listing the Bonneville trout as a threatened or endangered species.

A notice of the decision was published Tuesday in the Federal Register. The government was first asked to list the species as threatened in 1998.

The Center for Biological Diversity, an environmental group that sued the Fish and Wildlife Service over an earlier decision not to list the trout, may challenge the latest ruling.

Noah Greenwald, the group's science director, criticized the Fish and Wildlife Service for examining threats only to the trout's current habitat and not its much larger historic habitat prior to the West's settlement.

He also faulted the agency for not taking into account the long-term effects of climate change.

"I think that's naive at best and dishonest at worst," he said.

Adorned with spots and a colorful slash on its jaw, the Bonneville cutthroat once dwelled in prehistoric Lake Bonneville, which covered some 20,000 square miles of what's now Utah, Nevada and Idaho. After the lake spilled out and receded, many of the fish switched to streams.

The cutthroat thrived in the region until the late 1800s and early 1900s. With settlement of the region also came logging, grazing, irrigation and larger-scale development.

"We don't see the wide-scale habitat abuse that we did back then, but that's not to say everything's perfect," Abate said.

Just a few decades ago, biologists thought the Bonneville cutthroat might have winked out forever.

But more recently, federal officials say there's been signs of improvement.

Changes in land-use practices, more attention to saving important habitat and conservation plans in each of the states have helped the fish recover. So have restocking efforts in some waters and, in other places, decisions not to stock non-cutthroats where the Bonneville trout are established, said Eric Wagner, research director at the Utah Division of Wildlife Resources' Fisheries Experiment Station in Logan.

"Public land management agencies are doing a better job but there's still room for improvement," Wagner said, noting careful attention still needs to be paid to water withdrawal for irrigation, cattle grazing and riparian habitat.

Today, about 64 percent of the trout's habitat is in Utah. Idaho has 23 percent, Wyoming 12 percent and Nevada 1 percent.

"My sense is that they're doing well and that the populations are on an upward trend," Wagner said.

In its existing range, 5 percent of its habitat is rated as excellent, 47 percent as good and 28 percent as fair, according to the Fish and Wildlife Service.

The biggest wild card may be how the trout - which rely on cold water - respond to predicted temperature increases. Utah is expected to warm more than the global average in the coming years.

The Bonneville trout may be able to survive better than other cutthroat subspecies, according to the Fish and Wildlife Service, but it's difficult to predict.

"Climate change is definitely something we're going to have to keep tabs on," Abate said.