A study unraveling the genetics of Colorado’s state fish, the greenback cutthroat trout, has found that pure greenbacks exist only on a 4-mile stretch of a creek southwest of Colorado Springs.

This overhaul of what is defined as a greenback cutthroat may come as a blow to anglers who fished such high-country sweet spots as Rocky Mountain National Park and thought they caught the real thing.

“The only greenback cutthroat trout population that we have now is the one in Bear Creek,” said Jessica Metcalf, a University of Colorado post-doctoral researcher who conducted the study.

Federal, state and Trout Unlimited officials digesting the findings Monday said cutthroat trout previously tagged as greenback likely will be renamed.

The University of Colorado study using DNA analysis and historical specimens also is triggering a federal re-evaluation of the biological status of greenback cutthroats, which are listed as “threatened” rather than “endangered.”

And the findings are expected to bolster nascent efforts by conservationists to protect Bear Creek cutthroats in the face of threats from hikers, motorcyclists, homeless campers and partyers who use trails and fragile slopes along the creek — a tributary of the Arkansas River.

“We’ve known for some time that the trout in Bear Creek were unique,” said Doug Krieger, senior aquatic biologist for Colorado Parks and Wildlife. “But we didn’t realize they were the only surviving greenback population.”

Since the late-19th century, fish-stocking programs designed to restore depleted fisheries clouded the distribution of native cutthroat populations. For example, more than 50 million cutthroat from the Gunnison and White River basins were stocked across the state, including Front Range locations between 1889 and 1925.

CU-Boulder scientists, with support from a state-federal Greenback Cutthroat Trout Recovery Team, based their findings on historical fish-stocking data and on repeated sequencing of fish DNA — trying to clarify the lineage.

This study is among a handful using old specimens stored in alcohol for retrieval of DNA — applying the techniques used to understand Neanderthal man and charismatic Ice Age mega-fauna such as saber-toothed tigers.

Greenback cutthroats were native to the South Platte River basin. The Bear Creek population, estimated at about 750, survived in a headwaters stream in the Arkansas River basin because, according to the study, they were stocked there in the early 1880s by a hotel operator hoping to promote a tourist route up Pikes Peak.

CU scientists focused on six cutthroat trout lineages native to Colorado. Two are considered extinct — the yellowfin from the Arkansas and a lineage from the San Juan River basin. The others are the Colorado River cutthroat, Rio Grande cutthroat and a species referred to as “the Colorado/Gunnison lineage.”

The pure greenbacks in Bear Creek survived droughts and other hazards, Metcalf said. “It is very encouraging that this population has persisted, in isolation, for over 150 years.”

Determining that Bear Creek holds the last remaining population of the greenback cutthroat trout “makes the protection of habitat in Bear Creek even more important, and adds urgency to our efforts to get trail impacts addressed and trails relocated,” said David Nickum, executive director of Colorado Trout Unlimited.

Last week, the Center for Biological Diversity filed a lawsuit in federal court against the U.S. Forest Service — challenging under the Endangered Species Act the way the agency is handling possible harm to the fish.

Trout Unlimited officials called the litigation unfortunate because an emerging group of trail users has been gaining momentum in voluntary projects to rehabilitate degraded creek habitat. Motorized trail users, in particular, have mobilized to try to minimize harm from humans.

Forest Service officials have declined to comment on the issue.

A Forest Service study is exploring options for re-locating trails that erode into the creek and lead to harm of fish habitat. Colorado Springs Utilities owns land in the area.

The scientific findings “make this situation alarming. Until now, we thought there was some sort of ‘backup’ for this species. But this is the only stream with the state fish of Colorado. It has to be protected immediately,” Center for Biological Diversity staff attorney Tim Ream said.

“The center is calling on the Forest Service to immediately close the Bear Creek trails to all off-road vehicles,” he said. “We can’t let motorcycle riding and the erosion it causes threaten the last of this important part of Colorado’s heritage.”

Bruce Finley: 303-954-1700, twitter.com/finleybruce or bfinley@denverpost.com