As part of their ongoing search for remnant populations of indigenous cutthroat trout, U.S. Division of Wildlife Resources officials found a small group of cutthroats in tiny Beaver Creek on the east side of the La Sals a couple of years ago.
Because the Colorado River cutthroat was once common throughout the upper Colorado and Green River drainages, it’s possible that the fish would have come up (or down) the Dolores River and into the La Sals, DWR officials said. Paul Birdsey, who’s the regional aquatics manager for the DWR’s southeastern region, said it seems likely the La Sal Mountains held populations.
“We can presume that we did, but we don’t know for certain because, unfortunately, no samples were taken before the widespread stocking of non-native fish, brook trout and rainbow trout,” Birdsey said. “We always presumed that if we did have native cutthroat in that area, they would be Colorado River cutthroat, and that’s in fact what we were looking for.”
It can be tricky to identify subspecies of cutthroat, which include the state fish of Utah and Colorado, the Bonneville cutthroat and the greenback cutthroat, Birdsey said. The fish hybridize easily among the subspecies, as well as with rainbow trout. Pure strains are rare, so visual identification is problematic.
So the DWR sends its samples to Brigham Young University for DNA analysis. This one didn’t come back as the Colorado cutthroat, or the rare but indigenous Bonneville, or even as the widespread Yellowstone, which leaves its genetic markers throughout the West. Instead, the DWR had found Colorado’s state fish, the greenback cutthroat, a threatened species that has never been found in Utah, and, once nearly extinct, was on the endangered species list until 1978.
“This kind of surprised us, when it came back as greenback,” Birdsey said.
The fish was once limited to the front range in Colorado, in the drainages of the Arkansas and South Platte Rivers. However, it was raised in the first federal hatchery in Colorado in the late 1800s, and it’s not known how far it was distributed. Until recently, Colorado officials weren’t aware it had jumped its native bounds.
“The state of Colorado has been doing cutthroat trout work, just as we all have recently, and they are finding greenback cutthroat on the west side of the divide now, throughout many areas of the Dolores drainage,” Birdsey said. “We’re not sure how they got there, but they appear to be fairly widespread.”
BYU biology professor Dr. Dennis Shiozawa is one of the lead scientists on the project, and he hopes to be able to answer the question of how the fish got there. He said he has three main theories, all of which suggest the fish have been there for a while.
“It may have been stocked by man. If that happened, it probably occurred back in the early 1900s, would be my guess,” Shiozawa said. “It would be a very old event, it wouldn’t be something that happened in the last 20 years. There just hasn’t been a lot of movement of greenback around more recently.”
There had been movement back when the greenback made its first appearance as a hatchery fish (it’s being hatchery-raised again, as part of the recovery program), which supports Shiozawa’s timeline. However, his other theories put the greenback here much earlier.
“It’s possible that these fish are a relic of the initial fish that were in this area before the Colorado cutthroat came in and began to dominate the system,” Shiozawa said. “We may be looking at a trace of an ancient migration event of fish that were in the Colorado drainage, that eventually got up on the Rockies and spilled over on the other side to form the greenback.
“The other possibility is the greenback is formed maybe from the same trout that became the Colorado River cutthroat,” he said. “That they evolved over on the east slope of the Rocky Mountains, but then a stream capture brought them back into the Colorado drainage, and some of them moved downstream.”
So far, the mitochondrial DNA testing Shiozawa’s lab has conducted can’t make those connections. “Mitochondrial DNA is passed from mother to the offspring, passed through the mitochondria of the cell, so it doesn’t have the same dynamics as nuclear genes,” Shiozawa said. “It’s kind of like passing a last name on, that only goes from the mother’s side.”
Shiozawa plans to continue analyzing the data, including using more complex nuclear DNA, to tease out the distinctions.
“One of the things I’m interested in looking at is the amount of genetic diversity that we see in the populations we picked up in the Colorado basin, and comparing that to the populations that are over in the native habitat,” he said. “If what we’re seeing is an ancient trace of this invasion, the ones on the Colorado River side are probably going to have a lot of nuclear genes that they’re sharing with Colorado cutthroat trout.”
Birdsey said the creek’s inaccessible nature probably helped isolate the fish. “It has been a not very well-utilized area. It’s very brushy,” he said. “We’re hoping that this particular finding does not encourage a lot of people to go down there. There’s always the possibility that people are going to take in whirling disease or some other invasive species on their gear unintentionally. We’re encouraging people to respect the fact that we have a unique population here and not love it to death.”
Anglers should make sure their gear is exceptionally clean to avoid such contamination, Birdsey said. The stream has also been classified as catch-and-release only, and fish may only be caught on barbless artificial flies and lures.
For now, those are the only special regulations related to the discovery, but that could change if DNA evidence demonstrates that these are native fish, Birdsey said.
“Right now populations on the west slope [of Colorado] are not in the critical habitat areas as defined by the recovery plan for the greenback cutthroat, so this population does occur outside that critical habitat designation at the moment,” Birdsey said. “But the [U.S.] Fish and Wildlife Service can always amend that, based on this finding and the findings in Colorado, to say that these populations are indigenous to the area, and they need to be protected as such.”