Trout are delicate creatures, requiring clear, cold, unpolluted water to thrive. In recent years, however, a number of factors have called into the question the future of the fish. A 2015 report by Trout Unlimited notes that three of the United States' 28 native trout species and subspecies have already gone extinct, and that 13 of the 25 remaining species inhabit a quarter of their original range. Below is a closer look at seven trout populations from around the country that are of particular concern, along with the biggest threats they face.

Yellowstone Lake Cutthroat Trout

Threat: Lake Trout

Yellowstone Lake is home to North America's largest population of Yellowstone cutthroat trout. It's also home to a sizable population of invasive lake trout, which have threatened the native cutties since their being introduced there, in the early 1990s. Lake trout begin feeding on cutties when they turn four years old, and a mature fish can consume up to 50 cutthroats a year. What' more, lakers live two to three times longer than cutthroats, and can potentially devastate an entire population over their lifetimes. "A seven-year-old cutthroat is sort of an old fish," Pat Bigelow, a fisheries biologist with the Park Service, told Jackson Hole News & Guide. "If you have seven years of serious impact on the juveniles coming up, you could lose your whole population." But there is a reason to hope for the Yellowstone cutties: Park officials are optimistic that the lake trout population will collapse in the next 10 years, following two decades of removal efforts.

Cutthroat Trout of the Northern Rockies

Threat: Rainbow trout, owing to climate change

In the face of a changing climate, cutthroat trout of the Northern Rockies face an acute threat—rainbow trout. According to Dr. Clint Muhlfeld, a researcher at the University of Montana, warming stream temperatures encourage the encroachment of rainbow trout into frigid, high-elevation streams, where they interbreed with the native cutties. The effects of such hybridization can ripple throughout a cutthroat population, as hybrids reproduce with other hybrids and weaken the gene pool. "[Hybrids] don't survive as well as the native fish," Muhlfeld told NPR. "And hybrids that do survive continue to make more hybrids; there's no going back to making cutthroats again."

bull trout threatened
Loss of habitat is the biggest threat to bull trout.Courtesy FWS Pacific Region

Bull Trout of the Pacific Northwest

Threat: Habitat Degradation

Bull trout once ranged from California to the Bering Sea, but the salmonid is now listed as threatened, resulting from habitat loss that has decimated their populations. But bull trout face a myriad of threats, including rising water temperatures, poor water quality, and man-made barriers that restrain the large, migratory fish. These threats have led to at least one local extinction of bull trout; the fish was last seen in California in 1975. Mines, which can drain groundwater out of and contaminate critical streams, are a particular threat to bull trout. A federal judge recently ruled against the construction of a mine in the Cabinet Mountains Wilderness of Montana, fearing it would cause tremendous damage to trout spawning areas, as well as to local grizzly bear habitat.

California Steelhead

Threat: Dams

The bulk of West Coast steelhead are considered threatened, but California's populations were highlighted in a report this year warning of the potential demise of the state's salmonid populations. Though the U.S. Fish and Wildlife Service considers only one group of steelhead endangered, "The State of the Salmonids" listed half of the state's eight steelhead populations at a critical state. Though no one factor can be singled out for the decades-long population declines of these anadromous fish, dams, which block the spawning runs of mature steelhead and limit the migration of smolt to the sea, are no doubt a key contributor.

California Golden Trout

Threat: Habitat degradation and grazing

Though the federal government doesn't consider the California Golden Trout endangered or threatened, California has taken the steps to indetify it as a species of special concern. The fish has suffered significant population declines and exists in small enough numbers, owing, in part, to more than a century of livestock grazing. Grazing contributed to a reduction in riparian vegetation and made many streams shallower and wider, both of which negatively affected the golden trout. California's recent drought also threatened the fish, by way of stressing some streams and trout populations. A recent study by the University of California, Davis, and California Trout found that 23 of 31 salmonids in California are at risk of extinction in the next 100 years, including the golden trout. Some researchers fear the trout could be gone a half century from now, given its critical classification.

threatened species gila trout
The Gila trout is found only in the waters of two Southwestern states.Courtesy FWS

Gila Trout

Threat: Hybridization

Found only in New Mexico and Arizona, the Gila is one of the rarest North American trout, and, owing to the introduction of rainbows in the early 20th century, hybridization threatens to further curtail its populations. The desert salmonid evolved to survive in the extreme conditions of the American Southwest, and hybridization contaminates a unique and highly specialized gene pool, compromising the species. In addition, FWS has identified a number of threats to the Gila, including habitat degradation and forest fires. Unsurprisingly, the Gila is considered threatened throughout the entirety of its range. Interesting enough, reprieve for the Gila came in 2013, when the 300,000-acre Whitewater-Baldy blaze ripped through the Gila Wilderness, which resulted in sedimentation and ashfall eliminating large numbers of rainbows, easing the risk of hybridization.

All American Trout

Threat: Loss of stream protections

Admittedly, the last entry is a catch-all, but only because it's impossible to know, at this early stage, which populations will be affected by the threat. In early 2017, President Donald Trump ordered the Environmental Protection Agency to review the Clean Water Rule, as well as the Stream Protection Rule, with the intent of revising or scrapping the measure, which would roll back protections for certain streams and wetlands. Before the rule had taken effect, it was expanded to encompass river headwaters, and conservationists fear losing it would pose a grave threat to trout species living in both headwaters and in the larger waters they feed. In 2015, under the Obama Administration, the EPA released a report stressing the relationship between large rivers and the smaller streams the Clean Water Rule would protect, such as headwaters and seasonal streams. According to Trout Unlimited, these waters account for nearly 60 percent of America's streams. Thus, because of the relationship between big and small streams, trout could be at risk without the protections of the Clean Water Rule. Immediately following the executive order to review, and likely scale back, the Clean Water Rule, a group of sportsmen and conservationists released a statement condemning the decision.