View source for Threatened and endangered fish species

==Overview==
[[File:TamariskImage.jpeg|thumb|700px|Figure 1. (A) Tamarisk-dominated habitat in which the Southwestern Willow Flycatcher bred at St. George, Utah, prior to tamarisk defoliation. (B) The same habitat during defoliation in 2008. An active flycatcher nest eventually failed in this habitat. (Photos by P. Wheeler, Utah Division of Wildlife Resources. Modified from figure compilation by Paxton et al., 2011.)]]

The Colorado River Basin is home to at least 35 native fish species. At least 26 of these species are endemic to the basin, occurring nowhere else, including four key federally-listed threatened or endangered species: Colorado Pikeminnow (Ptychocheilus Iucius), Razorback sucker (Xyrauchen texanus), Bonytail (Gila elegans), and Humpback Chub (Gila cypha). The range and populations of all four species have declined dramatically since the early 1900s due to the combined impacts of dam construction and resulting changes in flow regimes, water temperatures, and water quality; channelization and backwater habitat loss; water diversions and depletions; and the introduction and expansion of predatory non-native coldwater species, and overfishing. The viability and status of these four species of fish–all finely adapted to pre-development river conditions–serves as a general indicator of the ecosystem health of the river system. In addition to the four major federally-listed species, the Woundfin (Plagopterus argentissimus) and Virgin River Chub (Gila seminuda) are also federally listed threatened or endangered species and are found in the Virgin River tributary to the Colorado. 
Under the Endangered Species Act (ESA), federal agencies must avoid actions that are likely to jeopardize the existence of listed species, and must develop programs to conserve critical habitat and recover the species, in partnership with states and other stakeholders. The need to meet these directives has resulted in changes to the operation of the basin’s dams and to water diversions, principally to meet species’ instream flow requirements. In 1988, the need to better coordinate research, monitoring, adaptive management, and recovery of the four species within the Upper Basin led to the creation of the Upper Colorado River Endangered Fish Recovery Program (UCR FRP), which in 1992 was supplemented by a recovery implementation program specific to the San Juan River, the San Juan River Basin Recovery Implementation Program (SJRB RIP).  The SJRB RIP focuses specifically on the recovery of the Razorback Sucker and the Colorado Pikeminnow. In 2005, the Lower Colorado River Multi-Species Conservation Program (LCR MSCP) was initiated to coordinate monitoring and recovery for the endangered fish, and ESA-listed bird species, in the Lower Basin. A Virgin River specific recovery program, the Virgin River Program (VRP), coordinates recovery and conservation efforts of native species in the Virgin River tributary. 
Three additional native fish species, sometimes collectively called the “three-species assemblage” since they often co-occur, are not listed under ESA but have declining populations: Flannelmouth Sucker (Catostomus latipinnis), Bluehead Sucker (Catostomus discobolus), and Roundtail Chub (Gila robusta). 

===Colorado Pikeminnow=== 

The Colorado Pikeminnow is the largest minnow in North America, historically growing up to 6 feet in length and living for up to 40 years. Large adults found today are more typically 2-3 feet in length,The Colorado Pikeminnow has a long, torpedo-shaped, green and gold body and is a top predator despite its lack of jaw teeth.  
The historic range of the Colorado Pikeminnow extended throughout the accessible warmwater reaches of the basin, which included the following:
* The mainstem from western Colorado down to the delta in Mexico
* The major Upper Basin tributaries (Green, Yampa, White, Gunnison, Dolores, San Juan, Uncompahgre, Animas)
* The Gila River and its tributaries
Colorado Pikeminnow are currently found only in the Upper Basin, in portions of the mainstem, and the Green, White, Yampa, Gunnison, and San Juan rivers. The Colorado Pikeminnow was listed as endangered by the U.S. Fish and Wildlife service in 1967, prior to the enactment of ESA in 1973.

===Razorback Sucker===
 
The Razorback Sucker can be identified by the sharp-edged hump, or razorback, behind its head. The only member of its genus, the Razorback Sucker can grow to 3 feet in length and live for 40 years. The historic range of the Razorback Sucker extended throughout  warmwater reaches of the basin, typically in calmer water. The Razorback Sucker was listed as an endangered species in 1991. Populations in the upper mainstem, Green, Yampa, Gunnison, and San Juan are maintained by hatchery augmentation. These are hatchery spawned fish that are stocked into these systems (usually once they reach sub-adult sizes ~ 300 mm). The Lake Mohave population consists of fish that were spawned in the wild, brought into captivity, reared to a size where they're large enough to avoid most predation, and stocked back to Lake Mohave. There is very limited wild recruitment outside of the Lake Mead population, which is small but self-sustaining. The Razorback Sucker was proposed for downlisting in 2021, but as of early 2024 its status remains “endangered”. 

===Bonytail===

Bonytails are the rarest of the four threatened and endangered fish species in the basin. It has large fins, a streamlined body that is pencil-thin near its tail, and gray or olive-colored back, silver sides, and a white belly. Bonytail can grow to about 2 feet in length and can live up to 50 years. 	
Bonytail habitat historically extended throughout much of the warmwater reaches of the basin. Currently, the few populations of bonytail–in limited reaches of the upper mainstem in western Colorado, the Green and Yampa, and in Lake Mohave–-are maintained only by ongoing stocking programs. Survival rates of stocked young bonytail are extremely low, and self-sustaining populations have not been established. The Bonytail was listed as endangered in 1980. 

===Humpback Chub===

The Humpback Chub, named for the prominent hump behind its head, is a minnow endemic to the warmwater reaches of the Colorado River Basin. Humpback Chub are able to navigate swift waters despite being relatively small (~20 inches maximum length size) by using its hump as a hydrodynamic foil. The historical range of the Humpback Chub was probably limited to the eddies below rapids in several canyon reaches of the mainstem, including the Grand Canyon, as well as in canyons in the Green, Yampa, and Little Colorado rivers. At the time of its listing as endangered in 1967, two of eight historical populations of Humpback Chub had been extirpated due directly to dams, and a third population is believed to have been extirpated by 2004. However, the Humpback Chub’s largest population, in the Grand Canyon, has expanded in numbers and range in recent years, leading to the downlisting of the species from Endangered to Threatened in 2021. 

===Three additional species of concern ===

The Flannelmouth Sucker, Bluehead Sucker, and Roundtail Chub are all warmwater fish historically distributed widely across the basin, including smaller tributaries and streams. The ranges and populations of all three species are declining; estimates suggest that each species occupies about half of its historic range in the Colorado River Basin. None of these species are federally listed as threatened or endangered.  The Lower Basin population of Roundtail Chub was a candidate for listing as a Threatened population under ESA, but in 2022 that proposal was withdrawn. At the state level, all three fish are listed as a species of concern, special concern, or endangered by two or more basin states. A multi-state conservation agreement, known as the three-species program, was signed in 2006. Conservation of the Flannelmouth Sucker is also covered under the Lower Colorado River Multi-Species Conservation Program.