Gila cypha

Gila cypha, G. robusta, and G. elegans exhibit confusing patterns of morphological and genetic variation, evidently resulting from historical and possibly contemporary introgressive hybridization.. Much recent research has attempted to document and explain these patterns. Available evidence indicates that the three taxa should continue to be recognized as distinct species that are "on their own evolutionary trajectories" (Douglas and Douglas 2007).

McElroy and Douglas (1995) examined morphological variation in Gila robusta and Gila cypha in the upper Colorado River basin and found that the two species were clearly distinct in sympatry and in allopatry. All sampled populations of both species differed significantly, and there was no relationship between morphological similarity and geographic proximity among populations of either species. McElroy and Douglas (1995) and Douglas et al. (2001) found that although G. robusta and G. cypha are morphologically distinct, in Desolation Canyon and Cataract Canyon each species phenotypically resembled one another more closely than either did with conspecifics elsewhere in the basin, presumably due to: (1) extensive introgressive hybridization, (2) similar selective pressures on distinct, but sympatric populations, or (3) retention of a high proportion of ancestral characteristics at both locations, such that morphological similarities reflect shared, ancestral traits (McElroy and Douglas 1995). Genetic analyses by Douglas and Douglas (2007) consistently grouped G. cypha and G. robusta from Desolation Canyon into a distinct cluster, consistent with the hypothesis of introgressive hybridization and confirming findings of morphological analyses. Introgressive hybridization among G. cypha, G. robusta, and G. elegans was also documented by Dowling and DeMarais (1993).

Gerber et al. (2001) found that Lower Colorado River basin populations of Gila robusta, G. elegans, and G. cypha exhibit distinct mtDNAs, with only limited introgression of G. elegans into Gila cypha, but most sampled upper basin fishes exhibit only Gila cypha haplotypes, with some individuals exhibiting mtDNA from G. elegans. The complete absence of Gila robusta mtDNA, even in populations of morphologically pure Gila robusta, indicates extensive introgression that predates human influence (Gerber et al. 2001). Results of genetic studies by Douglas and Douglas (2007) are consistent with this conclusion. Douglas and Douglas (2007) found that neither G. cypha nor G. robusta could be discriminated using mtDNA, although this marker was successful in separating these species from G. elegans. Douglas and Douglas (2007) proposed that "both species were reduced to very small populations by an end-of-Pleistocene warming event and were subsequently forced together into refugial (and shrinking) riverine habitat, thus becoming syntopic with one another. They then hybridized, possibly backcrossing (progeny to parental forms) over an extended temporal span." Subsequently they expanded their ranges, occupied their former niches, and reproduced with conspecifics, but not enough time has passed to allow each species to evolve distinctive mtDNA haplotypes. Historical gene flow would mask more recent instances of introgressive hybridization (Douglas and Douglas 2007).

In contrast to mtDNA results, msat DNA provided sufficient resolution to discriminate among Gila populations and basins, but upper basin G. cypha and G. robusta were too similar to one another to adequately differentiate them using this method (Douglas and Douglas 2007). Based on msat DNA data, Douglas and Douglas (2007) delineated six subgroups of Colorado River basin Gila: (1) G. elegans; (2) all Grand Canyon G. cypha 'aggregates'; (3) Desolation Canyon G. cypha

Restricted to the Colorado River system, where distribution and abundance are greatly reduced, due largely to the effects of dams; also threatened by exotic fishes; recently declining in the Upper Colorado River Basin, increasing in the lower basin; most populations are not self-sustaining.

This species formerly occurred throughout much of the Colorado River basin, from western Colorado and southwestern Wyoming to northern Arizona (and perhaps California), including not only the Colorado River, but also major tributary systems such as the Green River, lower Yampa River, and White River (Utah). Currently, six populations of humpback chub are known to exist. Five of the populations occur in the upper basin recovery unit: 1) Black Rocks, Colorado River, Colorado; 2) Westwater Canyon, Colorado River, Utah; 3) Yampa Canyon, Yampa River, Colorado; 4) Desolation/Gray Canyons, Green River, Utah; and 5) Cataract Canyon, Colorado River, Utah (USFWS 2011). The only population in the lower basin recovery unit occurs in the mainstem Colorado River in Marble and Grand Canyons and the Little Colorado River (USFWS 2011).

Only one self-sustaining populations of humpback chub is known to exist (USFWS 2011).

The endangered status of this species has been attributed primarily to the following factors: loss, fragmentation, and modification of habitat through impoundment (e.g., stream inundation, reduced water temperatures, reduced spring flows, and increased daily fluctuation in flows, resulting from construction and operation of Hoover Dam, Glen Canyon Dam, and Flaming Gorge Dam); introduced competitors and predators; and hybridization with G. elegans and G. robusta. However, genetic analyses suggest that hybridization in Colorado River basin Gila species "can be considered natural and not detrimental to the long-term survival of the species" whereas habitat alteration and effects of non-native fishes should be of greater concern (Douglas and Douglas 2007). Flow reductions and low water temperatures (Clarkson and Childs 2000) may curtail successful spawning/recruitment and increase competition with other species. The range expansion of the introduced Asian tapeworm is a serious threat (Clarkson et al. 1997). Populations in the Little Colorado River, Black Rocks/Westwater Canyon, and Yampa Canyon are restricted to relatively short river reaches that could be decimated by a catastrophic event (USFWS 1990). Effects of climate change on chub habitat, dam operations, and water use by irrigators need to be assessed and addressed (USFWS 2011).