Order: Cypriniformes
Family: Cyprinidae
Species: Gila cypha
Author and Year: Miller 1946
Common Name: Humpback chub
G. cypha is found in canyon reaches of the Little Colorado and mainstream Colorado rivers in northern Arizona,the Colorado river in southern Utah, and the Green and Yampa Rivers of northern Utah and Colorado. However, there is data that strongly suggests humpback chub are unable to spawn in the Colorado river due to frequent water-level and temperature fluctuations caused by the Glen-Canyon Dam. Humpback chub are successful in the Little Colorado river, and chubs found in the Colorado were probably spawned in the Little Colorado.
Size: Adult humpback chub generally range in total length from 230 mm to around 400 mm. Female and males are considered adults when they become reproductively mature, usually at around three years of age. Although it was thought the hump was a secondary sexual characteristic of males, it has been found that G. cypha are not sexually dimorphic in the size of the hump or body size.
Habitat: Gila cypha live only in the large freshwater rivers of the Colorado River Basin.They are most abundant in the lower Yampa River in Yampa Canyon and the Little Colorado river in eddy habitats in moderate to steep gradient reaches. Because shoreline eddy habitats are temporary, occuring during high-flow times, some fish move to nearby deep habitats during low-flow periods. Juvenile humpback chubs use shallow shore areas during darkness, but in daylight only during periods of great turbidity. The turbidity of this habitat makes it difficult to observe spawning, feeding, or social behavior.
Diet: The feeding habits of G. cypha are not well known. They have been collected in the areas between shoreline eddies and adjacent flows, suggesting they use this area for feeding on drift. Stomach contents show their main diet consists of algae, plant debris,fish remains, Mormen crickets, and other terrestrial insects. Bite marks and wounds found on humpback chub suggest they make up part of the diet of channel catfish, while common carp probably feast on the eggs of humpback chub.
Reproduction: Both males and females in breeding condition are silver in color and have gold flecks on the dorsal side. The males have additional orange coloration on the lower side of the head, opercles, and abdomen, and both males and females usually have some nuptial tubules on portions of the head, nuchal hump, opercles, and paired fins. Some fish may have nuptial tubules even though they are nonripe (not ready for breeding).
Females are only able to breed for a limited time, and spawning seems to occur for a short time following the highest spring flows when river temperatures are increasing. Ripe adults are found in eddies at temperatures between 14 and 23 C, which are near optimal egg incubation conditions, 20 C. Adult humpback chub remain in or near specific eddies for extended periods and return to the same eddy during the spawning season in different years. These eddies seem to be part of the breeding requirements for humpback chubs, although it's not known if they deposit eggs there or use them for resting and feeding.
Behavior: As mentioned above, spawning migration takes place in some humpback chubs that move from the Colorado to the Little Colorado to spawn. Little else is known about the behavior of humpback chubs because of the difficulty of observing them in their natural habitat.
The humpback chub is federally protected by the Endangered Species Act of 1973. Other species of fish in the Colorado River Basin are also endangered, and some are extinct, due to extensive modification of their river habitat by humans. The persistance of these fishes in the Little Colorado may be due in part to the unsuitability of this environment for other species and thus lack of competition. It's important that new species of fish that are compatible with this environment are NOT introduced into this habitat, as it could be devastating to the humpback chub population.
Colorado River Gila are the most morphologically diverse group of minnows in western North America. The evolution of Gila species in the Colorado River Basin can be traced to fishes isolated in the basin during the Miocene when the Colorado Plateau was uplifted and became higher than surrounding areas, then divided into an upper and lower basin. Large river fish, like the humpback chub, probably evolved in the upper basin, and became distributed throughout the Colorado basin when the Colorado River began its course through the Grand Canyon, creating a passageway to the lower basin. The present distribution of G. cypha suggests it may have evolved in the canyons of Northern Arizona. Genetic analyses of the genus Gila have discovered that while three species, G. cypha,G.elegans,and G.robusta are morphologically and sympatrically distinct, there has been a large amount of gene exchange among the distinct forms in their evolutionary history, which has contributed to the existing diversity of the genus. Colorado RiverGila are thus a good example of introgressive hybridization, in which genetically distinct species are capable of exchanging genetic material.
The bizarre morphology of the humpback chub has evolved in the unusually harsh conditions of a turbulent, abrasive, and highly silted riverine habitat. It is distinctive because of its large, abrupt hump at the nape of the head,leathery skin,embedded scales, and large, sickle shaped fins. The lower portion of the head and the abdomen are flattened, and the mouth is horizontal and beneath a protruding fleshy snout.
The hump seems to act as a barrier to swift currents flowing against the fish, and helps force the fishes' body against the bottom or on the sides of the river bed, where the flow is not as torrential as in mid-water. Lateral grooves on the hump and leading ventro-posteriorly to the upper part of the gill openings may allow water to irrigate the gills when the mouth and body are pressed downward. The mouth would then be usable because it would be protected from the onrushing water by the snout. Their relatively small eyes in comparison to G.robusta or G.elegans may have degenerated because of reduced light or excessive silt, or could be a result of the scouring action of the sediment rich waters.
Year-round low temperatures in the Colorado river because of the Glen-Canyon Dam result in nearly complete mortality of eggs and larvae of humpback chub. As a result, humpback chubs have been found to migrate from the Colorado to the Little Colorado to spawn.
There is very little published data on this species, in part because much of the research is done by government agencies publishing results as "gray" literature, and in part because the habitats of these chubs are difficult or impossible to access, which makes research on these animals extremely difficult and expensive. Its endangered status precludes collection of specimens, and because it has only been known for about 50 years, there are few specimens available for study.
The humpback chub is most likely doomed to extinction, but few conservation studies have been done, if any.
Douglas, M.E. 1993. Analysis of Sexual Dimorphism in an Emdangered Cyprinid Fish (Gila cypha Miller) Using Video Image Technology. Copeia 1993(2):343-351.
Douglas, M.E., W.L. Minckley, and H.M. Tyus. 1989. Qualitative Characters, Identification of Colorado River Chubs (Cyprinidae: Genus Gila) and the "Art of Seeing Well". Copeia 1989:653-662.
Dowling, T.E. and B.D. DeMarais. 1993. Evolutionary significance of introgressive hybridization in cyprinid fishes. Nature 1993(362):444-445.
Holden, P.B. and C.B.Stalnaker. 1979. Systematic Studies of the Cyprinid genus Gila, in the Upper Colorado River Basin. Copeia 1070(3):409-419.
Kaeding, L.R. and M.A. Zimmerman. 1983. Life History and Ecology of the Humpback Chub in the Little Colorado and Colorado Rivers of the Grand Canyon. Transactions of the American Fisheries Society 1983(112):577-594.
Karp, C.A. and H.M. Tyus. 1990. Humpback Chub(Gila cypha) in the Yampa and Green rivers, Dinosaur National Monument, with observations of Roundtail Chub (G.Robusta) and other sympatric fishes. Great Basin Naturalist 1990 50(3):257-264.
Minckley, W.L. 1973. Fishes of Arizona. Arizona Division of Game and Fisheries. pp 98-99.