Colorado, and Wyoming. Presently the species is restricted to six population centers in: 1) the Colorado and Little Colorado rivers in Grand Canyon, Arizona; 2) the Colorado River in Cataract Canyon, Utah; 3) the Colorado River in Black Rocks, Colorado, and Westwater Canyon, Utah; 4) the Green River in Desolation and Gray canyons, Utah; 5) the Green River in Dinosaur National Monument, Colorado and Utah; 6) and the Yampa River in Dinosaur National Monument, Colorado. The Humpback Chub is found in Arizona at and around Coconino County, Colorado and Little Colorado rivers in the Grand Canyon. Habitat In general, species persists only in turbulent, high gradient, canyon-bound reaches of large rivers in the Colorado River Basin. The young prefer shallow, low-velocity nearshore pools in the Little Colorado River, and progressively move to deeper, faster areas with increasing size and age. In the Colorado River in Grand Canyon, young-of-year are found in backwater and other near-shore, slow-velocity sites, with similar ontogenetic tendencies. Adults in the Colorado River in Grand Canyon and in the Upper Basin are associated with large eddy complexes. Humpback chub appear to be more active at night. Parasites and diseases The population in the Grand Canyon has been previously infested with the parasitic copepod Lernaea cyprinacea, and Asian tapeworm, Bothriocephalus acheilognathi. Kaeding and Zimmermann also reported 13 species of bacteria, six protozoans, and the fungus Saprolegnia to infect humpback chub. Population trends The historic range of the humpback chub is uncertain, but the distribution was presumably more contiguous than in the present. There were possible populations in and below the Flaming Gorge that were likely destroyed by the poisoning of the Green River associated with the construction of the Flaming Gorge Reservoir. The fish's distribution within the Grand Canyon has contracted since the construction of Glen Canyon Dam. For 2008 the total population of the humpback chub in the Grand Canyon is estimated at 6,000 to 10,000. This is a 50-percent increase over the estimation of 2001 and a reversal of the declining trend between 1989 and 2001. It seems that a combination of human causes and natural events stabilized the population, particularly the experimental flooding of the canyon and the increase in water temperatures due to draught conditions over the last decade. Management and conservation Threats: altered hydrology and cold tailwater releases from reservoirs; Predation by and competition with nonnative fishes; and, parasitism. Management Needs: ameliorate effects