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(Karp and Tyus 1990a). A single humpback was collected in 1980 at Cross Mountain Canyon of the Yampa River (USFWS 1980; Wick et al. 1981), and several specimens were collected in 1988 from the Little Snake River 6.25 miles upstream from the confluence with the Yampa River (Ed Wick, NPS, pers. comm.). STATUS This species is most often found in relatively inaccessible canyons, so it is not surprising that the species remained undiscovered until after World War II when the use of rubber rafts allowed easier access to canyon areas for sampling and research. The few remaining populations of humpback chub are limited to remote reaches in canyon areas of the Basin. This species decline, like the other Colorado River endangered fish, is attributed to streamflow alteration, competition, increased predation, in addition to hybridization with other members of the genus Gila (Stalnaker and Holden 1973; Rosenfeld and Wilkinson 1989). HABITAT REQUIREMENTS Populations of humpback chub are found in river canyons, where they utilize a variety of habitats, including pools, riffles, and eddies. Most of the existing information on habitat preferences has been obtained from adult fish in the Little Colorado River, the Grand Canyon, and the Black Rocks of the Colorado River (Holden and Stalnaker 1975a; Kaeding and Zimmerman 1983; Kaeding et al. 1990). In these locations, the fish are found associated with boulder-strewn canyons, travertine dams, pools, and eddies. Some habitat-use data are also available from the Yampa River Canyon where the fish occupy similar habitats, but also use rocky runs, riffles, rapids, and shoreline eddies (Karp and Tyus 1990a). This diversity in habitat use suggests that the adult fish is adapted to a variety of habitats, and studies of tagged fish indicated that they move between habitats, presumably in response to seasonal habitat changes and life history needs (Kaeding and Zimmerman 1983; Karp and Tyus 1990a). Spring peak flows, availability of shoreline eddy and deep canyon habitats, and competition and predation by nonnative fishes were reported as potential factors limiting reproduction of humpback chub in the Yampa River (Tyus and Karp 1989; Karp and Tyus 1990a). Spawning Habitat Humpback chub in reproductive condition are usually captured in May, June, and July, depending on location. Little is known about their specific spawning requirements, other than the fish spawn soon after the highest spring flows when water temperatures approach 68°F (Kaeding et al. 1990; Karp and Tyus 1990a; USFWS 1990b). The importance of spring flows and proper temperatures for humpback chub is stressed by Kaeding and Zimmerman (1983), who implicated flow reductions and low water temperatures in the Grand Canyon as factors curtailing successful spawn of the fish and increasing its competition with other species. 28