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3.26 The life-history phases that appear to be most critical for the Colorado squawfish include spawning, egg fertilization, and development of larvae through the first year of life. These phases of Colorado squawfish development are tied closely to specific habitat requirements. Natural spawning of Colorado squawfish is initiated at approximately 680F. Spawning, both in the hatchery and in the field, generally occurs in a 2-month timeframe between July 1 and September 1, although high flow water years may suppress river temperatures and delay spawning in the natural system until September. Field studies conducted over the past several years by the FWS (Kaeding and Osmundson, 1987; Miller, et al., 1982) document the use of the 15-mile reach between Palisade, Colorado, and the confluence with the Gunnison River by several life stages of Colorado squawfish (i.e., adults, larvae, postlarvai young-of- year). The Upper Colorado River Basin Biology Subcommittee (1984) identified RM 170-180 of the 15-mile reach above the Gunnison confluence as a "suspected Colorado squawfish spawning area." This determination was based upon: (a) the collection of ripe male and female fish in the reach above Gunnison; (b) the observation of radio-tagged fish in this reach during the spawning season; (c) the occurrence of larvae in and downstream of the reach; and (d) the availability of suitable physical habitat conditions in the reach. Two major populations of humpback chub occur in the upper Colorado River: at the Black Rocks area of Ruby Canyon (RM 135 to.137) and in Westwater Canyon (RM 116 to 124). In the Colorado River, according to recent collection efforts, humpback chub typically inhabit sections of river that are deep and narrow with jettied flow. Turbulent flows in these narrow areas during humpback spawning may serve to reproductively isolate the species from its common relative, the roundtail chub. Several fish biologists believe that humpback and roundtail chub hybridize when flows are no longer sufficient to isolate the species during spawning (Valdez and Clemmer, 1982). Spawning occurs from April to early July at water temperatures of about 51°F to 62°F. The current distribution of the bonytail chub in the upper Colorado River basin is based on four collection sites: (1) the lower Yampa River in 1979 (Holden and Crist, 1981); (2) Coal Creek Rapid on the Green River in 1981 (Tyus et al., 1982); (3) Black Rocks on the Colorado River in 1984 (Kaeding, 1985); and (4) Lake Powell in 1985 (Radant, 1985). The first 3 sites resulted in 1 specimen each being collected in deep, swift, and rocky riverine areas. Little is known about the biological requirements of the bonytail chub, since few