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Page 21 more prevalent in larger tributaries, but not necessarily in the mainstem habitats (Voeltz 2002). Historical literature suggests that these three fishes were common to all of their historical localities within the Colorado River Basin up until the 1960s (Jordan and Evermann 1902, Minckley 1973, Sigler and Miller 1963). There.had been no range-wide distribution or status assessments for any of these three species preceding the review of Bezzerides and Bestgen (2002); however, Voeltz (2002) offers a comprehensive status survey of roundtail chub in the lower basin. ROUNDTAIL CHUB Roundtail chub utilize slow moving, deep pools for cover and feeding. These fish are found in the mainstem of major rivers in addition to smaller tributary streams. They use a variety of substrate types (silt, sand, gravel, and rocks) and prefer murky water to clear (Brouder et al. 2000, Sigler and Sigler 1996). Roundtail chub partition habitat use by life stage [adult, juvenile, young-of-year (YOY)]. Juveniles and YOY are found in quiet water near shore or backwaters with low velocity and frequent pools rather than runs and ritlles. Juveniles avoid depths greater than 100 cm and YOY avoid depths greater than 50 cm. Juveniles use instream boulders for cover, while YOY are found in interstices between and under boulders or the slack-water area behind boulders (Brouder et al. 2000). Adults generally do not frequent vegetation and avoid shallow water cover types (overhanging and shoreline vegetation) (Brouder et al. 2000, Sigler and Sigler 1996). Adults are found in eddies and pools adjacent to strong current and use instream boulders as cover (Brouder et al. 2000, Sigler and Sigler 1996). Adults occupy depths. greater than 20 cm and select for velocities less than 20 cm/s. Adults commonly move 100 m or less over the course of a year, often in search of pool habitats (Brouder et al. 2000). Roundtail chub mature at five years of ageandlor 254 mm to 305 mm in length. Spawning begins in June to early July when water temperatures reach 18.30C. Eggs from one female may be fertilized by three to five males over gravel in water up to 9.1 m. A 305 mm female can produce 10,000 eggs, 0.7 mm in diameter. The eggs are pasty white and adhesive, sticking to rocks and other substrate or falling into crevices (Sigler and Sigler 1996). Roundtail chub are omnivorous, opportunistic feeders. Documented food items include aquatic and terrestrial insects, fish, snails, crustaceans, algae, and occasionally lizards (Bestgen 2000, Brouder 2001, Osmundson 1999, Sigler and Sigler 1996). Potential hybridization among Gila species in the Colorado River Basin has caused management agencies to carefully consider their conservation actions. In Utah, hybridization between humpback chub (Gila cypha) and bonytail (G, elegans) in Desolation and Gray Canyons of the Green River has been postulated by many observers (Douglas et al. 1998, Kaeding et al. 1990, Valdez and Clemmer 1982). Whether biologists and agencies recognize two species, two species and a hybrid form, three species, or some other combination has implications for how the fish are managed. Because roundtail chub are congeners with humpback chub and bonytail, the potential for hybridization between the species exists, though it has not been as well documented as humpback chub/bonytail hybrids. Valdez and Clemmer (1982) have suggested that hybridization is a result of dramatic environmental changes, while Dowling and DeMarais (1993) suggest that hybridization among these species has occurred continually over geologic time, providing additional genetic variability. Barriers to hybridization among some Gila species may illustrate that it is a paraphyletic genus (Coburn and Cavender 1992 and references therein). Roundtail chub in the Gila River drainage of New Mexico