Description
The following description, taken from Minckley (1973), Rinne (1976), and Sublette et al. (1990), is of Gila robusta robusta, with ranges of characters for the other subspecies given parenthetically.
Gila robusta is a relatively large (250 to 350 mm standard length [SL]) minnow. The number of dorsal, anal, and pelvic fin rays is usually nine (8-10). The body is completely scaled, and scales are small, thin, and only slightly embedded (scattered populations may not be completely scaled, ranging to the condition in G. seminuda, which sometimes has a naked back, breast, and belly). Basal radii on scales are absent to weakly developed. There are usually 81 (75- 99) or more scales in the lateral line and 43 to 49 total vertebrae. Barbels are absent and pharyngeal teeth are in two rows (2,4-5,2 with some variation). The length of the head divided by the depth of the caudal peduncle is typically 3.3 to 4.3 (3.25-5.0), but rarely greater than 4.0. Both sexes possess breeding tubercles although the distribution of tubercles is less extensive on females. The dorsum is dark, becoming lighter silvery white below (Sublette et al. 1990). Both sexes have an orange-red color on ventrolateral surfaces and on all fins except the dorsal fin (Muth et al. 1985). This coloration becomes more intense in males during the breeding season (Minckley 1973). During the breeding season the genital orifice protrudes to a greater degree in females. Larvae have been described by Snyder (1981).
Diagnostic Characteristics
Gila intermedia, the Gila chub, is a more robust, more darkly colored minnow; it usually has eight dorsal and anal fin rays; the scales are larger, thicker, and broadly imbricate, and basal radii are usually present; scales in the lateral line are usually less than 80, and total vertebrae number 38 to 45.
The Yaqui chub, Gila purpurea, and the Sonora chub, Gila ditaenia, have radii strongly developed on all fields of scales, the mouth is horizontal to oblique, and a basicaudal spot is present, albeit possibly indiscrete or diffuse. Gila elegans, the bonytail chub, and Gila cypha, the humpback chub, are distinctive as adults, each having a prominent predorsal hump; also, a line projected along the venter parallel with the base of the anal fin either intersects near the middle of the upper lobe of the caudal fin as in roundtail chub, falls on or near the dorsal margin of that fin as in the humpback chub, or passes across the caudal peduncle anterior to the caudal fin as in the bonytail chub. Other distinguishing characteristics are given in Douglas et al. (1989). See McElroy et al. (1997) for information on discriminating G. robusta and G. cypha (identification of a significant number of individuals in any sample likely will be problematic). Douglas et al. (2001) provided a geometric morphometric method for differentiating between Gila cypha and G. robusta in the Upper Colorado River basin.
Habitat
Habitat includes rocky runs, rapids, and pools of creeks and small to large rivers; also large reservoirs in the upper Colorado River system; generally this species prefers cobble-rubble, sand-cobble, or sand-gravel substrate. Adults are associated with the largest, most permanent water in streams (Minckley 1981), where a few deep (greater than 1 meter) pools with cover (boulders, woody debris) are intermixed with riffles, runs, and eddies (Bestgen and Propst 1989, Propst 1999, White 2005). Large populations often occur in pools behind irrigation diversions (Barber and Minckley 1966). Roundtail chubs are less prone to using cover than are Gila or headwater chubs, and they typically frequent open areas in the deepest pools and eddies of middle sized to larger streams (Minckley and DeMarais 2000).
Eggs are scattered over clean gravel or cobble substrates (sometimes sand and silt) of pools or moderate velocity runs generally at least 25 cm deep (Neve 1976, Minckley 1981, Sublette et al. 1990).
Ecology
The roundtail chub is associated with a native fish fauna that varies over its extensive range but would typically include speckled dace (Rhinichthyes osculus), one or more Catostomus sp., and one or more Pantosteus sp. Historically it was also associated with woundfin (Plagopterus argentissimus), bonytail chub (Gila elegans), humpback chub (Gila cypha), squawfish (Ptychocheilus lucius), and razorback sucker (Xyrauchen texanus), now extirpated over most of their former range (Minckley 1973).
Reproduction
In the lower Colorado River basin, breeding colors and tubercles in Gila species have been noted from December to June (primarily February to June), mature gonads from December to June (primarily April to June), and spawning activity from March to May (primarily April to May) (Neve 1976, Minckley 1981). In the upper Colorado River basin, Gila species spawn from mid- to late June or early July, when water temperatures reach 18 C (Vanicek and Kramer 1969).
Roundtail chubs mature between 2-5 years of age. A few may live to an age of up to 20 years (Scoppetone 1988), but the largest and oldest individuals are more commonly around 7-10 years of age and about 400 mm TL (Bezzerides and Bestgen 2002).