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2 Identification In view of Holden and Stalnaker's (1970, Table 2 and Figure 4) remarks with regards to extensive 'intergrada- tion' (their usage) in the Powell Reservoir area, we postponed our identification of the young and juveniles until we had sufficient materials for plotting scatter- grams of various measurements. Authors in various papers (Miller, 1946; Gaufin, Smith and Dotson, 1960; Miller and Lowe, 1964; Holden and Stalnaker, 1970; and Minckley, 1973) have mentioned or emphasized the nuchal hump as a distinguishing feature of the hump- back chub. We now view the nuchal hump as a highly variable character. The small eye, the inferior, nearly horizontal mouth and the combination of several other characters in addition to the hump has enabled us to identify the juvenile and adult specimens from the Grand Canyon area as Gila cYPha. Smith (in: Gaufin, Smith and Dotson, 1960) in the annotated list of fishes of the Flaming Gorge Reservoir Basin in reference to Gila cypha stated that, "Young specimens are difficult to identify because of the lack of development of the characteristic shapes of the nuchal area and caudal peduncle and the overlap in fin ray counts for the three types". Holden and Stalnaker (1970) said that, "Since general body morphology is very differ- ent between mature and immature fish, a minimum size of 210 mm standard length was enforced for most fish studied". Confronted with these various stated difficulties, we proceeded cautiously in our determinations. We decided that the logical way to determine the identity of the young and juveniles was to obtain a graded size series. During the early part of our study we considered the slim-bodied specimens to be elegans or hybrids, but as we accumulated more material and became familiar with the variability, we became more and more convinced that we had a single, highly variable form. Our belief was substantiated by our collection of juvenile Gila elegans form, Gila CYPha, and Gila robusta from the Green River in Utah (Figure 9). During the last few collecting trips in the Grand Canyon we were selective in our collections attempting to obtain the different body types as well as to fill the gaps in the graded size series. Our specimens range in standard length from 24.6 mm to 320 mm. The largest specimen is one of the sal- vaged specimens from Powell Reservoir. We have suffi- cient specimens ranging in size from 26.4 to 110 mm in standard length (Figures 1 to 8) but none between 110 and 164 mm in standard length. All of these up to 1I0 mm and the 164 mm specimen were obtained with a ten-foot seine. Another gap exists in our data between the 164 mm specimen and the 204 mm. A series of photo- graphs (Figures 10 to 17) illustrate the differences in body shape of the various size fish. Altho~gh we are primarily concerned with an analysis of Gila cypha from the Grand Canyon area, we present an illustration (Figure 9) of juvenile specimens of the elegans form (upper), cypha (middle) and robusta TULANE UNIVERSITY MUSEUM NATURAL HISTORY (lower). These three specimens are almost the same length (77.5, 71.2, and 70.3 mmin standard length, re- spectively) and thus are suitable for comparison. !he three specimens were collected from the Green RIver. The elegans and cYPha were taken from near the town of Green River, Utah, and the robusta specimen was taken from near the town of Jensen, Utah. Note the relatively small eye of elegans and cypha versus the large eye of robusta. The caudal peduncle is most slender in elegans and deepest in robusta. The caudal fin lobes are longest and most pointed in elegans and shortest and rounded in robusta. The mouth is essentially terminal in elegans and robusta and subterminal or inferior in cypha. This brief comparison is presented as background information for the review of the series of illustrated growth stages (Figures 10 to 17) of Gila cypha from the Grand Canyon area. These illustrations are arranged starting with smallest (upper illustration in Figure 10) and proceeding to the largest. Figure 10 illustrates three small specimens of approxi- mately 2.5 centimeters (upper) to three centimeters in standard length. The subterminal mouth is apparent even at this small size. Also the slight concave dip is apparent in the profile of the head. These t~ree. speci- mens exhibit relatively long pectoral and pelVIC fms. Figure II illustrates a transition from a nearly scale- less condition (upper) to visible lateral scales (middle and lower). The upper specimen has relatively short pectoral and pelvic fins. The middle and lower illustra- tions show two different snout shapes. The former has a rather blunt snout and a slightly curved outline from end of snout to nape whereas the latter has a rather hooked and pointed snout and the dorsal outline shows a cephalic dip. Perhaps this latter form represents the juvenile stage of the adult which has the extreme hooked snout and prominently developed hump. Figure 12 illustrates three different combinations of morphological features. The upper specimen has a blunt snout, long pectoral fins, and moderately deep body. The middle specimen has a moderately sharp snout with a cephalic dip, short pectoral fins and a rather slim body. The lower specimen illustrates a moderately sharp- snouted individual but lacks a marked cephalic dip. It has a moderately deep body and moderate-length pec- toral fins. Figure 13 also illustrates three combinations of mor- phological features. The upper specimen has a blunt snout, very little cephalic dip, short pectoral fins and a moderately deep body. The middle specimen illustrates a slightly sharper snout than the upper specimen but has about the same degree of cephalic depression. The pec- toral fins are long and obviously extend beyond the in- sertion of the pelvic fins. The body is less deep than the upper specimen but is not considered as a slender form. The lower fish has a sharper snout than either of the above and shows more of a cephalic dip. The body is moderately deep, and the pectoral fins are moderately long.