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Bonytail Chub GiZa eZegans Status Endangered on both federal and Colorado lists. Many recent studies clearly indicate that the bony- tail chub is the rarest of the Colorado River native fishes and the species nearest extinction. Distinguishing Features Large fins and a streamlined body with a very thin caudal peduncle (the thinnest part of the body just in front of the tail) distinguish the bonytail chub. The bonytail chub might be confused with both roundtail and humpback chubs. The body is more streamlined and the caudal peduncle much thinner in the bonytail chub than in the roundtail chub. Bony- tail chubs may develop a slight hump on the back, which would cause confusion with the humpback chub. The bonytail chub typically has 10 rays in both the dorsal fin and the anal fin, whereas the roundtail chub typically has 9 dorsal and anal fin rays; the humpback chub most frequently has 9 dorsal rays and 10 anal rays, but is more variable. Many unusual specimens collected in the 1960's suggested hybridi- zation between the bonytail and humpback chubs. The current consensus is that, although some of these specimens do represent hybrids, most merely represent normal variation in the humpback chub. Considerable confusion surrounds the identifica- tion and classification of bonytail chubs. The bony- tail and roundtail chubs were described as separate species in the 19th century, but later were consi- dered only as environmental modifications of a single species. That is, it was believed that a roundtail chub, leaving a tributary stream for life in the main river channel of the Colorado or Green River, would turn into a bonytail chub under the direct in- fluence of a different environment. When it was discovered that both roundtail and bonytail chubs were frequently found living together, with both of them maintaining distinctions from each other and not hybridizing, the two chubs were again recognized as separate species. Confusion also surrounds the common name. In former times, professional biologists typically used the name 'bonytail' for both roundtail and true bony- tail chubs. Consequently, many literature references to the bonytail chub refer, in fact, to the roundtail chub. Life History Until large dams were constructed, the bonytail chub was probably the most abundant species in the main river channels of the Colorado and Green rivers and in the lower reaches of the larger tributary rivers. The bonytail chub was most common in the open-river areas of large river channels, the hump- back chub in or near deepwater areas, and the round- tail chub in tributary streams. However, where 19