Life History and Ecology of the Humpback Chub in the Little Colorado and Colorado Rivers of the Grand Canyon

Humpback chubs Gila cypha in the Little Colorado River, a warm saline tributary to the Colorado River, grew rapidly to about 250–300 mm total length at 3 years of age, the onset of reproductive maturity for female fish. Fish spawned in April or May; annual reproductive success was greatest when spawning coincided with seasonal river runoff. Meristic characters of humpback chubs from the upper 8 km of the river differed from those of fish from the adjacent lower 5‐km river reach, and suggested that some genes of bonytail Gila elegans may occur in the latter population. Use of the physical habitat by age‐0 and juvenile humpback chubs was affected by light intensity; shallow littoral areas were used during darkness, but during daylight only when the water was turbid. No evidence of humpback chub reproduction was found in the Colorado River part of the study area; small humpback chubs collected there resulted from spawning in the Little Colorado River. Year‐round low temperatures in the Colorado River (the tailwater of Glen Canyon Dam) did not inhibit seasonal gonadal maturation of humpback chubs; however, laboratory studies have indicated that such low temperatures result in nearly complete mortality of embryonic and larval humpback chubs. The recapture of tagged fish, seasonal changes in rates of capture of adults from the Little Colorado River, and the distribution of adult humpback chubs in the Colorado River near its confluence with the Little Colorado River supported the hypothesis that some adult fish from the Colorado River enter the Little Colorado River to spawn. Stomach contents from humpback chubs were dominated numerically by immature Chironomidae and Simuliidae. Lernaea cyprinacea was the most conspicuous metazoan parasite of humpback chubs in the Little Colorado River but was rarely found on fish in the Colorado River. Persistence of the humpback chub in the Little Colorado River could be attributable, in part, to the unsuitability of that environment, and that of the Colorado River, for potential competitor or predator species in the drainage. Introduction to the Little Colorado River of such a species that might prosper under the physicochemical conditions of the river could have a devastating effect on the humpback chub in the Grand Canyon. Received November 24, 1982 Accepted June 1, 1983