Population Estimates/Population Movements of Gila cypha, an Endangered Cyprinid Fish in the Grand Canyon Region of Arizona

Douglas, Michael; Marsh, Paul

Gila cypha (the humpback chub) is a unique but endangered cyprind fish endemic to the Colorado River system in western North America. Its distribution within the system is patchy; occurrence is restricted primarily to narrow, canyon-bound reaches of these rivers. Greatest abundance is achieved at the confluence of the Colorado and Little Colorado rivers (= LCR) in northern Grand Canyon (Coconino County, AZ). This study defines the nature and extent of G. cypha's movements within the LCR, and tests the hypothesis that its duration of stay within that river is restricted to the reproductive period. During 1991-1992, adult G. cypha were captured and tagged during 19 6-14 day sampling periods in three separate reaches of the LCR. From these data, population estimates were derived for each reach on a monthly basis, by month for the entire LCR, and over the entire study period. Results indicate an upriver migration by some individuals in early spring, followed by a slow, protracted postreproductive movement downstream. Localized stasis by adults in the LCR, particularly summer through winter, is also strongly supported by the data. Movements by G. cypha in the LCR thus appears to be an amalgam of two processes: upriver movement in spring coupled with localized movements by overwintering adults. The latter suggests a possible alteration in life-history strategy for the species and is discussed in the context of Glen Canyon Dam, built in 1963 to impound Lake Powell at the northern extent of Grand Canyon.

Population Estimates/Population Movements of Gila cypha, an Endangered Cyprinid Fish in the Grand Canyon Region of Arizona

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