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to \q9~ Oou9\Q~ Cop,ia. 1996(1), pp. 15-28 7'6 toO Population Estimates/Population Movements of Gila cypha, an Endangered Cyprinid Fish in the Grand Canyon Region of Arizona MICHAEL E. DOUGLAS AND PAUL C. MARSH Gila cypha (the humpback chub) is a unique but endangered cyprind fish en- demic 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. THE Colorado River is "probably the most utilized, controlled, and fought over river in the world. It flows through lands of incom- parable beauty and includes nearly seven per- cent of the nation's contiguous land mass, in- cluding parts of seven states. From the time of early settlers to the present, the waters of the Colorado River have been the key to develop- ment of the arid region" (Crawford and Peter- son, I974:vi). Waters of the Colorado River basin are not only economically important but also contain the most distinctive ichthyofauna in North America, with species-level endemism ap- proaching 75% (Minckley, 1991; 93% if unde- scribed forms and subspecies are included, as in Carlson and Muth, 1989). The parallel im- portance of economic potential and ichthy- ofaunal diversity has initiated a classic and on- going confrontation between development and conservation (see Wydowski and Hamill, 1991). From the conservation viewpoint, at risk is a unique and endemic ichthyofauna of ancient origin, extending as far back as the Miocene (Miller, 1959; Minckley et aI., 1986). These fish- es possess remarkable adaptations to survive in a turbulent environment. Foremost are a suite of morphological and anatomical modifications which may act in concert to minimize the river's impact upon the phenotype of the fish while optimizing the abilities of the fish to negotiate boulder-strewn, high velocity rapids. Although alternative hypotheses may explain the evolu- tion of these phenotypes, morphological trends across numerous, unrelated taxa speak for com- monality in other than phylogeny, and the se- lective arena of the river seems reasonable (Minckley, 1991:128). The majority of these fishes are endangered (or candidates for such listing; see Minckley and Douglas, 1991) due to numerous recent habitat modifications by mod- ern humans. The specialized morphologies of the main- stream Colorado River fish fauna reach their culmination in the phenotype of humpback chub (Gila cypha; Fig. I), the most remarkably spe- cialized minnow in western North America and one of the most bizarre in the world (Miller, 1964; Minckley, 1991; Douglas, 1993; and ref- erences therein). It is known only from the Col- orado River and its major, swift-flowing tribu- taries (Holden and Minckley, 1980); it occurs only sporadically and is seldom locally abun- dant, particularly when compared to other in- digenous fishes. Gila cypha has been recorded from the gorge sections of the Green and Yam- pa rivers in Utah and Colorado (Green River Wilderness Area and Dinosaur National Mon- @ 1996 by the American SocielY or IchlhyologiSlS and Herpelo1ogisLS