The lower perennial corridor of the Little Colorado River in Grand Canyon, Arizona, is numerically dominated by endemic desert fishes and therefore significant for conservation of these species. From 2000 to 2012, the U.S. Fish and Wildlife Service conducted monitoring of native fishes in the Little Colorado River near its confluence with the Colorado River. The primary focus of these efforts was to estimate the spring and fall abundance of native fishes, especially the federally endangered humpback chub Gila cypha. Because humpback chub in Grand Canyon are influenced by operations of Glen Canyon Dam, our efforts provide managers of the Glen Canyon Dam Adaptive Management Program with abundance estimates and trends of humpback chub in the Little Colorado River, the most important tributary in Grand Canyon for spawning and production of this species. From 2001 to 2006, the spring abundance estimates of humpback chub ≥150 and ≥200 mm remained relatively low (≤3,419 and ≤2,002 fish, respectively), thereafter significantly increasing to highs of 8,083 and 6,250, respectively, by spring 2010. Also from 2000 to 2006, the fall abundance estimates of humpback chub were substantially below those abundances estimated after 2006. In addition, flannelmouth sucker Catostomus latipinnis and bluehead sucker Catostomus discobolus showed post-2006 increases in relative abundance, suggesting a systemwide event occurred that was beneficial to native fishes. Most of the increases of humpback chub occurred during the spring season in the reaches of the Little Colorado River between 5 and 13.57 km upstream from the confluence. Successful production of age 0 year classes of humpback chub may be partially driven by hydrograph dynamics of the Little Colorado River, whereas water temperatures and predation pressures in the mainstem Colorado River likely influence survivorship of native fishes into subadult and adult life stages.

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We used translocation as a conservation measure to promote the recovery of an endangered freshwater fish species. We collected juvenile Humpback Chub Gila cypha from the lower 9.6 km of the Little Colorado River (LCR), Arizona, and translocated them upriver above a natural travertine structure called Chute Falls where the species was absent. The translocated fish were released at river kilometer (RKM) 16.2 above the mouth of the LCR. We measured growth rates across 14 size-classes of Humpback Chub in three contiguous reaches of the LCR. Growth rates were usually highest in the 3.8-km-long reach above Chute Falls (translocation reach), slightly lower in a short 0.53km reach immediately below Chute Falls (Atomizer reach), and lowest in the lower 13.57 km of the LCR (lower reach). Most base flow of the LCR originates from relatively warm springs (20.6-22.8°C) that are located upriver from the translocation release site. The grand mean annual water temperature differences across eight years averaged 1.1°C/d warmer at RKM 16.2 than at RKM 1.05 and reflected a higher correlation to the growth rates of Humpback Chub in the translocation reach than in the lower reach. Moreover, this and other studies found that Humpback Chub's food base of prey fishes was also higher in the translocation reach than in the lower reach. High growth rates of juvenile Humpback Chub in the translocation and Atomizer reaches resulted in most reaching adulthood (200 mm TL) by age 2, a year earlier than most Humpback Chub in the lower reach did. Because higher growth rates of Humpback Chub in the translocation and Atomizer reaches provide a substantial head start for adult reproduction to commence, we suggest that translocating juvenile Humpback Chub to above Chute Falls is a management action that enhances recovery efforts of Humpback Chub in Grand Canyon, potentially at the population level. Blue Spring, RKM 20.74 km translocation reach RKM 16.20 lower reach Lower Atomizer Falls, RKM 13.57 Chute Falls, RKM 14.11 RKM 17.90 FIGURE 1. Map depicting the lower Little Colorado River (LCR), Arizona. Study reaches included the lower reach (lower 13.57 km), the Atomizer reach from the top of Lower Atomizer Falls (RKM 13.57) to the base of Chute Falls (RKM 14.11), and the translocation reach from above Chute Falls to RKM 17.90. 476 STONE ET AL.