Effects of Disparate Water Temperatures and Food Bases on Humpback Chub Growth Rates within the Little Colorado River, Arizona

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 0.53‐km 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.