Evidence of Young Humpback Chub Overwintering in the Mainstem Colorado River, Marble Canyon, Arizona, USA

ABSTRACT

Global climate change models predict that the southwestern United States will become warmer and drier, resulting in decreased reservoir volumes, increased water temperatures, and changes in fish distributions. Such conditions may also result from drought and water extraction. Our observations of humpback chub Gila cypha allowed us to test predicted impacts of increased water temperatures released from Glen Canyon Dam on this federally listed endangered cyprinid endemic to the Colorado River Basin. We modeled the potential that the young humpback chub we captured in 2006 and 2007 were hatched and reared up to 50 km upstream from the Little Colorado River, the natal origin of the majority of the humpback chub population below Glen Canyon Dam. Larval and adult humpback chub have been observed by others in our study reach in the post-dam era. Humpback chub assumed to be juveniles have been observed in our study area, but their ages were not confirmed. Our observations of humpback chub were coincident with not only increased water temperatures as a result of regional drought, but also decreasing rainbow trout population numbers and some constraints on dam release fluctuations. One or all of these factors could have contributed to increased survivorship of young humpback chub. The most parsimonious explanation for our observations was that the largest young humpback chub captured at our study site took advantage of warmer water temperatures to grow to one and two years of age. Continued favorable habitat conditions in this reach would likely support recruitment to adulthood and reproduction. Managers wish to increase the likelihood that a self-sustaining mainstem Colorado River population of humpback chub is established, distinct from the larger Little Colorado River population. Our results indicate that a separate population may be successfully established.