Evidence of young humpback chub, Gila cypha , overwintering in the mainstem Colorado River, Marble Canyon, Arizona

Authors: Matthew E. Andersen, Michael W. Ackerman, Kara D. Hilwig, A. Elizabeth Fuller, Angela A. Abel

Year: 2008 (xl)

Abstract

Global climate change models predict that the southwestern United States will become warmer and drier, resulting in decreased reservoir volumes and increased water temperatures. These physical changes to aquatic habitats are further predicted to cause changes to native and nonnative fish distributions. Our observations of humpback chub Gila cypha allowed us to test predicted impacts of habitat changes on this federally listed endangered cyprinid endemic to the Colorado River Basin. Recent drought conditions in the southwestern United States have caused a lowering of Lake Powell, the reservoir created by Glen Canyon Dam, upstream of our Colorado River study site. The lower reservoir has, in turn, resulted in the release of warmer water between 2004 and 2006 than the twelve year running average that included the study period. We used bioenergetics models, laboratory swimming performance data, and river flow velocities to evaluate the potential that the young humpback chub we captured in 2006 and 2007 were hatched and reared up to 45 km upstream from the mouth of the Little Colorado River, the location where the majority of the humpback chub population below Glen Canyon Dam is hatched and reared. We determined that the most parsimonious explanation for our observations, especially for the largest fish we captured, was that the young humpback chub captured at our study site appear to have been able to take advantage of warmer water released from the dam by growing larger and faster, allowing them to overwinter upstream from the area where most of their congeners live. These results suggest that increased water temperature may be allowing humpback chub below Glen Canyon Dam to reproduce and rear in the mainstem Colorado River, upstream of their traditional natal tributary, the Little Colorado River.