Population Estimate of Humpback Chub, Gila cypha, and Roundtail Chub, Gila robusta, in Westwater Canyon, Colorado River, Utah, USA, 2007-2008

Authors: Darek Elverud

Year: 2010 (xlii)

Abstract

Westwater Canyon on the Colorado River in eastern Utah contains the largest of the five remaining populations of endangered humpback chub, Gila cypha, in the upper Colorado River Basin. Westwater Canyon also contains a large population of roundtail chub, Gila robusta, which are listed as a species of concern throughout their range. Multiple pass population estimates of sympatric adult humpback chub and adult roundtail chub were calculated in 2007 and 2008 and have been calculated periodically in Westwater Canyon since 1998. Fishes were captured via trammel nets and cataraft mounted electrofishing and were implanted with PIT tags upon initial capture. Three sampling trips were completed in both 2007 and 2008. Closed-capture population estimates for adult humpback chub indicate significant decrease in the population from 1998, but no significant differences were found between the 2007-2008 sampling period and years after 1998. Trammel net catch per unit effort (CPUE) likewise show significant decrease in catch rate of adult humpback chub from 1998 and years prior to 1998, but exhibit no significant decrease from years after 1999. Adult roundtail chub population estimates and CPUE metrics alternately did not show significant change over time.