Colorado River fish monitoring in Grand Canyon, Arizona: 2002-2011 humpback chub, Gila cypha, aggregations

Authors: William R. Persons, David R. Van Haverbeke

Year: 2012 (xliv)

Abstract

Humpback chub, Gila cypha, is an endangered cyprinid species endemic to the Colorado River basin of western United States. The species was described in 1946 by R. Miller from a specimen taken near the mouth of Bright Angel Creek, Grand Canyon National Park, Arizona; and was listed as endangered in 1967. Long term fish monitoring in the Colorado River downstream of Glen Canyon Dam is a component of the Glen Canyon Dam Adaptive Management Program. Monitoring for humpback chub in the mainstem Colorado River in Grand Canyon has been conducted sporadically since the 1970’s, and has improved since the introduction of small motorized watercraft and the use of hoopnets and trammel nets. Nine humpback chub aggregations were originally identified based on fish collected during 1990 -1993, and closed population model abundance estimates were generated for six of those aggregations. An aggregation was defined as ‘a consistent and disjunct group of fish with no significant exchange of individuals with other aggregations, as indicated by recapture of PIT-tagged juveniles and adults and movement of radio-tagged adults’. An open population model has been developed to estimate the population size of the aggregation centered at the Little Colorado River and the adult humpback chub population is estimated between 9,000 and 12,000 fish. We estimated abundance of humpback chub at aggregations by applying a uniform set of capture probability estimates to annual catches within the aggregations to estimate humpback chub abundance. The adult humpback chub population at eight mainstem aggregations, exclusive of the Little Colorado River aggregation, is estimated between 1,000 and 1,800 fish. There appears to have been an increase in 2010 and 2011 in the 30-mile, Shinumo, Havasu, and Pumpkin Springs aggregations compared to previous years. Shinumo and Havasu aggregations appear to have benefited from an ongoing program of translocations of young humpback chub to those tributaries.