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C~ EXECUTIVE SUMMARY Humpback chub (Gila cypha) are listed under the Endangered Species Act of 1973, as amended. In accordance with recovery goals finalized in 2002, a population estimate was completed for adult humpback chub in Westwater Canyon on the Colorado River. This population estimate was conducted from 1998 to 2000 with the objectives of obtaining population estimates for humpback chub and roundtail chub in Westwater Canyon. Sampling occurred during September and October ~ throughout the three years of the study. Three passes were conducted annually with approximately one week between passes. The primary method of capture was via trammel netting with supplemental electrofishing on one pass per year. Population estimates were generated from recapture data using the null estimator (Mo) within Frogram CAPTURE. Separate estimates were generated for each year of the study. Results indicated a decline in the adult humpback chub ~ population between 1998-1999 with a retention in abundance between 1999-2000: 4,744 in 1998; 2,215 in 1999; and 2,201 in 2000. This overall declining trend was not statistically significant, but maybe of concern. The adult roundtail chub population in Westwater Canyon during this time period was relatively stable: 5,005 in 1998; 4,234 in 1999; and 4,971 in 2000. Length-frequency analysis for both species indicated shifts in the size class structure through the three years. Growth ~ of humpback chub was slower than roundtail chub based on recapture data from this and previous studies. Analysis of recaptures indicated more net movement from humpback chub than from roundtail chub. However, both species exhibited movement between Black Rocks and Westwater Canyon. This movement was frequent enough to consider Black Rocks and Westwater Canyon a single population for humpback chub and roundtail chub. Analysis of catch per unit effort (CPUE) data from this study and historic interagency standardized monitoring indicated a continued t declining trend in mean CPUE for humpback chub that was significant. Mean CPUE for roundtail chub also indicated a continued declining trend, but it was not statistically significant. The results of this study will provide valuable information for conducting future population estimates of chub in the upper Colorado River basin in addition to providing three point estimates that will be used to determine if humpback chub have met the recovery goals. A S ix