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EXECUTIVE SUMMARY The Colorado River in Black Rocks was sampled in 1998, 1999, and 2000 to estimate size and structure of the humpback chub population. Sampling occurred in late summer - earlyfall after water temperatures began to cool for the year. Three sampling trips were conducted in 1998 and 2000; four trips were made in 1999. Sampling was primarily done with multi-filament trammel nets (1-in inner mesh), although Gila captures were supplemented with electrofishing and trap nets in 1998 and with angling in all years. Trammel nets were set in shoreline eddies in early morning and late afternoon. Nets were run at 1 to 2 hr intervals with 2 hrs set as a maximum interval (mean 1.5 hr). All Gila were removed from the nets, placed in fresh water and transported to a central processing point. All Gila were identified as either humpback chub or roundtail, checked for a PIT tag, measured (total length, ± 1 mm), and weighed (± 1 g). Untagged fish were equipped with a PIT tag before release. After handling, Gila were placed in a 1.5% salt bath for 0.5-1 min, and released at the common location. About 9% of the total number of humpback chubs handled were placed in a live cage and held overnight to assess initial mortality - no overnight mortalities occurred. All fish appeared healthy when released. Catch rates declined markedly in 2000 compared with 1998 and 1999. A total of 184 humpback chubs were handled in 1998 (number does not include within year recaptures), 293 in 1999 (four trips compared with three trips in other years) and 68 in 2000. Within year recapture rates were about 10%, with overall recapture rates of 30 to 40% (includes multiple recaptures of the same fish during the same sampling trip, recaptures of fish tagged by other investigators or fish tagged in previous years of this study). Recaptures included a total of 14 humpback chubs that had originally been tagged in Westwater Canyon. In addition, three humpback chubs tagged in Black Rocks were subsequently recaptured in Westwater Canyon. Length-frequency distributions of humpback chub were bimodal in all years. Modes were at 23-25 cm and 31-34 cm. Growth of recaptured fish averaged 7 mm per year. There was no significant difference in growth when humpback chubs were partitioned into two size groups based on the length-frequency distribution. All humpback chub were sexed during the iv