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humpback chubs that were tagged in Black Rocks during this study were subsequently recaptured in Westwater Canyon. One of these fish was tagged in 1998 and two were tagged in 1999. The 1998 fish was recaptured within the 1998 sampling period. One of the 1999 fish was recaptured in 1999, but after the sampling period was complete. The other fish was recaptured in 2000. Black Rocks and the upstream end of Westwater Canyon are separated by about 10.5 river miles. Size Structure Humpback chubs captured in Black Rocks ranged from 18 to 40 cm TL (Figure 4). Only three humpbacks less than 20 cm (estimated minimum length for adults; USFWS 2002) were captured during the 3 year study and ali of those were captured in 1998. Size structure was bimodal in all three years, with modes at about 23-25 cm and 31-34 cm. The largest fish captured each year was 40 cm long. Growth A total of SS humpback chubs were captured in more than one year of the study. Growth of these fish ranged from 0 to 19 mm per year, with a mean of 7.58 mm (SE, 0.57). Recaptured fish were arbitrarily divided into two size groups to include the two modes evident in the size distribution. Average growth of the two size groups was virtually identical - <30 cm TL, 7.58 mm; z 30 cm TL, 7.59 mm. Sex Ratio A total of 97 humpback chubs were sexed during the last rotation in 1999. Three of those fish were recaptured during the same rotation and were assigned the same sex on both occasions. Of the fish that were sexed, 46 were male and 51 were female; the sex ratio was not significantly different than 1:1 (P >0.1). Population Estimate ' Population estimates for humpback chub in Black Rocks were highly variable depending on the model selected to make the estimate (Table A-1). For example, point estimates in 1999 ranged from 635 using the Schnabel estimator to 1,118 using model Chao Mh from CAPTURE. However, the model selection criteria from CAPTURE showed that model Mo 7