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DISCUSSION Capture-recapture data yielded population estimates that varied among years, but were not significantly different from each other because of wide confidence intervals. Coefficient of variation was lowest in 1999 when four discrete passes were made. Confidence intervals were also tighter in 1999 than in other years, which illustrates that implementation of a fourth pass can make a substantial improvement in the quality of the population estimate. However, catch rates dropped off dramatically in the third year of the study causing an increase in CV and a widening of confidence intervals. Because of the variation in point estimates among years and wide confidence intervals, the average value for the study period (741) may provide the best general number for reference to population size, even though comparisons among years may still be made. The relatively large shift in estimated population size far 1998 caused by leaving three fish out of the analysis emphasizes the unreliability of using a single number to reflect population size in small, difficult to sample fish populations. The models used by program CAPTURE are closed models that assume emigration or immigration does not occur during the estimation period. Although previous information documented that humpback chubs moved between Black Rocks and Westwater Canyon (e.g., Valdez and Clemmer 1982), the movement was believed to be limited and the short sampling period was intended to meet the assumption of closure. However, the capture of two fish in both Black Rocks and Westwater Canyon in the same year indicates that movement may occur more frequently than previously believed. Future data analysis should include use of open models to compensate for the more frequent movement. The known violation of closure in at least one year of a three year study is another reason that the three-year average value is probably the most representative estimate of population size. Nesler (2000) analyzed Black Rocks ISMP data from 1991-1997 and produced estimates for 1994 and 1997. These estimates were based on recaptures from one year to another and therefore could not be considered closed estimates. However, he compensated by making estimates based on survival rates of 59%, 76%, and 90%. Averages for 1994 and 1997 were 537 (59010 survival), 1,197 (7G%}, and 2,267 (90%) humpback chubs; 95% confidence intervals were generally wider than observed in this study. The 1994-1997 estimates !2