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Studies specifically directed at determining population size of Colorado pikeminnow in the Green River basin began in 2000 (T. Czapla, personal communication). Because that was the last year of ISMP no comparisons between mean CPE and an independent population estimate can be made for that basin. However, it may be possible to compare annual sampling effort with annual population estimates when the study is completed. Because sampling effort in the new study includes the entire range of occupied habitat and will collect the fish used in the estimate, the possibility of understanding the relationship between the two values will be enhanced. Bayley and Austen (2002) demonstrated the importance of determining catchability (fish capture efficiency; the proportion of fish caught from a known or accurately estimated population) of target species before using change in CPE to estimate change in abundance. Careful analysis of the population-estimation data may provide catchability estimates for Colorado pikeminnow. If catchability can be estimated under a wide range of environmental conditions, CPE maybe useful to monitor trends in Colorado pikeminnow abundance after mark-recapture population estimates are no longer required. Size structure of Colorado pikeminnow populations in the different rivers determined by ISMP corresponds with previous studies. Osmundson et al. (1998) and Valdez et al. (1982) also found that most small Colorado pikeminnow were found in the lower Colorado River. Osmundson et al. (1998) described upstream dispersal patterns of Colorado pikeminnow in the Colorado River as they grew and matured. In the Green River basin, Tyus (1991) reported that most small Colorado pikeminnow were found in the lower Green River and that the Yampa River population was almost entirely adult fish. However, the small numbers offish collected in some years (especially early in ISMP) meant that the annual length-frequency histograms were probably incomplete in some cases. This situation was partly remedied in later years when more fish were collected and by the addition of the alternate reaches in 1994. The alternate reaches added 16 to 115 extra fish per year for use in the length-frequency distributions which substantially improved sample size in some years. Upstream dispersal of Colorado pikeminnow as they mature may partially explain differences in annual trends between upper and lower portions of the Green and Colorado rivers. Upstream movement of maturing fish could drop catch rates in lower reaches and increase catch rates in upper reaches in some years, even though river-wide CPE was stable or mcreas~ng. Electrofishing is size selective (i.e. larger fish are mare susceptible to capture than smaller fish [Reynolds 1996]) and smaller Colorado pikeminnow may be under represented in ISMP samples even though they were captured in substantial numbers in some years. For example, the mode for the average length-frequency distribution for the lower Colorado River is larger than reported by Osmundson (2002) in most years. This may result from scare and snare being more efficient than shoreline electrofishing in capturing smaller Colorado pikeminnow. 20