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It may also reflect the larger sample sizes resulting from more samples being taken from more extensive reaches of the Colorado River. Smaller Colorado pikeminnow maybe under represented in samples from the other rivers as well. Alternately, very large Colorado pikeminnow can sometimes be more difficult to capture than smaller specimens because of their weight or violent reaction to the anode (personal observation). However, size structure determined by ISMP is accurate as possible using electrofishing as the primary collection technique. Data collected during ISMP and other studies were used to develop preliminary estimates of survival and population rates of change for the Colorado pikeminnow population in the upper basin (K. Bestgen, personal communication). Although outside the scope of the original intention of ISMP, further analysis of these two parameters would supplement the trend information provided by CPE and the population estimates currently being developed for the rivers of the upper basin. CONCLUSIONS Consistent increase in electrofishing CPE in all rivers sampled by ISMP indicates that the Colorado pikeminnow population in the upper Colorado River basin increased from 1986 to 2000. This increase was most dramatic in the Green and White rivers, but an increase also occurred in the Colorado and Yampa rivers. Developing CPE indices using shoreline electrofishing is a valid method of determining trends in Colorado pikeminnow populations. However, CPE values must change substantially for significant differences to be detected and their inherent variability often requires several years of data before an increase or decrease in relative abundance can be determined. In addition, an estimate of population abundance can not be made using CPE alone. Although all fish were marked before release, the study design did not allow development of robust population estimates using mark-recapture techniques. Shoreline electrofishing is a valid method of capturing Colorado pikeminnow for population estimates, but the study design should be developed specifically for that purpose. Alternately, studies that utilize one-pass electrofishing to develop relative abundance data can be reinitiated when relationships between CPE and population size are identified (i.e. fish capture efficiency) and mark-recapture population estimates are no longer necessary. Sampling reaches should encompass as much of occupied habitat as possible and should be selected randomly to provide an improved study design. 21