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northern lakes {Serns 1982, 1983). Few studies have related electrofishing CPE to population density of fish in rivers, however Paragamian (1989) found that electrofishing CPE of smallmouth bass M. dolomieui in rivers was significantly greater during nighttime than it was during daytime. In the upper Colorado River basin, mean CPE was poorly con elated with independent population estimates of Colorado pikeminnow in most cases. However, estimates made by Nesler (2000) for the Green and White rivers were correlated with ISMP catch rates for 1992-1997; no correlations were found for the Colorado and Yampa rivers. The correlations occurred in the two rivers with the greatest increase in mean CPE during the study period. Nesler's (2000) estimates were made using capture-recapture data collected during all studies funded by the Recovery Program. The data were collected with a variety of sampling designs and capture records from ISMP comprised a substantial portion of those data. However, the data were not collected using amulti-pass sampling methodology specifically designed to develop amark-recapture population estimate (e.g. Pollock et al. 1990). Osmundson and Burnham (1998) and Osmundson (2002) estimated population size of Colorado pikeminnow in the Colorado River during two periods encompassed by ISMP - 1991-1994 and 1998-2000 - using a multi-pass sampling design. ISMP data were included as portions of one pass in their studies, but considerable data were collected specific to the study. They noted a consistent increase in population size in the Colorado River which roughly corresponded with the increase in CPE recorded by ISMP. However, the two values were not correlated. Although CPE in the Colorado River increased from 1986-1998, it was relatively stable (~1 CPM/hr) during the years when population estimates were available. In addition, estimated population size continued to increase through 2000 (although not significantly) while CPE decreased during the last two years of ISMP. Osmundson (2002) also noted a decline in CPE (measured as CPM per trammel net set} in the later years of his study, even though the annual estimates increased slightly. Osmundson's primary collection technique involved trapping Colorado pikeminnow in backwaters using trammel nets (scare and snare; described in Osmundson and Burnham 1998); shoreline electrofishing was used to supplement the scare-and-snare data in reaches without sufficient backwaters. Scare and snare is more efficient than electrofishing because Colorado pikeminnow are attracted to backwaters during runoff and are more susceptible to capture. The different sampling technique may be partly responsible for the lack of correlation between ISMP data and the Colorado River population estimates. CPE should be related to population size in some manner, but the many variables associated with sampling in difficult environments such as the rivers of the upper basin suggest that the nature of the relationship will be difficult to identify. 19