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METHODS Sampling Reaches Sampling reaches were delineated based upon the Sensitive Areas Document (Biological Subcommittee 1984) which summarized river reaches important to the endangered fishes. Designation of the sensitive reaches was based on distribution and abundance data collected by earlier investigators (e.g. Wick et al. 1981, 1985a,b, 1986; Tyus et al. 1982b; Valdez et al. 1982; Archer et al. 1985). The reaches selected for ISMP contained more Colorado pikeminnow than other areas of the four rivers. Sampling was limited to flat-water sections because canyon-bound reaches required specialized equipment and extended sampling trips. Participants recognized that the ISMP reaches did not include the entire range of Colorado pikeminnow, but they felt that the reaches comprised a sufficient amount of habitat to monitor trends in abundance. When ISMP began 186 miles of four rivers in the upper basin were sampled -the Green (five reaches, 77 mi), Colorado (three reaches, 54 mi), White (two reaches, 30 mi), and Yampa (three reaches, 25 mi} rivers (Figure 1). These 13 reaches were sampled every year. Additional reaches were added in 1994 that were sampled in alternating years to expand the sampling program and increase the number of Colorado pikeminnow handled each year. The additional reaches added 189 mi to the total sampling area, but only 10-60 mi of the extra habitat were sampled in a given year. All sampling reaches and the years they were sampled are described in Table 1. Because ISMP reaches had different lengths (5- 22 mi), they were divided into at least two, but no more than five subreaches ranging between 2.5 and 6 mi Iong. These subreaches remained constant-after being established in the first year of ISMP. Sampling included both left and right shorelines of each reach (often sampled on different days). Sampling Protocol Colorado pikeminnow monitoring was conducted annually in April or May, prior to peak runoff. All sampling was conducted from rigid-bottom, outboard-powered electrofishing boats equipped with 4- or 5-kilowatt generators and WP-15s (or equivalent) to adjust the voltage transmitted to the water column. Electrofishing boats used two spherical anodes suspended from booms in front (use of spheres began in 1989, stainless steel cables were used 1986-1988) and stainless steel cables (cathodes) suspended from the sides. Electrode size, voltage, and amperage were adjusted to minimize the possibility of injuring a rare fish while maximizing electrofishing efficiency as much as possible. Adjustments were made to keep amperage at 6-8 amps to the extent possible. One or two netters were positioned on the front of the boat to capture fish. 2