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DISCUSSION Significant differences in mean CPE among years and significant positive relationships over time suggest that the Colorado pikeminnow population increased in reaches sampled by ISMP during 1986-2000. Based on the CPE data, the most dramatic increases occurred in the Green and White rivers, with a lesser increase in the Colorado and Yampa rivers. Relative abundance, as estimated by electrofishing CPE, dropped off in the Colorado River in the last two years of sampling; however, it continued to climb in the Green and White rivers. Relative abundance in the Yampa and Colorado rivers exhibited two or more plateaus rather than steady increases as occurred in the White and Green rivers. Mean CPE in the Colorado River was low in the late 1980s and high in the mid 1990s. In the Yampa River, mean CPE was lowest in the late 1980s then rose, but continued to fluctuate, through the 1990s. Mean CPE in the White River exceeded that of the Green River by the end of ISMP. However, that does not necessarily mean that there were relatively more Colorado pikeminnow in the White River than in the Green River. The White River is smaller than the Green River (e.g., mean annual flow (average of all days of the year] of the White River at Watson UT in water year 1998 was 1,089 cfs compared with $,402 cfs for the Green River at Green River UT [Herbert et al., 1999]) and electrofishing was probably more efficient in the smaller river. Nonetheless, there has been a dramatic increase in Colorado pikeminnow in the White River. Taylor Draw Dam was closed on the White River in 1984 and blocked fish movement into historical habitat upstream. Irving and Modde (2000) hypothesized that the barrier increased Colorado pikeminnow abundance below the dam because they were prevented from moving further upstream. They also .felt that a high prey base downstream from the dam may have artificially increased carrying capacity for large predators such as Colorado pikeminnow. Mean CPE was highly variable in all rivers and large changes were necessary to detect statistically significant differences among years. In general, significant differences were not detected over short periods and several years of increase were required for differences to be significant. Although ISMP reaches were representative of the rivers to be monitored, it is important to recognize that extrapolation of change in CPE to reflect the entire range of Colorado pikeminnow should be done cautiously. However, the increase in CPE in all four rivers provides substantial evidence that the entire upper Colorado River basin population has increased, but actual change in population size can not be estimated. Electrofishing is a common technique for estimating relative abundance of fishes (Reynolds 1996). Electrofishing CPE of largemouth bass Micropterus salmoides was significantly related to population density in mid-western and southern reservoirs (Hall 19$6; McInerny and Degan 1993) and to population density of walleye Stizostedion vitreum in 18