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00150a 1650 c. P. PAUKERT criticized for sampling bias (Hubert, 1996; Reynolds, 1996). Nonetheless, these gears are two of the most common and potentially effective gears in these large river habitats. Fisheries researchers, however, need to consider their target size of fishes when selecting the appropriate gear. The present results suggest that trammel net and electrofishing CPUE for adult (2:200mm Lr) native fishes was not related and conclusions about the population from one gear may not be the same as conclusions from the other gear. The disparity of fish sizes collected by electro fishing and trammel netting is probably due to the unique habitats where these gears are fished as well as the life stages of native fishes in these habitats. Electrofishing was conducted primarily in nearshore habitats whereas trammel nets were primarily used in deepwater pools and eddies (Valdez & Ryel, 1995). Juvenile humpback chub, tlannelmouth suckers and bluehead suckers (<200 mm Lr) typically use shoreline areas (Tyus et al., 1982; Minckley, 1991; Converse et al., 1999), possibly with more vegetation or other cover (Converse et al., 1999), which are more difficult to sample with trammel nets than electrofishing. Adults of these same species typically occupy deepwater eddies, tributary mouths and offshore habitats (Tyus et aI., 1982; Minckley, 1991; Valdez & Ryel, 1995; Valdez & Hoffnagle, 1999), which are more easily sampled with trammel nets than electro fishing. Therefore, the present results may not only be explained by gear bias, but also by location of sampling gears and fish life stages. Although trammel netting produced the lowest variability of adult CPUE for native fishes, variability was still very high and may have precluded the detec- tion of CPUE changes on an annual basis, and therefore CPUE may not be an appropriate index of abundance for these rare fishes. The sample sizes needed probably require a higher effort than would be possible (i.e. sample sizes encompassed the entire sampling universe of sample locations; Thompson, 2002). All three native fishes are relatively rare in the mainstem Colorado River when compared to the warmer water tributaries (e.g. Little Colorado River, Havasu Creek and Paria River; Keading & Zimmerman, 1983; Weiss et al., 1998; Douglas & Douglas, 2000). The CV estimates ranged from 128 to 575, suggesting very high variability in catches of these native fishes even when sampling concentrated on native fish habitats. Based on this high variability, at least 1057 trammel net sets would be needed in this 8 km reach to detect a 10% change in annual CPUE. This amount of effort is logistically not feasible and therefore suggests that using annual point estimates of relative abundance are not adequate to detect changes in native fish relative abundance. Given an appropriate sampling design, analysis of long-term CPUE trends using trammel nets may be more appropriate than electrofishing for adult native fishes in the mainstem Colorado River in the Grand Canyon. Trammel nets in the mainstem Colorado River may provide more precise estimates of relative abundance of adult native fishes when compared to electro- fishing. Both gears, however, are inherently biased to habitats and life stages of fishes where these gears can be effectively fished. Because the U.S. Government have developed criteria that determine 200 mm Lr is the minimum size of an adult humpback club (USFWS, 2002), the use of trammel nets is recommended over electrofishing in the mainstem as this gear collected a higher proportion of fish 2:200 mm Lr and variability of this gear was lower than electrofishing. As @ 2004 The Fisheries Society ofthe British Isles, Journal of Fish Biology 2004, 65, 1643-1652 (No claim to original US government works)