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UUlaa~ 1644 C. P. PAUKERT Hanchin et al., 2002; Tate et al., 2003), and variability in relative abundance within a gear type (Allen et al., 1999; Tate et al., 2003) can often mask year to year differences in relative abundance indices. Nonetheless, relative abundance estimates are often used in fisheries and are assumed to be an index of overall abundance (Hubert, 1996); however, this assumption is often not met (Harley et al., 2001). In the mainstem Colorado River, Grand Canyon, various sampling gears are used because of the biases inherent in each gear in different habitats. Trammel nets and electro fishing, however, are two of the most common gear types used to collect native fishes (Valdez & Ryel, 1995). There has been recent concern about the status of native fishes in the Grand Canyon, particularly the humpback chub Gila cypha Miller, which has been identified by the U.S. Government of being in danger of extinction (USFWS, 2002). The status and trends of other large-river native fishes, flannelmouth sucker Catostomus latipinnus Baird & Girard and bluehead sucker Catostomus discobolus Cope, are also monitored. Native fishes in the Grand Canyon have declined in abundance since impoundment of the Colorado River by the Glen Canyon Dam in 1963, and the fish populations currently may still be declining or are relatively stable at lower than historic abundances (Minckley, 1991). Recently, the U.S. Fish and Wildlife Service (USFWS) suggested that the humpback chub in the Grand Canyon could be downlisted if (in part) the trend in abundance of fish ~200mm did not decline over a 5 year period (USFWS, 2002). Therefore, sampling methodologies that allow for detection of trends in relative abundance and size structure (total length, Lr) over time would be beneficial (Tate et al., 2003). There is a need to determine the most reliable sampling gears for native fishes in the Grand Canyon so population trends can. be assessed and gear bias minimized. The objectives of this study were to determine the precision of trammel netting and electro fishing in the Colorado River to assess relative abundance and size structure of humpback chub, flannelmouth sucker and bluehead sucker. High variability in relative abundance indices, which is common in native fish populations with low abundance (Counihan et al., 1999; Paukert & Fisher, 1999) leads to low statistical power and reduces the utility of these indices to make management decisions (Peterman & Bradford, 1987). Therefore, this study was used to assess variability between gears in relative abundance indices, the length frequency for each native fish species and the samples size required to detect changes in relative abundance for each gear. MATERIALS AND METHODS Sampling was conducted as part of an ongoing monitoring programme of fish popula- tions in the Grand Canyon. This study focused on sampling conducted in ] 992 and 1993 in the Colorado River between dan 96,6 and 104,7 (36022' N; 111053' W), near the confluence of the Little Colorado River (LCR). This 8 Ian area was chosen because of its higher density of native Colorado River fishes compared to other areas (Valdez & Ryel, 1995). The LCR, the largest tributary to the Colorado River in the Grand Canyon, enters the Colorado River at rkm 98.7. This tributary is the primary spawning area for native fishes (Childs et al., 1998) and is the reason why there typically is a higher concentration of native fishes in the mainstem Colorado River in this area. Low and patchy catches of native fishes, however, are very common throughout the mainstem, @ 2004 The Fisheries Society of the British Isles, Journal of Fish Biology 2004, 65, 1643-1652 (No claim to original US government works)