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002490 60-72 and RM 185-205. Fluctuating river stage in late spring or early summer may affect survivorship of small, larval age-classes. Seasonal use of tributary mouths in the spring has been noted for this fish. Tributary mouths are used primarily by adult and juvenile FMS for feeding, and for staging areas for spawning in the spring. Movements into the Glen Canyon tailwater occurred throughout the year, but appeared to increase during late fall and winter and decline in spring and sununer, and results suggest that peak seasonal migration into the Lees Ferry reach occurred from downstream locations. Timing of seasonal increase in the Lees Ferry reach corresponds with pre-spawning migration to the Paria River and its confluence with the mainstem Colorado River and is coincident with decline in numbers in the Little Colorado River and its confluence area (McKinney et al. In press). Bluehead suckers Fast current runs and sidechannels appear to be preferred habitats for adult bluehead suckers. Highest capture data indicate that these fish are found in greatest numbers below National Canyon to Diamond Creek (RM 165-205) and below the LCR over rubble substrates and in the absence of vegetation. Sub-adults have been caught most often in runs. Substrate preferences vary with reach but YOY bluehead suckers are often found with YOY FMS in soft substrates (sand/silt), consuming benthic invertebrates. At about 50-80 mm, they develop a cartilaginous scraper and feed on algae and diatoms from rocks. Humpback Chub The endangered humpback chub (HBC; Cyprinidae: Gila cvoha) is an endemic fish species in the Colorado River basin (Valdez and RyelI997). The HBC was taxonomically described by Miller (1946), and was listed as an endangered species in 1968. The Grand Canyon supports the only successfully reproducing HBC population in the lower Colorado River basin (Kaeding and Zimmerman 1983, Valdez 1995, Valdez and Ryel1997). Valdez (1995) identified nine distinct aggregations in the mainstream Colorado River downstream from Glen Canyon Dam, including: 30-Mile, the Little Colorado River (LCR) confluence area, Lava/Chuar to Hance Rapids, Bright Angel Creek mouth, Shinumo Creek mouth, Stephens Aisle, Middle Granite Gorge, Havasu Creek mouth and Pumpkin Spring. From 3000 to 3500 adult HBC occupy the mainstream Colorado River, and these are largely concentrated within:!: 4.2 miles of the mouth of the Little Colorado River (Mile 61), the largest sub-population. The mainstream HBC in the LCR aggregation use the LCR for spawning, while other HBC appear to be resident in the LCR. The distribution of HBC in the mainstream has not changed over the past two decades (Valdez and Ryel1997); however, HBC density may have declined in the LCR during the past decade (Douglas and Marsh 1996). Habitat use by HBC varies between age classes and by time of day. Young HBC in the mainstream have been collected in return current channels and other low velocity environments (Maddux et ai., 1987, Arizona Game and Fish Department 1996, Valdez and Ryel1997). Young- of-year and subadult HBC in the Colorado River mainstream often use irregular shorelines as habitat, and adult HBC often occur in or near eddies (Valdez 1995, Valdez and Ryel 1997). Adult radio-tagged HBC demonstrated a consistent pattern of greater near-surface activity during