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<br />. <br /> <br />. <br /> <br />From 3,000 to 3,500 adult HBC occupy the mainstream Colorado River, and these are <br />largely concentrated within :t 4.2 miles of the mouth of the Little Colorado River (Mile <br />61), the largest sub-population. The mainstream HBC in the LCR aggregation use the <br />LCR for spawning, while other HBC appear to be resident in the LCR. The distribution <br />of HBC in the mainstream has not changed over the past two decades (Valdez and <br />RyeI1997); however, HBC density may have declined in the LCR during the past <br />decade (Douglas and Marsh 1996). <br /> <br />Habitat use by HBC varies between age classes and by time of day. Young HBC in the <br />Colorado River mainstream commonly use return current channels and other backwater <br />habitats (Maddux et aI., 1987, Arizona Game and Fish Department 1996, Valdez and <br />RyeI1997); however, HBC use of backwater habitats in Grand Canyon has been <br />compromised by fluctuating flows and cold-stenothermic releases which reduce <br />warming and create unstable conditions. In addition, backwater habitat area has been <br />reduced and backwaters have aggraded through siltation under Interim Operations in <br />Grand Canyon (McGuinn-Robbins 1997). <br /> <br />Young-of-year and subadult HBC in the Colorado River mainstream often use irregular <br />shorelines as habitat, and adult HBC often occur in or near eddies (Valdez 1995, <br />Valdez and RyeI1997). Adult radio-tagged HBC demonstrated a consistent pattern of <br />greater near-surface activity during the spawning season and at night, and day-night <br />differences decreased during turbid flows (Valdez 1995, Valdez 1997). Turbid water <br />may provide cover for humpback chub. AGFD has found lower catches in shallow <br />nearshore areas during periods of clear water, and increased catches during daylight <br />hours in turbid water (Valdez and RyeI1997). <br /> <br />Life Requisites <br /> <br />The life history and ecology of HBC in Grand Canyon has been intensively studied <br />(Suttkus and Clemmer 1979, Kaeding and Zimmerman 1983, Carothers and Minckley <br />1981, Maddux et aI., 1987, Gorman 1994, Valdez 1995, Douglas and Marsh 1996, <br />Valdez and RyeI1997). A key issue is the lack of recruitment to the adult population, <br />which is reflected by low survivorship of young fish (Valdez, 1995). Individual adult <br />HBC demonstrate high microsite fidelity (Valdez 1995), but young HBC may drift for <br />relatively long distances (TuegeI1995). <br /> <br />Mainstream Colorado River HBC in Grand Canyon spawn primarily in the lower nine <br />miles of the LCR from March through May. Adult fish initially stage for spawning runs in <br />large eddies in February and March, and make spawning runs that average 17 days <br />into the LCR from March through May, as LCR flows decrease, warm and clear (Valdez <br />1995). Spawning runs of up to 25 miles have been reported for this species. After <br />spawning, many adult chub apparently return to specific microsites in the mainstream. <br /> <br />Young HBC remain in the LCR, or move into the mainstream where mortality due to <br />thermal stress (Lupher and Clarkson 1993) and predators (Valdez 1995) appears to be <br /> <br />1998 GCD Beach/Habitat Building Flow 13 <br /> <br />Biological Assessment <br />