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<br />002491 <br /> <br />the spawning season and at night, and day-night differences decreased during turbid flows <br />(Valdez 1995, Valdez 1997).. <br /> <br />Life Requisites <br /> <br />The life history and ecology of HBC in Grand Canyon has been intensively studied (Suttkus and <br />Clemmer 1977, Kaeding and Zimmerman 1983, Carothers and Minckley 1981, Maddux et al., <br />1987, Gorman 1994, Valdez 1995, Douglas and Marsh 1996, Valdez and Ryel1997). A key <br />issue is the lack of recruitment to the adult population, which is reflected by low survivorship of <br />young fish (Valdez, 1995). Individual adult HBC demonstrate high microsite fidelity (Valdez <br />1995), but young HBC may drift for relatively long distances (Tuegel 1995) during periods of <br />high turbidity, particulary during monsoon flooding in the LCR. In this case, it is unclear if <br />drifting is active or passive and survivorship is estimated as being low. Mainstream Colorado <br />River HBC in Grand Canyon spawn primarily in the lower nine miles of the LCR from March <br />through May, other possible spawning sites include Havasu Creek and upstream of RM 44. Adult <br />fish initially stage for spawning runs in large eddies in February and March, and make spawning <br />runs that average 17 days into the LCR from March through May, as LCR flows decrease, warm <br />and clear (Valdez 1995). Spawning runs of up to 25 miles have been reported for this species. <br />After spawning, many adult chub apparently return to specific microsites in the mainstream. <br />Young HBC remain in the LCR, or move into the mainstream where mortality, due to thermal <br />stress (Lupher and Clarkson 1993) and predators (Marsh and Douglas 1997; Valdez 1995) <br />appears to be extremely high. During the summer, the young HBC that survive in the mainstream <br />tend to occupy low-velocity, vegetated shoreline habitats; however, low winter survivorship <br />virtually eliminates the young-of-the-year HBC in the mainstream. Therefore, few if any HBC <br />spawned during the previous year are present in the mainstream in the following spring. <br /> <br />Limited spawning of HBC occurs among other sub-populations in the Colorado River. Valdez <br />(1995) documented limited spawning success at 30-Mile Spring in upper Marble Canyon, and <br />rare young HBC have been documented at Kanab Creek, also in Shinumo and Bright Angel <br />Creeks. However, presence of individual juvenile chub in those tributaries does not confirm <br />spawning in the tributaries. These sightings are rare compared with the reproductive success of <br />those HBC that spawn in the LCR. <br /> <br />Dietary analyses reveal HBC to be opportunistic feeders, selectively feeding on aquatic <br />and terrestrial invertebrates and other fish (Valdez 1995, Valdez and Ryel 1997; Dennis Stone, <br />pers. com). HBC diet changes over the course of the year in response to food availability and <br />turbidity-related decreases in benthic standing biomass over distance downstream from Glen <br />Canyon Dam (Stevens et al1997, Valdez and Ryel1997). Non-native Gammarus lacustris <br />occasionally comprise a large proportion of HBC diet, especially after high mainstream flow <br />events (Valdez 1995, Valdez and Ryel1997; AGFD 1996). <br /> <br />High flows, such as the 1996 BHBF, had little detectable effect on the movement patterns or <br />distribution of adult HBC, and the 1996 BHBF did not appear to serve as a spawning cue <br />(Valdez 1997). The increased drift associated with that BHBF resulted in an increase in <br />Gammarus in HBC gut contents, an effect which is not surprising given the opportunistic <br />foraging behavior of this species (e.g., Tyus and Minckley 1988). Given that this species evolved <br />under the highly variable flow regimes that characterized the pre-dam Colorado River, it is <br />unlikely that short-duration BHBF flows of 45,000 cfs could be demonstrated to affect sub adult <br />