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<br />SUBADULT HUMPBACK CHUB IN THE COLORADO RIVER <br /> <br />281 <br /> <br />Talus Cobble <br />Change in angle of repose and substrate size Homogeneous angle of repose and substrate <br />result in less cover at higher Q result in no cover change at higher Q <br /> <br /> <br />Qhigh <br /> <br />Qlow <br /> <br /> <br />- <br /> <br />Q <br />D------<58oc::> <br /> <br />Figure 9. Channel cross section demonstrating difference of how cover changes with discharge between geologically dependent and <br />independent shoreline types <br /> <br />~ <br /> <br />at all discharges encountered in this study (Figure 9). However, this relationship must be sinusoidal over <br />a larger range of discharge, because cobble, sand and vegetation will be exposed at some discharge lower <br />and inundated at some discharge higher than those observed in this study. <br />Although food abundance was not considered in this study, regulation by Glen Canyon Dam has <br />altered food availability and the food web dynamics in the Colorado River through Grand Canyon (Blinn <br />and Cole, 1991; Angradi, 1994). Glen Canyon dam traps allochthonous debris, thereby shifting the food <br />base from allochthonous to primarily autochthonous material. Such alterations may alter or limit food <br />availability in channel margins. However, vegetated shorelines may provide allochthonous debris from <br />riparian vegetation and, macroinvertebrates may be associated with such inputs. Fish may select <br />vegetation for this reason only, or more likely, they may be attracted to a combination of habitat <br />conditions that include greater food availability and specific physical conditions. <br />Loss of historic habitat. Another possible explanation for subadult humpback chub occurring along <br />vegetated shorelines rather than natural shoreline types is that certain habitat types have been lost with <br />the onset of flow regulation. Because flow and sediment transport regimes in the Colorado River through <br />Grand Canyon have been altered, sand deposits are less extensive and structurally more simple (less <br />sinuous perimeter and less complex bedforms) than those that occurred in the pre-dam era (Graf et al., <br />1987; Schmidt and Rubin, 1995). Sand deposits may have historically provided complex, sinuous shoreline <br />habitat, such as backwater habitat, that no longer exists or is currently infrequent and ephemeral. <br />Shoreline complexity in sand deposits (e.g. backwater habitat) is known to provide lower depths and <br />velocities for small fish and greater protection from predators (Tyus, 1991, 1991; Jurajda, 1995). The few <br />backwaters that are permanent in Grand Canyon can have very high densities of young native fish, but <br />fish presence in backwaters depends on high turbidity conditions (personal observation; Arizona Game <br />and Fish Department, 1994; Valdez and Ryel, 1995). <br /> <br />, <br />. <br /> <br />.. <br /> <br />Implications of flow regulation on survival of subadult humphack chub <br /> <br />When compounded with other changes in the river ecosystem, decreased habitat quality may limit <br />survival of subadult humpback chub in Grand Canyon. In addition to a general reduction in physical <br />habitat quality, the dam has altered water temperature. Historically, temperatures ranged from 2 to 180C. <br />The Colorado River temperature now averages from 9 to 110C at the LCR (US Department of Interior, <br />Bureau of Reclamation, 1995; Valdez and Ryel, 1995). Consequently, growth, reproduction and survival <br />of native fish have almost certainly been affected. Bulkley et al. (1981) showed extreme compromises in <br />swimming abilities and growth rates of humpback chub in temperatures ranging from 5 to 150C. In <br />response to these changes in the wild, young fish may be limited to very different environments where they <br />expend less energy to compensate for reduced growth efficiency imposed by suboptimal temperatures. For <br />example, subadult humpback chub may be forced to occupy a portion of the channel margin nearer to <br />shore, which has more cover and refuge from high velocities to reduce energy expenditure, or young fish <br />may shift to naturalized vegetated shorelines that consistently provide these conditions at a range of <br />discharges. <br /> <br />@ 1998 John Wiley & Sons, Ltd. <br /> <br />Regul. Rivers: Res. Mgmt. 14: 267-284 (1998) <br />