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<br />CIBCharge Curing S.mpllng (m'/sec)
<br />
<br />Figure 9. Effects of discharge on relative suilable shoreline habitat availability (slandardized to 226 ml/s) compared to percentage of 190 elec.
<br />lrofishing samples collected in 1993 ~tween river krn 99 and 107
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<br />01198 I
<br />
<br />HUMPBACK CHUB HABITAT MODELLING
<br />
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<br />DISCUSSION
<br />
<br />Converse et ai, (1998) concluded that the effect of artificially elevated base flows due to impoundment on native
<br />fish habitat has not rcc~ived adequate attention in studies related to GCD. and our simulation study is the first effort
<br />to address this deficiency, Researcher> working in Grand Canyon have hypothesized that reductions in the
<br />frequency of low.flow periods. coupled with reduced waler temperature, have resulted in reduced habitat quality
<br />and an extremely detrimental posl-dam environment for juvenile humpback chub (Valdez and Ryel. 1995:
<br />Converse et ai., 1998), While we cannot comment on the effects of temperature, on the basis of our results, the
<br />effect of dam operations on suitable fish habitat is extremely variable across seasons and reaches, and the effect is
<br />not always negative. Dam operations have decreased suitable habitat availability in the pre-dam low-flow months
<br />by increasing discharge, but have increased it in the spring (April-June) by attenuating the freshet (Figure 6.
<br />Table V), Furthermore, pre- and post.impoundment suitable habitat differences were highly variable among
<br />reaches. and one questions whether there are any meaningful biological effects of impoundment on habitat avail.
<br />ability in the less-sensilive reaches (e,g. PAL, R3, R5) even though the differences were oflen statistically signifi-
<br />cant. Because many years of habitat estimates were computed in pre-impoundment and post-impoundment
<br />periods, the number of degrees of freedom in the ANOVAs was high, This resulted in statistically significant dif-
<br />ferences among means even when some of these differences were quite SInalL
<br />Conclusions on'lhe overall effect of flow-driven changes in habitat availability on juvenile humpback chub sur-
<br />vival in the mains tern are difficult to make because we do not have an adequate model to integrate the seasonal and
<br />spatial variation documented in our study and. more importantly, to translate the overall habitat effect into a popu-
<br />lation response. Dam operations have increased suitable shoreline habitat availability in Ihe spring but reduced it in
<br />most reaches from August to February. Is the overall impact therefore negative because operations have reduced
<br />suilable shoreline habilat in more monlhs Ihan they have increased it, or has increased suitable shoreline habitat
<br />availability during the period of dispersal from the LCR (early to mid-summer) resulted in a net beneficial effect?
<br />Similar questions can be asked in a spatial context. Are potentially negative effecis of the decreased frequency of
<br />low-flow periods on suitable habitat availability in highly sensitive reaches compensated, or at least minimized, by
<br />relatively constant suitable shoreline habitat availability at insensitive reaches? Further fe.linement of our numer~
<br />ical habitat model, by adding more details to the biological component of Ihe calculaiions. or by increasing the
<br />spatial extent of the study area, wi II not address these questions, The interaction between habitat and ecosystem
<br />processes like competition and predation are highly uncertain. Ultimately. questions regarding the effects of dam
<br />
<br />Copyright @ 2004 John Wiley & Sons. Lid.
<br />
<br />River RtJ. Applic. 20: 379-400 (2004)
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