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.,d .~ ~;'" . Obiectives The objectives of this part of the study are to determine any effects of the high discharge ,event on fish populations in Grand Canyon, and, if so, whether different life stages respond differently, Another objective is to determine how responses of the species and different life stages vary in relation to different stages of the high-discharge hydrograph. Justification Adult and sub-adult indigenous fishes occupying unregulated canyon-bound Arizona streams ofvarying sizes (mean discharges, <1.0 to 25 mJ/sec) successfully resist floods three to four orders of magnitude greater than mean discharge (Minckley and Meffe 1987). The same age-dasses of non-indigenous fishes were, in contrast, significantly reduced in abundance by floods exceeding only one order of magnitude in the same systems, and often totally removed at higher discharges. The same patterns existed in regulated rivers subjected to uncontrolled releases of high-volume discharges; controlled, low-volume releases had no demonstrable effects. Larval and juvenile fishes, whether indigenous or non-indigenous, are more suscepbble to high discharges (Harrell 1978, Schlosser 1985), and may be expected to suffer downstream displacement by floods, assuming no "refuge(s)" are available. Based on these data, a proposed release of about 1,190 mJ/s (42,000 fWs) into the Colorado River downstream from Glen Canyon Dam (only 3 times the mean discharge of 400 mJ/s since closure of the dam and comparable to the minimum flood-flows recorded for many years of pre-dam gaging may be predicted to have no discermble effect on distribution or abundance of adults and subadults of indigenous and non-indigenous fishes in Grand Canyon. It is possible, however, that the high-discharge event may effect changes in distrIbution or population sizes of larval and juvenile individuals. The question is, how can such hypotheses be tested in a large and complex system so difficult of access and resistant to effective sampling? Hvnotbeses Adult Fisbes - Null hypothesis 1- HO 1: There is no distributional response of sub-adult and adult indigenous and non- indigenous fishes to the experimental flood. It is not possible with present technology (or if,.fA....~) to determine whether flood flows will affect adult fishes, either indigenous or non-indigenous. It is possible, however, to test some ideas relative to how adults of indigenous fishes maintain their position in canyon- bound rivers during flood by examining their distnbutional responses. Secondary null hypothesis- HO I a: Indigenous and non-indigenous adult and sub-adult fish remain in the channel and are displaced to retunl as they do not resist displacement during flood. They do not seek refuge from high discharges in tributary mouths or other low velocity locations. 22