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12 CLAllKSON ET AL. Reproductive strategies of the native fishes are intimately integrated with the patterns of hydrology, sediment transport, water temperature, and trophic processes. Spawning by native fishes in the major rivers of the upper Colorado River basin is initiated when temperatures rise above 16 C (Vanicek and Kramer 1969, SeethaIer 1978, Nesler et aI. 1988, Tyus and Haines 1991), aIthough razorback sucker can utilize Iower temperatures for reproduction (Tyus 1987, Bozek et aI. 1991). Excepting razorback sucker (Tyus and Karp 1989, 1990), the major period of mainstem spawning typically coincides with the descending limb of the peak flow, usuaIIy in May-August (Vanicek and Kramer 1969, Tyus and Karp 1989, Karp and Tyus 1990, Tyus 1990). The predictable flood puIse patterns of large rivers favors the development of ethological and other adaptations by aquatic organisms (Junk et aI. 1989). , Spring-summer flow patterns in combination with temperature serve as an environmeniaI cue to cUlminate gonadal maturation and release of gametes in Colorado squawfish (Vanicek and Kramer 1969, NesIer et aI. 1988), and association of spawning with specific hydrograph patterns for other native fish species suggests a more general influence. However, most CoIorado River species can reproduce successfully when confined in the absence of hydrological stimuli (Johnson and Jensen 1991). Photoperiod undoubtedly plays a large roIe in the sequences of reproductive physioIogy and behavior of native fishes (Honte1a and Stacey 1990). Scouring (cleaning) of substrates following flood events may aIso serve as an environmental cue to trigger spawning activity (John 1963, Mueller 1984), as may reduced turbidity levels (Deacon and Minckley 1974). Although the mechanisms that historically stimulated native fishes toward reproduction (gonadal maturation, staging, release of gametes) are not precisely known, the ichthyofauna did respond, and persisted for millennia under the natural hydrograph and associated conditions. '--',:.. ;", .' " .".. .', Reproduction and survival of native fishes in Grand Canyon are enhanced by seasonal interactions between the discharge patterns of the Colorado River and its tributaries. Tributary reproduction peaks in April-June in most tributaries (Suttkus and CIemmer 1976, Kaeding and Zimmermann 1983, Maddux et aI. 1987, Maddux and Kepner 1988, Clarkson anlRobinson 1993, Robinson et aI. 1994). This feature historically allowed,utilizationof canyon-bound tributary mouths backed up by high main stem flows in May-July by rearing ~ and early post-larvae. . Larvae drifting (VaIdez et aI. 1985, Angradi et aI. 1992, Robinson et al. 1994) into lower reaches of tributaries encountered conditions of warmer, clearer water with slowing currents that deposited drifting invertebrates and particulate organic matter. These conditions provided"an excellent rearing environment for the young native fishes. By the time mainstem flows began to recede in July, young-of~year fishes Were" of sufficient size to increase their probability of survival as they passed into the "inamstem. . \