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300 C. Differences in optimum food abundance for growth (342 nauplii fish-1 day-I) and survival (235 fish-l day-I) suggests that growth continued to in- crease with increasing food abundance, but that sur- vival reached a high plateau at a lower food abun- dance. # Starvation resistance , Starvation has often been hypothesized as a factor important in regulating recruitment in fish popula- tions (Miller et al. 1988). Starvation resistance in fish larvae is generally a function of body size at hatching; larger fish are more resistant than smaller ones (Miller et al. 1988). Colorado squawfish larvae denied food for up to 17.5 d after feeding was first possible (23.5 d after hatching) were resistant to starvation, but survival was low for fish denied for 20 d (26 d after hatching). Time to 50% mortality was estimated to be between 17.5 and 20 dafter feeding could begin, or 23.5 to 26 d after hatching. Ecological applications , Results of this study indicate that maximizing tem- perature and food abundance in backwater habitat should benefit growth and survival of Colorado squawfish in early life. Because relatively small changes in growth and survival rates may cause sub- stantial changes in annual recruitment levels of fish- es (Houde 1987, Crowder et al. 1992, Rice et al. 1993), enhancing temperature and food abundance should also benefit recruitment of Colorado squaw- fish. Number and area of backwaters, and growth and abundance of age-O Colorado squawfish are all negatively correlated with discharge levels in upper Green River (Tyus & Haines 1991), so lower sum- mer flows in regulated streams should benefit growth and survival oflarvae. It is important to note that these backwaters are ones within the bankfull channel that are formed as high spring flows recede and should not be confused with inundated flood- plain lowlands that result from overbank flooding. Backwaters that have relatively low exchange rates with cooler main channels have greatest potential 207 to provide warm, food-rich environments, but are also the most susceptible to drying under fluctuat- ing flow regimes. Blinn et al. (1995) found that a sin- gle 12 h desiccation event, which was caused by arti- ficial flow fluctuations of the Colorado River down- stream of Glen Canyon Dam, Arizona, reduced benthic macroinvertebrate mass by ;;:: 85%. Thus, non-natural river fluctuations such as those caused by power-production water releases from dams should be minimized in order to maximize backwa- ter temperature and productivity. Diel temperature fluctuations in backwaters may motivate movements of early life stages of Colora- do squawfish from warm backwaters during the day to relatively warm main channels at night when backwaters are cool (Tyus 1991b). Such movement patterns may maximize diel degree day accumula- tion and growth potential of Colorado squawfish. Growth of Colorado squawfish larvae may be limit- ed in river reaches where main channel temper- ature regimes are < 220 C. Starvation resistance times for Colorado squaw- fish larvae are probably shorter in the wild than in the laboratory because of increased energetic costs of negotiating large and swift rivers, avoiding pred- ators, and securing suitable nursery habitat. High and extended spring runoff, floods, and river fluctu- ations caused by dams are among the factors that may affect ability of larvae to find and maintain po- sitions in productive backwater habitat and avoid starvation. Nevertheless, relatively high resistance of Colorado squawfish larvae suggests that direct starvation mortality may be unimportant in wild populations except during rare events. However, reduced growth may result in increased mortality by increasing the amount of time larvae are suscep- tible to predators. Further research is needed to link results of growth, survival, and starvation resistance studies to annual, seasonal, or daily discharge and temper- ature regimes in main channels of the Colorado River Basin. Interactions between feeding and growth rates of early life stages of Colorado squaw- fish under various environmental conditions, and competition and predation are largely unstudied but may playa significant role in structuring recruit-