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206 showed none (references in Vondracek et al. 1989, Woiwode & Adelman 1991). If fluctuating temper- ature has real effects, they may be life-interval and species-specific, or dependent upon specific condi- tions associated with cycles of feeding and temper- ature change or the amplitude of oscillations. Be- stgen & Williams (1994) found that survival of en- dogenously feeding 7 d old Colorado squawfish lar- vae was 10-31 % higher, and swim bladder inflation and first feeding occurred sooner, in fluctuating than in constant temperatures. The observed ef- fects may have been due to life interval specificity for certain environmental conditions. Lack of an ef- fect of fluctuating temperatures on growth or sur- vival of Colorado squawfish larvae in this study may be due to fluctuations that were too small or to physiological differences in slightly older and exog- enously feeding larvae. Optimal temperatures for growth of larvae and juveniles of Colorado squawfish appear to be dif- ferent. Growth of Colorado squawfish larvae in this study was optimal at 310 C and generally high at temperatures above 260 C. However, Black & Bulkley (1985) found maximum growth of 0.26 mm TL d-I for yearling Colorado squawfish (45 mm TL) at250 C, but lower growth of 0.06, 0.19, and 0.17 mm TL d-I at 15, 20, and 300 C, respectively. Such differ- ences in temperature optima during ontogeny (i.e. larva and juvenile) are not uncommon and may be a mechanism for habitat segregation in fishes (Mag- nuson et al. 1979, McCauley & Huggins 1979). Data and model solutions suggested that growth of Colorado squawfish larvae declined at temper- atures below 220 C, even when abundant food was available. This may suggest that feeding rates, di- gestion rates, or food conversion efficiency was low at these temperatures. Basal metabolic rates and food requirements in fishes are typically higher at higher temperatures. This generalization was sup- ported here as growth of Colorado sq uawfish larvae was low at 300 C when coupled with low food abun- dance levels of 12.5, 28, and 64 nauplii fish-I day-I, compared to all other test temperatures. Survival response Survival of Colorado squawfish larvae was 90% or more in most treatments with food abundance of 142 and 320 nauplii fish-I day-l at all test temper- atures. Survival declined slightly at 180 C, the lowest temperatures tested, even with high food abun- dance, and was lowest overall at low food abun- dance levels and high temperatures. Elimination of a single and unexpected survival observation of 30% at 180 C and 320 nauplii fish-I day-I increased mean survival to 88% for the remaining observa- tions in that treatment. The outlier observation is partly the cause of decreased survival at higher food abundances in the model response surface (Fig. 3) with temperature apparently responsible for the re- mainder of the effect. Survival of Colorado squawfish larvae during this laboratory study was higher than the 28-36% sur- vival observed by Hamman (1989) for similar-sized fish reared in earthen hatchery ponds for 48-51 d at 19.4-23.80 C. Low food density caused by chemical treatment for eubranchiopod infestations may have been the cause for lower survival in that study. Comparison of survival for razorback sucker Xy- rauchen texanus, another endangered Colorado River Basin cypriniform fish, and Colorado squaw- fish suggested highest rates for larvae of both spe- cies at highest food abundance levels [> 58 nauplii fish-I day-1, for razorback suckers (conversion of their 50 nauplii t1) and ~ 142 nauplii fish-1 day-I for Colorado squawfish] at 180 C, the only temperature tested by Papoulias & Minckley (1990). Survival of Colorado squawfish was 67% at 12.5 nauplii fish-I day-I compared to 27% for razorback suckers at 11.7 nauplii fish-1 day-I. Lower survival for razorback sucker larvae was likely due to longer test duration (50 vs. 42 d) and higher food requirements of those larger larvae compared to Colorado squawfish. Highest mortality for both species occurred 20 d or more after initiation of experiments. Differences observed in optimum temperatures for growth (310 C) and survival (26.20 C) were not expected, but were probably not biologically im- portant because the response surface model solu- tions and experimental observations suggested sur- vival was high at temperatures ranging from 20 to ~ ..