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18 WESTERN NORTH AMERICAN NATURALIST [Volume 68 TABLE 2. Parameters for a response sUlface model to estimate total length as a function of water temperature (1'), time (D, days post-hatch; fish were 9 days post-hatch when experiments began and 37 days post-hatch at the end), and the squared and interaction tellns for those effects. The main effect fi)r T was not included (P = 0.39), The overall model was statistically significant (F 4,23 = 174.2. P < 0,0001, R2 = 0,968). Parameter Estimate s:;: t-value Pr > t Intercept 13,055 2.768 4.72 <0,0001 D -0,591 0.207 2.85 0,009 T'T -0,010 0.005 2.02 0.055 D'D 0,008 0,003 2,99 0,007 D'T 0,038 0,007 5,16 <0.0001 regression relationship suggested that time (days post-hatch), the sqmu'ed terms for time and temperature, and the time X temperature interaction all importantly affected growth of razorback sucker larvae; the interaction was the most significant effect (Table 2). Based on mean growth rates for each treat- ment, time required for razorback suckers to exceed 25 mm TL was 30 days (post-hatch) at 25SC, 33 days at 22SC, 36 days at 19.50C, and 41 days at 16.50C. Time required for slow- growth razorback suckers to exceed 25 mm TL under the simulated 200C temperature (0.29 mm . d-1) was 52 days. DISCUSSION Water temperature had a strong, positive effect on growth of early life stages of razor- back sucker, particularly when t~mperatures were 19SC or higher. A threshold was appar- ent, because change in mean length of razor- back suckers reared at 19SC was 50% greater than for those reared at 16.50C over the dura- tion of the experiment; bTfOWth rates at higher temperatures increased at a lower rate. Differ- ences in mean weight were even greater. Razorback suckers reared at 19.50C were more than \:\\.ice as heavy as those reared at 16.50C. I did not test colder temperatures because my main interest was determining more optimal , growth conditions and because some informa- tion describing growth of early life stages of razorback sucker at cooler temperatures \vas already known (Clarkson and Childs 2000). These findings support those of Clarkson and Childs (2000) and others who found faster de- velopment and higher growth rates of early life stages of razorback sucker and other native fishes of the Colorado River Basin at higher water temperatures (Marsh 1985, Bestgen and Williams 1994, Bestgen 1996). GrO\vth rates of razorback suckers at the highest water tem- peratures tested are relevant because water temperatures of the Green River floodplain exceeded 200C in early June and reached or exceeded 250C by late June (Modde et al. 2001, Christopherson et al. 2004, Modde and I-Iaines 2005), a period just after razorback suckers hatch in the. Green River (Modde et al. 2001, Bestgen et al. 2002). Growth rates of razorback sucker larvae observed in this study were generally Ulster than those in most other studies, perhaps due to unlimited food rations or warmer tempera- tures. Laboratorv-reared razorback suckers grew about 0.25 ~m' d-1 (Bundy and Bestgen 2001), pond-reared ones grew about 0.21-0.27 mm . d-I (Papoulias and Minckley 1992), and wild fish from the Green River grew about 0.27-0.35 mm . d-1 (Muth et al. 2000). The faster growth rate of razorback suckers reared at 19SC in this study (0.58 mm TL . d-1) compared to those reared at 200C (0.29 mm TL . d-1) in Clarkson and Childs (2000) is inexplicable because both studies used the same Artemia diet, food was provided ad libi- tum, and razorback sucker densities in tanks were similar (2.5 fish' L-l in this study, 1.3- 2.6 fish' L-I in Clarkson and Childs 2000). High growth rates of the few juvenile razor- back suckers documented in the wild in the Green River Basin suggested that fast growth and survival may be linked (Gutemmth et aL 1994, Modde 1996, Modde et al. 2001). Differences in time for razorback sucker larvae to achieve the hypothetical predation threshold of 25 mm TL under different grO\vth rates suggested that water temperature and food resources may playa role in determining intensity of size-dependent mortality processes and survival rates in the wild. I do not imply that survival is assured once razorback suckers reach 25 mm TL but suggest that fast growth may be important when the early life stages of this species are in the presence of large macro- invertebrates or abundant small-bodied preda- ceous fishes known to prey upon cyprinifoml fish larvae in nursery habitats (Minckley 1973, Ruppert et al. 1993, Horn et al. 1994, Bestgen et al. 2006, Markle and Dunsmoor 2007). Increased availability of nursery areas for razorback sucker larvae that promote fast growth and are relatively free of predators