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16 ,,. '. WESTERN NORTH AMERICAN NATURALIST [Volume 68 could reduce the time larvae are susceptible to size-dependent mortality from abundant predators and may enhance survival and recruitment (Marsh and Langhorst 1988, Hice et al. 199.3, Bestgen et al. 1997, 2006, Modde et al.2001). Primary factors affecting fish growth are water temperature and the quality and quan- tity of food (Weatherly and Gill 1987). Growth of razorback sucker larvae fed different diets and rations has been described (Papoulias and Mickley 1990, 1992), temperature preference of juveniles and adults is known (Bulkley and Pimentel 1983), and effects of 100, 140, and 200C water temperatures on development and growth of early life stages has been investi- gated (Marsh 1985, Clarkson and Childs 2000). I assessed growth of early life stages of razor~ back sucker at warmer water temperatures (I6.50-25.50C) and ad libitum food abundance to simulate gmwth conditions in Green River floodplain wetlands, where water temperatures exceed 200 -250C by late June and where food is abundant (Modde et aI. 2001, Christopher- son et aI. 2004, Modde and Haines 2005). A response SUIT.'lCe model qmmtifies the relation- ship between water temperature and grmvth of early life stages of razorback sucker, infor- mation which may assist in tlle recovery of this endangered species. METHODS Razorback sucker embryos were obtained in May 1996 from a U.S. Fish and Wildlife Service hatchery in Grand Junction, Colorado. Embryos were transported to Colorado State University, and they hatched 6 or 7 days post- fertilization in flow-through Heath trays. Fish were maintained at 180C and reared in fine mesh cages. Newly hatched brine shrimp nau- plii (Anemia sp.) were offered ad libitum twice per day beginning 5 days post-hatch, and 1st feeding was noted at 9 days post-hatch. Five healtllY 1st-feeding larvae were allocated to each of 12 tanks: 3 tanks were assigned at ran- dom to each of the 16.50, 19.50, 22S, and 25.50C treatments. Larvae were acclimated over a period of several hours (2 hours per 10C change from 180C rearing conditions) to treat- ment temperatures lmd placed in aerated 2-L flow-through containers. Treatment tempera- tures generally were chosen to reflect the range of conditions that newly emerged razorback sucker larvae may encounter in Upper Col- orado. River Basin streams or the floodplain (Muth et aI. 2000, Modde et aI. 2001). Water temperatures in treatment tanks were moni- tored several times per day throughout the test period and were maintained within 0.20C of the target; only nominal temperatures were used in analyses. Total lengths (TL, nearest 0.1 mm) of an additional 8 razorback sucker larvae (few fish were available) were measured at 1st feeding (9 days post-hatch) with an ocular micrometer fitted to a dissecting microscope. Their mean length was the benchmark upon which length changes at 23 and 37 days post-hatch (14 and 28 days post-exogenous feeding, respectively) were compared for each treatment. On day 23 post-hatch, all larvae were lightly arJaes- thetized with MS-222 (25 mg . L-l), mea- sured, and returned to tlleir respective tanks. On day 37 post-hatch, all larvae were sacri- ficed ,,-ith an overdose of MS-222, similarly measured, and weighed to the nearest 0.001 g. Growth experiments were discontinued after 28 days post-exogenous feeding because lar- vae were approaching the size at which small tank size mayhave limited growth. Mean total length of razorback sucker lar- vae in tarlks at 9, 23, arId 37 days post-hatch (0, 14, and 28 days, respectively, post-lst- feeding) and mean weight were used as exper- imental response variables. To avoid potential pseudoreplication, I used tanks as the experi- mental units rather than individual fish, even though a statistical tank eHect was not evident. Mean TL of razorback sucker in each tank (i.e., growth) was compared to water tempera- ture and days post-hatch (including squared and interaction ternls) to estimate a response surface function. A quadratic regression model was used to explore different water tempera- ture-growth scenarios over the entire 16.50- 25.5"C range. All statistical analyses were per- formed with SAS (Statistical Analysis Systems, version 8.0, SAS, Inc., Cary, NC). I also solved the response surface equation described above for growth as a function of water temperature and days post-hatch to de- termine the number of days required for razor- back suckers to achieve 25 mm TL, a size at which razorback sucker larvae may be immune to predation from some small-bodied, gape- limited cyprinid predator fishes (Bestgen et al. 2006, Markle and Dunsmoor 2007). I also