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70 <br />at very low plankton densities, larvae grew to a mean TL of about 16.0 <br />mm in 60 d. In cmqparison, larvae at Dexter achieved 16.0 nun TL in <br />unfertilized ponds 49 d from hatching. In laboratory experiments, <br />larvae grew to 16.0 mm in 57 d from hatching when fed at a rate of 50 <br />Artemia Salina nauplii./L (Papoulias 1988). Perhaps density of larvae <br />in the backwater was very low and, if as suggested above, razorback <br />sucker larvae are efficient in finding scarce prey, they were capable <br />of searching out enough food to survive and grow. The slightly slower <br />growth to 16.0 mm compared with that at Dexter (60 vs. 49 d) may <br />support this. <br />Papoul.ias (1988) concluded that larvae reared in the laboratory <br />at 1800 passed through a "critical period" (May 1974) between 8 and 19 <br />d of age when shifting from endogenous (yolk) to exogenous <br />(zooplankton) nutrition. Larvae in ponds at Dexter NFH still carried <br />yolk 14 d after hatching; none had yolk at 21 d. Water temperatures <br />in the ponds averaged nearly 60C lower than in laboratory experiments, <br />explaining the slower yolk assimilation (Houle 1974). Temperatures in <br />Lake Mohave during hatching and larval occurrence are comparable to <br />those in ponds at Dexter NFH (9 to 150C; Marsh 1985). Larvae could <br />therefore survive longer in ponds or in lake Mohave with less food <br />because of cooler water temperatures. The critical period would also <br />begin later, and perhaps extend longer than the total of 11 d <br />indicated under laboratory conditions. Under controlled experiments <br />(at 180C), larvae reared at food concentrations of 5 and 10 Artemia <br />sal nauplii/L apparently did not obtain sufficient food during the