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FEEDING BY LARVAL RAZORBACK SUCKERS at 18°C passed through a "critical period" (May 1974) when shifting from endogenous (yolk) to exogenous (zooplankton) nutrition between 8 and 19 d of age. Under these controlled experiments, larvae reared at food concentrations of 5-10 Ar- temia salina nauplii/L apparently did not obtain sufficient food during the critical period, and high mortality resulted 20-30 d after hatching. Larvae in ponds at Dexter National Fish Hatchery still carried yolk 14 d after hatching; none had yolk at 21 d. Water temperatures in ponds averaged near- ly 6°C lower than in laboratory containers, ex- plaining slower yolk assimilation (Houde 1974). Temperatures in Lake Mohave during hatching and larval occurrence are comparable to those in ponds at Dexter (9-15°C: Marsh 1985). Larvae presumably could survive longer in ponds or in Lake Mohave with less food because of cooler wa- ter. The critical period would also begin later, and perhaps extend longer than the 1 l d indicated un- der laboratory conditions. Nonetheless, larvae of the same sizes and probable ages occurring under conditions of lower concentrations of potential foods in Lake Mohave proper must be within or just past the critical period, and their disappear- ance may in part be attributable to nutritional stress. Larvae achieving 16.0 mm TL in the cutoff bay at even lower levels of prey density, however, must have attained full dependence on exogenous foods despite an inadequate supply, an explana- tion for which is unknown. Percentages of razorback sucker larvae with empty stomachs provide additional evidence that zooplankton abundance in Lake Mohave may be inadequate. In 1985, digestive tracts of 67% of 124 larvae (mean TL, 10.6 ± 0.3 mm) from Lake Mo- have proper and 42% of 75 fish (mean TL, 13.9 ± 2.9 mm) from its cutoff bay were empty (Lang- horst and Marsh 1986; Marsh and Langhorst 1988). In Dexter ponds, only 20% of larvae of sizes comparable to those from Lake Mohave had empty guts, and no larvae of these sizes in the cutoff bay were empty. Further, although sizes of food items were similar in larvae from Lake Mo- have and Dexter, Lake Mohave fish contained only half as many prey as Dexter fish (13 versus 25 per 10.8-mm larvae; 50 versus 100 per 13.9-mm fish). The problem of recruitment failure in Lake Mo- have remains unexplained. Based on larval sur- vival relative to zooplankton standing crops in ponds at Dexter National Fish Hatchery and un- der laboratory conditions (Papoulias and Minck- ley 1990), year-class failure of razorback suckers in Lake Mohave may well be attributable to nu- 353 tritional deficiency at the lowest recorded densi- ties of reservoir zooplankton. However, starva- tion or food-related problems do not seem likely at higher zooplankton densities, unless only a small percentage of the organisms are of appropriate sizes to be eaten. Acknowledgments Personnel of Dexter National Fish Hatchery, especially Buddy L. Jensen and Sharon Coates, provided facilities, assistance, and encourage- ment. Richard O. Anderson, Denny R. Buckler, Paul C. Marsh, and two anonymous reviewers read and improved the manuscript. This study was supported by funding from the U.S. Fish and Wildlife Service, Office of Endangered Species, Al- buquerque, New Mexico, and Arizona State Uni- versity, Department of Zoology. References APHA (American Public Health Association), Ameri- can Water Works Association, and Water Pollution Control Federation. 1980. Standard methods in the examination of water and wastewater, 15th edi- tion. APHA, Washington, D.C. Arthur, D. K. 1976. Food and feeding of larvae of three fishes occurring in the California Current, Sardinops sagax, Engraulis mordax, and Trachurus symme- tricus. U.S. Fish and Wildlife Service Fishery Bul- letin 74:517-529. Baker, J. R., and L. J. Paulson. 1983a. The effects of limited food availability on the striped bass fishery in Lake Mead. Pages 551-561 in V. D. Adams and V. A. Lamarra, editors. Aquatic resource manage- ment of the Colorado River ecosystems. Ann Arbor Science Publications, Ann Arbor, Michigan. Baker, J. R., and L. J. Paulson. 1983b. Limnological data for Lake Havasu intake structure (Granite Reef Aquaduct). Lake Mead Limnological Research Center, University of Nevada, Las Vegas. Burke, T. A. 1977. The limnetic zooplankton com- munity of Boulder Basin, Lake Mead, in relation to the metalimnetic oxygen minimum. Master's thesis. University of Nevada, Las Vegas. Collins, J. P., and J. R. Holomuzki. 1984. Intraspecific variation in diet within and between trophic morphs in larval tiger salamanders (Ambystoma tigrinum nebulosum). Canadian Journal of Zoology 62:168- 174. Edmondson, W. T., and G. G. Winberg, editors. 1971. A manual on methods for the assessment of sec- ondary productivity in freshwater. IBP (Intema- tional Biological Programme) Handbook 17. Eldridge, M. B., J. A. Whipple, D. Eng, M. J. Bowers, and B. M. Jarvis. 1981. Effects of food and feeding factors on laboratory-reared striped bass larvae. Transactions of the American Fisheries Society 110: 111-120.