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lvlGr? lh *-- 1 ijnq nc rr ?a ?) ?1 Ai? t h Pnvironmental Biolo,;, o: Fishes Vol. 21, No. 1. cq-_?'. i9 R ?Gt ?? Lt pp. 0 Dr W. Junk Publishers. Dordrecht. Feeding and fate of wild larval razorback sucker Paul C. Marsh & Daniel R. Langhorst Center for Environmental Studies and Department of Zoology, Arizona State University, Tempe, AZ 85287, U. S..A. Received 29.10.1986 Accepted 28.4.1987 Key words: Xyrauchen texanus, Imperiled fishes, Colorado River, Zooplankton, Food selection, Predation, Conservation Synopsis a? r V, The razorback sucker (Xyrauchen texanus) is disappearing throughout its native range in the Colorado River basin of western North America. The largest remaining wild population in Lake Mohave, Arizona-Nevada, has shown no recruitment since the 1950s. Although annual spawning is successful and larvae are seasonally abundant, no juveniles have been collected in recent decades. To evaluate the potential role of food availability in determining fate of larvae, fish and zooplankton samples were taken in 1985 from the reservoir and an adjacent, isolated backwater in which larvae were naturally produced. Food availability and primary dietary constituents were similar in both habitats. Reservoir larvae selected Bosrnina spp. (Cladocera) and apparently avoided Copepoda, while larvae from the backwater selected Bosmina, but avoided Rotifera. Larvae from both places showed evidence of selection for certain sizes of zooplankters, but preferred sizes differed between habitats. These differences were neither attributable to larval size nor zooplankton community structure. Nutritional factors such as type, number, or size of available foods do not explain L disappearance of larval razorback suckers from Lake Mohave, since larvae survive to far greater ages and o size in the backwater. Predation by introduced fishes appears a significant cause of larval mortality. Introduction The razorback sucker, Xyrauchen texanus (Ab- bott), is a large catostomid fish endemic to the Colorado River system of western North America. Once widely distributed and abundant (Holden 1980, McAda & Wydoski 1980, Minckley 1983), its numbers and range are now drastically diminished. The largest remaining population in Lake Mohave, a mainstem Colorado River reservoir in Arizona- Nevada, is comprised of old individuals, with no evidence of successful recruitment since the 1950s (Minckley 1983, McCarthy 1986). Although annual winter-spring spawning produces an abundance of larvae, these disappear for unknown reasons at approximately 12 mm total length (TL) by late spring. Explanations for this phenomenon include mortality, in-reservoir migration to unknown places, and transport out of the system. We exam- ined the role of potential nutrient constraints, which could result in high mortality, by comparing food availability and diets of razorback sucker lar- vae in Lake Mohave, from which they disappear, and an adjacent, isolated backwater in which they survived far longer and to a larger size. Methods Lake Mohave (Fig. 1) is immediately downstream