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Transactions of the American Fisheries Society 121:340-355, 1992
<br />© Copyright by the American Fisheries Society 1992
<br />Effects of Food Availability on Survival and Growth of
<br />Larval Razorback Suckers in Ponds
<br />DIANA PAPOULIASI AND W. L. MINCKLEY
<br />Department of Zoology, Arizona State University
<br />Tempe, Arizona 85287-1501, USA
<br />Abstract. -The reasons for consistent failure of razorback sucker Xyrauchen texamis year-classes
<br />in Lake Mohave, Arizona-Nevada, are not entirely understood. To date, egg and larval predation
<br />by nonnative fishes is considered the most likely explanation, but food limitation may influence
<br />larval mortality. Our experiments in rearing razorback sucker larvae in ponds fertilized at three
<br />levels ("high," "medium," and "low") with mean invertebrate densities of 43.3, 23.7, and 12.5
<br />organisms/L, respectively, did not result in significant differences in larval survival (77.0, 89.8,
<br />and 67.4%, respectively). However, total larval growth was greater at the two higher invertebrate
<br />densities. Biovolume of food but not numbers of food organisms in guts increased linearly with
<br />growth of larvae. Numbers and biovolumes of food items did not differ relative to fertilization
<br />treatment. First foods of larval razorback suckers were diatoms, detritus, and algae. Soon thereafter,
<br />small rotifers and chironomids were taken, and larger organisms, primarily cladocerans, were
<br />selected by larger larvae. Year-class failure of razorback sucker in Lake Mohave may be attributable
<br />to nutritional deficiency at the lowest recorded densities of reservoir zooplankton. However, star-
<br />vation or food-related problems do not seem likely at the higher Lake Mohave zooplankton
<br />densities, which are at least as great as the invertebrate densities in our low-fertilization treatment
<br />ponds.
<br />McAda and Wydoski (1980), Minckley (1983),
<br />Tyus (1987), Lanigan and Tyus (1989), and
<br />Minckley et al. (in press) have addressed popu-
<br />lation declines of endemic razorback sucker Xy-
<br />rauchen texanus in the Colorado River basin of
<br />the western USA. Lake Mohave, a mainstream
<br />reservoir in Arizona-Nevada, supports the largest
<br />known population of this unique species, but the
<br />fish are old (average, >30 years; McCarthy and
<br />Minckley 1987) and there is no evidence of re-
<br />cruitment. Nonetheless, annual reproduction oc-
<br />curs and larvae are produced abundantly only to
<br />disappear when less than 12 mm total length (TL)
<br />(Marsh and Langhorst 1988). Reasons for consis-
<br />tent failure of year-classes are unknown, although
<br />predation by nonnative fishes on eggs and larvae
<br />has been considered the likely explanation
<br />(Minckley 1983; Marsh and Langhorst 1988).
<br />In marine systems, insufficient food is often sug-
<br />gested as a cause of larval fish mortality, either
<br />directly through starvation or indirectly due to
<br />predation on slow-growing or weakened individ-
<br />uals (Hjort 1914, 1926; O'Connell and Raymond
<br />1970; Theilacker 1986). Kashuba and Matthews
<br />(1984) demonstrated nutritional mediation of poor
<br />t Present address: U.S. Fish and Wildlife Service, Na-
<br />tional Fisheries Contaminant Research Center, 4200 New
<br />Haven Road, Columbia, Missouri 65201, USA.
<br />?No
<br />year-class production by shad (Dorosoma spp.) in
<br />Lake Texoma, Texas-Oklahoma. Cellular atrophy
<br />and deterioration of midgut and digestive organs
<br />occurred in a period when zooplankton density
<br />was considered low (about 100 organisms/L).
<br />There is evidence that threadfin shad D. petenense
<br />experienced similar year-class failure in Lake
<br />Mead, Arizona-Nevada, immediately upstream
<br />from Lake Mohave, under conditions of low nu-
<br />trient inputs (Baker and Paulson 1983a). A pos-
<br />sibility thus exists that consistently low zooplank-
<br />ton in Lake Mohave (Paulson et al. 1980)
<br />contributes to recruitment failure of razorback
<br />suckers.
<br />We experimentally evaluated the effects of in-
<br />adequate nutrition on razorback sucker larvae in
<br />the laboratory (Papoulias and Minckley 1990) and
<br />in ponds. This paper describes larval survival,
<br />growth, and patterns of feeding in artificial ponds
<br />in which different levels of food availability were
<br />induced by an array of fertilization regimes.
<br />Methods
<br />In our experiments, we used 12 earthen ponds
<br />at Dexter (New Mexico) National Fish Hatchery
<br />that varied from 0.04 to 0.10 (mean, 0.06 ± 0.01
<br />SE) hectares in surface area and averaged 1 m
<br />deep. Each pond was assigned to one of three
<br />groups according to a stratified block design (Sokal
<br />and Rohlf 1967) to reduce within-treatment ef-
<br />340
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