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<br />Introduction <br />Starvation early in life has been suggested as a potential cause <br />of significant mortality in fishes, and thus a major factor in <br />reducing recruitment to adult populations (Hjort 1914, 1926, May 1974, <br />Cushing 1976). Larval fishes are especially vulnerable at the time of <br />switching from endogenous yolk reserves to exogenous sources of <br />rrutrition. During this transition, termed the "critical period" (May <br />3 <br />1974), they must encounter and capture foods of the proper size, <br />quality, and in sufficient quantity to sustain life and prmnote growth <br />and development. <br />The razorback sucker, Xyrauchen texarnis (Abbott), a large, endemic <br />species of the Colorado River basin, seems to be disappearing due to <br />chronic Year-class failure QkAda & Wydoski 1980, Minckley 1983, Tyus <br />1987, Lanigan & Tyus in p_ress). In Lake Mohave, Arizona Nevada, mean <br />size of captured larval razorback suckers is 10.6 mm total length <br />(TL), stamachs are mostly empty, lake zooplankton densities tend to be <br />F <br />low and variable at the time of razorback sucker hatching (Paulson et <br />al. 1980, Langhorst & Marsh 1986), and no recruitment to the adult <br />population has been detected despite extensive sampling (Minckley <br />1983, McCarthy & Minrkley 1987). Three laboratory experiments were <br />thus performed to investigate effects of food availability (absence, <br />delayed presentation, and quantity) on mortality and growth of larval <br />razorback sucker during and following the transition period fray <br />endogenous to exogenous feeding.