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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />~ <br /> <br />1:i <br /> <br />18 <br /> <br />(paulson et al. 1980; Marsh and Langhorst 1988). Many larvae in Lake Mohave survive <br /> <br /> <br />past the yolk stage, but often have empty guts when captured, indicating they may not be <br /> <br /> <br />surviving the "critical period" during the transition from endogenous to exogenous <br /> <br />feeding (Hjort 1914; Stewart 1926; May 1974; Marsh and Langhorst 1988). This is in <br /> <br /> <br />sharp contrast to isolated backwaters where larval fish achieve rapid growth (Marsh and <br /> <br />Langhorst 1988). <br /> <br /> <br />As early as the 1940's it was realized that the second hypothesis, predation as a <br /> <br /> <br />result of the wide variety of introduced fishes, may play an important role in the decline <br /> <br /> <br />of the razorback sucker, which historically inhabited an environment with few piscine <br /> <br /> <br />predators (Di1l1944, Minckley et al. 1991). Bluegill and green sunfish prey heavily on <br /> <br /> <br />larval suckers, while common carp and channel catfish captured near razorback spawning <br /> <br /> <br />areas often have their stomachs 'filled with eggs (Minckley et al. 1991). In backwater <br /> <br /> <br />habitats used to rear razorback suckers, all larvae disappeared within a few days <br /> <br /> <br />following an invasion of larvivorous fishes. If larval fish are nutritionally deficient the <br /> <br /> <br />effects are magnified because of an increased vulnerability to predators (Rice et al. 1987). <br /> <br /> <br />Minckley (1983) and Minckley et al. (1991) hypothesized predation as the most <br /> <br /> <br />important contributor to the decline of this species. <br /> <br /> <br />Since larval suckers have the propensity to actively seek currents they are prone to <br /> <br />transport into sub-optimal habitats (Harold Tyus, University of Colorado, personal <br /> <br /> <br />communication), thus the third hypothesis of transport. Little is known about larval <br /> <br /> <br />sucker drift. In Lake Mohave there are strong subsurface density currents, which may <br /> <br /> <br />entrain larvae and physically transport them out of the system, though Marsh and <br /> <br />t~ <br />., <br /> <br />i <br />