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<br />DJ2482 <br /> <br />razorback sucker. Recruitment failure has been linked with loss of early life-history <br />stages of the fish, and is the result of predation by nonnative fishes (reviewed by <br />Minckley et al. 1991; Johnson et al. 1993). Marsh and Langhorst (1988) reported that <br />larval razorback suckers in Lake Mohave survived longer and grew larger in the <br />absence of predators. Loudermilk (1985) observed that young razorback sucker larvae <br />exhibited little defensive behavior in the presence of potential predators. Johnson et al.. <br />(1993) compared predator avoidance of razorback sucker larvae with that of northern <br />hog sucker (Hypentelium nigricans) and concluded that "larval razorbacks are not likely <br />to survive in habitats that support high numbers of nonnative fishes" (Johnson et al. <br />1993). Smaller species such as red shiner and fathead minnow may attack or display <br />agonistic behavior toward razorback sucker larvae (Karp and Tyus 1989). Young of <br />some of the more aggressive game fish are highly agonistic ( Sabo et al. 1996) and <br />can be expected to consume the relatively naive young suckers (Tyus and Karp 1989). <br /> <br />Razorback suckers are susceptible to predation from several nonnative fishes, <br />Green sunfish, common carp, and flathead and channel catfish have been observed <br />feeding on eggs and/or larval razorback suckers (Medel-Ulmer 1983; Minckley 1983; <br />Brooks 1985; Langhorst 1987, Marsh and Langhorst 1988; Marsh and Brooks 1989). <br />An experiment performed by Karp and Tyus (1989) demonstrated voracious <br />consumption of razorback sucker larvae by several nonnative fishes in 4-minute trials <br />(green sunfish, consumed 90% present; red shiner, 50%; and redside shiner, 10%). A <br />field experiment in Lake Mohave provided indirect evidence of the predation effect by <br />monitoring larvae in habitats with and without predators. Razorback sucker larvae in a <br />predator-free environment grew to 30 mm during the trial, while the size distribution of <br />those exposed to predation was truncated at 10-12 mm (Brooks 1985; Langhorst 1987; <br />Marsh and Brooks 1989). Competition with introduced fishes for food also may be a <br />factor limiting the success of the razorback sucker as suggested by Papoulias (1988) <br />and Papoulias and Minckley (1990), who demonstrated that some recruitment failure of <br />young razorback suckers in Lake Mohave could be caused by starvation. Minckley et <br />al. (1991) provided conclusive evidence that predation of nonnative fishes on young <br />razorback suckers was the primary factor responsible for the near complete recruitment <br />failure of this species. <br /> <br />Direct observations, including stomach content analyses, of predation by <br />nonnative fishes have been reported for many species native to the Colorado River <br />basin, which include the endangered big river fishes (Table 1). The table is <br />supplemented by reports of humpback chub with characteristic bite marks that have <br />been attributed to channel catfish. These marks could not have been made by native <br />cyprinids or catostomids which lack jaw teeth (Kaeding and Zimmerman 1983, Karp and <br />Tyus 1990). The list is extensive and should leave no doubt that predation by. <br />nonnatives is a powerful force. The number of predator species is great, especially for <br />the early life history stages of the razorback sucker. Part of the difficulty in <br />documenting predation in early studies is that the rapid digestion of some of the <br /> <br />10 <br />