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I I I f ,\ ] I j ~ :~ 1 , 13 Hendrickson and Brooks (1987) documented predation by yellow bullhead and largemouth bass on Colorado squawfish stocked in the Verde River, Arizona. Marsh and Brooks (1989) reported that catfish predation significantly reduced number of stocked razorback sucker (45-168 mm TL) during a 3-year stocking program in the Gila River, Arizona, even though channel and flathead catfish Pylodictis olivaris were a small portion of the total fish fauna. Other forage available included red shiner and other native suckers. Razorback sucker were probably vulnerable to predation because of disorientation from transport, stress, and novel surroundings. Marsh and Minckley (1989) suggested that the hypothesis that lack of recruitment is attributable to predation is enhanced by high survival of razorback sucker in predator-poor canal habitats. Marsh and Langhorst (1988) concluded predation was the cause for loss of young razorback sucker in an isolated backwater on the shores of Lake Mohave. High water levels and storm-driven waves breached the isolated backwater allowing invasion of nonnative fishes including threadfin shad, common carp, channel catfish, largemouth bass, green sunfish, and bluegill. Predation was documented with the discovery of an average of four razorback sucker larvae in nearly 40% of the green sunfish collected during a 24-hour period. One month after the invasion of nonnatives, razorback sucker larvae could no longer be collected; total mortality was attributed to predation. Marsh and Brooks (1989) suggested that self-sustaining populations of razorback sucker were unlikely in areas of abundant, nonnative predators. However, once beyond the size range of abundant smaller predators like green sunfish, razorback sucker probably attain capabilities to avoid predation from larger predators like largemouth bass and catfish (Marsh and Minckley 1989). Brooks (unpublished data'in Marsh and Langhorst 1988) found a direct relationship for predation of razorback sucker and density of green sunfish at Dexter National Fish Hatchery. In Lake Mohave, common carp were observed feeding in redds immediately after razorback sucker had spawned, but of 201 common carp stomachs examined, none contained eggs. Razorback sucker eggs were found in four of 63 channel catfish stomachs; no other species examined contained either eggs or larvae. Species examined included largemouth bass, rainbow trout, cutthroat trout, and striped bass. Egg predation by common carp was considered insignificant given the large number of eggs available and the lack of eggs in any common carp stomach. The lack of eggs in predator stomach samples may also have been due to mastication, rapid digestion, or regurgitation (Bozek et al. 1984). Competition for food. -- U (1979) identified three requirements for competition to occur between two species: (1) the resource must be limited, (2) the resource must be shared, and (3) the sharing of the resource results in a negative influence on one of the two species. Competition for food is often identified by measuring the amount of dietary overlap of two species and then determining the ayailability of the food If food is in limited supply and overlap is high, then competition may be occurring. Field documentation of food competition is often difficult to obtain. Miller and Hubert (1990) identified several recent