The Fall Diet Of Smallmouth Bass and Walleye In The Middle Columbia River: Influence Of Juvenile American Shad and Implications For Predation On Juvenile Salmon

American shad Alosa sapidissima in the middle Columbia River (MCR)—a nonindigenous high energy food available in the summer and fall—may be contributing to the increased growth and enhanced condition of nonnative piscivores. To test this hypothesis, we quantified the late summer and autumn diets of smallmouth bass Micropterus dolomieu and walleye Sander vitreus in the three lowermost reservoirs on the Columbia River—Bonneville, The Dalles, and John Day. The diet of smallmouth bass (SMB) was fairly similar among locations, with crustaceans and fish being the dominant prey groups by percent mass. Fish were always the dominant prey item for walleye (WAL) at all areas sampled. Juvenile American shad composed a significant portion of the diet of SMB and WAL in some areas at certain times.  We evaluated temporal changes in condition of SMB and WAL relative to their diet by determining their relative weights, liver somatic indices, gonadal-somatic indices, and fat content.  Collectively, our results are the first to describe the diet and condition of SMB and WAL over a large spatial area in the MCR during late summer and fall. We hope to determine whether juvenile American shad in the diet of these predators contributes significantly to their condition prior to the onset of winter, perhaps making them more effective predators on juvenile salmon the following spring. Our results will help fill some gaps defining the food web and should be useful for future discussions regarding predation and fish management in the Columbia River.