Food Habits of Walleyes in Lake Oahe, South Dakota

Food habits of young-of-the-year (YOY} walleyes were determined in Okobojo Bay of Lake Oahe, South Dakota, from June through September, 1991. Walleyes initially fed on zooplankton. Copepods, Diaptomus and Cyclops, were initially important for zooplanktivorous walleyes with Diaptomus being elected for and Cyclops elected against. Daphnia became important on 17 June and were elected for through September. Walleyes became piscivorous by 17 June. Smallmouth buffalo were initially the most important prey fish, but rainbow smelt became important as walleyes moved from the littoral zone of the bay to open water. Macroinvertebrates were not a substantial portion of the diet of YOY walleyes. The only macroinvertebrates found in walleye stomachs were ephemerids and chironomids, which were also found to be prevalent in Ekman grab samples. Walleye food habits were also determined at nine stations throughout the South Dakota portion of Lake Oahe in August, 1991. Walleyes were primarily piscivorous; however, food habits differed by walleye size and throughout the reservoir. Consumption of rainbow smelt was limited to three sites in the lower portion of Lake Oahe in August due to water temperatures which restricted smelt distribution. Above the Cheyenne River, rainbow smelt became less important and the diet of walleyes was primarily comprised of freshwater drum, yellow perch, and white crappies. Macroinvertebrates were not a substantial portion of the diet of walleyes in August. Walleyes preyed predominately on YOY fishes except at Peoria Flats and Cow Creek where adult rainbow smelt were consumed. Most prey fishes were consumed in approximately the same proportion as in the environment (P=0.0001, r=0.60). The exceptions were walleyes, spottail shiners, and goldeyes which were always elected against and rainbow smelt which were positively elected. Growth of walleyes (age 2-4) was generally highest for sites on lower Lake Oahe and decreased at sites further up the reservoir. Growth may have been influenced by the predominance of rainbow smelt in the diet of walleyes at the lower sites, but could also be related to temperature differences or prey fish availability. Young-of-the-year walleyes had slower growth rates at sites where zooplankton were prevalent in the diet