The effects of water level on the early life history of fish in Lake Francis Case, South Dakota, were studied by comparing the abundance, growth, diet, and food supply (zooplankton) of selected species in a low-water year (1974) and a high-water year (1975). Young of 21 fish species were collected. Yellow perch (Percaflavescens), white bass (Morone ch•7sops ), buffaloes (two Ictiobus species), and centrarchids were significantly more abundant in net catches in 1975 than 1974. Flooded shoreline vegetation in 1975 apparently enhanced spawning success by providing a more suitable substrate and protective cover for early life stages. First-summer growth of the four most abundant fishes (yellow perch, buffaloes, and white bass) did not change significantly between years. Standing crops of the larger zooplankton (calanoids and daphnids) decreased during the high-water year, perhaps because of predation by age-0 fish. Standing crops of the smaller zooplankton species such as Bosmina longirostris and Chydorus sphaericus increased during the high-water year, as did the overall diversity of the zooplankton community. We conclude that management of water levels in Missouri River main-stem reservoirs to include one highwater year out of every three would greatly enhance the lake's fishery resources. Lake Francis Case, South Dakota, is one of six main-stem Missouri River reservoirs constructed and managed by the United States Army Corps of Engineers for flood control, hydroelectric power, irrigation, downstream navigation, and other purposes. Reproduction of fish in Lake Francis Case has varied widely among years. A direct relation has been observed between water levels in spring and early summer and the numbers of age-0 fish in summer seine catches (Gasaway 1970; Walburg 1977). Reproductive success appears to increase when spring water levels are high, and to decrease when they are low. The objective of this study was to investigate in more detail the effects of spring and summer water levels on the 1 Reprints should be requested from C. Walburg. 2 Present address: Columbia National Fishery Research Laboratory Field Research Station, United States Fish and Wildlife Service, Post Office Box 139, Yankton, South Dakota 57078. s Present address: Southeastern Reservoir Investigations, United States Fish and Wildlife Service, Highway 123, Clemson, South Carolina 29631. 4 Present address: East Central Reservoir Investigations, United States Fish and Wildlife Service, Federal Building, Bowling Green, Kentucky 42101. early life history of fish by comparing the abundance, diet, growth, and food supply (zooplankton) of selected species in a low-water (1974) and a high-water year (1975). At summer pool elevations, Lake Francis Case has a length of 161 kin, an average width of 2.1 km, a surface area of 377 km 2, and a mean depth of 15 m. Physical, chemical, and biological characteristics of the lake were described by Benson (1968), Gasaway (1970), Martin and Novotny (1977), and Walburg (1977). Methods Previous studies have shown that mos...

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