A multifaceted approach for assessing potential competition between Smallmouth Bass Micropterus dolomieu and Walleye Sander vitreus in Lake Oahe, South Dakota

Smallmouth Bass Micropterus dolomieu is a popular sportfish introduced to many waterbodies outside of its native range to increase angling opportunities. Increases in Smallmouth Bass abundance has overlapped with Walleye Sander vitreus populations, an economically important harvest-oriented sportfish native in the Midwest. Walleye have experienced declines throughout their range corresponding with increasing Smallmouth Bass abundance, prompting concerns regarding competition. Our objectives were to assess potential competition between Smallmouth Bass and Walleye by evaluating diet overlap, community isotopic niche overlap, and historical changes in Walleye isotope signatures associated with increasing Smallmouth Bass abundance. We collected predator diets and isotope samples May-September 2019 and 2021 from Lake Oahe, SD and used Morisita's index to estimate diet overlap, isotopic niche breadth, and core niche overlap. Smallmouth Bass and Walleye exhibited moderate diet overlap (0.48) that varied temporally, with lower (0.33) diet overlap during June and higher overlap (0.67) during September. Walleye primarily consumed fishes and Ephemeroptera whereas Smallmouth Bass consumed a greater diversity of prey and relied more on Hemiptera. Periods of higher diet overlap coincided with higher consumption of Gizzard Shad by both species but body condition was not related to diet overlap. Smallmouth Bass and Walleye shared no isotopic core niche overlap during 2019 but shared low core niche overlap (0.18) during 2021. Historically, Lake Oahe Walleye 615N values were higher and 613C reflected a shift in feeding from the pelagic to littoral zone in 2007-2009 compared to 2001-2002 that was unrelated to Smallmouth Bass relative abundance. Isotope values in 2019 and 2021 were similar to those in 2001-2002 that may be the result of the cyclical nature of pelagic prey populations within Lake Oahe. Our results indicate resource partitioning between Smallmouth Bass and Walleye across multiple prey assemblages, suggesting population changes of either species are unlikely due to competitive interactions.