Interactions between Walleye Sander vitreus and Smallmouth Bass Micropterus dolomieu in Missouri River reservoirs
Walleye Sander vitreus is a popular sportfish economically beneficial to the fisheries they inhabit but have experienced recent declines throughout their range. As a result, supplemental stocking is common. Smallmouth Bass Micropterus dolomieu have been introduced to many waterbodies outside of its native range to increase angling opportunities. However, expansion of Smallmouth Bass has overlapped with declining Walleye populations, prompting concerns regarding potential competition and predation. Yet, little research has assessed interactions between Smallmouth Bass and Walleye. My study helped to increase the understanding of these interactions by 1) assessing adult Smallmouth Bass and Walleye predation on stocked age-0 Walleye, and 2) comparing diet and stable isotope overlap in Smallmouth Bass and Walleye in systems of varying prey assemblages to assess potential competitive interactions. My first research chapter assessed post-stocking predation on age-0 Walleye by conducting a two-summer intensive diet analysis of Smallmouth Bass and Walleye. We collected predator diets in May (pre-stocking) and June-September (post-stocking) 2019 and 2021 from three bays in Lake Oahe, SD, two were stocked with Walleye (30-32 mm; 255-1,649 Walleye/ha) whereas one served as a reference. We estimated predator population abundance using Schnabel capture-recapture models and used bioenergetics to estimate the percent of stocked age-0 Walleye consumed. We found age-0 Walleye in up to 11.4% of Smallmouth Bass and 14.6% of adult Walleye diets post-stocking. Daily mean percent composition by weight (± 95% CI) of age-0 Walleye in diets peaked at 43.2% (± 35.1%) on 3 days post-stocking (DPS) for Smallmouth Bass and 49.8% (± 97.7%) on 14 DPS in adult Walleye. Following peaks, age-0 Walleye percent composition by weight rapidly declined and approached 0% after 25 DPS. Smallmouth Bass consumed up to 5.7% ± 1.8% of stocked age-0 Walleye whereas adult Walleye consumed up to 0.5% ± 0.4%. My second research chapter assessed diet and isotopic niche overlap between Smallmouth Bass and Walleye in Lake Oahe and Lake Sharpe. We collected predator diets and isotope samples May-September 2019 and 2021 from Lake Sharpe and Lake Oahe, SD, used Morisita’s index to determine diet overlap, and estimated isotopic niche breadth and core niche overlap using Bayesian models. Historical changes in Lake Oahe Walleye isotope values were assessed using general linear models and pairwise comparisons tests. Smallmouth Bass and Walleye exhibited moderate diet overlap (0.48) that varied temporally and was not related to condition. Smallmouth Bass and Walleye shared no isotopic core niche overlap in either lake during 2019 but shared low core niche overlap in Lake Sharpe (0.29) and Lake Oahe (0.18) during 2021. Historically, Lake Oahe Walleye δ13C and δ15N values fluctuated reflecting shifts in diet. Our results are the first documenting the importance of Smallmouth Bass predation on stocked Walleye. Smallmouth Bass and Walleye consume age-0 Walleye frequently within a few days post-stocking. However, overall consumption occurs at a low percent. Managers should consider up to 6.2% loss in stocking fingerling Walleye from resident Smallmouth Bass and adult Walleye predation and may consider alternative stocking densities, locations, or timing to reduce potential predation. Additionally, our results indicate that while Smallmouth Bass and Walleye can occasionally predate on similar prey, they ultimately partition resources, suggesting declines in either species are unlikely due to food competition.
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