Walleye and yellow perch resource use in large lakes invaded by spiny water fleas and zebra mussels

Walleye (Sander vitreus; WAE) and yellow perch (Perca flavescens; YEP; collectively percids) are freshwater fishes threatened by multiple stressors, including aquatic invasive species. Zebra mussels (Dreissena polymorpha; ZM) and spiny water fleas (Bythotrephes cederstromii; SWF) are aquatic invasive species that reduce pelagic zooplankton biomass, an important food resource for both age-0 percids and the prey fish of adult percids. Both percid species are generalist consumers, and it is unknown how they respond to reduced pelagic energy resources associated with invasions. We examined pelagic and littoral energy use in percids from nine large north temperate lakes which vary in invasion status. We sampled adult and age-0 percids from each lake in 2017 or 2018 and analyzed muscle tissue for delta C-13 and delta N-15 isotope ratios. We characterized isotope baselines with littoral and pelagic invertebrates to allow cross-lake comparisons. We estimated the proportion pelagic reliance using Bayesian mixing models and determined the variance contribution of ZM and SWF presence to estimates using model comparison. Pelagic reliance of percids sampled from ZM-invaded lakes was consistently lower than uninvaded lakes, while pelagic reliance of percids sampled from SWF-invaded lakes was greater than uninvaded lakes, although neither effect was statistically distinguishable from zero. Model comparison indicated pelagic reliance of adult WAE, age-0 WAE, and age-0 YEP was influenced by ZM presence, while pelagic reliance of adult YEP was influenced by SWF presence. If percid populations persistently rely on pelagic resources, despite those resources being reduced, there may be negative consequences to growth, survival, or recruitment.