Stable Isotopes and Movement of Walleye Change Following Ecological Shifts Driven by Dreissenid Mussels

The Great Lakes support some of the most productive fisheries for Walleye Sander vitreus in North America, although system-level change from overexploitation, pollution, and invasive species has dramatically changed these fisheries over time. More recently, the widespread establishment of dreissenid mussels (zebra mussel Dreissena polymorpha and quagga mussel D. bugensis) has strongly altered energy flow by sequestering nutrients in nearshore benthic habitats, thereby influencing primary, secondary, and prey fish production with implications for Walleye. While the impacts of dreissenids on the open-water food web are well established, little is known about their influence on fish populations occupying nearshore or embayment habitats. In this study, we tested the hypothesis that ecosystem changes resulting from dreissenid mussels have altered the stable isotope ratios and displacement of Walleye in Little Bay de Noc, an embayment of Lake Michigan. Retrospective isotope analysis of fin spines over a 31-year time series coincident with the invasion of dreissenid mussels suggests that alterations in energy dynamics have increased Walleye reliance on nearshore benthic energy. From reports of jaw-tagged Walleye caught over the same time period, we found significant temporal changes in Walleye displacement distances from the tagging site that were strongly size dependent following the invasion of dreissenid mussels. These results potentially suggest that the spatial distribution of large Walleye shifted in response to prey availability or habitat conditions. Additionally, sex-specific differences in Walleye displacement indicate that females move greater distances than males, possibly driven by higher energetic and reproductive demands. Our findings suggest that the largest Walleye are seasonally leaving Little Bay de Noc, potentially altering the dynamics of the sport fishery in Green Bay and impacting management programs that occur across state lines.