Invasive spiny water flea (Bythotrephes cederströmii) and their impacts on young-of-year walleye (Sander vitreus) growth

Abstract

Invasive species are a major threat to aquatic ecosystems, costing an estimated $3.6 billion in impacts across Ontario. In particular, the predatory cladoceran Bythotrephes cederströmii (Bythotrephes hereafter) has had significant trophic impacts on invaded ecosystems, such as decreasing zooplankton density and biomass. Changes to the zooplankton community can impact ecosystem energy rates and dominant pathways of transfer to other organisms in a “middle-out” trophic cascade, altering predator-prey dynamics for both algae and fish. Here, I used relevant zooplankton abundances characteristic of invaded and non-invaded lakes and a bioenergetics model to evaluate whether the Bythotrephes-induced changes to native zooplankton is expected to alter larval walleye consumption (and therefore rates of growth). I then used observational data to evaluate the impact Bythotrephes has on the zooplanktivorous young-of-year (YOY) walleye. I used a back-calculation model and linear mixed effect modeling to evaluate differences in YOY walleye growth between invaded and non-invaded waterbodies, as well as within invaded waterbodies pre- and post-invasion. In the first data chapter (chapter 2), a functional response and bioenergetics model revealed larval walleye grow slower in invaded mesotrophic lakes compared to non-invaded mesotrophic lakes, and models suggest they are unlikely to survive in invaded oligotrophic lakes based on available zooplankton. In the second data chapter (chapter 3), I found that relative to similar sized non-invaded lakes, YOY walleye grow slower in small, invaded lakes, whereas large lakes show less severe effects. Overall, Bythotrephes has differing impacts on YOY walleye growth depending on lake characteristics such as lake size and trophic state. As growth rates are ultimately linked to reproduction, recruitment, and production, understanding growth rates post-invasion is essential for fisheries managers to develop and anticipate adaptive management strategies.