Seasonal Open-Water Diet Composition of Non-Native Yellow Bass in Six Iowa Natural Lakes

Many species within the family Moronidae possess biological traits that facilitate their success as invasive species in freshwater ecosystems. In Iowa, USA, non-native Yellow Bass (Morone mississippiensis) have expanded their range into at least 19 glacial natural lakes, yet their trophic interactions in these complex systems remain poorly understood. From 2018 to 2020, we evaluated the open-water diet composition of 1300 Yellow Bass across six Iowa natural lakes to quantify diet composition, feeding intensity, and ontogenetic dietary shifts. While zooplankton numerically dominated diets across most systems (>80% by number) biomass was driven primarily by benthic invertebrates and fish. Feeding intensity was not uniform, characterized by a distinct suppression of foraging during late spring followed by intense feeding in early summer. Overall, we found that Yellow Bass foraging is highly plastic but heavily constrained by spatial (lake identity, season, and year) and biological (ontogeny, age, and sex) filters. Spatial heterogeneity was the primary driver of diet composition (R2=0.407), with individual lakes explaining the largest portion of variance (R2=0.126). The interaction between lake size and population history (R2=0.054) was also significant, highlighting that the ecological impact of Yellow Bass is context-dependent, differing among established populations in small lakes versus recent invasions in large lakes. We identified distinct ontogenetic breakpoints at 114 mm and 252 mm; fish < 114 mm were obligate zooplanktivores, while significant piscivory was restricted to large adults (>252 mm). These results suggest that the successful colonization of Yellow Bass is supported by high dietary plasticity, which may lead to intensive resource competition with native juveniles. Our findings provide a critical baseline for fisheries managers to assess the ecological risks associated with Yellow Bass expansion and emphasize the importance of monitoring trophic shifts to preserve the integrity of native fish communities in the Midwest.