No Fish No Problem: Diet plasticity as an invasion driver in Japanese smallmouth bass

Diet plasticity of invasive apex predatory fish is thought to potentially amplify their negative effects on native species. Smallmouth bass ( Micropterus dolomieu ), an invasive sport fish introduced globally, are a known fish predator that have caused native fish decline in a host of invaded ecosystems. In Japan, there is currently concern as smallmouth bass continue to expand their range and predate on native fish. Although smallmouth bass diets have been studied in detail and show variability between fish, crayfish, and insect prey, an investigation of diets in severely prey-fish limited ecosystems is lacking, and the ability of smallmouth bass to persist in prey-limited ecosystems is unclear. To address this knowledge gap, smallmouth bass diets and growth rates were studied for 2-years in Lake Katsura, Japan, which is currently thought to have nearly no prey-fish availability. Smallmouth bass diets were comprised primarily of small freshwater shrimp and terrestrial insects. Only four out of 446 stomachs contained fish prey, all of which were con-specific smallmouth bass. A size influenced dietary shift was observed as smallmouth bass size showed a positive effect on terrestrial prey ratio in both years. Smallmouth bass growth rates were below the 40 th percentile for all age classes and showed especially reduced growth in older fish. These results represent the first know smallmouth bass population in which prey-fish predation was not observed. This indicates that invasive smallmouth bass populations are able to shift their diet when prey-fish become scarce, and can persist in prey-fish limited environments when alternative prey is available.