Size-dependent ontogenetic diet shifts to piscivory documented from stable isotope analyses in an introduced population of largemouth bass

Piscivorous largemouth bass (Micropterus salmoides) have been introduced in several regions outside of their native range in North America, resulting in significant disturbance to native fish communities. This species exhibits an ontogenetic diet shift from zooplanktivory to piscivory as juveniles. An early switch to piscivory allows 0+ bass to increase their growth rate prior to winter, resulting in reduced mortality. However, little is known about the dietary switch at the population level during the first year. We used carbon stable isotope analyses to examine the diets of age 0+ individuals in Lake Izunuma, Japan. The onset of the shift to piscivory occurred at a smaller size than in native or other non-native areas [> 40 mm total length (TL)]. We found a positive correlation between TL and delta C-13 throughout summer and autumn. Small individuals had delta C-13 values that were similar to those of zooplankton, whereas large individuals had delta C-13 values that were similar to those of cyprinid prey species. This suggests that the smaller 0+ individuals remain zooplanktivorous until late autumn, whereas the larger individuals shift to piscivory as early as June, soon after the breeding season ends. Our results also suggest that a significant number of 0+ bass failed to switch to piscivory until the winter of their first year, despite the smaller size threshold for the onset of piscivory.