Examining Ontogenetic Diet Shifts, Growth Rates and Reservoir Characteristics for Insights Toward an Understanding of Variable Largemouth Bass Recruitment

For young fish of many species, the progression from planktivory to piscivory improves survival by increasing growth and reducing mortality risk. To examine the ontogenetic diet shift we combined information about growth rates (from analysis of otoliths) and diets (from analysis of stable isotopes) to characterize the timing of the switch to piscivory in juvenile Largemouth Bass (n=240; 27 – 150 mm total length) collected from 6 Ohio reservoirs during 2012.  Patterns observed in these data reinforced the importance of large scale, watershed level mechanisms at work in the responses of our study species.  These data indicate that highly productive reservoirs located within agricultural watersheds produce juvenile bass that display a discrete jump in δ15N signature at the onset of piscivory, and thus indicated a truly distinct ontogenetic switch.  Juveniles in these highly productive reservoirs also displayed faster growth than conspecifics in our less productive study systems. Juveniles in less productive systems, i.e. reservoirs characterized by more forested watersheds, displayed δ15N signatures that increased linearly with fish size.  This suggests a gradual shift and a continued reliance on a mixed diet of benthic macroinvertebrates, large zooplankton and some fish in the diets of juvenile bass from systems with lower productivity.