Abstract – Body sizes of young-of-the-year (YOY) perch (Perca fluviatilis) at the end of their first summer are extremely variable and range in different studies between 4 and 15 cm. To analyse whether size divergences in YOY perch may be attributed to alternative use of food resources, adult perch were stocked into two previously fishless ponds and growth, size distribution and food intake of the YOY perch were recorded. In addition to perch, adult bream (Abramis brama) were introduced to produce juvenile bream that could serve as a food resource for YOY perch. The body sizes of YOY perch at the end of the experiment ranged from 32 to 168 mm with a bimodal size distribution. The combination of stomach content analyses and stable isotope signatures revealed that the small size cohort were planctivorous/benthivorous while the large size cohort was piscivorous/cannibalistic. Results implicated that different feeding behaviour contributed to the size divergences in YOY perch and that the extreme growth of the large size cohort was induced by piscivory.