MECHANISM OF ESTABLISHING BIMODALITY IN A SIZE DISTRIBUTION OF AGE-0 PIKEPERCH, STIZOSTEDION-LUCIOPERCA (L) IN THE SULEJOW RESERVOIR, CENTRAL POLAND

The distribution and food of age-0 pikeperch were investigated in the lowland Sulejow Reservoir in Central Poland. Samples were taken weekly , from mid June to mid August 1994, both in pelagic and littoral zones . Up to mid-July, age-0 pikeperch were found to be divided into two di stinguished groups. The first group was located in the pelagic zone an d consisted of planktivorous specimens; the second group seemed to be restricted to the littoral zone and were predominantly piscivorous alr eady at a very small size (TL similar to 30 mm). These differences in feeding between the two groups of age-0 pikeperch were due to prey fis h availability and resulted in a slower growth of pelagic fish, as com pared with that of littoral fish. In mid-July, after reaching mean len gth of about 50 mm, pelagic pikeperch invaded the littoral zone, but t hey were not able to shift from planktivory to piscivory, as available prey fish were already too big. As larger littoral pikeperch continue d foraging on fish, previously established size differences were furth er strengthened.