ZOOPLANKTON FEEDING IN COMMON BREAM (ABRAMIS-BRAMA), WHITE BREAM (BLICCA-BJOERKNA) AND ROACH (RUTILUS-RUTILUS) EXPERIMENTS, MODELS AND ENERGY-INTAKE

Three models of the sieving mechanism of the branchial sieve were used to predict the ability to retain zooplankton of three sympatric cypri nids: common bream, white bream and roach. The model predictions were tested with filter-feeding experiments, using three size classes of ea ch species. Results of experiments in darkness corroborated closely wi th the reducible-channel model for common bream (retention in the medi al) channels on the gill arches; the diameter of these channels can be reduced with the lateral rakers), rather well with the unreducible-ch annel model for white bream (the channel diameter cannot be reduced) a nd possibly with the saw-tooth model for roach (retention on the gill slits). Common bream can adjust the mesh size of its branchial sieve, thus achieving a higher flexibility in food uptake than the other two species. In light experiments, roach and the small common and white br eam switched to particulate intake, characterized by a lower retention ability and a higher filtering rate than during gulping. The retentio n ability was used to calculate the percentage of the available zoopla nkton energy that the three cyprinids can retain as a function of the fish's length. This retained energy percentage decreases sigmoidly wit h increasing fish length. At any length between 10-50 cm, common bream has the highest retained energy percentage, white bream the lowest an d roach is intermediate. The population of common bream will therefore be at an advantage in the competition for food when zooplankton is a major food source, like in eutrophic lakes.