EFFECTS OF ROACH RUTILUS-RUTILUS ON 2 PERCIDS, PERCA-FLUVIATILIS AND GYMNOCEPHALUS-CERNUA - IMPORTANCE OF SPECIES INTERACTIONS FOR DIET SHIFTS

Biomass of cladocerans decreased with increasing roach density in September but not in October, probably due to the lower biomass of cladocerans in October. Biomass of copepods was not related to roach density. Growth of perch and roach decreased with increasing roach density, whereas growth of ruffe did not change with roach density. Roach density did not affect the diet of perch, whereas roach consumed a higher proportion of plant material at high roach densities. Diet of ruffe was not affected by roach density. A high density of a planktivore (roach) should be able to force perch to shift from zooplankton to benthic macroinvertebrates. To explain the lack of a diet shift the author developed a hypothesis about how the simultaneous effects of 2 competitors (roach and ruffe) on a 3rd species (perch) influence the expected diet shift of perch. -from Author