GAPE-SIZE DEPENDENT FEEDING OF AGE-0 PERCH (PERCA-FLUVIATILIS) AND AGE-0 ZANDER (STIZOSTEDION-LUCIOPERCA) ON DAPHNIA-GALEATA

Based on fish consumption estimates, the midsummer decline of Daphnia has been attributed frequently to predation by age-0 fish. From studie s on food selectivity, however, little evidence exists that small, gap e-limited age-0 fish strongly influence Daphnia populations. Therefore , we measured gape sizes and compared gape-size dependent feeding in a ge-0 fish of two related percid species, perch (Perca fluviatilis) and zander (Stizostedion lucioperca). We sampled fish and Daphnia in the biomanipulated Bautzen reservoir (Germany) during spring and early sum mer over 2 years. The small gape size of newly hatched fish prevented predation on daphnids until mid-May. Later, feeding intensity on daphn ids by fish (expressed as the index of relative importance IRI) closel y corresponded to the Daphnia density, independent of gape size of fis h. However, the gape size of fish predicts both species-selective and size-selective feeding on daphnids in perch and zander. Only perch and zander > 20-25 mm TL fed on larger-than-average daphnids and positive ly selected daphnids, independent of Daphnia density. Although growth of fish and Daphnia densities differed between both years studied, slo pes and intercepts of the linear relationships between gape size and s electivity for daphnids did not differ. Consequently, only in those ye ars in which age-0 fish are sufficiently large to consume large, matur e Daphnia in early summer, can we expect a midsummer Daphnia decline.