FOOD COMPOSITION AND SELECTIVITY OF JUVENILE PERCH (PERCA FLUVIATILIS L.) AGAINST FOOD RESOURCES IN THE MOUTH OF THE ODRA RIVER.

In the lower reaches of large rivers considerable densities of zooplankton, including large crustaceans, can successfully develop. We studied food composition and selectivity of juvenile Perca fluviatilis. The material was collected near the left bank of the western branch located in the estuarine part of the Odra. Quantitative and qualitative food composition of perch fry caught in June and July was assessed based on the content of their alimentary tracts. Also zooplankton samples were collected from the water column. Both in June and July, copepods prevailed in the plankton of the shore zone in the river. In the diet of fry in June, however, the most common were cladocerans, while in July - copepods again. In the diet, the group of cladocerans were dominated by female Daphnia of 1.11 to 1.50 mm in length, whereas in situ smaller Daphnia prevailed. Among copepods, in June, mature stages were most common; in July, however, juvenile copepodits prevailed. Feeding juvenile perch have a narrow food selectivity. They preferred Daphnia and females of large copepods, avoiding Bosmina, Diaphanosoma, small Thermocyclops, and copepodits. [ABSTRACT FROM AUTHOR]

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