ASSESSMENT OF NATURAL DIETS IN THE ADULTS OF TWO SMALL BODY NATIVE FISHES AND FINGERLINGS OF TWO EXOTIC SPECIES INHABITING THANA LAKE, LUSHNJA-ALBANIA

Thana lake (Albania) is populated with 16-18 fish species, from which 9-11 are native species and 7 non-native species. Three non native species were introduced in the lake spontaneously whereas four other species were introduced in consciousness, in the framework of repopulation programs for the purpose of fishery activities. For a period of 12 months we have studied the diets and some ecological feeding indices for two native species, bleak and common roach adults and two non native species, bighead carp and grass carp fingerlings. We proved that the relative content of zooplankton in bleak diet was 76.81% when in bighead carp diet it was 73.98%. Average value of diet overlap index between these two species was aab = 0.661+0.156, showing the existence of food competition. In the diet of common roach, macrozoobenthos had the highest value of frequency (40.97%). The frequency of occurrence of zooplankton was 29.44% when the vegetal component 13.3%. In the diet of grass carp fingerlings the relative frequency of algae was 61.38%, when for the macrophyta was 18.24%. Zoobenthos frequency was 6.62%. The average value of diet overlap frequency between common roach and grass carp was aab=0.464+0.119, showing moderated competition between them.