Spataru, P., Fish and Aquaculture Research Station, Dor, Mobile Post, Hof Hacarmel 30-820 Israel
Wohlfarth, G.W., Fish and Aquaculture Research Station, Dor, Mobile Post, Hof Hacarmel 30-820 Israel
Hulata, G., Fish and Aquaculture Research Station, Dor, Mobile Post, Hof Hacarmel 30-820 Israel

Natural food components were analysed for six species of fish grown in nine polyculture ponds. The ponds were stocked with common carp (Cyprinus carpio L.), tilapia hybrids (Sarotherodon (Oreochromis) niloticus L. × S. (O.) aureus Steindachner), grass carp (Ctenopharyngodon idella Val), and either silver carp (Hypophthalmichthys molitrix Val.) or bighead carp (Aristichthys nobilis Rich.) or their inter-specific hybrid. The natural food examined and defined was collected both from the guts of fish sampled and from pond water. The proportional amount of phytoplankton found in the guts of silver carp, bighead carp and their hybrid was 88-95%, 0-2% and 28-55%, respectively. The parallel zooplankton values were 4-7%, 75-95% and 32-63%. In ponds stocked with silver carp, bighead carp and their hybrid, the proportional amount of phytoplankton in the pond water was 12-33%, 54-99% and 17-77% respectively. This demonstrates empirically the inverse relationship between the type of plankton preferentially consumed by a given fish and the predominant plankton type present in ponds stocked with that fish. The mechanism of this preferential feeding is presumably associated with the diameter of the filtering net meshes of the gills of these fishes: 36 μm in silver carp, 84 μm in bighead carp and 56 μm in their hybrid. © 1983.