MILSTEIN, A., Agricultural Research Organization, Fish and Aquaculture Research Station, Dor, Israel
HEPHER, B., Agricultural Research Organization, Fish and Aquaculture Research Station, Dor, Israel
TELTSCH, B., Mekoroth Water Co., Central Laboratory, Nazareth, Israel

Abstract. Interactions between fish species and their effect on the ecological conditions in the fish pond were studied in ten ponds of 0.1 ha each. The ponds were stocked with bottom‐feeding fish: common carp, Cyprinus carpio L., and male hybrid tilapia Oreochromis niioticus (L.) x O. aureus (Steindachner). and a filter‐feeder: silver carp, Hypophthalmichtys molitrix (Valenciennes), in various combinations. This paper deals with the effect of different assemblages of these fishes on the phytoplankton populations in the ponds. A Principal Component Analysis of the data showed that only 20% of the phytoplanktonic variability is accounted for by the fish combination present in the pond (treatment). The first principal component (PCI) groups the species of phytoplankton which responded to treatment. This component is formed by small size species; Scenedesmus spp., small Chlorophytes (mainly Chlorella), Selenastrum minutum, Ankistrodesmus setigerus, Merismopedia minima and Diatoms of the order Pennales. The presence of silver carp led to an increase in total phytoplankton numbers, concurrently with a decrease in their dominant size. This was due to the predominance in the water of the small size species of the PCI group, which could not be retained by the gill filtering apparatus of the fish. Reduction in zooplankton abundance by silver carp also contributed to this situation. The presence of bottom‐feeding fish resulted in a decrease in total phytoplankton numbers, and in the importance of the small‐size species of the PCI group, and hence in the dominance of larger algae. The interactions between these two trophic types of fish and algae size are discussed. Copyright © 1985, Wiley Blackwell. All rights reserved