The effect of fish species combination in fish ponds on plankton composition

Abstract. In continuation of an experiment carried out in 1983, the effect of polyculture of bottom-feeding fish (common carp. Cyprinus carpio L.; tilapia hybrids, Oreochromis nilolicus (L.) × O. aureus (Steindadner); and grey mullet, Mugil cephalus (L.) and a filter-feeding fish (silver carp. Hypopluhalmichtys molitrix (Valenciennes)) on the nannoplankton and netplanklon has been studied. This effect was analysed through principal component analysis. The results confirmed the linding of 1983 that the presence of silver carp in a pond, either alone or with bottom-feeding fish, results in a decrease in netplankton (phylo- and zooplankton) and an increase in nannoplankton which passes through the tiller apparatus of the fish. The effect of the silver carp seems to be related more to its numerical density than to its individual weight, since the effect diminished with the decrease in density during 1983–1985. It became least pronounced when the density was 400/ha. Bottom-feeding fish enhance the development of the larger nannoplankton by bringing up nutrients from the bottom, but reduce the number of the netplankton (mainly zooplankton) as compared to the control pond without lish. This effect was more apparent at a higher density of tilapia. but was masked when silver carp was present.