Impact of silver and bighead carp (Cyprinidae) on plankton and water quality in fish ponds

The effect of the filter-feeding fishes - silver carp, Hypophthalmichthys molitrix and bighead carp, Aristichthys nobilis - in polyculture on the trophic state and plankton communities of experimental fish ponds was studied in the course of two years (1993-1994). All ponds were highly eutrophic. Nevertheless, control fish ponds had higher phytoplankton biomass and density then carp treatments. Zooplankton density was reduced in the presence of filter-feeding fishes. During 1993, numbers of microfilter-feeders (Cladocera) and macrofilter-feeders (Copepoda) decreased markedly in carp treatment with higher stocking rate. As zooplankton is considered effective in controlling phytoplankton, zooplankton feeding, together with fast circulation of phosphate, is a key factor in the failure of planktivorous carps to control algae. More work needs to be done to determine the most appropriate fish stocking density for water quality control.