Effect of Free-Floating Macrophyte, Azolla pinnata, on WaterPhysico-Chemistry, Primary Productivity and the Production of NileTilapia, Oreochromis niloticus, L., and Common Carp, Cyprinus carpioL., in Fertilized Earthen Ponds

摘要

In subtropical fish ponds many ecological processes depend on aquatic macrophytes, especially free-floating plants. The hypothesis that different free-floating macrophytes would be linked with different biological mechanisms especially out-competing plankton was tested here. The effect of free-floating plant, Azolla pinnata, cover on water physico-chemistry, phytoplankton biomass, zooplankton abundance, and fish growth in fertilized fish ponds was compared experimentally. In 250 m super(2) fish ponds, Azolla, plants covered 0%, 25%, 50%, or 75% of pond surface area and the ponds were stocked with 250 Nile tilapia, Oreochromis niloticus (L.) (15.5 plus or minus 1.0 g), and 100 common carp, Cyprinus carpio L. (25.6 plus or minus 1.5 g), for 165 days. The dissolved oxygen, pH, water conductivity, phosphate, and nitrate concentrations decreased with increasing Azolla cover, especially at 50% and 75% covers. Chlorophyll a content, phytoplankton numbers and zooplankton density decreased significantly with increasing Azolla cover. Competition of Azolla plant with phytoplankton involved shading and nutrient removal from the water. There was no significant (P > 0.05) difference in the production of Nile tilapia and common carp at control and 25% Azolla cover (746 and 905 kg/ha and 706 and 874 kg/ha, respectively). The lowest fish production was obtained at 75% Azolla cover (P < 0.05). It is recommended that the fish pond could be used to culture and produce beneficial free-floating plants and fish. Moreover, free-floating macrophytes could be used to control nutrient enrichments and eutrophication. In all cases, the cover of free-floating macrophytes should not exceed 25% of pond surface area to obtain a balanced fish culture ecosystem.