Effect Of A Mixed Culture Of Silver Carp (Hypophthalmichthys Molitrtix) And Nile Tilapia (Oreochromis Nilotica) On Phytoplankton Community And Eutrophication In A Tropical Reservoir: A Large Enclosure Experiment

In fishery of tropical reservoirs, the mixed culture of silver carp (Hypophthalmichthys molitrtix) and Nile tilapia(Oreochrornis nilotica) is a popular culture model. Tounderstand the phytoplankton and their relationship to fish culture in tropicalreservoirs, we examined the effects of three mixed culture biomass levels (0, 16.7, 50g.m^-3) on phytoplankton communities and water quality in a tropical eutrophicreservoir, Gancun Reservoir, located in ZhanJiang, China, through a large enclosureexperiment. Silver carp and tilapia were cultured with a biomass ratio of 2:3. Fishdensity for biomass for mixed culture 16.7 g.m^-3 was corresponding to the actualdensity in the reservoir. Each enclosure with a water volume of 340m³, each treatmentand control have three replicates. In total, 9 large enclosures were used for theexperiment.In all the experimental enclosures, the phytoplankton community was a type ofChlorophyta-Cyanophyta-Bacillariophyta, and the Chlorophyta was the primarydominated algal group. Specific composition and dominanted species were notmodified by treatments. Water transparent was negatively correlation withphytoplankton biomass and Chl.a concentration. The water transparent in treatmentenclosures was improved with cultured fish density. The precipitation was anotherfactor affecting water transparent. The mixed culture of silver carp and Nile tilapiasignificantly reduced pH value and chlorophyll a concentration,, and the suppressionof chlorophyll a concentration finally resulted in decrease in trophic state. The mixedculture of fish didn't significantly affect TN and TP concentrations at the beginningperiod of the experiment, however, an increase of TN and TP concentrations withcultured fish was observed in the treatments two month later. The phytoplanktonbiomass was not significantly different between the treatment and control at thebegining, but the phytoplankton biomass in high-fish density treatment decreasedlargely in contrast to no-fish and low-fish density treatment in the late period. In thetreatments, the size composition of phytoplankton biomass was significantly differentfrom that in controls. In the treatments, the biomass of picophytoplankton that theirsize is less than 20μm increased significantly with fish biomass, and the large-sizephytoplankton that their size is larger than 100μm decreased largely. This variation ofsize fraction biomass for phytoplankton resulted from selective filtration or grazing ofsilver carp and Nile tilapia. As the growth of cultured fishes was limited by food in the high density treatments, their bodymass showed a slight decrease. Therefore, fishculture in tropical eutrophic reservoirs can improve the water quality and stimulatesmall size phytoplankton to dominate the community structure, the reseasonalbiomass density for fishery is less than 50 g.m^-3.