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

Zooplankton links the fish and algae in the process of Top-down and Bottom-up, and plays a key role in sustaining the ecosystem health. Heavy stocked fish can suppress largely zooplankton communities, and consequently break down zooplankton based food chains in reservoirs. Tilapia and silver carp are the most common fishes in tropical reservoirs; their mixed culture is a typical mode for fishery tropical reservoirs. To study impact of this typical culture mode on zooplankton, we used a large enclosure experiment with a mixed culture of nile tilapia and silver carp in a tropical reservoir with high fishery yield, Gancun Reservoir (21°16'N, 110°30'E), from March 27^th to June 30^th, 2006. The fishes were stocked with a biomass ratio of 3:2 and three treatments of different biomass levels (No fish: 0 g/m³, Low density:16.7 g/m³, High density: 50 g/m³) with three replicates of each treatment. The fish biomass level of 16.7 g/m³ was corresponding to the actual density of Gancun reservoir. The water samples were collected every week to analyze species composition, body sizes composition, abundance and biomass of zooplankton.The mixture of nile tilapia and silver carp resulted in decrease in both abundance and species numbers of zooplankton, and significantly modified size distribution of abundance and biomass. Zooplankton was primary composed by small size zooplankton such as rotifers. 39 rotifer species, 4 species for cladocera and 4 species for copepod were observed. Large body size species such as Daphnia did not found. 18 rotifers, 2 copepods and 1 cladocera appeared with more than 50% appearance frequency. The most common rotifers were Anuraeopsis Brachionus, Trichocerca, and Keratella, for copepods were Thermocyclops taihokuensis and Mongolodiaptomus birulai and their larva, for cladocera was only Diaphanosoma dubium. Among all 47 species we observed, the dominated ones have sizes smaller than 200μm; 21 species were＜100μm and 17 species were 100～200μm, and only 3 species were larger than 1 mm.The dominant species of rotifer showed an evident temporal succession. In the first 6 weeks, Anuraeopsis fissa, Keratella cochlearis, Polyarthra vulgaris and Trichocerca rousseleti were primary dominant species in abundance. Their abundance sharply decreased aider the week 6. And Brachionus angularis, Conochilus dossnarius, Trichocerca stylata and Epiphanes sp. increased quickly, despite that their increase extent were suppressed by fish predation in the fish treatments. Asplanchna priodonta is the primary dominant species in biomass which not much affected by fish predation. Thermocyclops taihokuensis and its larva were dominant in abundnace of copepods; Diaphanosoma dubium was the only dominant species in cladocera. The two larger crustacean dominant species were also affected by fish significantly.Changes in zooplankton abundance and biomass were significant related to fish numbers and density. In the high density treatments, zooplankton abundance were persistently below 1500 ind./L, and biomass were below 700μg/L. In the no fish treatment, there were two visible climaxes in week 3 and week 8; the abundance were always higher than 1500 ind./L before week 9, and the maximum was 3850ind./L; the biomass were persistently higher than 700μg/L and the maximum was higher than 2000μg/L. The zooplankton abundance and biomass presented an evident gradient phenomenon between different treatments in our enclosure experiment. The abundance of rotifers which were smaller than 200μm contributed to the total abundance in a high percent, but the total biomass was more contributed by mesozooplankton with the body size 200～400μm. Fish predation also influenced the abundance and biomass sized distributions of zooplankton. Fish predation significantly affected zooplankton with a body size 600～1000μm. Abundance and biomass of zooplankton with a body size larger than 1mm were affected by fish predation to less extent due to their extreme low abundance. For small sized zooplankton, their abundance was also influenced by fish. The mean abundance of＜100μm zooplankton in the high density, low density, and no fish treatments were 480ind./L, 680ind./L and 841 ind./L respectively. And the mean abundance of 200～400μm zooplankton in the high density, low density, and no fish treatments were 257ind./L, 432ind./L and 797 ind./L respectively.