THE PREDATOR EFFECTS OF SLIVER CARP ON DIFFERENT ALGAL SPECIES INSIDE AQUARIUM

To study the ingestion and digestion of different algal species by sliver carp, we conducted feeding and feces experiments in aquarium microcosms. Sliver carps were fed with three species of algae (Cyclotella, Chlorella and Microcystis aeruginosa), and the ingestion rates per fish weight for the three species were 2.70104, 1.99104 and 1.06104 cells/(gh) respectively. In feces, the ratios of the complete cells to the imperfect cells were 20.04%, 7.13% and 1.97% for the three species respectively. Microcosm experiments were conducted in aquariums filled with water from the Donghu Lake for two weeks. Each aquarium had 20 fish total weight (4.50.021) g inside, and there were three replicates in one experiment. We found that after 20 days the cell density in the aquariums was significantly reduced compared to the control. Planktolyngbya accounted for 87% of the phytoplankton density in feces, and Merismopedia sp., Pseudoanabaena sp., Oscillatoriaceae sp. and Microcystis sp. accounted for 11%, but Raphidiopsis sp., Scenedesmus spp., Coelastrum sp. only formed a very small portion. The morphology of Oscillatoria sp., Spirulina sp., and Pediastrum sp. appeared to be incomplete, while Synedra sp., Navicula sp. and Cyclotella sp. had a nearly vacant-shell shape. We collected the feces to feed the hungry sliver carps and did not observe any Pediastrum sp., Cyclotella sp. or Synedra sp. in the new discharge. However, we still found Planktolyngbya sp., Anabeana sp., Pseudoanabaena sp., Merismopedia sp., Microcystis sp., Raphidiopsis sp., Oscillatoriaceae sp., Microcystis sp., Scenedesmus spp., Oocystis sp., Pandorina morum sp., Coelastrum sp., Chlamydomonas sp. and Cymbella sp. After a 10d culture of the feces in the medium, we observed the revival of Planktolyngbya sp., Oscillatoriaceae sp., Merismopedia sp., Anabeana sp., Pseudoanabaena sp., Microcystis sp., Scenedesmus spp., Chlamydomonas sp. and Oocystis sp. The level of chlorophyll a decreased slightly at first and then increased in fish groups. Our study suggested that sliver carp could filter phytoplankton with the size of 250 m, and that sliver carp might mainly feed on diatoms, of which the ingestion and digestion was easier than that of green algae and blue-green algae. The results also indicated that Cyclotella sp. and Navicula sp. were sensitive to the predator stress. Therefore silver carp could be an efficient tool to regulate the algal population and the phytoplankton communities.