Effects Of Stocking Density And Feeding Level On Water Qualityand Plankton Community Structure Of The Yellow Catfish(Pelteobagrus Fulvidraco) Rearing Enclosure Ecosystem

The primary productivity of aquacultural pond was inhibited because of constant and periodical occurence of excessive zooplankton during the rearing of yellow catfish, which would reduce the feeding and growth performance of juvenile yellow catfish, even result in some pathological response. The stocking density and feeding level were controlled to reveal its effects on water quality and plankton community structure of the enclosure ecosystem reared yellow catfish. Consequently, we expected to find the reasons of the zooplankton-phytoplankton community structure unbalance during the rearing of yellow catfish, and provide some improvement suggesting in aquaculture of this species. Our main results were summarized as follows: 1 Effect of stoking density on growth of yellow catfish, water quality and plankton community structure of the enclosure ecosystemA control group(D0) and 4 treatment groups(D1, D2, D3, D4) were set with the corresponding stocking densities being 0, 40, 50, 60, 70 ind./m3. The experiment lasted for 24 days and the water quality was monitored every 3 days. At the end of the experiment, growth performance of yellow catfish in each group was measured to understand the effect of stocking density on growth performance. The results showed that the final body weight, weight gain and specific growth rate of yellow catfish decreased with the increase of stocking density, and the significant difference were found among group D1, D2 and D3(P<0.05). The condition factor of D1 and D2 was significantly bigger than that of D3 and D4(P<0.05). The content of nitrite nitrogen in the water was significantly affected by stocking density(P<0.05), and was increased with the increasing stocking density, while the contents of total nitrogen and nitrate nitrogen were not affected by stocking density(P>0.05). During the first 18 days, the contents of ammonia nitrogen in all groups were stable at a low level, but then increased significantly until the end of the experiment. Contents of total phosphorus and phosphate fluctuated within a certain range during the experiment period and were also not affected by stocking density. Increase of stocking density would reduce the growth rate of yellow catfish and increase the harmful substances in the water which would accelerate the deterioration of water quality.The species number of phytoplankton decreased with the increase of stocking density.The fluctuation amplitude of phytoplankton diversity index in each group was slight during the early stage of experiment, and increased after the twelfth day of the experiment. The stability of the phytoplankton community structure was not significantly affected by the stoking density. At the beginning of the experiment, the phytoplankton densities among different groups were similar, while its difference among each groups were increased gradually, and the phytoplankton density decreased with the increasing of the stoking density. The phytoplankton biomass fluctuated within a certain range, and the levels of group D2 and D4 were larger than group D1 and D3.With the increasing of stocking density, the number of zooplankton species were increased during the early stage and then decreased. The changes of zooplankton diversity index was similar with phytoplankton diversity, the slight fluctuation was found in early stage and then became larger. The density and biomass of zooplankton increased and the decreased with the increasing of culture time. In most of the sampling time, both the zooplankton density and biomass of group D0 were the highest. According to the results, the appropriate stocking density of yellow catfish should be 70 ind/m3 based on the optimum plankton community structure. 2 Effect of feeding level on growth of yellow catfish, water quality and plankton community structure of the enclosure ecosystemThe experiment was conducted to determine the effects of four feeding levels(100%(satiation)(F1)、80%(F2)、60%(F3)、40%(F4) satiation per day respectively) on the growth of yellow catfish, water quality and plankton community structure of the enclosure ecosystem. The stocking density was 50 ind/m3, and the feeding amount was corrected every three days. The results showed that the growth performance of yellow catfish was significantly affected by the feeding level. The final weight, weight gain rate, specific growth rate and condition factor were decreased with the decreasing of the feeding level. There were no significant differences in the values of final weight between the each adjacent groups(P>0.05). The significant differences were found in the weight gain rate and specific growth rate among different groups(P<0.05), while there was no significant difference in condition factor among different groups(P>0.05). With the increasing of culture time, the content of NO2-N in water increased gradually and was unaffected by the feeding level(P>0.05). The contents of NH4-H and NO3-N were significantly influenced by the feeding level, and the values in group F3 were lower than that in other three groups. The content of TN in water decreased with the decreasing of feeding level. The content of PO4-P in water decreased gradually with the increasing of the culture time and feeding level, and it valus in group F1, F2 and F3 were significantly different(P<0.05), and the differences between F3 and F4 groups was not significant(P>0.05). With the decreasing of feeding level, the TP content in the water showed an increasing trend, the differences among group F1, F2 and F4 were significant(P<0.05), and the difference was not significant between group F3 and F4(P>0.05).The species number and diversity index of plankton were not significantly affected by feeding level. The density and biomass of phytoplankton increased with the decreasing of feeding level. The density of zooplankton increased with the decreasing of feeding level. The rank of zooplankton biomass was: F3>F4>F1>F2, and the significant differences were observed between group F3, F4 and group F1, F2(P < 0.05). Consequently, the appropriate feeding level should be 80% of satiation based on the optimum plankton community structure.