Nitrogen And Phosphorus Recycling Driven By Silver Carp And Bighead Carp In The Three Gorges Reservoir Water Ranch

Recent years, stocking plankton-consumer silver carp and bighead carp in water ranch of Three Gorges Reservoir as an ecological fishery model has achieved economic and social success. The research regarding major nutrients kinetics in water ranch will help to elucidate that this fishery production model not only get marketing fish products of silver carp and bighead carp, but also absorb nitrogen and phosphorus (N and P) via fish body grow-up from water, then drive nutrients cycling in ecosystem, finally, manage to control water eutrophication and improving water quality.Ganjing River water ranch is located in the Three Gorges Reservoir area, Zhongxian County, Chongqing Municipality. Specific barriers were constructed formed water ranch. Fishery production adopts alternation of stocking and catching. In this study, we employed ecological stoichiometry approaches to analyze the effect of ecological fishery on process of the main nutrients in Ganjing river ecosystem. From October,2013 to October,2014, we determined N and P element contents in silver carp and bighead carp at six different times. In addition, we measured the N and P excretion rate of bighead carp and silver carp in situ. And we quantitative estimated the two carps on driving N and P recycling in Ganjing River. The main results are as follows:The element average composition of silver carp body dry mass are N 9.58±0.77% and P 3.49±0.22%, with a mean N:P molar ratio of 6.08:The element average body composition of bighead carp dry mass are N 9.50±0.65% and P 3.46±0.60%, with a mean molar N:P ratio of 6.08:1.We derived models about nutrients of N and P excretion rate of fish body based on bighead carp and silver carp in different fish mass and temperature. The specific excretion rate of N and P is significantly negative correlation with water temperature; bighead carp release of N and P is Log10N excretion (μg g-h-1)=2.115×Log (T)-2.112, Log 10N excretion (μg g-1 h-1)=1.583×Log(T)-1.641; Silver carp release rates of nitrogen and phosphorus is Log10N excretion (μg g-1 h-1)=3.736×Log (T)-4.333, Log 10P excretion (μg g-1 h-1)=2.817×Log (T)-3.477.The average excretion rates of zooplankton are N 161.57μgL-1 d-1 and P 2.56 μg L-1 d-1 from July 2013 to June 2014 in Ganjing River. The average release rate of silver carp was N 0.00682 μg L-1 d-1 and P 0.001924 μg L-1 d-1. The average release rate of bighead carp are N 0.006872 μgL-1 d-1 and P 0.002499 μg L-1 d-1. The excretion rate of zooplankton is much higher than that of silver carp and bighead carp. The excretion rate of the carps is much lower than the content of nitrogen and phosphorus in the water (1.76 mg N/L,0.13mg P/L). The contribution of nitrogen and phosphorus to aquatic ecosystems by silver carp and bighead carp was very low.The demands of N and P for phytoplankton primary production separately are 196.45 μg L-1 d-1 and 6.13 μg P L-1 d-1. We estimate that the percent of algal nutrient (July 2013 to June 2014) demand supplied by fish were 0.382%o for N and 0.722%o for P. Thus, the release rate of nitrogen and phosphorus by the silver carp and bighead carp is insufficient to the changes in primary productivity of Ganjing River. The results indicate that the ranching fishery production did not accelerate the eutrophication in Ganjing River.During investigation period, fish element contains approximately N 0.232μg L-1 (total N retention 3339.9 kg in fish body) and 0.084μg P L-1 (1209.3kg) for silver carp, and 0.115 μg N L-1 (1656.9kg) and 0.041 μg N L-1(603.4kg) for bighead carp. Catches of silver carp are 118535kg and 100364kg for bighead carp, taken away 3642.58kg N and 1322.33kg P. Fish nutrient contains and amount of nitrogen and phosphorus released are lower than TN and TP in the water. Given their high P content and low released rates, silver carp and bighead carp appear to be important P pools in Ganjing River.About all, the ecological fisheries model that stocking and catching plankton consumer silver carp and bighead carp expands way of nutrient circulation (nutrient-algae-silver carp and bighead carp) in Ganjing River water ranch and accelerate flowing of nutrient from lower trophic level to higher level. The fishery production way does not result in eutrophication; on the contrary, it can remove N and P from water through fish products retention and suppress eutrphication process.