Study On The Biological Recycling Of Nutrients In Polyculture Ponds Of Grass Carp And Silver Carp

The present study assessed nutrient pollution derived from intensive farming of grass carp (Ctenopharyngodon idella) by means of determining the carbon, nitrogen and phosphorus budgets and the physiological mechanisms of nutrient budgets by using the enrichment-dilution approach of stable isotopes. Based on the stable isotopic mixing model, we studied the food sources of silver carp (Hypophthalmichthys molitrix) and uptake of farming wastes in the pelyculture ecosystems, to optimize the aquaculture mode in accordance with an economic and environmental win-win resolution scheme. Subsequently, the nutrient budgets of silver carp in the optimized mode and phytoplankton fixation of the carbon excretion by grass carp and silver carp were studies. Finally, the biological cycling of nutrients in polyculture ponds were constructed, which evaluated the feasibility and capacity of using silver carp as a biofilter to remove the farming wastes of grass carp. The results are summarized as follows:Carbon, nitrogen and phosphorus budgets-ograss carp and assessment of nutrient pollution in polyculture ponds. We-developed mesocosmic enclosures and studied the main nutrient budgets of grass carp in monoculture and polyculture comprising grass carp and silver carp. The results showed that the net budget rate in terms of scope for growth (SFG) and total assimilation of carbon, nitrogen and phosphorus varied during the culture period. We estimated that for each ton of grass carp production,355～385kg C,30～40kg N and3～4kg P, on average, were released into the water column and approximately81.7%C,65.6%N and67.5%P in feed were not retained by fish, which could become nutrient pollution in the form of uneaten pellets, feces and excretion.Turnover and fractionation of carbon and nitrogen stable isotopes in tissues of grass carp. Based on the enrichment-dilution approach, we conducted a diet-switch experiment using grass carp cultured using artificial feed with distinct stable isotopic compositions, to assess turnover and fractionation of carbon and nitrogen stable isotopes in different tissues, including liver, muscle and gill. The results revealed that the turnover rates exhibited significant differences between tissues and stable isotopes, and increased in the sequence of gill<muscle<liver and nitrogen<carbon. Stable isotopic turnover was driven by both tissue replacement in the form of metabolism and net tissue increase in the form of growth, the proportional contributions of which between tissues exhibited significant discrepancy. Stable isotopic fractionation also differed significantly between tissues and nitrogen fractionation was higher than that of carbon isotope.Uptake of farming wastes by silver carp in polyculture ponds of grass carp. Based on the stable isotopic mixing model, we developed mesocosmic enclosures combing grass carp and silver carp with different proportions, to analyze the food sourcess of silver carp and evaluate the feasibility and capacity of using silver carp as a biofilter to remove the farming wastes. The results showed that silver carp can efficiently reduced total particulate matter and organic content in water column. Particulate organic matter was the most principal food source of silver carp, followed by feed residues, then fish feces. Filter-feeding silver carp can directly take up feed residues-and fish feces in water to remove farming wastes. On the other hand, the high filtration rate in phytoplankton demonstrated that silver carp had great potential to remove-dissolved farming wastes, i.e. nitrogen and phosphorus excretion derived from grass carp.Carbon, nitrogen and phosphorus budgets of silver carp:implication, for removal of nutrients. We assessed the feasibility and capacity of silver carp.as a biofilter to remove excess nutrient pollution through carbon, nitrogen and phosphorus budgets in optimized polyculture mode. The results revealed that the metabolic acquisition and expenditure of silver carp exhibited considerable temporal fluctuation during the culture period, owing to changes in exogenous environmental conditions and autogenous physiological status. Silver carp can efficiently remove nutrient pollution in polyculture pond by taking up particulate organic matter, feed residues and fish feces. We estimated that for each ton of silver carp production,73.65kg C,18.84kg N and1.57kg P were removed from water and the total uptake of feed residues and fish feces reached185.01kg.Study on the phytoplankton fixation of the carbon excretion by the grass carp and silver carp. The phytoplankton Synedra sp. fixation of the carbon excretion by the grass carp and silver carp was studied by using carbon stable isotope technique. The results revealed that the respiration activities by fishes significantly altered the concentration and carbon stable isotope value of dissolved inorganic carbon (DIC). The growth and reproduction of phytoplankton coold effectively utilize the carbon excretion by fish, and exhibited strong preferential selectivity. In polyculture ecosystem, calculation with stable isotopic moxing model showed that the proportional contributions of carbon excretion by grass carp and silver carp to the phytoplankton fixation were51.66%and14.27%, respectively, while that of DIC was34.07%, which indicated that there were mutually beneficial mechanisms between fishes and phytoplankton.