A Study Of Nitrogen And Phosphorus Budgets, And The Comparison Of Water And Sediment Quality Variation In Grass Carp Polyculture Ponds

Grass carp (Ctenopharyngodon idellus) is one of the main species in freshwateraquaculture in china. Though the intensive aquaculture has been technically improved,its increasingly aggravated water organic pollution leads to a frequent occurrence ofall kinds of diseases, and therefore a negative effection on the healthy development ofgrass carp aquaculture. The polyculture of different species is crucial to achieve thesustainable development of grass carp culture, which functions not only in increasingits breeding production but also in reducing the pollution of the environment. Westudied the nitrogen and phosphorus budgets of the grass carp, silver carp(Hypophythalmichthys molitrix) and carp (Cyprinus carpio) polyculture system.Meanwhile, the variation of water quality and sediments in two different culturepatterns of grass carp, silver carp and prawn（Litopenaeus vannamei）(the whole pondpolycuture and separatory polyculture) was also studied. The main research resultswere listed as follows:1. A study of nitrogen budget in different grass carp polyculture systemsThis study was conducted to investigate the total nitrogen budget in the grass carpmonoculture group (G)，the grass carp and silver polyculture group (GS), the grasscarp and carp polyculture group (GC), grass carp, silver carp and carp polyculturegroup (GSC) in land based enclosure.Results of present experimental showed were asfollows:(1). fish feed was the main source of N input, and the proportion of which intotal N input was85.54%~93.38%, being highest in group G and lowest in groupGSC2. No significant difference was observed in contents of N input among differenttreatments (P>0.05).2. The N output of cultured organisms accounted for the most proportion of total nitrogen output, being62.80%~77.15%.3.The proportion ofaccumulated sediments in total N output (15.19%~27.60%) was only lower than thatof cultured organisms, among which the GSC1and GSC2groups were significantlylower than G, GS and GC groups (P<0.05). Grass carp, silver carp and carppolyculture group could significantly reduce the accumulation of nitrogen in thesediments, and the N utilization of GSC2was significantly higher than that of all theother groups, making GSC2to be a better polyculture mode for grass carp.2. A study of phosphorus budget in different grass carp culture systemsThe method of land based enclosure experiment was chosen to study thephosphorus budget in the grass carp monoculture group (G), the grass carp and silvercarp polyculture group (GS) and the grass carp, carp polyculture group (GC), grasscarp, silver carp and carp polyculture group (GSC1and GSC2). The mainexperimental results showed that:(1) the fish feed was the main source of total P input,and the proportion of which was82.60%~84.26%; The phosphorus input of rainwater,water and the living organims accounted for a smaller part of the total phosphorusinput, being4.04%~4.12%,7.38%~7.81%and3.66%~5.55%respectively.(2) Inthe total P output, the phosphorus output of accumulated sediments (76.37%~79.96%) accounted for the most. The accumulated phosphorus output of water(13.07%~15.23%) was only lower than that in the sediment output part. Amongdifferent treatments, GSC1and GSC2groups were significantly lower than othergroups, while GS and GC groups were significantly lower than G group (P<0.05).Grass carp, silver carp and carp polyculture group could significantly reduce theaccumulation of phosphorus in the aquaculture water. The phosphorus utilization inGSC2and GS groups were significantly higher than that in G, GC and GSC1groups.Based on the above information, it could be seen that the polyculture of grass carp andsilver carp could reduce the accumulation of phosphorus in sediments and GSC2wassupposed to be the better polyculture mode.3. The comparative study between water quality and sediment in two different grasscarp polyculture systemsFrom Jun. to Sep. in2012, we studied the variation of cultural water and sediment with cultural cycle in the whole pond polyculture and separatory polyculture systemsof the grass carp, silver carp and prawn and the difference between the two differentpolyculture systems were also compared. The main experimental results showed that:1. The concentration of ammonia nitrogen in the pond number9(whole pondpolyculture) was higher than that in the pond number5(separatory pond polyculture)from June to September.The variation amplitudes of the nitrite nitrogen concentrationwere drastic were drastic in the whole pond polyculture system and steady in theseparate pond polyculture. In June and July, The nitrte nitrogen contents in the waterof the separatory pond polyculture was significantly higher than that in the wholepond polyculture(P＜0.05). The active phosphorus contents in the pond number9ranged from0mg/L to0.0042mg/L, being higher than that in the pond number5.2.The total nitrogen contents in the whole ployculture pond ranged from1.65mg/L to3.57mg/L, and the highest value of which was observed in August and significantlyhigher than that in the separatory polyculture pond(P＜0.05). The total nitrogencontents of the separatory polyculture pond ranged from0.84mg/L to2.05mg/L,with the minimum value in August. The total phosphorus contents in the separatepolyculture pond were lower than that in the whole pond polyculture from July toSeptember.3. By comparing the total N and P contents in culture water and sediments,the whole pond polyculture had a significant higher value than the separatepolyculture system (P＜0.05).Therefore, as can be seen from the aspect of reducingthe pollution of cultural environment, the separate polyculture mode can significantlyreduce the pollution of cultural water and sediment compared with the wholepolyculture mode.