| Swimming crab-shrimp polyculture is an important pond culture mode in coastal areas of China.Although the polyculture mode has greatly improved the economic benefits and nutrient utilization rate compared with the swimming crab monoculture mode,there is still room for further optimization and improvement.Filter-feeding bivalves are considered to be ecosystem ’engineers’.They can affect the material transfer path of the ecosystem through physiological activities such as filtration and biological deposition,and change the nutrient utilization efficiency of the system.They are ideal tool species for optimizing the structure of aquaculture systems.Based on this,this study takes Portunus trituberculatusMarsupenaeus japonicus in the coastal area of Zhejiang Province as the research object and intends to adopt the method of land-based enclosure experimental ecology.By coculturing four different densities of Sinonovacula constricta,the density from low to high is 25 g·m-2(PMS1),50 g·m-2(PMS2),100 g·m-2(PMS2),200 g·m-2(PMS4),the effects of S.constricta on the economic benefits,water environment,plankton particle size structure,and nitrogen and phosphorus utilization of P.trituberculatus-M.japonicus culture system were explored.The results could provide references and a basis for the optimization of the swimming crab culture system from the perspective of aquaculture water environment and nutrient element utilization characteristics.The results are as follows:1.Effects of polyculture of S.constricta on yield and economic benefits of swimming crab-shrimp aquaculture systemThe results showed that the survival rate and yield of PMS1 and PMS2 were the highest,which were significantly higher than those of PM,PMS3,and PMS4(P<0.05).The survival rate of M.japonicus in PMS1 was the highest(60.54±0.76)%,which was significantly higher than that in other aquaculture systems(P<0.05).The yield of PMS1,PMS2,and PMS3 was the highest,which was significantly higher than that of PM and PMS4(P<0.05).The survival rates of S.constricta were the highest in PMS1 and PMS2,which were(65.72±3.20)%and(62.71±3.18)%,respectively,which were significantly different from PMS3 and PMS4(P<0.05).The yield increased with the increase in polyculture density,and the difference between each culture system was significant(P<0.05).From the perspective of economic benefits,the total income of different aquaculture systems was the highest in PMS2,followed by PMS1,which was significantly higher than PM,PMS3,and PMS4(P<0.05).The net profit and return on investment of PMS1 and PMS2 were significantly higher than those of PM,PMS3,and PMS4(P<0.05).The results showed that the high polyculture density of S.constricta had a significant negative impact on the growth,survival rate,and yield of P.trituberculatus and M.japonicus.Under the conditions of this study,S.constricta could obtain higher economic benefits at 25～50 g·m-2 polyculture density.2.Effects of polyculture of S.constricta on water quality indexes of swimming crabshrimp culture systemThe results showed that there was a significant difference in water pH between different culture systems(P<0.05),and it gradually decreased with the increase of polyculture density.COD was the highest in PMS3(2.51 ± 1.37)mg·L-1,followed by PMS4(2.34±1.25)mg·L-1,which was significantly higher than PM,PMS1 and PMS2(P<0.05).The differences of chlorophyll a,TN,and suspended particulate matter among different culture systems were similar,which were the highest in PM,which were(38.02±13.74)mg·m-3,(2.88±0.57)mg·L-1 and(44.35±15.72)mg·L-1,respectively.PMS1 and PMS2 were the second,and PMS3 and PMS4 were the lowest,which were significantly lower than other mixed culture systems(P<0.05).The contents of NH4+-N and PO43--P were the highest in the high polyculture density group(PMS3,PMS4),which were significantly higher than those in PM,PMS1,and PMS2(P<0.05).The water transparency and NO3--N content were the highest in the high-density polyculture system of S.constricta(PMS3,PMS4),followed by PMS1 and PMS2.The water transparency of PM in the control culture system was the lowest,which was significantly lower than that in the polyculture system of S.constricta(P<0.05).The contents of TP and Zn2+ were the lowest in PMS1 and PMS2,which were significantly lower than those in other polyculture systems(P<0.05).The results of this experiment showed that S.constricta polyculture could significantly reduce the pH,suspended particulate matter,TN,TP,chlorophyll a,and other indicators of the water body of P.trituberculatus-M.japonicus culture system,and increase the transparency of the water body.From the density effect of S.constricta,when the polyculture density was 25～50 g·m-2,the indexes of NH4+-N,NO3--N,PO43--P,TP,and COD in the aquaculture water were significantly reduced,so the optimum stocking density was obtained under this experimental condition.3.Effects of polyculture of S.constricta on the size structure of plankton in swimming crab-shrimp culture systemThe plankton was divided into four particle sizes:macro-plankton(>200 μm),microplankton(20～200 μm),nano-plankton(2～20 μm),and pico-plankton(<2 μm)by grading filtration.The chlorophyll a content,primary productivity,and respiration of plankton with different particle sizes were measured and analyzed.The results showed that,in general,the contribution rate of nano-phytoplankton to chlorophyll a and primary productivity was the largest,which was 50.24%～54.92%and 40.89%～52.16%,respectively.The second was micro-phytoplankton,with contribution rates of 32.19%～39.93%and 25.40%～28.20%,respectively.The contribution rate of pico-phytoplankton to respiration was the largest,which was 32.57%～44.82%.Followed by nanophytoplankton(24.80%～30.17%).After comparing different aquaculture systems,it was found that the total content of chlorophyll a in PMS3 and PMS4 was the lowest,which was significantly lower than that in PMS1 and PMS2(P<0.05).The content of chlorophyll a in macro-,micro-,and nano-phytoplankton was the lowest in PMS3 and PMS4,which was significantly lower than that in other aquaculture systems(P<0.05).The chlorophyll a in PM was the highest(2.12±0.49)mg·m-3,followed by PMS4(1.81±0.35)mg·m-3,and PMS1 and PMS2 were the lowest,which were significantly lower than those in other aquaculture systems(P<0.05).There was a significant difference in total primary productivity between different aquaculture systems(P<0.05),and it decreased with the increase in polyculture density.The primary productivity and total primary productivity of macro-,micro-,and nano-phytoplankton showed similar differences,which decreased with the increase in polyculture density.On the contrary,the productivity of pico-phytoplankton increased significantly with the increase in polyculture density(P<0.05).The water respiration of PMS4 was the highest,followed by PMS3,which was significantly higher than PMS1 and PMS2(P<0.05).The respiration and total respiration of macro-,micro-,and nano-phytoplankton showed similar differences.The respiration of pico-plankton and bacterioplankton increased significantly with the increase in polyculture density(P<0.05).The results showed that,in general,the polyculture of S.constricta with higher density may significantly reduce the biomass of macro-,micro-,and nano-plankton and increase the biomass of pico-plankton through strong filter feeding,so that the chlorophyll a content,primary productivity and respiration of plankton with different particle sizes were found to change accordingly.4.Effects of polyculture of S.constricta on nitrogen and phosphorus budgets in swimming crab-shrimp culture systemThe results showed that feed was the main source of N and P in the aquaculture system,followed by N and P in the water layer.During the experiment,the N input from the feed in different aquaculture systems was 551.81±0.46 g·25 m-2,accounting for 81.75%～84.64%of the total input.The N input from the water layer was 59.70±0.79 g·25 m-2,accounting for 8.73%～9.37%of the total input.The P input from feed was 77.03±0.19 g·25 m-2,accounting for 96.64%～98.13%of the total input,and the P input from the water layer was 1.36±0.09 g·25 m-2,accounting for 1.56%～1.68%of the total input.The output of N and P was mainly in sediment accumulation.The output of N and P from sediment in different aquaculture systems was 414.87～524.14 g·m-2 and 46.30～50.99 g·25 m-2,accounting for 57.65%～66.07%and 49.05%～58.52%of N and P expenditure,respectively.Followed by P.trituberculatus and water layer accumulation.Among them,the N and P output of P.trituberculatus were 94.09～120.75 g·25 m-2 and 10.88～12.90 g·25 m-2,respectively.The percentage of P.trituberculatus in the total N and P output was 12.40%～15.39%and 11.42%～15.94%,respectively.The N and P accumulated in the water layer were 82.05～98.55 g·25 m-2 and 17.79～25.65 g·25 m-2,accounting for 9.95%～13.13%and 20.97%～27.40%of the total output of N and P,respectively.From the utilization rate of N and P in the breeding system,the utilization rate of N in the PMS2 breeding system was the highest(28.25%),which was significantly higher than that in other breeding systems(P<0.05),and the utilization rate of PM was the lowest,which was significantly lower than that of PMS1,PMS3,and PMS4(P<0.05).The utilization rate of P was the lowest in the PM group(14.60%),which was significantly lower than that in the S.constricta polyculture system(P<0.05).But there was no significant difference with other S.constricta polyculture systems(P>0.05).Therefore,considering the utilization of nitrogen and phosphorus by aquaculture organisms and aquaculture systems,the optimal polyculture density range of S.contrita was 25～50 g·m-2 under the conditions of this study.In summary,from the perspective of economic and ecological benefits,the lowdensity polyculture of S.constricta can significantly increase breeding yield and breeding profits.At the same time,the levels of NH4+-N,PO43--P,TP,and COD in aquaculture water were significantly reduced,the water quality was improved,and the utilization rate of nitrogen and phosphorus in the aquaculture system was significantly increased.Therefore,S.constricta 25～50 g·m-2,P.trituberculatus 2 ind.·m-2,M.japonicus 30 ind.·m-2 was the best ratio. |
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