| The green development of aquaculture requires the reduction of pollution emissions while ensuring safe production and stable supply.Cultural industry of Chinese mitten crab(Eriocheir sinensis)is one of the pillar industries of agriculture in Suzhou,which is related to the livelihood of millions of people.Therefore,it is necessary to explore the green development path for the cultural industry of Chinese mitten crab in Suzhou.Aquatic grass is plant in crab ponds in order to increase the survival rate of crab,which also plays a role in purifying water quality.Therefore,aquatic grass is closely related to the crab yield and the water quality in the pond.The water quality of crab pond was monitored and the mode of field management was investigated in this study in order to explore the countermeasures of controlling aquaculture pollution,preventing the premature senescence of aquatic grass,increasing the yield and reducing the death rate.The research results could provide a technical reference for the green development of crab culture in Suzhou.1.Water quality characteristics of crab pond and its response to aquatic grass stateThe difference between the water quality of crab pond and that of source river was compared.Water quality of more than 16 ponds and more than 7 water source rivers was monitored in the sunny days each month from April to September 2019.The water source rivers are connected with Yangcheng Lake or Taihu Lake.According to the "discharge requirements of pond aquaculture water in Taihu lake basin",eight indexes including pH,total nitrogen(TN),total ammonia(TAN),total phosphorus(TP),total suspended sediment(TSS),chemical oxygen demand(CODMn),copper(Cu),and zinc(Zn)were monitored.The results showed that the average pH value of crab pond water was 8.75 ± 0.42,TN was 2.37 ± 0.50 mg/L,TP was 0.15 ± 0.03 mg/L,TAN was 0.24 ± 0.16 mg/L,CODMn was 9.85± 0.81 mg/L,TSS was 33 ± 20 mg/L.In the same period,the pH of water source river was 7.87 ± 0.22,TN was 1.73± 0.31 mg/L,TP was 0.09 ± 0.03 mg/L,Tan was 0.34± 0.18 mg/L,CODMn was 6.19 ± 0.79 mg/L,TSS was 13 ± 7 mg/L.The Cu was<0.01 mg/L and Zn was<0.2 mg/L in both pond and source river.The above results showed that the pH,TN,TP,CODMn and TSS of the pond water were higher than those of source rivers,while the TAN was lower in general.All 8 indexes meet the first to third level of the "discharge requirements of pond aquaculture water in Taihu lake basin",indicating is could be discharged directly in theory.The effect of aquatic grass state on water quality was explored.The aquatic grass was classifed into three grades from good(Ⅰ)to worse(Ⅲ)according to its functions in crab pond.The water quality of the ponds with different grass state was monitored in June,July,August and September 2017.More then 4 ponds each grade were monitored.The results showed that TN concentration was significantly lower in grade Ⅰ than grade Ⅲ in August(P<0.05).The lowest TP concentration was observed in grade Ⅱ from Jun.to Aug.(P<0.05).The CODMn was the lowest in grade Ⅱ in Jun.and the lowest in grade Ⅰ in Aug.(P<0.05).TSS was the lowest in grade Ⅰ in Sep.(P<0.05).TAN was low in grade Ⅱ as well as in grade Ⅲ in Jul.(P<0.05).These results suggested that the water quality of crab pond could be improved by proper grass management.However,the rising range of nutrient concentration in water body was relatively small after grass premature senescence,and even a significant decrease in TAN(P<0.05).It might be attributed to the absorption and utilization of nutrients by microalgae.Therefore,it is not suitable to judge the pollution intensity of crab culture industry only by the nutrient level of water body,but also to estimate the discharge of tail water.1.Analysis of the causes of the premature senescence of aquatic grass:a case study of Changshu and ZhangjiagangThe aquatic grass in crab pond not only affects the water quality,but also has a decisive influence on the yield and quality of crab.In this study,43 farmers in Changshu area and 17 farmers in Zhangjiagang area were surveyed.Pearson correlation and principal component analysis were used to analyze the factors inducing premature senescence of aquatic grass.The results showed that the highest correlation with the premature senescence of aquatic grass in Changshu was the number of times of using herbicide in pond bank,and the correlation coefficient was r=0.468(P<0.01).It was followed by the range of grass treatment(r=0.384,P<0.05)and stocking density of silver and bighead carp.All the above factors were positively correlated.The significant negative correlation factors with the premature senescence of aquatic grass in Changshu included the sterilization frequency(r=-0.363,P<0.05),grass planting season(r=-0.309,P<0.05)and the average maximum water depth(r=-0.302,P<0.05).In addition,experience(r=-0.290)and aeration intensity(r=-0.259)were also highly correlated(P>0.05).The highest correlation with the premature senescence of grass in Zhangjiagang was the crab stocking density(r=0.499,P<0.05)and water quality regulation freguency(r=-0.403,P<0.05).In addition,the high correlation coefficient was also noticed in the variety of grass(r=-0.387),the input amount of animal feed(r=0.360),the input amount of snail(r=-0.300),water exchange frequency(r=0.318),and the water bloom(r=-0.311).Nine principal components contributed to the premature senescence of grass in Changshu were ontained by principal component analysis.The contribution rate of variance was from 14.05%to 6.65%,and the cumulative contribution rate of variance was 74.92%.Eight principal components of the premature senescence of aquatic grass in Zhangjiagang were screened.The contribution rate of variance was from 12.90%to 8.60%,and the cumulative contribution rate of variance was 85.59%.These results showed that each principal component for the premature senescence of aquatic grass had similar action intensity,suggesting that the management of aquatic grass in crab pond was a systematic project,and any absence of management could cause the senescence.2.Analysis of the factors affecting the crab yield:a case study of Changshu and ZhangjiagangThe field management factors affecting the yield of crab were studied by the same research method as that of the analysis of the premature senescence of aquatic grass.In view of the fact that the income of crab culture is not only related to the yield,but also to the crab size and stocking patterns,standardized yield of crab was defined in this study.The results showed the standardized crab yield was significantly positive correlated with the aerotion intensity(r=0.305,P<0.05)in Changshu,and significantly negative correlated with the stocking crab size(r=-0.334,P<0.05).In addition,the crab stocking density(r=0.274),economical fish stocking density(r=0.237),input maize amount(r=0.257)and snail amount(r=0.287)were also showed a relative high correlation coefficient.There was no significant correlation between the standardized yield and any index in Zhangjiagang,however,the correlation coefficients with the amount of input formulated feed(r=0.409),the premature senescence of grass(r=-0.410),and the pond area(r=0.413)were higher.Through the principal component analysis,nine principal components affecting the standardized crab yield in Changshu were found.The contribution rate of variance was from 9.61%to 6.76%.The cumulative contribution rate of variance was 73.64%.Six principal components affecting the standardized crab yield in Zhangjiagang was obtained,the contribution rate of variance was from 23.30%to 9.10%,and the cumulative contribution rate of variance was 83.32%.These results showed that each principal component of crab yield had similar action intensity,suggesting that the yield of crab should be improved from every aspect,and the yield could be reduced by any aspect of management absence.The results also suggested that the yield could be increased by artificial aerotion and other measures.3.Analysis on the causes of death of pond crab in ChangshuFor many years,the crab cultivation in Changshu area has been plagued by enteritis.In response to the urgent needs of the majority of crab farmers,the causes for the crab death in Changshu were analyised.From May to July 2018,we visited crab farmers and 29 effective field management records were obtained.The death crabs were always noticed after floating,and the cause of disease could not be identified,therefore the loss per unit area was investigated but not the type of disease.The dead crab amount was reported by the farmers themselves.It was defined as 1 when the daily death amount reached 10 or more,or the cumulative death amount was more than 100 crabs per hm2 in the 30 days before the survey,otherwise it was defined as 0.The results showed visceral input amount was significantly positive correlated with crab death(r=0.409,P<0.05);the formulated feed input amount(r=-0.370,P<0.05)and aerotion intensity(r=-0.584,P<0.01)was negative correlated with crab death.There was no significant correlation between other indexes and the crab death.By principal component analysis,eight principal components affecting the crab death were obtained.The variance contribution rate of each principal component was from 12.02%to 8.61%,and the cumulative variance contribution rate was 80.66%.These results suggested that the amount of visceral input amount should be reduced,the amount of formulated feed should be increased,and the artificial aerotion should be strengthened.However,prevention and control of mortality was a systematic project,and the absence of management in each link may increase the mortality. |
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