| Chinese mitten crab,Eriocheir sinensis(H.Milne Edwards,hereinafter referred to as crab),also known as river crab or crab,is nutritious and delicious,and thus of high economic value.Crab is one of the unique freshwater varieties and important foreign exchange earning aquatic products in China,which has always been favorable delicacies of Chinese consumers.In recent years,the rearing area of crab has been expanding,from Heilongjiang Province in the north to Guangdong and Guangxi Provinces in the south.The aquaculture model has constantly been innovated and the production is also increasing recently.However,it still cannot meet the growing needs due to the concentrated marketing time and short supply period.Therefore,the issues of scattering marketing time and extending supply period has attracted more and more attention from practitioners,so the effective overwintering cultivation can be one potential solution to reach out.At present,there are few studies on the overwintering cultivation of crab,how to improve the survival rate and quality during overwintering cultivation has become an urgent problem.As the stocking density is the main factor affecting the growth,survival,yield and other production parameters of crustaceans.The temporary stocking density has also been the most concerned issue of farmers in the overwintering cultivation technology of E.sinensis.In order to explore the optimal stocking density and time for overwintering rearing,net cages deployed in a pond circulating aquaculture system were applied to culture crabs temporarily,and then the growth performance,mortality,and physiological status of E.sinensis under different temporary stocking densities were analyzed,as well as the changes of nutritional quality and flavor of the edible tissues after overwintering cultivation.The results are as follows:1.The effect of stocking density and rearing time on the survival rate of E.sinensisRegardless of time,the mortality of medium density(MD)group was decreased by 0.90%than that in the low density(LD)group and 1.17%than that in the high density(HD)group,although without significant differences(P>0.05).On the contrary,overwintering rearing time had shown significant effects on mortality across stocking densities(P<0.05).Results showed that,the mortality was less than 5%in first two months,while increased significantly after 90 days.After rearing for 105 days and 120days,the mortality reached up to average 21.37%and 33.04%,respectively.After rearing for 90 days,the cumulative survival rates of all three stocking densities were still higher than 85%.Fifteen days later,it decreased to about 70%in LD and MD groups,while about 62.44%in HD group.After 120 days rearing,the cumulative survival rates were lower than 50%except the MD group.Taken together,the temporary survival rate of MD group was better than the other two groups.2.The effect of stocking density and rearing time on growth performance of E.sinensisThere was no significant change in the body weight of crabs during the overwintering period(P>0.05).Overall,the meat yield(MY)and total edible yield(TEY)of the HD group were significantly higher than those of the LD group.Regardless of stocking densities,in the first 105 days of rearing,The total edible index(TEY)and gonadal index(GSI)showed an increasing trend over time,while the hepatopancreas index(HSI)showed a decreasing trend.After rearing for 105 days,TEY,GSI and HSI all decreased significantly.In addition,the rearing time of overwintering had no significant effect on other growth parameters such as carapace length and carapace width of E.sinensis.3.The effect of stocking density and rearing time on antioxidant capacity and expression of HSP70 in E.sinensisWith the prolongation of temporary cultivation time,the content of malondialdehyde(MDA)in the crab hepatopancreas firstly increased and then decreased,while the activity of catalase(CAT)fluctuated and decreased regardless of stocking densities.E.sinensis from MD and HD group had obviously fluctuant SOD activity along rearing time.The activity of GSH-Px decreased dramatically after 30days rearing in all three densities groups,while it showed an increased trend only in MD group along the rearing time.In summary,the hepatopancreas antioxidant system of E.sinensis reared at three densities performed well during overwintering period,especially those in the MD group.The HSP70 level of E.sinensis first increased(before mid-February)and then decreased with the extension of rearing time,which is consistent with the characteristics of oogenesis,suggesting that HSP70 plays an important role in the oogenesis of E.sinensis.The HSP70 levels in the hepatopancreas of different stocking density groups fluctuated with rearing time,suggesting that E.sinensis were under different degrees of stress during the overwintering period.4.The effect of overwintering rearing on the quality of E.sinensisE.sinensis was in a stable growth status during the temporary overwintering period,decreased HSI went along with increased GSI,resulting no obvious"cream losing"phenomenon.The total edible tissue content and the crude lipid content in ovaries of E.sinensis after temporary rearing was significantly decreased compared with that before temporary rearing(P<0.05),while no significant changes among other nutrient.These results indicated that there was a certain degree of nutrient loss in the edible tissues of E.sinensis after temporary cultivation.After 105 days of overwintering cultivation,the total content of free amino acids(T)decreased(P<0.05),while the ratio of essential amino acids to free amino acids(E/T)increased significantly(P<0.05),and the ratio of non-essential amino acids to free amino acids(N/T)decreased significantly(P<0.05),the ratio of essential amino acids to non-essential amino acids(E/N)increased significantly(P<0.05).After overwintering cultivation,Adenosine(AMP)and inosinic acid increased significantly(P<0.05),while hypoxanthine and inosine decreased significantly(P<0.05).These results indicate that the quality of crab has been improved after temporary rearing.After overwintering rearing,the polyunsaturated fatty acids,such as C20:2(eicosadienoic acid),C20:4(arachidonic acid)and C20:5(eicosapentaenoic acid),and the PUFA/SFA ratio were increased significantly,indicating the quality and flavor of edible tissues have been improved to some extent.The main volatile substances of the E.sinensis after temporary culture were 1-octene-3-alcohol,nonanal,hexanal and2-pentylfuran,suggesting the volatile flavor of the edible tissue was relatively better than that of the E.sinensis before temporary cultivation.In summary,under the experimental conditions,the overwintering cultivation has a certain effect on the survival rate,growth performance,antioxidant enzyme status,nutritional quality and flavor of E.sinensis.Considering the mortality and economic benefits,the optimal stocking density is around 21.88 kg/m~2when the overwintering cultivation is less than 90 days,while a medium density of 15.63 kg/m~2 is more suitable for longer term of overwintering cultivation(more than 90 days).For long-term overwintering,this study suggested that the optimal overwintering rearing of E.sinensis is not beyond the first ten days of April.Although there is certain loss in nutritional compositions of E.sinensis after overwintering rearing,its quality and flavor have been improved to a certain extent. |
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