Study On Community Structure And Environmental Factors Of Crustaceans In The Yangtze River Estuary

The Yangtze River estuary is the largest estuary in China,located in the east coast of China.The unique geographical conditions and hydrological characteristics make it nurture rich fishery resources.The Yangtze River estuary provides spawning ground and feeding ground for many types of fish,shrimp,crab and other species.It is also a migration channel for Acipensersinensis,Coilia nasus and other important fish.For a long time,due to the impact of major water conservancy projects,overfishing,environmental pollution and other human activities,its ecosystem is sensitive and fragile,and the traditional economic fishe resources are in serious decline.Crustaceans such as shrimp and crabs have gradually become the dominant group in the Yangtze River estuary.At present,most studies on the fishery resources of the Yangtze River estuary have focused on plankton,macrobenthos and fishes,while studies on the crustacean community of the Yangtze River estuary are relatively weak.In recent years,fish stocks in the Yangtze River estuary have been severely reduced.As an important part and supplementary resource of the Yangtze River estuary ecosystem,it is of great significance to further investigate and study the community structure of crustaceans.In order to improve the understanding of crustacean community,trawl fishery resource surveys were conducted in the Yangtze River estuary.Based on the seasonal survey data of the year 2020 and 2021,the community structure characteristics of crustaceans in the Yangtze River estuary were analyzed,including the species composition,dominant species change,community similarity and heterogeneity of crustaceans.Based on the data of crustacean biomass（mass）,the particle size spectrum of the crustacean community in the Yangtze Estuary was preliminarily constructed,and the diversity index,replacement index and ABC curve were combined to analyze the diversity,stability and disturbance status of the crustacean community in the Yangtze estuary.Finally,the spatial and temporal distribution of crustaceans resources in the Yangtze River Estuary was comprehensively studied,and the environmental factors affecting the crustacean community were explored combined with the environmental data of the Yangtze River estuary.The results showed that:（1）A total of 20 species（11 families and 2 orders）were captured during the four voyages,including 11 species of shrimp,8 species of crab and 1 species of mantis shrimps.Eurytherm and euryhaline were dominant species in autumn,while the species adapted to low salinity mainly appeared in the other three seasons.Among them,four dominant species were identified,including Exopalaemon annandalei,Palaemon gravieri,Exopalaemon carinicauda and Portunus trituberculatus.There were obvious seasonal alternations of dominant species.The replacement rate of dominant species was 100%and 75%from spring to summer and summer to autumn,respectively.The results of clustering and multi-dimensional scale analysis showed that the crustacean community in the Yangtze Estuary were divided into 3 groups in spring and summer with 57.10%and42.71%community similarity,4 groups in autumn with 51.62%community similarity and 2 groups in winter with 42.05%community similarity.Percentage similarity analysis showed that,The dominant species or common species contributed more to the similarity within groups and the heterogeneity between groups.（2）The ranges of shannon-Wiener diversity index（H’）,Pielou evenness index（J’）and Margalef richness index（D）of all stations of the crustacean community structure were 0～1.38,0～0.90 and 0～1.41,respectively.These three indices in autumn were higher than those in the other three seasons,and the highest values of diversity index in each season were located in the northern branch of the Yangtze River estuary.The results of seasonal replacement index（AI）and migration index（MI）showed that the AI was the lowest in autumn and MI was close to 0,while the AI was the highest in winter and MI was both negative.The range of size classes of crustaceans was greatest in autumn（-3～9）,and the peak biomass of each season located in the range of 0～4 size class.The fitted curves for the normalizedsize spectrum were all dome-shaped,and the curvature ranged from-0.13 to-0.04.The W statistics representing the ABC curve ranged from-0.21 to-0.10,all of which were negative values,indicating that the crustaceans in the Yangtze River estuary were seriously disturbed and their community structure was in an unstable state.（3）A total of 9720 crustaceans with a total weight of 13.81 kg were collected.The average annual resource density of crustaceans was 1242.90 kg?km^-2 and 874.74×10³ ind.?km^-2.There were significant seasonal differences in the resource density of crustaceans（P<0.05）,with the highest in summer and the lowest in winter.As for the spatial distribution of resource density,the northern branch of Yangtze River estuary was higher than the southern branch on the whole.And compared with the survey results before the year of total ban（2017～2018）,the density of crustacean resources in the Yangtze Estuary showed a multiple increase in the early period of the ban.The result of CCA showed that temperature,salinity,dissolved oxygen,depth and p H played an important role in the distribution of crustacans in the Yangtze River estuary.The main environmental factors were salinity in spring,salinity and dissolved oxygen in summer,salinity,dissolved oxygen and p H in autumn,and depth,temperature and salinity in winter.The main environmental factors in all seasons were significant（P<0.05）except spring.The study investigated the community structure characteristics,diversity and stability,resource density distribution and environmental factors of crustaceans in the Yangtze River estuary during the early period of fishing ban in key waters of the Yangtze.And It can provide a preliminary basis for the assessment of aquatic living resources in the Yangtze Estuary before and after the ban on fishing.