Primary Studies On The Community Structures And Trophic Structures Of Nekton In The South-central East China Sea

Affected by factors such as depth and water mass,the south-central East China Sea has been afformed a good place for a variety of fishery organisms to multiply,feed and overwinter,and it is also one of the most productive areas of fishery resources in China.However,influenced by multiple factors such as overfishing,marine pollution,and environmental changes,the composition of the catches in this sea area has changed greatly,which is characterized by the proportion increased of the species with small individuals and low economic value.Although the implementation of some policies has alleviated the recession crisis of fishery resources in the East China Sea,relevant studies have found that the current situation of fisheries resources in this area was still griming and more attention should be paid.Based on the survey data of bottom trawling in the south-central East China Sea in autumn 2018 and spring 2019,the species composition,dominant species,diversity characteristics,and community structure of nekton in this sea area were studied.A total of 181 nekton species were caught in autumn and 134 in spring,respectively.For dominant species,Benthosema pterotum,Trichiurus lepturus,Bregmaceros mcclellandi,Harpadon nehereus,and Acropoma japonicum were determined in autumn,while Trichiurus lepturus,Scomber japonicus,Benthosema pterotum,Setipinna tenuifilis,Abralia multihamata and Bregmaceros mcclellandi were determined in spring.The abundance curves were all above the biomass curves,which indicated that the species with smaller individuals accounted for a higher proportion of the catch.In autumn,the range of Margalef’s species richness index（D）,Shannon Wiener diversity index（H′）,and Pielou’s evenness index（J′）was 0.37 to 5.94,0.38 to 2.64,and 0.14 to 0.71,respectively.While the value of D,H′and J′ranged from 0.08 to 4.55,0.29 to 2.62,and 0.15 to 0.83,respectively,in spring.According to the results of the cluster,the nekton communities’structure of the south-central East China Sea was divided into 2 groups which were bounded by 90 m isobath.The results of SIMPER analysis showed that the variations among the groups were mainly caused by Benthosema pterotum,Pampus echinogaster,Harpadon nehereus,Setipinna taty,and Bregmaceros mcclellandi in autumn.While in spring,the differences were mainly caused by Setipinna taty,Harpadon nehereus,Scomber japonicus,Benthosema pterotum and Engraulis japonicus.The results of BIOENV analysis showed that the water depth was most closely related to the community structure of nekton,and the correlation coefficient was 0.55 in autumn and 0.53 in spring,respectively.In addition,the combination of the water depth and temperature also had a great influence on the community structure.According to the results of the community structure analysis,the species of IRI>10was taken as the main research object on the nutritional structure of the south-central East China Sea.The stable isotope analysis（SIA）was used to study the trophic characteristics and feeding habits of the functional groups.In autumn,the mean value ofδ¹³C ranged from-24.3‰to（-15.8±0.5）‰,andδ¹⁵N ranged from（7.0±1.1）‰to（13.8±0.8）‰.In spring,the mean value ofδ¹³C ranged from（-21.1±0.8）‰to（-16.4±0.3）‰,andδ¹⁵N ranged from（6.6±1.4）‰to（12.8±0.4）‰.Marked difference has been confirmed in stable isotope signatures and trophic levels among sampling periods,which may be closely related to the seasonal variations of the environmental factor,such as the nitrogen fixation ability of primary producers,the abundance of food organisms,or the feeding habits of predators.The stable isotope niche constructed byδ¹³C andδ¹⁵N values was used to represent the relationship among various nekton items,and the results showed that there was a certain competitive relationship among fish,crustacean,and cephalopod.Importantly,our results found that the trophic niche overlapped more in autumn,which suggests a more complex competition among the items,and maybe resulted in the varies of primary productivity and food sources among sampling periods.It was also found that the structure of the food web in the south-central East China Sea tends to be simplified,which is characterized by zooplankton playing an important role in the feeding ecology of cephalopods,crustaceans,and fish,with the dietary contribution ranging from 6.0%to84.4%.Using stable isotope analysis（SIA）,we investigated the trophic ecology of Bombay duck in the south-central East China Sea in two periods.The results showed that theδ¹³C andδ¹⁵N values ranged from-18.6‰to-16.0‰and 10.7‰to 14.0‰in autumn,and ranged from-20.2‰to-16.7‰and 8.5‰to 13.2‰in spring,respectively.Significant differences inδ¹³C andδ¹⁵N values were found among sampling periods and samples’body length,while the stable isotope signatures were less affected by the offshore distance.The trophic levels（TLs）of Bombay duck ranged from 3.1 to 4.1 in autumn and 2.6 to 3.9 in spring,respectively,which was characterized by the increase in body length and fluctuated sharply in the same size classes.Furthermore,marked differences were confirmed in the feeding habits of the Bombay duck among individuals and temporal,characterized by the variations of zooplankton and fish dietary contribution rates to predators.Such a shift of diet expanded the food sources of the population and satisfied the nutritional needs of the Bombay duck at different growth stages.The ingestion rate of fish and shrimp changed among different periods,which can be related to the biomass varies of food organisms in the sea areas.High trophic plasticity has been confirmed in Bombay duck by stable isotope evidence,which enables it to adjust the feeding habits according to the growth needs or environmental changes.And this may one of the reasons for the success of this species in competition with other fish.