Stable Isotope-Based Community Trophic Structure Of Fishery Organisms In The Offshore Waters Of Central And Southern Zhejiang

The offshore waters of central and southern Zhejiang belong to the core waters of the central and southern East China Sea.Affected by multiple water masses such as alongshore current and warm current,the offshore waters of central and southern Zhejiang has become an excellent bait yard and spawning ground for various economic species.As one of the main producing areas of important economic fishes,shrimps and crabs in the East China Sea,due to the influence of human fishing activities,marine environmental pollution and global climate change,the fishery structure shows that low trophic level species instead of high trophic level species have become the main fishing targets,the catch structure shows a miniaturization and low age trend,and fishery biodiversity decreases.Improving the resilience and stability of the ecosystem can effectively mitigate the adverse effects of environmental disturbance,and make the system stable enough to cope with the impact of man-made fishing,and to effectively manage and protect the dynamic ecosystem,it is necessary to thoroughly understand the trophic structure of the food web in the ecosystem and the characteristics of the trophic relationship among organisms.In this paper,stable isotope technique is used to study the trophic structure and its spatio-temporal changes of fishery communities in offshore waters of central and southern Zhejiang,analyze the trophic diversity and trophic redundancy of community,and explore the internal stability mechanism of community structure,in order to provide scientific references for future resarch on the material cycle,energy flow and ecological restoration of offshore ecosystems.In my paper,based on the fishery surveys in the offshore waters of central and southern Zhejiang in February,May,August and November 2016,stable isotope analysis was used to determine the?¹³C and?¹⁵N values of the muscles of main fishery organisms,such as fishes,crustaceans,cephalopods and gastropods,to quantitatively analyze the trophic structure characteristics of feeding types,trophic functional groups and continuous trophic spectra,and spatio-temporal dynamics of fishery community in the survey area.Isotopic niche was used to analyze the trophic structure and interspecific and intraspecific trophic interactions of important fish community.The main conclusions are as follows:?1?The stable carbon and nitrogen isotope ratios(?¹³C,?¹⁵N)of 33 species of fishes and 18 species of invertebrates caught in the offshore waters of central and southern Zhejiang were calculated by stable isotope analysis.The range of isotope ratios of the fishery community in offshore waters of central and southern Zhejiang was wide,and the?¹³C values in the community ranged from-19.71‰to-14.01‰,with the span of 5.70‰,and the range of?¹⁵N values was from 7.05‰to 13.69‰,with the span of 6.64‰.The major fishery organisms had great variations in composition of stable carbon and nitrogen isotopes,especially fish species,with statistically difference P<0.01.The range of carbon and nitrogen isotopes of fish was the largest(?¹³C:-19.07‰?-15.53‰;?¹⁵N:7.57‰?12.87‰).Compared with fish,invertebrates have poorer?¹⁵N values and richer?¹³C values(?¹⁵N:8.79‰?11.17‰;?¹³C:-17.30‰?-14.58‰).?2?Based on?¹³C and?¹⁵N cluster analysis and feeding literature,it is determined that there are five feeding types of main fishery organisms in the offshore waters of central and southern Zhejiang,including the feeding habits of zooplankton of Japanese anchovy Engraulis japonicus,kammal thryssa Thryssa kammalensis and skinnycheek lanternfish Benthosema pterotum,omnivorous habits of spotted velvetfish Erisphex pottii,mimika bobtail squid Euprymna morsei and Plesionika izumiae,benthic feeding habits of crustaceans and gastropods such as Solenocera crassicornis,swimming crab Portunus trituberculatus and Murex trapa,the mixed feeding habits of small yellow croaker Larimichthys polyactis,bombay duck Harpadon nehereus,largehead hairtail Trichiurus lepturus and spadenose shark Scoliodon laticaudus,and the feeding habits of swimming animals such as silver croaker Pennahia argentata,yellow goosefish Lophius litulon and blackspot threadfin Polydactylus sextarius,among which the number of mixed species was the most.?3?Using filter-feeding bivalves as baseline organisms,it was estimated that the avegrage trophic levels of the main fishery organisms in the offshore waters of central and southern Zhejiang were between 2.66 and 4.21,in which the ranges of fish,crustacean,cephalopod,gastropod were 2.66 to 4.21,3.08 to 3.72,2.83 to 3.49,and3.54 to 3.62,respectively.The major fishery organisms were mainly in the range of 3.0to 4.0,accounting for 84.31%of the total,most of which were primary and intermediate carnivorous species.According to the divided standard of trophic level,the trophic structure of food web in offshore waters of central and southern Zhejiang could be divided into four trophic groups.The primary consumers?2 species,2.5-3.0?were omnivorous species,and its representative organisms including Erisphex pottii and Euprymna morsei,while the secondary consumers?14 species,3.0-3.5?were mainly small fish,shrimps and cephalopods,and its representative organisms including Engraulis japonicus,Champsodon snyderi,Plesionika izumiae and Sepiella inermis.The intermediate consumers?28 species,3.5-4.0?were mainly benthic crabs,gastropods and mixed-feeding fish,and its representative organisms including swimming crab Portunus trituberculatus,small yellow croaker Larimichthys polyactis,Amblychaeturichthys hexanema and Trichiurus lepturus,while senior consumers?7species,?4.0?were ferocious carnivorous fishes,and its representative organisms including Dysomma anguillare and Lophius litulon,etc.Based on the stable isotope analysis,the types of feeding habits and the division of community trophic groups in the offshore waters of central and southern Zhejiang were analyzed,which laid a parameter foundation for the construction of ecosystem energy model in the future.?4?Based on the isotopic niche breadth and overlap quantification index of?¹³C-?¹⁵N two-dimensional coordinate system,the trophic structure,interspecific trophic interaction and seasonal variation of 14 species of fishes with different trophic levels were analyzed.The results showed that there were significant seasonal differences in the trophic structure of fish community?P<0.01?,especially the?¹³C values in summer were significantly different from those in other seasons?P<0.01?.The?¹³C values of fish community in summer mainly ranged from-16.0‰to-15.0‰,while those in other seasons ranged from-17.0‰to-16.0‰.In addition,it was found that there were seasonal differences in?¹³C and?¹⁵N values of 13 species of fish except moray eel Muraenesox cinereus.However,the interseasonal fluctuations of?¹³C values?1.15‰?1.52‰?and?¹⁵N values?1.02‰?3.18‰?of mixed feeding habits animals representing Trichiurus lepturus,Chelidonichthys spinosus and Larimichthys polyactis etc were higher than those of benthic feeding habits animals including moray eel Muraenesox cinereus,Dysomma anguillare,Johnius belangerii and Amblychaeturichthys hexanema etc?0.34‰?0.84‰?.0.38‰?0.86‰),indicating that the carbon and nitrogen stable isotope characteristics of planktonic food sources were higher than those of benthic food sources.The trophic niche breadth of fish was different among seasons.The trophic niche breadth of fish in spring was generally higher than that in other seasons?average SEAc=0.835‰²?.In summer,the trophic niche breadth of most fish is relatively smallest?average SEAc=0.455‰²?.Largehead hairtail had a wider trophic niche width than other fish in each season,showing a strong ability to occupy and utilize food resources.Compared with other fish,the width of trophic niche of large yellow croaker Pseudosciaena Crocea was smaller,and its ability to occupy resources was relatively weak.There was a significant trophic niche overlap among fish species,and there was a strong overlap among fish species with similar trophic positions,such as large yellow croaker had a high trophic niche overlap with similar trophic position of species such as Chelidonichthys spinosus?99.95%?and Amblychaeturichthys hexanema?77.78%?in spring,Harpadon nehereus?55.44%?and small yellow croaker?43.50%?in summer,samll yellow croaker?63.39%?in fall and Johnius belangerii?77.33%?and Stromateoides argenteus?76.71%?in winter.?5?The stable isotope characteristics and trophic niche breadth and overlap of 10dominant fish species at different ontogenetic stages were analyzed to predict the potential differentiation of intraspecific resources competition caused by ontogeny.The results showed that there was no significant difference in?¹⁵N and?¹³C values among body length groups of Amblychaeturichthys hexanema and Setipinna tenuifilis?P>0.05?,and there was no significant difference in?¹³C values among different body length groups of Pennahia argentata,Dysomma anguillare and Larimichthys polyactis?P>0.05?,but there were significant differences in?¹⁵N and?¹³C among length groups of other six species of fish such as largehead hairtail?P<0.05?.Using nonlinear local polynomial regression to fit the variation trend between?¹⁵N or?¹³C and body size,the results showed that the?¹⁵N values of Pennahia argentata,Trichiurus lepturus,Chelidonichthys spinosus,Johnius belangerii,Harpadon nehereus and Dysomma anguillare increased with ontogeny,which indicated that the food intake tended to be of higher trophic level species.The?¹³C values of Johnius belangerii,Harpadon nehereus and Trichiurus lepturus increased with the increase of body size,indicating a shift in feeding habits to benthos,while the small individuals of Chelidonichthys spinosus and small yellow croaker increased first and then decreased when they growed to large individuals,which indicated that the feeding habits changed from benthic to pelagic food,which reflected the great fluctuation of feeding habits.Among the 10species of fish,the trophic levels of Pennahia argentata,Chelidonichthys spinosus,Johnius belangerii,Dysomma anguillare and Trichiurus lepturus increased gradually with ontogeny.Ontogeny contributes to wider distribution and less overlap of isotopic niche of fish.It was found that the isotopic niche overlap ratio of fish under size-pooled was 82.22%,while that of size-separated was only 51.72%.In terms of the average overlap percentage,the average overlap percentage of the niche ellipse of the size-pooled was 19.64%,while that of the size-separated was 17.12%,which significantly reduced the overlap of isotopic niche.Comparing the intraspecific trophic niche differences,there were significant differences in the size classes of seven species of fish except Amblychaeturichthys hexanema,Setipinna tenuifilis and Larimichthys polyactis?P<0.05?,which indicated that the ontogeny of fish could reduce the competition for intraspecific resources and help to maintain the stability of the trophic structure of community.?6?Layman community-wide metrics were used to reflect the important aspects of thophic structure,and compare the temporal and spatial changes of the trophic structure of fishery community in the offshore waters of central and southern Zhejiang.The results showed that there were significant temporal and spatial differences in the composition of main fishery organisms in the offshore waters of central and southern Zhejiang.The fishery biodiversity in the waters of southern Zhejiang was higher than that in central Zhejiang,and the fishery biodiversity in summer was significantly lower than that in other seasons.The analysis based on the carbon and nitrogen stable isotope characteristics of fishery organisms showed that there was no significant difference in the trophic structure of the main fishery community in the waters of southern Zhejiang and central Zhejiang?P>0.05?,and the spatial structure of trophic niche of the community was highly overlapping,with the average overlap percentage of 93.43%,and the trophic length also was similar,and the range of biological trophic levels were basically from 3.0 to 4.5;There were significant seasonal differences in the trophic structure of community between the waters of southern and central Zhejiang,which showed that the?¹³C values of the community in summer were significantly different from those in other seasons,and the?¹⁵N values in spring were significantly different from those in other seasons.The values of CR,NR and CD of fishery community in the waters of southern Zhejiang were generally higher than those in central Zhejiang,and had higher trophic diversity than those in waters of central Zhejiang.The values of CR,NR and CD of fishery community in summer were lower than those in other seasons,and the trophic diversity was lower,while the trophic diversity and ecological space utilization rate of the fishery community were higher in spring,with high trophic redundancy.Based on the results of convex polygon analysis in isotopic two-dimensional coordinate system,the key vertex species of community structure in the waters of central and southern Zhejiang were divided into three categories,namely,secondary consumers of mid-upper zooplankton feeding habits,including Engraulis japonicus,Benthosema pterotum and Collichthys lucidus,and secondary or intermediate consumers of benthic crustaceans,such as Portunus trituberculatus,Solenocera crassicornis and Charybdis bimaculata,and senior consumers with carnivorous feeding habits,such as Parargyrops edita,Dysomma anguillare,Scomberomorus niphonius and Lophius litulon and so on,which ensured the stability of food web structure.