Analysis Of The Trophic-ecological Relationships Of The Major Cephalopods In The Northwest Pacific Ocean

The Northwest Pacific Ocean is the largest marine ecosystem on Earth and is located where two strong western boundary currents（the Kuroshio Current and Oyashio Current）converge,which brings abundant nutrients,and provides a baits rich feeding ground for many pelagic fish,cephalopod and shrimp and so on.In addition,cephalopods are considered to play an important structural role in the pelagic marine ecosystem,serving as an important feeding resource for many top predators,as well as the main predators of lower trophic organisms.Meanwhile,the long-distance migration movement during their life cycle also brings the spatio-temporal transfer of a large amount of biomass in the ecosystem.At present,stable isotope technology has been widely used in the study of individual,interspecific and community trophic structure of marine species.By selecting different types of soft and hard tissues,the feeding,habitat variations and migration movement of selected research objects can be studies.In this study,the catch and zooplankton collected by Shanghai Ocean University’s oceanic fisheries resources survey vessel“Song Hang”in March and September 2019 and the Ommastrephes bartramii were obtained by Chinese jigging vessels in the North Pacific Ocean from May to November 2020.Firstly,from the perspective of community,we studied the pelagic trophic structure of catch in the Northwest Pacific Ocean,determining the trophic position of cephalopods in the community structure preliminarily and comparing the resource utilization among other species.By measuring the carbon and nitrogen stable isotope values of catch muscles（including fish,cephalopod,shrimp）and zooplankton species to calculate trophic levels of each fish species,comparing trophic niche among fish,cephalopod and shrimp.In addition,we also compared the trophic niche of several catch with relatively large samples.Secondly,to understand the role of oceanic cephalopods in the marine food web,we selected two major cephalopods,Enoploteuthis chunii and Abralia similis,to study their interspecific niche changes:comparing their carbon and nitrogen stable isotope values among different sites and the niche variation in different ontogenetic stages.In addition,from the aspect of individual study,we mainly selected the important economic species,O.bartramii to research.Through the measurement of carbon and nitrogen stable isotope values in continuous eye lens segments,combining with eye lens segment diameter and statolith increments back calculation,we investigate the ontogenetic changes in terms of trophic ecology of O.bartramii in the Northwest Pacific Ocean.The results are as follows:（1）Studies of the trophic structure of catch in the Northwest Pacific Ocean have shown that Significant differences ofδ¹³C andδ¹⁵N values were detected among fish,cephalopod and shrimp groups withδ¹³C value ranges of-21.9‰to-18.7‰,-21.3‰to-17.7‰and-20.4‰to-19.5‰,respectively and the range ofδ¹⁵N values was 7.0‰to 12.4‰,8.2‰to 12.2‰and 7.6‰to 10.6‰,respectively.Using copepods as the baseline for estimating the average trophic level（TL）of micronekton,the TLs ranged from 2.67 to 4.80 and the average TLs for cephalopods,fishes and shrimp were 3.3±0.3,3.7±0.6 and 4.0±0.3,respectively.Myctophidae and Stomiidae occupied higher TLs and cephalopods were widely distributed at different TLs.In addition,Oplophorus gracilirostris,Enoploteuthis chunii and Abralia similis had wide isotopic niches.Our results show that Myctophidae,Stomiidae,O.gracilirostris,and cephalopods play important roles in maintaining the stability of the Kuroshio-Oyashio ecosystem in the Northwest Pacific Ocean.（2）Studies of niche variation of two major cephalopods,E.chunii and A.similis have shown that theδ¹³C,δ¹⁵N andδ¹⁵N_b values of E.chunii were significantly different among sites（P<0.05）,while theδ¹³C andδ¹⁵N_b values of A.similis were significantly different（P<0.05）and theδ¹⁵N values were not significantly different（P>0.05）.During ontogenetic development,for E.chunii,the correlations betweenδ¹³C andδ¹⁵N values and MLs were not significant（P>0.05）,while the correlation betweenδ¹⁵N_b and MLs were significant（P<0.05）;for A.similis,the correlations betweenδ¹³C,δ¹⁵N andδ¹⁵N_b values and MLs were not significant（P>0.05）.Comparative analyses showed that the adult of E.chunii had a higher niche width（SEAc=1.78‰²）and a low overlap between the juvenile and the adult stages（0.19）;the SEAc of the juvenile（SEAc=0.68‰²）and the adult（SEAc=0.39‰²）stages of A.similis were less variable with a medium overlap between them（0.37）.Also there was a medium overlap beweeen E.chunii and A.similis in the juvenile stage（0.33）and a low overlap between them in the adult stage（0.20）.The analysis suggested that theδ¹³C andδ¹⁵N values of E.chunii were influenced by the isotopes baseline spatial variation and the feeding effect,while theδ¹³C andδ¹⁵N values of A.similis were mainly influenced by the spatial variation of baseline isotope values.（3）Studies of ontogenetic changes in terms of trophic ecology of O.bartramii in the Northwest Pacific Ocean have shown thatδ¹³C andδ¹⁵N values gradually increased with ontogenetic growth of the squid.From the paralarval to the juvenile stage,the trophic niche breadth increased,which might be the reason why the swimming and feeding ability improved as they entered the juvenile stage.Meanwhile,interactions between different ecosystems led to a greater diversity of food sources;thus,their feeding targets were no longer limited to plankton but shifted toward small fish and other cephalopods.From the juvenile to the adult stage,the trophic niche breadth decreased,which can be explained by that O.bartramii had a selective preference for certain prey as ontogenetic growth proceeded,and they seemed to focus more on larger prey in the adult stage.Furthermore,the small amount of overlap between early and later life cycles suggested a significant trophic niche separation among different trophic ecologies and spatial ecologies.