| The central western South China Sea is one of the main fishing grounds for fishery development.In order to understand the trophic relationships of the major fishery organisms in the central western South China Sea,we applied stable isotope techniques to analyze and determine carbon and nitrogen stable isotope ratios of major fishery biological samples in the area.The trophic levels of the main fishery biota were calculated and a continuous trophic spectrum of the main fishery organisms in the area was drawn.On this basis,the changes in carbon and nitrogen stable isotope of the dominant species in the sea area,the different long-term groups of Sthenoteuthis oualaniensis,and the differences in nutrient niche between the two Sthenoteuthis oualaniensis populations were further analyzed.In this study,we had preliminarily drawn the spectrum of the continuous trophic levels of the major fishery bio-nutritional levels in the central western South China Sea,providing a theoretical basis for the food web structure and future fishery resource utilization in the area.The following main findings were obtained:1.The results showed that the major fishery isotope ratios exhibited a wide range of changes in the central western South China Sea.The δ13C and δ15N values of the fish ranged from20.00‰ to-16.51‰ and from 7.94‰ to 11.81‰,respectively.The δ13C and 815N values of the cephalopods ranged from-18.84‰ to-17.60‰ and from 10.10‰ to 12.85‰,respectively.The corresponding trophic levels of each species were calculated by using zooplankton as the baseline organism.The trophic levels of fish ranged from 2.41 to 3.53,and those of cephalopods ranged from 3.03 to 3.84.Among these organisms,the average trophic level of cephalopods was higher than that of fish.Comparison of trophic levels in different mantle lengths of Symplectoteuthis oualaniensis,Thysanoteuthis rhombus,Decapterus russelli,Decapterus lajang,and Thunnus albacares revealed that the trophic level showed a correspondingly increasing trend as body length increased.The δ13C range and δ15N range values of spring biomes in the central western South China Sea were 3.49 and 4.91,respectively.The total area and mean distance to centroid values were 9.48 and 120,respectively,mean nearest neighbor distance and standard deviation of nearest neighbor distance values were 1.69 and 0.74,respectively.These results indicate that the food source in the waters of central western South China Sea has a single food source,a low nutrient level,a small niche breadth,a high nutritional redundancy layer,and a seasonal change in the nutrient structure.2.Based on the samples collected from the catch of Sthenoteuthis oualaniensis in the South China Sea from April to May 2017,we studied the relationship between feeding intensity and feeding level of purple flying squid and purple flying squid mantle length by using carbon and nitrogen stable isotope techniques to examine the values of carbon and nitrogen stable isotopes and the trophic levels of different squid groups.The results showed that the feeding levels of flying squid were of mainly grades 3 and 4,and the feeding grades of female and male individuals were increased with the increase of sputum length.The empty stomach individuals only appeared in the low sorghum group,both female and male squid.The empty stomach rates(VI)were 3.33%and 3.14%,respectively.The δ13C values of female and male squid ranged from-18.31‰ to-17.56‰,and from-18.52‰～-17.88‰ while the δ15N values range from 9.67‰ to 11.41%。,and 9.41‰ to 10.86‰,respectively.The values of δ13C and δ15N varied significantly with the length of mantle length(P<0.01).The trophic levels of female and male flying squid were 2.90～3.41,and 2.83～3.25,and the average trophic levels were 3.16 and 3.07,respectively.The δ15N value and trophic level showed that δ15N value and trophic level of S.oualaniensis varied significantly with its body size.When the squid was more than 130 mm long,the sapphire squid was higher than 3 in both male and female.The 815N value and trophic level of squid were increased with the increase of mantle length.Dietary shift may be the main factors contributing to the difference.3.In order to understand the relationship between the characteristics of carbon and nitrogen stable isotopes and trophic niche among different populations of the purple flying squid Sthenoteuthis oualaniensis,we collected the medium and dwarf forms of the purple flying squid in the central-western South China Sea in August 2017.The data of carbon and nitrogen stable isotopes were used to analyze the carbon and nitrogen stable isotope values and the changes in trophic levels of the two purple flying squid populations,and the differences trophic niche of the two purple flying squid populations were compared.The results showed that the δ13C value of the medium forms group of the purple flying squid ranged from 19.54‰～18.10‰ while the δ15N value ranged from 7.79‰～9.45‰,the average trophic level was 2.90 and ranged in 2.72～3.21,the total niche area(TA)was 1.21,the average centroid distance(CD)was 0.52;the δ13C values of the dwarf-form group were in the range of 19.69‰～18.43‰(CR value were 1.26),and the δ15N value ranged 8.02‰～8.99‰(NR value were 0.97),the average trophic level was 2.91 and ranged in 2.79～3.08,the total niche area(TA)was 0.82,and the average centroid distance(CD)was 0.38.The δ13C values of the medium and dwarf forms of purple flying squid were not significantly different whereas their δ15N values were significantly different.The mantle length had significant impacts on δ13C and δ15N,and the value of δ15N was increased with the mantle length.The trophic niche breadth and the trophic level of the medium-form of purple flying squid were more diverse than those of dwarf-form. |
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