Trophic Interactions In The Northern Slope Of The South China Sea Through Fatty Acid And Stable Isotope Analyses

Mesopelagic fishes are important components of marine food webs, studying as the core of marine food trophic structure in marine ecosystem energy flow and circulation of materials have important significance. The feeding relationships and trophic structure in the Northern Slope of the South China Sea were studied, through carbon and nitrogen stable isotope analysis and fatty acid components analysis, based on the survey samples collected. Feeding rhythm of mesopelagic fishes as the core of the food web of energy flow and material cycle has important significance, but little is known.Fish fatty acids contents of saturated fatty acid is higher, more than40%.Analysising of fish fatty acid composition, polyunsaturated fatty acids is mainly composed of n- 3 and n- 6 series unsaturated fatty acids. The main dominant species have feeding relationships or the same food sources because of the similar fatty acid kinds and compositions in the Northern Slope of the South China Sea.According to main component analysis of fatty acid compositions, all species distributing in factor score map principal component 1 is upward. It shows a typical characteristic of marine phytoplankton. Meanwhile, different DHA value indicates different nutrition level in the food web of those species, the higher the value of the relative levels of the trophic level increases.The δ13C andδ15N of fish is significantly higher than that of zooplankton. The value of δ13C range from-19.5 ‰ to-18.2 ‰ by stable carbon isotope analysis,showing a typical characteristic of marine phytoplankton in vivo enrichedδ13C.Nitrogen stable isotope analysis measured δ15N, concentrating in the range of7.5‰~10.8‰. These mean wider range of diets.The value of δ15N is accumulating as the nutrition level rises, while the relative amount of enrichment of each trophic level is constant. There are 3 groups:zooplankter(Rhincalanus sp.、Euchaeta sp.、Scolecithrix sp.、Calanus sp1.、Calanus sp1.)、secondary consumer(Diaphus chrysorhynchus、Diaphus luetkeni、Myctophum obtusirostris、Champsodon capensis、 Apogonichthys sp. and Priacanthus tayenus) 、 Senior consumers(Nemipterus virgatus、Priacanthus tayenus、Priacanthus tayenus).Diaphus chrysorhynchus sampled length range of 28 mm ~ 110 mm, mainly in the 50 ~ 59 mm length group. There were not significant differences between length and feeding stages(P=0.054,n=366). Diaphus chrysorhynchus were feeding at night,stomach fullness index(SFI)of which varied from 1.50 to 2.57, increasing of food intake in the evening around 18:00 and reaching a peak feeding about 05:00. It showed a rising trend at night, the 01:00~02:00 time period of which does not appear empty stomach phenomenon, that there is a continuous feeding behavior. Analysis of the correlation between feeding stages of each feeding period, there are significant differences which in 18:30 ~ 19:30 time period with 4:45 ~ 05:50 and 06:30 ~ 7:30gastric feeding grade difference was very significant(P = 0.000; P = 0.007). Diaphus chrysorhynchus less food intake in the evening, food grade was low as well as, while feeding mainly at night, higher saturation index stomach.According to the analysis method of fatty acids and carbon and nitrogen stable isotope ratio, it shows that the biological feeding channel and relationships among marine food chain by matter and energy transfer from the growth of marine phytoplankton primary consumers(Zooplankton) to the secondary consumers(Mesopelagic fishes), and finally by a senior consumer(Symplectoteuthis oualaniensis,Nemipterus virgatus, Priacanthus tayenus, Decapterus maruadsi) uptake. Meanwhile,it reveals energy flow and transformation among the high nutrient levels in marine food web and the irreplacable significance of mesopelagic fishes in marine ecosystem in the Northern Slope of the South China Sea.