Fatty acids and stable isotopes of a marine ecosystem: Study on the Japanese anchovy (Engraulis japonicus) food web in the Yellow Sea

In recent years, the decline of the fish resource in the Yellow Sea has been dramatic. The fish community structure is changing. We determined the stable isotope and fatty acids composition in the Japanese anchovy (Engraulis japonicus) (dominant species) food web in the Yellow Sea, China, to study the ecological variation especially in the shifting trophic level. The major fatty acids found in the species were 14:0, 16:0, 16:1n-7, 18:0, 18:1n-7, 18:1n-9, 20:5n-3, and 22:6n-3, which made up 60-80% of the total fatty acids (FA). Scomberomorus niphonius and Conger myriaster were richer in 18:1n-9. A higher percentage of saturated FA and lower percentage of polyunsaturated FA were found in Pseudosciaena polyactis, Chelidonichthys kumu, and Priacanthus macracanthus. A special feature of the grassfish was the high level of 20:4n-6. An enrichment of stable carbon and nitrogen isotopes was observed from phytoplankton and zooplankton to fishes; the stable nitrogen isotope varied in the range 2.9-13.9‰ and the mean values of stable carbon isotopes of fishes tended to vary within a limited range (-19.4 to -17.9‰). Both δ¹⁵N and specific fatty acids (such as 18:1n-9, DHA, EPA) showed a good relationship with trophic level. A distinct change in the fatty acids signature and stable isotopes was observed for the Japanese anchovy with the increase in body length, which was independent of the variety of geographic locations. Egg and postlarva samples were richer in 16:1n-7, 18:1n-9, and C22:6n-3. The major fatty acids were 16:0 and 18:1n-9, followed by 20:5n-3, 22:6n-3, and 20:4n-6, in young and adult Japanese anchovy. The shift in diet between the different life stages could be the explanation for this observation. Variations in trophic levels can be proven using stable isotope and fatty acids compositions, but this should be done with caution, such as with the application of principal components analysis in this study.