Approach to determine individual trophic level and the difference in food sources of Japanese anchovy Engraulis japonicus in Sagami Bay, based on compound-specific nitrogen stable isotope analysis of amino acids

Abstract

Compound-specific stable isotope analysis of amino acids (AA method) has recently been used to estimate the trophic level (TL) of animals. The individual TL (TLGlu/Phe) of Japanese anchovy Engraulis japonicus was precisely determined using the AA method without sampling or analysis of primary producers. Previously, TLs of organisms were determined using bulk nitrogen stable isotopic composition (δ15Nbulk) of tissues (bulk method). The δ15Nbulk for Japanese anchovies varied widely because of their migratory behavior, and the TL of anchovies in Sagami Bay remained ambiguous. We showed that the TLGlu/Phe of anchovy was almost 3 and that food-web structures differed between inshore and offshore waters. Furthermore, there was little difference in TLGlu/Phe for representative Japanese anchovies in Sagami Bay, despite the large intra-sample variation in δ15Nbulk for anchovies ranging over a trophic shift of 3–4 ‰ in Sagami Bay. We speculated that this variation in δ15Nbulk resulted from differences in food sources among the different food webs. Estimation of individual TLs in some simple food chains using the AA method may enable the determination of the food-web structure of migratory fishes, which is difficult to determine using the traditional bulk method.