Trophic hierarchy of coastal marine fish communities viewed via compound-specific isotope analysis of amino acids

Coastal marine ecosystems are very complex and composed of myriad organisms, including offshore, coastal, and migratory fish occupying diverse trophic positions (TPs) in food webs. The illustration of trophic hierarchy based on the TP and resource utilization of individual organisms remains challenging. In this study, we applied compound-specific isotope analysis of amino acids to estimate the TP and isotopic baseline (i.e. delta N-15 values of primary resources at the base of food webs) for 13 fish and 1 squid species in a coastal area of Sagami Bay, Japan, where a large diversity in the isotopic baseline is caused by an admixture of ocean currents and artificial nitrogen inputs. Our results indicate that the TP of fish and squid varies between 2.9 and 3.9 (i.e. omnivorous, carnivorous, and tertiary consumers), with low variation within individual species. Moreover, the delta N-15 values of phenylalanine revealed the diversity of isotopic baselines between and within species. Low values (7.8-10.3 parts per thousand) and high values (18.6-19.2 parts per thousand), with a small variation (1 sigma < 1.0 parts per thousand), were found in 2 offshore species and 3 coastal species, respectively. In contrast, highly variable values (9.8-19.7 parts per thousand), with large variation within species (1 sigma > 1.0 parts per thousand), were found for the remaining 9 migratory species. These results represent evidence of differential trophic exploitation of habitats between offshore and coastal species, particularly among individuals of migratory species, that were all collected in a single area of Sagami Bay.