Trophic structure and food resources of epipelagic and mesopelagic fishes in the North Pacific Subtropical Gyre ecosystem inferred from nitrogen isotopic compositions

We used bulk tissue C-13 and N-15 values and N-15 values of individual amino acids (AA) to characterize the trophic structure of a pelagic fish assemblage from the North Pacific Subtropical Gyre (NPSG) ecosystem. We focus on energy flow between fishes inhabiting distinct epipelagic, mesopelagic, and upper bathypelagic habitats and on predatory fish foraging across and within these depth habitats. Trophic positions (TPs) estimated from a combination of trophic and source AA N-15 values (TPTr-Src) spanned a narrow range of 0.7 TP for 10 species of large fishes, including tunas, billfishes, and gempylids (TPTr-Src 4.3-5.0). Similarly, 13 species of small micronekton fishes encompassed a range of 1.2 TP (TPTr-Src 2.6-3.8). The N-15 values of three source AAs were found to increase with increasing depth of capture across the 13 micronekton fish species (N-15(Phe) range=6.6 parts per thousand; N-15(Gly) range=13.4 parts per thousand; N-15(Ser) range=13.6 parts per thousand), indicating that some species from epipelagic, mesopelagic, and upper bathypelagic communities access distinct food resources, such as suspended particles. These isotopic depth trends are consistent with previous observations in particulate organic matter and zooplankton from the NPSG, providing new evidence that large pelagic and micronekton fishes access a food web fueled by particles formed in surface waters but that are highly modified by microbes as they slowly settle to remote depths. On the contrary, no significant relationships between the N-15 values of source AAs and habitat depth were observed in the large predator fish group, of which many species move and forage across large depth gradients.