Largehead hairtail (Trichiurus japonicus) is one of the most commercially important fishes in the western North Pacific. To identify the dietary changes across ontogeny and seasons, we investigated its stomach contents and stable isotope signatures (δ13C and δ15N) in the South Sea of Korea. The index of relative importance revealed that the dietary composition of largehead hairtail consisted mainly of Pisces (1.6%–84.8%) and Euphausiacea (3.0%–93.0%). However, the proportions of dietary items differed by season and anal length (ALs), suggesting a trophic adaptability response to seasonal food availability and size-related trophic variation, respectively. Our isotopic results showed that the largehead hairtail is placed intermediately between pelagic and benthic fishes and was more 13C and 15N-depleted than other carnivorous fishes, suggesting a high consumption of pelagic species. Similarly, seasonal variations were observed in the δ13C and δ15N values of the largehead hairtails (PERMANOVA, p= 0.001). Interestingly, their δ13C values in spring decreased significantly with ALs (r2 = 0.594, p< 0.001), suggesting an ontogenetic change due to the increased proportion of Euphausiacea to the nutrition of largehead individuals during this specific season. Based on a combination of stomach contents and stable isotope signatures, overall results provide new insights into the feeding characteristics of the largehead hairtail, which improves our understanding of its trophic ecology depending on season and body size.

The common squid Todarodes pacificus is a dominant species within epipelagic communities and an important commercial species in the South and East seas of Korea and in the East China Sea. In this study, to examine the dietary composition, trophic position, and ontogenetic dietary shifts of the common squid, we analyzed stomach contents and stable isotope values (δ13C and δ15N) in muscle tissue across different body sizes (mantle length, ML) and seasons (winter, spring, and summer 2021) in the South Sea of Korea. Regardless of the season, the diet of the common squid predominantly comprised Pisces (37.9–94.0%) and Cephalopoda (6.0–61.0%). However, in the smallest individuals (ML < 10 cm), the stomach contents in August primarily comprised Crustacea (95.5%), thereby indicating a seasonal difference in dietary composition and ontogenetic dietary shifts only during summer. Similarly, our isotope results revealed seasonal isotopic variation among sampling periods and significant positive correlations between ML and both δ13C and δ15N values during the summer. These results reveal a seasonal variability in major diet components among the common squid of different size classes and that their feeding strategies are probably adjusted in response to the temporal availability of prey. Collectively, the findings of this study enhance our understanding of the feeding ecology of T. pacificus and thereby provide valuable information that will contribute to the ecological fishery management of this commercially important species.

The aim of this study was to assess seasonal variation in the food-web structure of fish assemblages in the East (two sites) and the South (one site) Seas of Korea, and to compare the isotopic niche areas between the regions. To do this, we analyzed the community structures and the δ13C and δ15N values for fish assemblages, and their potential food sources collected during May and October 2020. There were spatial differences in the diversity and dominant species of fish assemblages between the two seas. The fish assemblages in the South Sea had relatively wide ranges of δ13C and δ15N (−22.4‰ to −15.3‰ and 7.4‰ to 13.8‰, respectively) compared to those (−22.1‰ to −18.0‰ and 9.8‰ to 13.6‰, respectively) in the East Sea. The δ13C and δ15N values of suspended particulate organic matter, zooplankton, and fish assemblages differed significantly among sites and between seasons (PERMANOVA, p < 0.05, in all cases). Moreover, isotopic niche indices were relatively higher in the South Sea compared to those in the East Sea. Such differences in food-web characteristics among sites are likely due to the specific environmental effects (especially, major currents) on the differences in the species compositions and, therefore, their trophic relationships. Overall, these results allow for a deeper understanding of the changing trophic diversity and community structure of fish assemblages resulting from climate variability.