Assessing the trophic interactions between two pelagic fishes in the northwestern Pacific based on stable isotopes

Scomber japonicus (S. japonicus) and Sardinops melanostictus (S. melanostictus) are widely distributed off the coast of Japan and in the northwestern Pacific. To investigate the trophic interactions between these two species, samples of S. japonicus and S. melanostictus were taken from the high seas of the northwestern Pacific Ocean (38°59'N-43°00'N, 150°30'E-161°48'E) from June to July 2021. We utilized stable isotope analysis to study the intraspecific and interspecific differences of them. For intraspecific variability, there was no significant difference in δ¹³C or δ¹⁵N values between females and males for either S. japonicus or S. melanostictus (P > 0.05). Significant differences in the δ¹³C and δ¹⁵N values of S. melanostictus were found among the body length groups (P < 0.01). The δ¹³C values of S. japonicus significantly differed among body length groups (P < 0.05). However, there was no significant difference in the δ¹⁵N values of S. japonicus (P > 0.05). For interspecific variability, there were significant differences in the δ¹³C and δ¹⁵N values between S. japonicus and S. melanostictus (P < 0.01). According to the GAM results, both the δ¹³C and δ¹⁵N values of S. japonicus gradually increased with body length, whereas both the δ¹³C and δ¹⁵N values of S. melanostictus tended to decrease and then increase. The niche overlap between S. japonicus and S. melanostictus decreased with increasing growth. Compared to those of S. melanostictus, the δ¹⁵N levels of S. japonicus were greater, while the niche width was smaller. The intraspecific niche overlap and width for both S. melanostictus and S. japonicus tended to increase and then decrease with growth. We suggest that the average trophic level of S. japonicus is greater than that of S. melanostictus. However, S. melanostictus has a greater diversity of prey items, and these two species coexist by reducing inter- and intraspecific competitive pressures through dietary differentiation.