Geographical differences in the stable isotope ratios of Pacific saury in the North Pacific Ocean

We collected samples of Pacific saury Cololabis saira (249–331 mm in knob length; ages 0 and 1) in the North Pacific Ocean from 154°E to 165°W during their northward migration in the early summers of 2013, 2014, and 2015, and measured the stable nitrogen and carbon isotope ratios (δN and δC) of their muscle tissues. A hierarchical cluster analysis based on δN and δC yielded three groups (G1–G3). G1 (mean δN: 12.9%; mean δC: –20.3%) had the highest δN and occurred only in the eastern area of 170° W in 2013 and 2015. G2 (δN: 9.3%; δC: −20.1%) and G3 (δN: 7.3%, δC: −20.9%) occurred in all years mainly in eastern and western areas of 170° W, respectively. The latter two groups presented reasonable δN and δC considering the trophic enrichment and potential prey such as Neocalanus copepods that exist in each area. In contrast, the substantially enriched δN of G1 was characteristic of organisms at higher trophic levels in the same area. Thus, this group is most likely an immigrant from outside the survey area. This study showed that Pacific saury utilize different ecosystems east and west of 170° W during their northward migration.