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

Abstract

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 (δ15N and δ13C) of their muscle tissues. A hierarchical cluster analysis based on δ15N and δ13C yielded three groups (G1–G3). G1 (mean δ15N: 12.9%; mean δ13C: –20.3%) had the highest δ15N and occurred only in the eastern area of 170° W in 2013 and 2015. G2 (δ15N: 9.3%; δ13C: −20.1%) and G3 (δ15N: 7.3%, δ13C: −20.9%) occurred in all years mainly in eastern and western areas of 170° W, respectively. The latter two groups presented reasonable δ15N and δ13C considering the trophic enrichment and potential prey such as Neocalanus copepods that exist in each area. In contrast, the substantially enriched δ15N 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.