Preliminary analysis of beak stable isotopes (δ13C and δ15N) stock variation of neon flying squid, Ommastrephes bartramii, in the North Pacific Ocean

Stable isotopes (13C and 15N) have been confirmed as a useful tool for understanding trophic position and related dietary variation in squid. In this study, we performed isotopic analysis on Ommastrephes bartramii beaks to examine differences between the eastern and western stocks in the North Pacific Ocean. Isotopic values of the upper beak (UB) and the lower beak (LB) were also compared. A generalized additive model (GAM) was used to select variables that explain stock variation. Isotopic signatures (δ13C and δ15N) were significantly different between the two stocks whereas there was no difference in C/N ratios. All isotopic values were significantly different between the UB and LB. Trophic niche width was distinct between two stocks with little overlap. The δ13C signature increased with latitude and mantle length (ML) with greater variability in the eastern stock. None of the variables could explain the variation in δ13C values for the western stock. The δ15N signature increased rapidly with ML in the eastern stock, whereas δ15N gradually increased with latitude and ML in the western stock. The isotopic variations can be successfully explained by different migration patterns and feeding behaviors of the two stocks. The δ13C and δ15N values of the UB were lower than those of the LB, possibly due to variable chitin/protein ratios at different developmental stages. Future studies should account for a greater sample size and the functionality of the UB.