We quantified the δ¹³C and δ¹⁵N values in the lower beaks of Humboldt squid, Dosidicus gigas, collected from international waters off Costa Rica, Ecuador, Peru and Chile by Chinese squid jigging vessels during 2009, 2010 and 2013. There was a significant difference in the isotopic values among regions with the lowest value off Ecuador and the highest off Chile, which were interpreted as a function of trophic effects as well as baseline values. However, constant trophic level of D. gigas across its geographic range showed that spatial variation in the baseline of primary production is the main driver responsible for the observed geographic isotope variability. Inter-regional difference and intra-regional convergence of isotope values indicated squid off Costa Rica, Ecuador and Chile belong to different geographically segregated populations, which were previously proved by integrated population identifying method. In contrast, the higher variations in δ¹³C and δ¹⁵N values in a given size group suggest the squid off Peru move and forage in different places. Moreover, potential population exchange could be responsible for the overlap of the isotope values between the squid off Peru and off Chile. On the whole, the spatial difference in isotopic values of Humboldt squid beaks improves our understanding of potential geographic population connectivity and movement.

The migration route of oceanic squid provides critical information for us to understand their spatial and temporal variations. Mark-recapture and electronic tags tend to be problematic during processing. Cephalopod hard structures such as the beak, containing abundant ecological information with stable morphology and statolith-like sequences of growth increments, may provide information for studying spatio-temporal distribution. In this study, we developed a method, which is based on elemental concentration of beaks at different ontogenetic stages and sampling locations, to reconstruct the squid migration route. We applied this method to Ommastrephes bartramii in the North Pacific Ocean. Nine trace elements were detected in the rostrum sagittal sections (RSS) of the beak using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). For those elements, significant differences were found between the different ontogenetic stages for phosphorus (P), copper (Cu) and zinc (Zn). Sodium (Na), P and Zn were chosen as indicators of sea surface temperature (SST) and a regression model was estimated. The high probability of occurrence in a particular area represented the possible optimal squid location based on a Bayesian model. A reconstructed migration route in this study, combining all the locations at different ontogenetic stages, was consistent with that hypothesized in previous studies. This study demonstrates that the beak can provide useful information for identifying the migration routes of oceanic squid.