Isoscapes reveal patterns of δ13C and δ15N of pelagic forage fish and squid in the Northwest Pacific Ocean

Isoscapes of stable isotope ratios of carbon (delta C-13) and nitrogen (delta N-15) in pelagic fish and squid were generated using 1967 measured values of forage fish and squid in the Northwest Pacific Ocean. We then used generalized additive models (GAMs) to assess the explanatory variables that best predicted regional fish and squid delta C-13 and delta N-15 values. A total of 522 squid and 1445 forage fish were analyzed for delta C-13 and delta N-15. The explanatory variables/variates used in GAM analyses were geographical parameters (longitude and latitude), body mass, season, category of organism (fish or squid), and vertical habitat. The resulting isoscapes of delta C-13 showed higher values in the sub-tropical region and lower values in the sub-arctic region. The isoscape patterns of delta(15) N were more complex; higher values were found in both tropical and temperate regions, and lower values were observed in sub-tropical and sub-arctic regions. These heterogeneities in isoscapes of delta C-13 and delta N-15 in the Northwest Pacific could be caused by productivity dynamics, ocean currents and upwelling, and the degree of atmospheric fixation of carbon and nitrogen. These new isoscapes can be used in conjunction with predator delta C-13 and delta N-15 values to evaluate movements and trophic dynamics of predatory marine animals, including tunas, billfish, sharks, seabirds, and marine mammals, in the Northwestern Pacific Ocean.