Contrasting C and N isotope ratios from sperm whale skin and squid between the Gulf of Mexico and Gulf of California: effect of habitat

To investigate feeding variation between populations of an apex oceanic predator, stable isotope ratios of carbon (delta C-13) and nitrogen (delta N-13) have been compared in skin of female and immature male sperm whales (Physeter macrocephalus) from the northern Gulf of Mexico (GoM) and the Gulf of California (GC). Whale sexes were determined genetically. The delta C-13 and delta C-13 values from squid muscle were used from the GC, and from inshore and offshore sites in the GoM. We documented contrastingly lower delta C-13 and delta N-13 from whales and squid of the GoM compared with those from the GC. While this difference may be associated with variation in trophic position, geographic variation in biochemical cycling influenced significantly the contrasting isotope values between gulfs. Within the northern GoM, the highly distinct delta N-13 values of neritic squid versus mesopelagic squid provide further evidence of habitat specificity in delta N-13.