Distributions of Arctic and Northwest Atlantic killer whales inferred from oxygen isotopes

Killer whales (Orcinus orca) are distributed widely in all oceans, although they are most common in coastal waters of temperate and high-latitude regions. The species' distribution has not been fully described in the northwest Atlantic (NWA), where killer whales move into seasonally ice-free waters of the eastern Canadian Arctic (ECA) and occur year-round off the coast of Newfoundland and Labrador farther south. We measured stable oxygen and carbon isotope ratios in dentine phosphate (δ¹⁸O_P) and structural carbonate (δ¹⁸O_SC, δ¹³C_SC) of whole teeth and annual growth layers from killer whales that stranded in the ECA (n = 11) and NWA (n = 7). Source δ¹⁸O of marine water (δ¹⁸O_marine) at location of origin was estimated from dentine δ¹⁸O_P values, and then compared with predicted isoscape values to assign individual distributions. Dentine δ¹⁸O_P values were also assessed against those of other known-origin North Atlantic odontocetes for spatial reference. Most ECA and NWA killer whales had mean δ¹⁸O_P and estimated δ¹⁸O_marine values consistent with ¹⁸O-depleted, high-latitude waters north of the Gulf Stream, above which a marked decrease in baseline δ¹⁸O values occurs. Several individuals, however, had relatively high values that reflected origins in ¹⁸O-enriched, low-latitude waters below this boundary. Within-tooth δ¹⁸O_SC ranges on the order of 1-2‰ indicated interannual variation in distribution. Different distributions inferred from oxygen isotopes suggest there is not a single killer whale population distributed across the northwest Atlantic, and corroborate dietary and morphological differences of purported ecotypes in the region.