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 (δ 18 O P ) and structural carbonate (δ 18 O SC , δ 13 C SC ) of whole teeth and annual growth layers from killer whales that stranded in the ECA (n = 11) and NWA (n = 7). Source δ 18 O of marine water (δ 18 O marine ) at location of origin was estimated from dentine δ 18 O P values, and then compared with predicted isoscape values to assign individual distributions. Dentine δ 18 O 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 δ 18 O P and estimated δ 18 O marine values consistent with 18 O-depleted, high-latitude waters north of the Gulf Stream, above which a marked decrease in baseline δ 18 O values occurs. Several individuals, however, had relatively high values that reflected origins in 18 O-enriched, low-latitude waters below this boundary. Within-tooth δ 18 O 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.