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.