Amino acid δ15N differences consistent with killer whale ecotypes in the Arctic and Northwest Atlantic.

Ecotypes are groups within a species with different ecological adaptations than their conspecifics. Eastern North Pacific (ENP) killer whale (Orcinus orca) ecotypes differ in their diet, behavior, and morphology, but the same is not known for this species in the eastern Canadian Arctic (ECA) and Northwest Atlantic (NWA). Using compound-specific stable isotope analysis (CSIA) of amino acids (AAs), we compared δ15N patterns of the primary trophic and source AA pair, glutamic acid/glutamine (Glx) and phenylalanine (Phe), in dentine collagen of (1) sympatric ENP killer whale ecotypes with well-characterized diet differences and (2) ECA/NWA killer whales with unknown diets. δ15NGlx-Phe was significantly higher in the ENP fish-eating (FE) than mammal-eating (ME) ecotype (19.2 ± 0.4‰ vs. 13.5 ± 0.7‰, respectively). Similar bimodal variation in δ15NGlx-Phe indicated analogous dietary divisions among ECA/NWA killer whales, with two killer whales having higher δ15NGlx-Phe (16.5 ± 0.0‰) than the others (13.5 ± 0.6‰). Inferences of dietary divisions between these killer whales were supported by parallel differences in threonine δ15N (-33.5 ± 1.6‰ and -40.4 ± 1.1‰, respectively), given the negative correlation between δ15NThr and TP across a range of marine consumers. CSIA-AA results for ECA/NWA whales, coupled with differences in tooth wear (a correlate for diet), are consistent with ecotype characteristics reported in ENP and other killer whale populations, thus adding to documented ecological divergence in this species worldwide.

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 (δ18OP) and structural carbonate (δ18OSC, δ13CSC) 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 (δ18Omarine) at location of origin was estimated from dentine δ18OP values, and then compared with predicted isoscape values to assign individual distributions. Dentine δ18OP values were also assessed against those of other known-origin North Atlantic odontocetes for spatial reference. Most ECA and NWA killer whales had mean δ18OP and estimated δ18Omarine 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 δ18OSC 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.