| Summary: | Southern hemisphere humpback whale ( Megaptera novaeangliae , SHHW) breeding populations follow a highfidelity Antarctic krill ( Euphausia superba ) diet while feeding in distinct sectors of the Southern Ocean. Their capital breeding life history requires predictable ecosystem productivity to fuel migration and migration -related behaviours. It is therefore postulated that populations feeding in areas subject to the strongest climate change impacts are more likely to show the first signs of a departure from a high-fidelity krill diet. We tested this hypothesis by investigating blubber fatty acid profiles and skin stable isotopes obtained from five SHHW populations in 2019, and comparing them to Antarctic krill stable isotopes sampled in three SHHW feeding areas in the Southern Ocean in 2019. Fatty acid profiles and delta 13 C and delta 15 N varied significantly among all five populations, however, calculated trophic positions did not (2.7 to 3.1). Similarly, fatty acid ratios, 16:10)7c/16:0 and 20:50)3/22:60)3 were above 1, showing that whales from all five populations are secondary heterotrophs following an omnivorous diet with a diatom -origin. Thus, evidence for a potential departure from a high-fidelity Antarctic krill diet was not seen in any population. delta 13 C of all populations were similar to delta 13 C of krill sampled in productive upwelling areas or the marginal sea -ice zone. Consistency in trophic position and diet origin but significant fatty acid and stable isotope differences demonstrate that the observed variability arises at lower trophic levels. Our results indicate that, at present, there is no evidence of a divergence from a high-fidelity krill diet. Nevertheless, the characteristic isotopic signal of whales feeding in productive upwelling areas, or in the marginal sea -ice zone, implies that future cryosphere reductions could impact their feeding ecology.
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