| Summary: | Southern hemisphere blue ( Balaenoptera musculus intermedia ) and fin ( Balaenoptera physalus ) whales are the largest predators in the Southern Ocean, with similarities in morphology and distribution. Yet, understanding of their life history and foraging is limited due to current low abundances and limited ecological data. To address these gaps, historic Antarctic blue (n = 5) and fin (n = 5) whale baleen plates, collected in 1947–1948 and recently rediscovered in the Smithsonian National Museum of Natural History, were analyzed for bulk (δ 13 C and δ 15 N) stable isotopes. Regular oscillations in isotopic ratios, interpreted as annual cycles, revealed that baleen plates contain approximately six years (14.35 ± 1.20 cm yr –1 ) of life history data in blue whales and four years (16.52 ± 1.86 cm yr –1 ) in fin whales. Isotopic results suggest that: 1) in the 1940s, blue and fin whales fed at the same trophic level but in slightly different habitats, 2) fin whales appear to have had more regular annual migrations, and 3) fin whales may have migrated to ecologically distinct sub-Antarctic waters annually while some blue whales may have resided year-round in the Southern Ocean. These results reveal differences in ecological niche and life history strategies between Antarctic blue and fin whales during a period when their populations were more abundant than today, and before major human-driven climatic changes occurred in the Southern Ocean. Funding provided by: National Science Foundation Crossref Funder Registry ID: https://ror.org/021nxhr62 Award Number: 1927742
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