Stable Isotope Analysis ( δ 13 C and δ 15 N ) of Baleen Plates Reveals Feeding Patterns in Stranded Humpback Whales ( Megaptera novaeangliae ) Along the Brazilian Coast

Abstract

Humpback whales ( Megaptera novaeangliae ) from breeding stock A migrate annually between Antarctic feeding grounds (~65° S) and Brazilian breeding areas (~17° S). Traditionally considered to fast during migration, recent feeding observations and increased strandings along southeastern Brazil (~23° S) raise questions about possible shifts in foraging behavior. We hypothesize that some individuals are modifying their feeding strategies due to changing Antarctic conditions, leading to opportunistic feeding along the migration route. To investigate this, we analyzed δ 13 C and δ 15 N values along the baleen plates of four stranded calves and juveniles. Three whales exhibited enriched δ 13 C values toward the base of the baleen (most recently formed tissue), consistent with subtropical prey, along with elevated δ 15 N values, suggesting foraging at higher trophic levels. One individual maintained Antarctic‐like isotopic signatures throughout. These findings provide direct isotopic evidence that some calves and juveniles may be feeding outside Antarctic waters—either directly on prey or indirectly through maternal milk—possibly in Brazilian coastal environments. This trophic behavior may contribute to increased net entanglements and stranding events. Our study underscores the ecological value of well‐preserved baleen samples and highlights the need for broader sampling to assess whether such changes reflect population‐level shifts in migration or foraging due to climate‐driven environmental change.

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