Ecological Competition Between Fin Whales, Blue Whales, and Their Hybrids

Abstract

Hybridization among cetaceans has been documented across multiple species. Within the Balaenopteridae family, most documented cases involve blue and fin whales, raising concerns about impacts on species recovery, particularly that of the highly endangered blue whale. Here, we examined baleen morphology, migratory phenology, distribution, and stable isotope profiles along baleen plates of five fin whales, two blue whales, and three hybrids sampled in Icelandic waters to evaluate ecological overlap among parental species and their hybrids. Nearly all hybrids had been misidentified and harvested as fin whales, suggesting bias toward fin whale‐like individuals, as blue whale‐like hybrids likely evaded capture. Stable isotope values indicated that both fin and blue whales relied primarily on krill, resulting in substantial trophic overlap, although fin whales tended to exhibit higher δ 15 N values consistent with the occasional use of higher‐trophic prey. Although the overlap of hybrids in both diet and habitat use occurred with both parental species, hybrids more closely resembled blue whales, possibly due to cultural transmission through the maternal lineage, as most of them have blue whale mothers. Although differences in habitat use or migratory behavior may reduce direct ecological competition, the substantial trophic overlap observed between blue and fin whales may favor unidirectional hybridization and potentially contribute to reduced blue whale reproductive output, with implications for the recovery of this species.

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