Stable Isotope Composition of Skin Layers Confirms Trophic Differentiation of Baird's and Sato's Beaked Whales (Berardius spp.) in the Western North Pacific

Abstract

Rationale: Closely related whale species can appear similar yet occupy distinct ecological niches. Sato's beaked whale (Berardius minimus) was only recently recognized as a separate species and was previously grouped with Baird's beaked whale (Berardius bairdii), limiting our understanding of their ecological differences. Clarifying their trophic ecology is essential for accurate species-specific conservation and management. Methods: Skin biopsy and necropsy samples of B. bairdii and B. minimus were collected in the western North Pacific in 2017–2025. Carbon (δ13C) and nitrogen (δ15N) stable isotope ratios were measured from three separately analyzed epidermal layers of skin samples using isotope ratio mass spectrometry. Results: B. bairdii exhibited significantly higher δ13C values than B. minimus, whereas δ15N values were broadly similar, indicating comparable trophic positions but distinct foraging habitats. Higher δ13C values in B. bairdii suggest greater reliance on benthic or coastal food webs, whereas lower δ13C values in B. minimus reflect stronger associations with offshore ecosystems. Isotopic variation of different epidermal layers revealed temporal dietary shifts within individuals. Conclusions: These results demonstrate clear ecological differentiation between B. bairdii and the recently described B. minimus. Historical data attributed to B. bairdii should be re-evaluated to ensure accurate species-specific interpretations of diet and habitat use, improving ecological understanding and conservation assessments.