Aerial photogrammetry and tag-derived tissue density reveal patterns of lipid-store body condition of humpback whales on their feeding grounds

• Published in: Proceedings of the Royal Society B: Biological Sciences
• Main Authors: Aoki, Kagari, Isojunno, Saana, Bellot, Charlotte, Iwata, Takashi, Kershaw, Joanna, Akiyama, Yu, Martín López, Lucía M., Ramp, Christian, Biuw, Martin, Swift, René, Wensveen, Paul J., Pomeroy, Patrick, Narazaki, Tomoko, Hall, Ailsa, Sato, Katsufumi, Miller, Patrick J. O.
• Other Authors: Grant-in-Aid for Young Scientists B, Environment Fund, SERDP, Office of Naval Research
• Format: Article in Journal/Newspaper
• Language: English
• Published: The Royal Society 2021
• Subjects: Canada; Norway; Megaptera novaeangliae
• Online Access: http://dx.doi.org/10.1098/rspb.2020.2307; https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2020.2307; https://royalsocietypublishing.org/doi/full-xml/10.1098/rspb.2020.2307

Monitoring the body condition of free-ranging marine mammals at different life-history stages is essential to understand their ecology as they must accumulate sufficient energy reserves for survival and reproduction. However, assessing body condition in free-ranging marine mammals is challenging. We cross-validated two independent approaches to estimate the body condition of humpback whales ( Megaptera novaeangliae ) at two feeding grounds in Canada and Norway: animal-borne tags ( n = 59) and aerial photogrammetry ( n = 55). Whales that had a large length-standardized projected area in overhead images (i.e. whales looked fatter) had lower estimated tissue body density (TBD) (greater lipid stores) from tag data. Linking both measurements in a Bayesian hierarchical model to estimate the true underlying (hidden) tissue body density (uTBD), we found uTBD was lower (−3.5 kg m −3 ) in pregnant females compared to adult males and resting females, while in lactating females it was higher (+6.0 kg m −3 ). Whales were more negatively buoyant (+5.0 kg m −3 ) in Norway than Canada during the early feeding season, possibly owing to a longer migration from breeding areas. While uTBD decreased over the feeding season across life-history traits, whale tissues remained negatively buoyant (1035.3 ± 3.8 kg m −3 ) in the late feeding season. This study adds confidence to the effectiveness of these independent methods to estimate the body condition of free-ranging whales.