Aerial photogrammetry and tag-derived tissue density reveal patterns of lipid-store body condition of humpback whales on their feeding grounds
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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.
Aoki , K , Isojunno , S , Bellot , C , Iwata , T , Kershaw , J L , Akiyama , Y , Martín López , L M , Ramp , C , Biuw , M , Swift , R J , Wensveen , P , Pomeroy , P , Narazaki , T , Hall , A J , Sato , K & Miller , P 2021 , ' Aerial photogrammetry and tag-derived tissue density reveal patterns of lipid-store body condition of humpback whales on their feeding grounds ' , Proceedings of the Royal Society of London Series B: Biological Sciences , vol. 288 , no. 1943 , 20202307 . https://doi.org/10.1098/rspb.2020.2307
Proceedings of the Royal Society of London Series B: Biological Sciences
Copyright © 2021 The Authors. Open Access. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.
DescriptionThis work was supported by the SERDP award (grant no. RC-2337). Additional support for fieldwork was provided by the US Office of Naval Research under the 3S collaborative research project. Analysis and writing were partly supported by the Grant-in-Aid for Young Scientists B (grant no. 17K12813), The Mitsui & Co. Environment Fund (grant no. R16-0044), JSPS Bilateral Open Partnership Joint Research Projects and NERC National Capability funding.
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