Abstract
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.
Highlights
Accumulating sufficient energy from the environment affects both survival and breeding success for many animal species, and thereby influences the dynamics of entire populations [1]
We successfully cross-validated two independent metrics of lipid-store body condition in a free-ranging cetacean: (i) tissue body density (TBD) estimated by fitting the hydrodynamic glide model with high-resolution tag data, and (ii) projected surface area-to-length ratios (LSSAI) estimated using Unmanned aerial vehicles (UAVs)
The correlation between these metrics demonstrated the validity of using the hydrodynamic glide model to determine individual and temporal variations in the TBD of humpback whales
Summary
Accumulating sufficient energy from the environment affects both survival and breeding success for many animal species, and thereby influences the dynamics of entire populations [1]. We investigate lipid-store body condition patterns in humpback whales in two geographically distinct feeding grounds in the Atlantic Ocean (figure 1): eastern Canada (northwest Atlantic) and Norway (northeast Atlantic). Most humpbacks in both locations probably breed in the West Indies during winter [43]. This model avoided multiple testing and accounted for measurement error inherent to both metrics Another advantage is that all data obtained using either approach could be used to estimate the effect of the covariates on body condition as the model predicted any missing observations of TBD or LSSAI based upon the specified relationships with uTBD. See the electronic supplementary material R script for further details
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