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

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.

Automated summary

This study cross-validated two independent methods, animal-borne tags and aerial photogrammetry, to estimate the body condition of humpback whales in Canada and Norway, showing lower tissue body density in pregnant females and higher in lactating females. Whales in Norway exhibited more negative buoyancy during the early feeding season compared to Canada, possibly due to longer migration distances. Despite a decrease in tissue body density over the feeding season, whales remained negatively buoyant in the late feeding season.