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

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.

Keywords: UAV; animal-borne sensor; cetacean; feeding season; neutral buoyancy; tissue body density.

Figure 1.

Study locations (green) on two separate feeding grounds: Canada and Norway, with their breeding locations (orange diagonal lines: West Indies and possibly Cape Verde Islands). The field seasons are shown. The figure was modified from the map on the NOAA website (https://www.fisheries.noaa.gov/species/humpback-whale) and the NAMMCO website (https://nammco.no/topics/humpback-whale/). (Online version in colour.)

Figure 2.

Length-standardized surface area index (LSSAI) calculated from aerial photogrammetry (see Material and methods). (a) The total length between the tip of the rostrum and fluke notch is divided into 20 equal length sections. (b) Grey shadows (body sections 7–17) indicate LSSAI. (c) Variation in each body section across whales. (Online version in colour.)

Figure 3.

Changes in gliding patterns corresponded with tissue body density of the same individuals (ID H584). High values of dorsoventral accelerations indicate periods of fluke-strokes (dive depth in green), while low values indicate gliding periods (dive depth in purple). (Online version in colour.)

Figure 4.

Animals with a large length-standardized surface area index show lower estimated TBD from hydrodynamic analyses (see table 1 for detailed information of each individual). The linear regression line is shown in black; 95% confidence intervals are shown as dashed black lines. (Online version in colour.)

Figure 5.

Underlying TBD (uTBD) across sex, age classes and reproductive status in Norway and Canada. Each symbol shows individual uTBD estimated from either TBD and/or LSSAI data, coloured by its reproductive status. Both dashed and solid lines indicate decreasing uTBD over feeding seasons predicted by the model (see details in Material and methods). The black up arrow shows differences of uTBD between Norway and Canada during the early feeding season. (Online version in colour.)