Sampling cetacean blubber using retrievable projectiles (biopsy hereafter) has facilitated a wealth of knowledge on inter alia: migration, life-history, trophic ecology, population structure and pollutant burden. Although observable behavioural responses in humpback whales (Megaptera novaeangliae) are usually mild or moderate (Noren and Mocklin, 2012), biopsy sampling is invasive and ought to be employed judiciously. Christiansen et al. (2020) compellingly argue that blubber lipid-content measurements could inform health and life-history assessments of humpback whales. They aimed to determine a relationship between blubber lipid-content (biopsy) and body shape (aerial photogrammetry). This is a laudable goal: validation of aerial photogrammetry for measuring internal body condition by proxy, would present a less-invasive alternative to biopsy sampling. However, it has previously been shown that the lipid content of biopsy samples is not representative of blubber in situ (Ryan et al., 2013). Therein, the blubber lipid content of a dead fin whale (Balaenoptera physalus) was estimated, comparing biopsy samples with control samples excised by scalpel. Percentage lipid content (mean±s.d.) was significantly lower and more variable for biopsies (37±6.6%, n=15) than controls (81±1.2%, n=3). The large discrepancy is likely a result of the biopsy dart rupturing adipocytes, precluding the use of biopsies for estimating blubber lipid-content (Ryan et al., 2013). I argue that this sampling effect could explain why Christiansen et al. (2020) did not find a relationship between metrics of internal and external body condition. Moreover, I discourage further use of biopsy sampling for measuring blubber lipid content due to sampling biases (Kershaw et al., 2019; Krahn et al., 2004; Ryan et al., 2013). This is especially relevant to longitudinal studies where repeat sampling increases the potential for adverse effects on the welfare of individual whales.

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