Blubber cortisol levels in humpback whales (Megaptera novaeangliae): A measure of physiological stress without effects from sampling

•Blubber cortisol is an indicator of humpback whale stress levels.•Remote biopsy sampling has no influence on concentrations in live whales.•Live whales display slight seasonal variation in blubber cortisol levels. Baleen whales are vulnerable to environmental impacts due to low fecundity, capital breeding strategies, and their reliance on a large amount of prey resources over large spatial scales. There has been growing interest in monitoring health and physiological stress in these species but, to date, few measures have been validated. The purpose of this study was to examine whether blubber cortisol could be used as a measure of physiological stress in humpback whales. Cortisol concentrations were initially compared between live, presumably ‘healthy’ whales (n = 187) and deceased whales (n = 35), which had died after stranding or entanglement, or washed ashore as a carcass. Deceased whales were found to have significantly higher cortisol levels (mean ± SD; 5.47 ± 4.52 ng/g) than live whales (0.51 ± 0.14 ng/g; p