Humpback Whale (Megaptera novaeangliae) Blubber Steroid Hormone Concentration to Evaluate Chronic Stress Response from Whale-Watching Vessels

A booming whale-watching industry in Juneau, Alaska, is raising concerns over potential impacts to humpback whales (Megaptera novaeangliae) and the sustainability of this growing industry. In this study, we investigated the physiological response of these whales to chronic vessel disturbance by measuring hormone concentrations (cortisol, progesterone, testosterone, and estradiol) that have been sequestered in blubber throughout the whale-watching season. We focused our analysis on cortisol, a steroid hormone associated with stress response, and hypothesized that cortisol in biopsy samples would be positively correlated with the amount of vessel traffic in the 3 to 4 months prior to sampling. Humpback whales in the Juneau area were compared with whales from control areas with far less vessel traffic in both Southeast Alaska and the western Gulf of Alaska using biopsies collected late in the tour season. We did not find elevated cortisol in whales sampled in the Juneau area relative to the Southeast Alaska control area (p = 0.14) or sites in the western Gulf of Alaska, which had higher cortisol levels (p < 0.001). The cause of the regional cortisol differences is not known but could be representative of regional differences in baseline hormone concentrations or be linked to predator or nutritional stressors. The lack of elevated cortisol in Juneau-area whales suggests high vessel traffic is not resulting in chronic cortisol sequestration in whales and may be indicative of whales near Juneau being habituated to vessel traffic.