Changing the Menu: Humpback Whale (Megaptera novaeangliae) Diet Switching in Senyavin Strait, Chukotka

Humpback whales have the most diverse diet of all rorquals. Their ability to use different prey depending on local availability makes them a promising indicator species for ecosystem dynamics. During five summer–fall seasons in 2017–2021, regular observations of humpback whale feeding aggregations were conducted as part of multi‐year cetacean monitoring in Senyavin Strait, on the eastern Chukotka Peninsula. The study included assessing the trophic level of the whales using stable isotope analysis, as well as collateral observations of feeding behavior, spatial distribution, and surface activity records. We found that the spatial distribution, daytime activity patterns, and trophic levels of the whales differed significantly between 2017 and 2018–2021. These differences, combined with some additional observations of whale feeding activity, suggest that the whales fed on fish, most likely polar cod, in 2017 and switched to preferentially krill in later years. We suggest that our observations in 2017 coincided with a sporadic schooling event of polar cod. We show that a single humpback whale population can abruptly change its feeding habits and switch from one prey type to another under optimal conditions. The high degree of behavioral plasticity may be one of the keys to the evolutionary success of this species.