Extreme diel variation in the feeding behavior of humpback whales along the western Antarctic Peninsula during autumn

Abstract

Most humpback whale Megaptera novaeangliae populations partition their time between prey-rich feeding and prey-deficient breeding/calving regions. How these whales feed and optimize the consumption of prey resources prior to long-distance migrations and fasting is largely unknown. We deployed multi-sensor tags on humpback whales around the western Antarctic Peninsula to describe their daily activity patterns late in the feeding season to test the hypothesis that feeding behavior varies over the diel cycle so as to maximize energy intake and limit energy expenditure. Dives were assigned to a behavioral state (feeding, resting, traveling, exploring) to determine hourly rates and to build an ethogram of activity patterns. Our results show a distinct diel pattern of whales feeding exclusively at night. Feeding depth was deeper around sunrise/sunset and shallower (~50 m) at night, consistent with diel vertical prey movement. Shallow feeding dives typically contained a single feeding lunge, a strategy known to increase feeding efficiency and maximize intake rates by maintaining proximity to the surface and reducing the energetic costs of deep diving. The lack of feeding during daytime may indicate prey being too deep for efficient foraging. Our results add information where currently there is a paucity of data describing how baleen whales optimize feeding behavior, specifically in relation to prey distribution and movement, to fuel their extraordinary energetic requirements necessary for growth, migration, and reproduction. Understanding behavioral patterns and predator/prey dynamics in rapidly changing marine environments, like the Antarctic Peninsula, is critical for understanding how these changes will affect ecosystem structure and function.

Winter distribution and size structure of Antarctic krill Euphausia superba populations in-shore along the West Antarctic Peninsula

“…Previous work in these coastal embayments has also suggested their importance as winter habitat for krill (Zhou et al 1994, Zhou & Dorland 2004, 2008, Friedlaender et al 2013. As early as 1938 the importance of these areas as krill habitat were noted, with 'E.…”

Does optimal foraging theory predict the foraging performance of a large air-breathing marine predator?

“…We used an automatic lunge detector in the software program TrackPlot (Ware, Arsenault, & Plumlee, 2006) to identify putative feeding lunges executed by the whales from the tag data (see Friedlaender et al, 2013;Tyson, Friedlaender, Stimpert, Ware, & Nowacek, 2012;Ware et al, 2011). The detector locates distinct fluctuations in the flow noise (speed) recorded by the tag's hydrophone that are characteristic of the rapid accelerations and decelerations associated with lunging (Goldbogen et al, 2006;Ware et al, 2011).…”

“…We calculated surface times (s), dive times and maximum dive depths of feeding dives (defined as any submergence greater than 3 m with at least one lunge; Friedlaender et al, 2013) for each whale in MatLab (Mathworks, Natick, MA, U.S.A.) (Note: a description of symbols used throughout the paper (in order of appearance) can be found in the Appendix, Table A1). In addition, we divided each dive into its respective foraging time (t) and travel time (t) for comparison with the OFT model predictions.…”

“…<25 m). While Ware et al (2011) did not discuss the duration of these shallow dives, these whales are physiologically capable of executing several lunges per dive but only do so at greater depths (Friedlaender, Tyson, Stimpert, Read, & Nowacek, 2013).…”

“…For example, humpbacks in this region adjust their foraging behaviours according to the depth and density of their primary prey, Antarctic krill, Euphausia superba (Friedlaender, Johnston, Tyson, Kaltenberg, et al, 2016). These whales forage almost exclusively at night, when krill are near the surface, and they rest or travel near the surface during the day, when krill are at depth (Friedlaender et al, 2013). In addition, these whales modulate their feeding depth with respect to their prey, targeting denser prey patches on deeper dives (Friedlaender et al, 2016).…”