Large krill and large patches lead to increased foraging opportunities for environmentally-selective humpback whales in the West Antarctic Peninsula

David E. Cade and Ari S. Friedlaender, Institute of Marine Sciences, UC Santa Cruz, Santa Cruz, CA, Shirel R. Kahane-Rapport and Jeremy A. Goldbogen, Biology, Stanford University, Pacific Grove, CA, Ben Wallis, Ocean Expeditions, Sydney, Australia

Background/Question/Methods
Effectively describing the availability of patchy resources to foragers is a difficult but important challenge in ecology since analyzing prey density and distribution at spatial scales relevant to foraging predators can elucidate critical patterns and motivations for predator behavior. In March 2020 we compared humpback whale (Megaptera novaeangliae) behavior and Antarctic krill (Euphausia superba) distribution in two Antarctic fjords. Though of similar geographic characteristics, Charlotte Bay included an estimated 150-200 humpback whales, while Wilhelmina Bay contained an estimated 30-50 whales, despite this area’s status as a documented foraging hot spot for large groups of whales. We deployed bio-logging tags on whales in both bays to study foraging behavior and collected active acoustic data to correlate whale behavior with the surrounding prey fields. We used acoustic differencing techniques and a newly developed whale-scale approach to describe the aggregate biomass density and size classes of krill within patches at the depths and locations of foraging whales.
Results/Conclusions
All tagged whales spent more than 80% of their time at night foraging, and whales in both bays demonstrated similar foraging rates at night (48.1 ± 4.0 vs 50.8 ± 16.4 lunges/hr). However, whales in Charlotte Bay utilized deep (280-450 m) daytime foraging dives to additionally feed for 58% of their daylight hours compared to only 22% in Wilhelmina Bay. Daytime krill biomass density peaked between 200-300 m depth. However, patch size and krill size both increased below 300 m, coincident with the observed foraging depths. Large, mature, lipid-rich krill have been similarly observed at these depths in previous studies, likely feeding on detritus while hiding from predators. The ability of humpback whales to selectively forage on these larger krill suggests the use of hierarchical decision making and an enhanced ability to locate not only high-quality foraging environments but also high-quality parts of the broader environment. Overall, our work refines techniques and highlights the importance of measuring patchiness at scales that are relevant to the foraging strategies of individual species as a supplement to broader-scale approaches.