The Patchy Distribution of Krill Within Temporally Transient Large Swarms Drives Collective Exploitation By Super Groups of Rorqual Whales

Engulfment ram filtration feeding, the unique feeding style employed by rorqual whales, is effective because it combines the energetic advantages of filter feeding in bulk, with the ability of raptorial feeders to discriminate among discrete parcels of variable quality. The distribution of resources within a large patch is thus a critical, though previously neglected factor of rorqual whale habitat. We describe spatially limited, temporally transient, and numerically extraordinary aggregations of rorqual whales in two environments: humpback whales (Megaptera novaeangliae) off the west coast of South Africa and blue whales (Balaenoptera musculus) in Monterey Bay, California, USA and report on the high quality of prey associated with these groups. The unique foraging style of rorqual whales inspired new analyses of hydroacoustic prey data at species-specific spatial scales. We show that these aggregations form proximal to prey patches that are vertically extensive and include high density regions of consistent quality. We then compare foraging data from whales tagged within super groups to a multi-ecosystem meta-analysis of tag data from 112 blue whales and 42 humpback whales to show that whales associated with super groups profit from the distribution of resources within a patch by shortening the interval of time and the distance traveled between lunges, allowing for higher overall feeding rates than in other ecosystems. The prey patches we describe in Monterey Bay are spatially associated with surface vorticity regimes that change on time scales consistent with super group durations, suggesting that the ephemerality of these extensive patches is driven by environmental processes more than predator depletion. Ephemeral patch exploitation by social foragers is an untested aspect of social foraging theory, and we use multiple lines of evidence, including increased calling behavior concurrent with super group formation, to suggest that rorqual whales are benefitted overall by communicating the location of foraging hot spots, implying that these large aggregations may have been an integral aspect of historical behavior (when numbers were high before anthropogenic depletion) that enabled more efficient exploitation of prey in a temporally and spatially heterogeneous environment.