Predator‐scale spatial analysis of intra‐patch prey distribution reveals the energetic drivers of rorqual whale super‐group formation

Abstract

Animals are distributed relative to the resources they rely upon, often scaling in abundance relative to available resources. Yet, in heterogeneously distributed environments, describing resource availability at relevant spatial scales remains a challenge in ecology, inhibiting understanding of predator distribution and foraging decisions. We investigated the foraging behaviour of two species of rorqual whales within spatially limited and numerically extraordinary super‐aggregations in two oceans. We additionally described the lognormal distribution of prey data at species‐specific spatial scales that matched the predator's unique lunge‐feeding strategy. Here we show that both humpback whales off South Africa's west coast and blue whales off the US west coast perform more lunges per unit time within these aggregations than when foraging individually, and that the biomass within gulp‐sized parcels was on average higher and more tightly distributed within super‐group‐associated prey patches, facilitating greater energy intake per feeding event as well as increased feeding rates. Prey analysis at predator‐specific spatial scales revealed a stronger association of super‐groups with patches containing relatively high geometric mean biomass and low geometric standard deviations than with arithmetic mean biomass, suggesting that the foraging decisions of rorqual whales may be more influenced by the distribution of high‐biomass portions of a patch than total biomass. The hierarchical distribution of prey in spatially restricted, temporally transient, super‐group‐associated patches demonstrated high biomass and less variable distributions that facilitated what are likely near‐minimum intervals between feeding events. Combining increased biomass with increased foraging rates implied that overall intake rates of whales foraging within super‐groups were approximately double those of whales foraging in other environments. Locating large, high‐quality prey patches via the detection of aggregation hotspots may be an important aspect of rorqual whale foraging, one that may have been suppressed when population sizes were anthropogenically reduced in the 20th century to critical lows. A free Plain Language Summary can be found within the Supporting Information of this article.

Asset Tracking Whales—First Deployment of a Custom-Made GPS/GSM Suction Cup Tag on Migrating Humpback Whales

“…These systems are fitted with 4 silicon suction cups, a galvanic release mechanism and a VHF transmitter for retrieval. They have extensively been used on a variety of whales [37,40,44]. The galvanic release was set to release within approximately 4 h after tag deployment to ensure timely retrieval.…”

Rorqual Lunge-Feeding Energetics Near and Away from the Kinematic Threshold of Optimal Efficiency

“…One is the patch prey density ( ρ prey ), which, per the results shown here, could constrain lunge feeding speeds ( U open ) to lower values to maintain high efficiency wherever the density is low, as during feeding near the surface. More generally, correlating a whale’s trajectory with actual prey density data obtained from echo sounding ( Goulet et al 2019 ; Cade et al 2021 ) would be preferable to estimation through a geographical average ( Nemoto 1983 ), as done here and elsewhere ( Goldbogen et al 2011 ; Cade et al 2016 ; Guilpin et al 2019 , 2020 ). This would go a long way in not only determining the efficiency on a lunge-to-lunge basis, but also in documenting the minimal patch densities that rorquals are known to avoid ( Hazen et al 2015 ).…”

Acoustic signature reveals blue whales tune life history transitions to oceanographic conditions

“…While these trends hold for the range of upwelling scenarios observed during the study period, only weak to moderately-strong cumulative upwelling occurred in Monterey Bay over 2015-2020 (Figure 3B). Extremely strong upwelling can lead to offshore advection, causing offshore transport of planktonic organisms (Bakun et al, 2015;García-Reyes & Largier, 2012), and leading to less krill available along the shelf break (Harvey et al, 2021), where the combination of bathymetric and oceanographic features are amenable to the dense krill aggregations on which blue whales depend during their intensive summer foraging (Benoit-Bird et al, 2019;Cade et al, 2021). Continued monitoring of this population's behavioral phenology during years of strong to extreme upwelling will provide greater insight into whether or not the relationships between ecosystem phenology and blue whale behavior extend to years characterized by stronger upwelling conditions and associated offshore advection of krill prey.…”

“…This endangered population migrates seasonally between higher-latitude foraging grounds off California and the northern reaches of the California Current Large Marine Ecosystem (CCLME) in summer and fall, and lower-latitude breeding grounds near Baja California and the Costa Rica Dome in the winter and spring (Bailey et al, 2010;Stafford et al, 1999Stafford et al, , 2001. These migrations are tightly linked with the seasonal and episodic formation of dense aggregations of blue whales' obligate prey, krill (specifically Thysanoessa spinifera and Euphausia pacifica; (Benoit-Bird et al, 2019;Cade et al, 2021;Croll et al, 2005)) resulting from wind-driven upwelling (Figure 1A). Upwelling ecosystems display temporal lags between upwelling, increases in primary productivity, and subsequent increases in the abundance of zooplankton (including krill) and higher trophic level predators such as blue whales (Barlow et al, 2021;Croll et al, 2005).…”