Evidence for Size-Selective Predation by Antarctic Humpback Whales

Abstract

Animals aggregate around resource hotspots, but what makes one resource more appealing than another can be difficult to determine. In March 2020 the Antarctic fjord Charlotte Bay included >5× as many humpback whales as neighboring Wilhelmina Bay, a site previously known for super aggregations of whales and their prey, Antarctic krill. We used suction-cup attached bio-logging tags and active acoustic prey mapping to test the hypothesis that whale abundance in Charlotte Bay would be associated with higher prey biomass density, and that whale foraging effort would be concentrated in regions of Charlotte Bay with the highest biomass. Here we show, however, that patch size and krill length at the depth of foraging were more likely predictors of foraging effort than biomass. Tagged whales spent >80% of the night foraging, and whales in both bays demonstrated similar nighttime feeding rates (48.1 ± 4.0 vs. 50.8 ± 16.4 lunges/h). However, whales in Charlotte Bay foraged for 58% of their daylight hours, compared to 22% in Wilhelmina Bay, utilizing deep (280–450 m) foraging dives in addition to surface feeding strategies like bubble-netting. Selective foraging on larger krill by humpback whales has not been previously established, but suggests that whales may be sensitive to differences in individual prey quality. The utilization of disparate foraging strategies in different parts of the water column allows humpback whales to target the most desirable parts of their foraging environments.

Trade-offs in prey quantity and quality in gray whale foraging

“…Therefore, we posit that while N. rayii abundance did not reach the threshold at which foraging is beneficial in our study, this threshold can be achieved across the PCFG foraging range, leading PCFG whales to forage in areas of lower N. rayii abundance than nearby areas of higher H. sculpta abundance. This trade-off between prey quantity and quality has also been detected in humpback whales foraging in Antarctica that feed at depths deeper than where the densest krill patches occur in order to exploit less dense krill patches that are likely composed of larger, gravid krill (Cade et al 2022). While it is unclear how baleen whales differentiate between prey species or reproductive stages, several mechanisms have been suggested, including visual and auditory identification (Torres 2017) or detection via vibrissae (Pyenson et al 2012).…”

“…However, there is increased recognition that prey quality is also an important factor to consider when assessing a predator's ecology and habitat use (Spitz et al 2012), and marine predators do show a preference for higher quality prey items (e.g. Haug et al 2002, Meynier et al 2008, Spitz et al 2010, Cade et al 2022. Moreover, negative impacts of low quality prey on the health and breeding success of top marine predators (Österblom et al 2008(Österblom et al , Ludynia et al 2010, including marine mammals (Rosen & Trites 2000, Trites & Donnelly 2003, have been documented.…”

Seasonal gain in body condition of foraging humpback whales along the Western Antarctic Peninsula

“…While we found similar levels in expected BAI between immature and mature whales by April (Figure 4), it is also plausible that prolonged residency times along the WAP further increases BAI for mature whales. Indeed, large aggregations of humpbacks have been documented along the WAP in late autumn (Nowacek et al, 2011;Cade et al, 2022), and while our sample size is small, we observed only mature individuals in late autumn (n = 10 in June) that showed continued increases in BAI (Table 1, Figure 3). A study examining the number of barrels of oil from humpback whales taken from commercial whaling found that whales caught in late season had greater total body lipid stores compared to individuals caught earlier in the season, possibly reflecting differences in residency times between age-classes in the Antarctic foraging grounds (Irvine et al, 2017b).…”

“…Humpback whale distribution is linked closely to areas of high krill abundance in which whales do not forage below a minimum threshold of prey density (Friedlaender et al, 2011;Tyson et al, 2016). In the early austral summer, humpback whales distribute themselves broadly across the continental shelf, becoming more concentrated in bays and fjords during autumn (Curtice et al, 2015;, where they may exploit super-aggregations of krill (Nowacek et al, 2011;Johnston et al, 2012;Cade et al, 2022). Humpback whales adjust their foraging behavior based on depth, density, and size of their prey, and follow the diel vertical migration of krillforaging mostly at night and resting during the day, especially late in the season (Friedlaender et al, 2016;Cade et al, 2022;Nichols et al, 2022).…”

“…In the early austral summer, humpback whales distribute themselves broadly across the continental shelf, becoming more concentrated in bays and fjords during autumn (Curtice et al, 2015;, where they may exploit super-aggregations of krill (Nowacek et al, 2011;Johnston et al, 2012;Cade et al, 2022). Humpback whales adjust their foraging behavior based on depth, density, and size of their prey, and follow the diel vertical migration of krillforaging mostly at night and resting during the day, especially late in the season (Friedlaender et al, 2016;Cade et al, 2022;Nichols et al, 2022). Humpback whale foraging behavior also reflects seasonal differences in krill behavior; in the summer, humpback whales forage predominately on shallow prey in all light conditions and at high feeding rates, whereas in autumn, feeding becomes progressively deeper, nocturnal, and at lower rates (Nichols et al, 2022).…”