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.
Highlights
The fjords along the West Antarctic Peninsula host a large diversity and biomass of marine organisms
The West Antarctic Peninsular fjords known as Charlotte and Wilhelmina Bays were explored from 12–16 March 2020 on board the RV Australis, a 23 m, steel-hulled, single-masted motor sailor with a relatively low environmental footprint
Given the null model Akaike information criteria (AIC), the high effect sizes of the length variables, the effects of temporal autocorrelation, and the low sample sizes overall, the modeling approach did not provide sufficiently strong evidence given the current sample size for a relationship between krill patch characteristics and whale foraging at depth. Both globally in the present day and historically within the West Antarctic Peninsula there appear to be more baleen whale species coexisting than would be expected (Perrin, 1991), especially given that many baleen whales have nearly global ranges and all feed in a similar manner and can feed on similar prey
Summary
The fjords along the West Antarctic Peninsula host a large diversity and biomass of marine organisms. When ecosystems appear to be relatively homogeneous, e.g., dominated at the mid-trophic level by a single species, yet support a variety of predator taxa, it generates an apparent contradiction with the competitive exclusion principle which posits that two or more species cannot coexist in space and time by exploiting the same prey (Gause, 1934; Hardin, 1960) In many cases, this contradiction can be resolved with a better understanding of how an ostensibly homogenous environment demonstrates heterogeneity— creating the possibility of niche separation—at scales relevant to the organisms involved (e.g., the paradox of the plankton, Hutchinson, 1961). Though feeding on the same prey using a similar technique (engulfment filtration feeding) as other rorqual whale species in the region, humpback whales (Megaptera novaeangliae) have been shown to occupy different spatial habitats, which may contain different quality prey, than Antarctic minke whales (Balaenoptera bonaerensis) (Friedlaender et al, 2009b, 2021) or fin whales (Balaenoptera physalus) (Santora et al, 2010)
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