The characteristics of krill swarms in relation to aggregating Antarctic blue whales

E J Miller,B S Miller,J M Potts,M C Double,R Leaper,P A Olson,S Calderan,M J Cox,R L O’Driscoll

doi:10.1038/s41598-019-52792-4

Abstract

We model the presence of rare Antarctic blue whales (Balaenoptera musculus intermedia) in relation to the swarm characteristics of their main prey species, Antarctic krill (Euphausia superba). A combination of visual observations and recent advances in passive acoustic technology were used to locate Antarctic blue whales, whilst simultaneously using active underwater acoustics to characterise the distribution, size, depth, composition and density of krill swarms. Krill swarm characteristics and blue whale presence were examined at a range of spatiotemporal scales to investigate sub meso-scale (i.e., <100 km) foraging behaviour. Results suggest that at all scales, Antarctic blue whales are more likely to be detected within the vicinity of krill swarms with a higher density of krill, those found shallower in the water column, and those of greater vertical height. These findings support hypotheses that as lunge-feeders of extreme size, Antarctic blue whales target shallow, dense krill swarms to maximise their energy intake. As both Antarctic krill and blue whales play a key role in the Southern Ocean ecosystem, the nature of their predator-prey dynamics is an important consideration, not only for the recovery of this endangered species in a changing environment, but for the future management of Antarctic krill fisheries.

Highlights

We model the presence of rare Antarctic blue whales (Balaenoptera musculus intermedia) in relation to the swarm characteristics of their main prey species, Antarctic krill (Euphausia superba)
The spatial distribution of vocalising blue whales was highly concentrated into aggregations which could be heard from hundreds of kilometres away (Fig. 1)
A total of 310 sonobuoys were deployed throughout the voyage providing over 520 hours of passive acoustic recordings containing 42,489 detections of blue whale calls

Summary

Introduction

We model the presence of rare Antarctic blue whales (Balaenoptera musculus intermedia) in relation to the swarm characteristics of their main prey species, Antarctic krill (Euphausia superba). Results suggest that at all scales, Antarctic blue whales are more likely to be detected within the vicinity of krill swarms with a higher density of krill, those found shallower in the water column, and those of greater vertical height These findings support hypotheses that as lungefeeders of extreme size, Antarctic blue whales target shallow, dense krill swarms to maximise their energy intake. A variety of methods have been used in these studies to assess whale presence and behaviour, including visual surveys[25,26,29], suction tags[23,24,27] and satellite tags[28] Those that collected concurrent data on krill distribution did so using either active acoustics (scientific echosounders), allowing finer spatial sampling of individual swarms[24,26,29] or net hauls, resulting in coarser spatial sampling but direct measurements and identification of krill[25,26]. Other studies did not collect data on krill directly, but tracked whale lunges to assess feeding behaviour[23,27]

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Conclusion