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From sea ice to blubber: Linking whale condition to krill abundance

Janelle Braithwaite, Jessica Meeuwig | Apr 02, 2015

Janelle Braithwaite, Jessica Meeuwig

Apr 02, 2015

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Humpback whales travel thousands of km between their feeding and breeding grounds every year.

Photo: Jon Shaw and Scott Last.

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CITATION

Braithwaite JE, Meeuwig JJ, Letessier TB, Jenner CS, Brierley AS. 2015. From sea ice to blubber: Linking whale condition to krill abundance using historical records. Polar Biology, 38(8): 1195-1202.

HIGHLIGHTS

  • Humpback whales do not actively feed during migration, relying entirely on stored energy to complete what is considered one of the longest seasonal mammalian movements in the animal kingdom.
  • Balancing energy gain with energy use is therefore crucial for the animals’ survival.
  • Whales rely heavily on krill stocks during the summer feeding season, such that annual changes in the body condition of whales can be linked to fluctuations in krill abundance, a relationship mediated by the extent of sea ice.
  • Historical whaling records can be used to understand the effects of environmental conditions on body condition, with implications for predicting the impacts of human disturbance on the whales’ migration routes.

ABSTRACT

Krill (Euphausia superba) are fundamentally important in the Southern Ocean ecosystem, forming a critical food web link between primary producers and top predators. Krill abundance fluctuates with oceanographic conditions, most notably variation in winter sea ice, and is susceptible to environmental change. Although links between local krill availability and performance of land breeding, central place foragers are recognised, the effects of krill variability on baleen whales remain largely unclear because concurrent long-term data on whale condition and krill abundance do not exist. Here, we quantify links between whale body condition and krill abundance using a simple model that links krill abundance to sea ice extent.

Body condition of humpback whales (Megaptera novaeangliae) caught in west Australian waters between 1947 and 1963 was estimated from oil yields in whaling records. Annual estimates of krill abundance in the Southern Ocean where those whales foraged (70°–130°E) were correlated significantly with contemporary annual winter sea ice extent. We hindcast sea ice extent for the whaling period from reconstructed temperature data and found that whale body condition was significantly correlated with hindcasted winter sea ice extent, supporting the hypothesis that variations in body condition were likely mediated by associated krill fluctuations. As humpback whales migrate and breed on finite energy stores accrued during summer foraging in the Antarctic, changes in sea ice and concomitant changes in krill abundance have long-term implications for their condition and reproductive success.

FUNDING & ACKNOWLEDGEMENTS

We thank the numerous contributors to the KRILLBASE database and to Angus Atkinson, Volker Siegel, and Evgeny Pakhomov for making these data available to us via the ICED website. We also thank Cherry Allison at the International Whaling Commission for providing the historical whaling database, and Jan Erik Ringstad and Dag Ingemar Børresen at the Whaling Museum, Norway, for insight into whaling history and culture. JE Braithwaite thanks the University of Western Australia for providing a PhD scholarship.

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ANTARCTIC KRILL

Antarctic krill (Euphausia superba) are an important energy source for foraging humpback whales.

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MORE ICE, FATTER WHALES

Body condition is positively linked with SIE (sea ice extent), as the latter promotes higher krill abundance.