Southern Ocean humpback whale trophic ecology. I. Combining multiple stable isotope methods elucidates diet, trophic position and foraging areas

Sarah J. Bury; Katharina J. Peters; Amandine J. M. Sabadel; Katie St John Glew; Clive Trueman; M. B. Wunder; Matthew R. D. Cobain; Natalie Schmitt; David Donnelly; Sarah Magozzi; Kylie Owen; Julie C. S. Brown; Pablo Escobar-Flores; Rochelle Constantine; Richard L. ODriscoll; Mike Double; Nick Gales; Simon Childerhouse; Matthew H. Pinkerton

doi:10.3354/meps14532

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MEPS 734:123-155 (2024) - DOI: https://doi.org/10.3354/meps14532

Southern Ocean humpback whale trophic ecology. I. Combining multiple stable isotope methods elucidates diet, trophic position and foraging areas

Sarah J. Bury^1,*, Katharina J. Peters^2,3,4, Amandine J. M. Sabadel^1,5,6, Katie St John Glew⁷, Clive Trueman⁷, M. B. Wunder⁸, Matthew R. D. Cobain⁹, Natalie Schmitt^10,11, David Donnelly^10,12, Sarah Magozzi¹³, Kylie Owen¹⁴, Julie C. S. Brown^1,15, Pablo Escobar-Flores¹, Rochelle Constantine¹⁶, Richard L. O’Driscoll¹, Mike Double¹⁰, Nick Gales^10,17, Simon Childerhouse^18,19, Matthew H. Pinkerton¹

¹National Institute of Water & Atmospheric Research (NIWA), Hataitai, Wellington 6021, New Zealand
²Marine Vertebrate Ecology Lab, Environmental Futures, School of Earth, Atmospheric and Life Sciences, University of Wollongong, Wollongong, NSW 2522, Australia
³Cetacean Ecology Research Group, College of Sciences, Massey University, Auckland 0632, New Zealand
⁴Evolutionary Genetics Group, Department of Anthropology, University of Zurich, 8057 Zurich, Switzerland
⁵Department of Zoology, University of Otago, Dunedin 9016, New Zealand
⁶Department of Environmental Science, Auckland University of Technology, Auckland 1010, New Zealand
⁷Ocean and Earth Sciences, University of Southampton Waterfront Campus, Southampton SO14 3ZH, UK
⁸Department of Integrative Biology, Denver, CO 80204, USA
⁹Department of Biological and Environmental Science, University of Jyväskylä, 40014 Jyväskylä yliopisto, Finland
¹⁰Australian Antarctic Division, Department of Agriculture, Water and the Environment, Kingston, TAS 7050, Australia
¹¹Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, Ontario L8S 4K1, Canada
¹²Killer Whales Australia, 17 Eric Crescent, Mornington, VIC 3931, Australia
¹³Department of Integrated Marine Ecology, Stazione Zoologica Anton Dohrn, Fano Marine Centre, 61032 Fano (PU), Italy
¹⁴Department of Environmental Research and Monitoring, Swedish Museum of Natural History, Stockholm 104 05, Sweden
¹⁵Bloomsbury Environmental Isotope Facility, Department of Earth Sciences, University College London, London WC1E 6BT, UK
¹⁶School of Biological Sciences, University of Auckland, Auckland 1010, New Zealand
¹⁷30 Power Road, North Bruny, TAS 7150, Australia
¹⁸Cawthron Institute, Nelson 7010, New Zealand
¹⁹Environmental Law Initiative, Epworth House, Te Aro, Wellington 6011, New Zealand
²⁰Present address: Department of Earth, Environment and Life Sciences (DISTAV), University of Genoa, 16132 Genoa, Italy
²¹Present address: National Biodiversity Future Centre (NBFC), 90133 Palermo, Italy

*Corresponding author: sarah.bury@niwa.co.nz

ABSTRACT: Southern Ocean humpback whales Megaptera novaeangliae are capital breeders, breeding in the warm tropics/subtropics in the winter and migrating to nutrient-rich Antarctic feeding grounds in the summer. The classic feeding model is for the species to fast while migrating and breeding, surviving on blubber energy stores. Whilst northern hemisphere humpback whales are generalists, southern hemisphere counterparts are perceived as krill specialists, but for many populations, uncertainties remain regarding their diet and preferred feeding locations. This study used bulk and compound-specific stable isotope analyses and isoscape-based feeding location assignments to assess the diet, trophic ecology and likely feeding areas of humpback whales sampled in the Ross Sea region and around the Balleny Islands. Sampled whales had a mixed diet of plankton, krill and fish, similar to the diet of northern hemisphere humpback whales. Proportions of fish consumed varied but were often high (2-60%), thus challenging the widely held paradigm of Southern Ocean humpback whales being exclusive krill feeders. These whales had lower ¹⁵N values and trophic position estimates than their northern hemisphere counterparts, likely due to lower Southern Ocean baseline ¹⁵N surface water values and a lower percentage consumption of fish, respectively. Most whales fed in the Ross Sea shelf/slope and Balleny Islands high-productivity regions, but some isotopically distinct whales (mostly males) fed at higher trophic levels either around the Balleny Islands and frontal upwelling areas to the north, or en route to Antarctica in temperate waters off southern Australia and New Zealand. These results support other observations of humpback whales feeding during migration, highlighting the species’ dietary plasticity, which may increase their foraging and breeding success and provide them with greater resilience to anthropogenically mediated ecological change. This study highlights the importance of combining in situ field data with regional-scale isoscapes to reliably assess trophic structure and animal feeding locations, and to better inform ecosystem conservation and management of marine protected areas.

KEY WORDS: Feeding ecology · δ¹⁵N · δ¹³C · Amino acids · MixSIAR · Isoscapes · Antarctica · UNSDG14 Life Below Water

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Cite this article as: Bury SJ, Peters KJ, Sabadel AJM, St John Glew K and others (2024) Southern Ocean humpback whale trophic ecology. I. Combining multiple stable isotope methods elucidates diet, trophic position and foraging areas. Mar Ecol Prog Ser 734:123-155. https://doi.org/10.3354/meps14532

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