Understand feeding habits of Humpback Whales

OUR ENVIRONMENT Understand feeding habits of Humpback Whales

Nuclear techniques helping our marine creatures

ANSTO participated in an important study to determine the feeding ecology and migratory behaviour of the Southern Hemisphere Humpback Whale because of reported changes to their physical condition and migratory feeding habits.

A new application for radiocarbon analyses was published in Scientific Reports. This application is expected to help understand the feeding ecology of migratory species in the Southern Ocean, because it provides an extra tracer of location.

Southern Hemisphere Humpback Whales have some of the longest migration patterns of any mammal. Each year, the two Australian populations of Humpback Whales journey from breeding grounds in the Southern Pacific region to summer feeding grounds around Antarctica. In winter the whales make their way back to tropical breeding grounds off the east and west coasts of Australia.

The classical model for the whales’ feeding ecology identifies them as krill obligates, meaning they exclusively prey on Antarctic krill. As they are also capital breeders, (breeding and migrating while the animal is fasting), they are reliant on their summer feeding to replenish large blubber stores that will support up to seven months of fasting. Amidst climate change concerns, declining Antarctic krill stocks would put the whales at a significant risk of being unable to support their own nutritional needs and those of their calves during migration.

Three significant signs have been reported relating to the feeding behaviour of Humpback Whales that may be indicators of such changes in the Antarctic sea-ice ecosystem.

The first indication is that the number of stranded whales around the Southern Hemisphere has greatly increased during the past decade. Secondly, controversial findings reported by the Japanese scientific whaling research community, suggest that the blubber thickness of Minke Whales has decreased by 10 per cent over the past two decades and the stomach content has decreased by 30 per cent. Humpback Whales consume the same food resources as Minke Whales. Finally, there has been an increase in reports of Southern Humpback Whales feeding along the migration route, possibly indicating a change in feeding behaviour.

Analyses of Humpback Whale baleen plates were done to provide temporal baselines of prey choice and to study possible changes in their feeding behaviour. Baleen plates are composed of keratin and grow throughout the life of a whale much like hair, although the ends are eventually ground down. Between 270–400 plates hang from the top jaw, acting as filters for krill-feeding.

As the plates grow, the nutrients absorbed through feeding are deposited in succession, thus reflecting temporal changes in food. Baleen plates are routinely collected opportunistically by the Southern Ocean Persistent Organic Pollutants Program (SOPOPP) and archives are maintained by SOPOPP, the Victoria Museum, the Australian Museum, the Queensland Museum and Murdoch University. For this study, most samples were collected between 2009 and 2014.

These plates were analysed for both stable isotopes of carbon and nitrogen at the Isotope Ratio Mass Spectrometry facility at Griffith University. ANSTO contributed with radiocarbon analysis of the plates using accelerator mass spectrometry. The use of stable isotope analysis of 13C and 15N from the baleen plates of numerous species of whales has been validated by marine researchers as an efficient way to trace feeding location and type of prey. Radiocarbon provided an ‘additional tracer’ between Antarctic and Australian waters and complemented the data acquired by stable isotope analysis by resolving ambiguous supplementary feeding signals.

Because of the natural and human variations of 14C in the world’s oceans, it can be used as a way to trace travelling bodies of water. The Southern Ocean is one of the few places where there is an ‘old’ 14C signature, because of the upwelling of deep water that replaces surface water. The surface water there is low in 14C compared to Australian surface waters. Since the whales travel through these two different bodies of water, variations in the amount of 14C in the baleen plates are directly related to feeding events in these two locations.

ANSTO measured the 14C concentration from 24 samples taken from seven baleen plates, as well as six skin samples

using the Star accelerator. This allowed the research team to compare isotope profiles from a whale feeding according to the traditional model to a whale showing signs of supplementary feeding. Results from the bulk stable isotope analysis suggested unexpected individual variation in feeding ecology, such as higher trophic level feeding and / or feeding within Australian waters.

This wide spectrum of feeding strategies was confirmed by the radiocarbon analysis, with some whales seemingly feeding opportunistically during migration, and some sticking to the classical model and fasting the whole way. Very few seem to be feeding extensively along Australian waters.

Contact

debashish.mazumder@ansto.gov.au

COLLABORATORS

1

ANSTO RESEARCHER TEAM

Miss Pascale EISENMANN

Prof Brian FRY

2

2

Dr Geraldine JACOBSEN

1

Assoc Prof Susan Bengtson NASH 2

Dr Debashish MAZUMDER

1

2

Griffith University

RESEARCH FACILITY / TECHNIQUE

Centre for Accelerator Science (CAS)

Star

2MV tandetron accelerator

www.ansto.gov.au/cas/star

PUBLICATIONS

Krill

Scientific Reports 2017