CSIRO’s Marine National Facility, RV Investigator, spent seven weeks in the Southern Ocean, looking at the complex relationship between Antarctic blue whales and krill, and their roles in maintaining the health of the Southern Ocean (more details in Australian Antarctic Magazine 35: 14–15, 2018).
The first challenge for the 28 scientists on the 13,000 kilometre journey was to find the rare Antarctic blue whales in the vast Southern Ocean.
Lead whale acoustician, Dr Brian Miller, said more than 250 underwater listening devices or ‘sonobuoys’ were deployed during the voyage to detect the whales’ low frequency calls.
Sonobuoys can detect blue whale calls up to 1000 kilometres away and can be used to triangulate the whales’ precise location as the ship closes in.
“We monitored over 750 hours of underwater recordings and measured over 33,000 bearings to blue whale calls, which enabled us to home in on whale ‘hotspots’,” Dr Miller said.
Over 300 hours of search effort led to 36 encounters with blue whales and 25 individual identifications. One was a whale that had previously been sighted on an expedition six years ago.
Antarctic blue whales almost exclusively eat krill and can eat up to four million krill, or more than three tonnes, in one day.
Australian Antarctic Division krill biologist, Rob King, said krill swarms can be deep or shallow, dense or diffuse, but little is known about the different swarm types and whether some are more attractive to blue whales than others.
To find out more, the ship’s echo sounders, which bounce sound off organisms in the water column, were used to locate and characterise krill swarms. The krill team was able to construct three-dimensional pictures of giant krill swarms, with several swarms extending over one kilometre in length and hundreds of metres across. Each swarm contained many millions of krill.
“This voyage has provided much more information about the fine-scale three-dimensional structure of krill swarms, so we can start to get a better idea of the sort of swarms Antarctic blue whales hunt,” Mr King said.
Drones were also used in more than 130 missions across a range of scientific applications, including size measurements and ‘blow’ (exhalation) sampling of blue whales, and a new method for trace metal surface water sampling.
Voyage Deputy Chief Scientist, Dr Elanor Bell, said biogeochemists conducted experiments on these surface water samples, and other samples from various depths, to test the theory that whale faeces is an important source of iron in the Southern Ocean.
“It’s thought the whales’ iron-rich faeces may stimulate the growth of phytoplankton on which krill feed, which are in turn whale food,” Dr Bell said.
The voyage’s multidisciplinary research will contribute to the improvement of ecosystem-based management of the Antarctic krill fishery and the conservation of endangered species, including Antarctic blue whales.
The research was supported by a grant of sea time on RV Investigator from the CSIRO Marine National Facility. A large number of collaborating institutions and partners also supported the voyage, including the University of Tasmania, Murdoch University, University of Technology Sydney, University of Washington, University of Liverpool, Texas A&M University, University College Cork, the International Whaling Commission’s Southern Ocean Research Partnership, and the World Wildlife Fund — Australia.
Australian Antarctic Division