Krill 'super-swarm' surprises scientists

Dr Andrew Constable (left) examines a dish containing salps. Other researchers sort through trays of fish and krill samples.
Dr Andrew Constable (left) examines salps collected from nets deployed for krill, zooplankton and phytoplankton samples. (Photo: Nick Roden)
Scientists and crew empty the krill net during the K-Axis voyage.  This map shows part of the voyage track of the Aurora Australis (orange line) from Hobart (not shown) towards the K-Axis study area  between the southern Kerguelen Plateau and the Antarctic continent  followed by a resupply trip to Davis station. Ocean colours indicate depth.

Super-swarms of krill and a highly productive ecosystem were among the discoveries of the Australian Antarctic Division’s marine science voyage to the Kerguelen Axis*, between January and February this year.

The 8850 km ‘K-Axis’ voyage, led by Dr Andrew Constable of the Australian Antarctic Division and Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC), surveyed nine habitat gradients across the Antarctic continental shelf, deep ocean and on to shallow banks, to understand the drivers of the krill-based food web in the southern part of the K-Axis, and the fish- and copepod-based food web in north.

In each habitat the team surveyed krill, fish, zooplankton and phytoplankton, and a range of chemical and physical properties including iron concentration, ocean temperature and salinity, and sea ice extent.

“Our preliminary results indicate that the system in East Antarctica is much more productive than we thought,” Dr Constable said.

“For example, we found that chlorophyll, which is a marker for phytoplankton, was regularly much more abundant in water deeper than the surface waters measured by satellites. The abundance of grazers, such as salps [floating forms of sea squirts] and small zooplankton, was much greater than expected in areas where Antarctic krill were expected to dominate. And we found that fish occurred in great abundance throughout the ecosystem, and that they represent a different but inextricably linked food chain to krill.”

The most surprising discovery was that krill – including a rare super-swarm – were found further north than expected.

“Most studies have found krill concentrated south of the southern boundary of the Antarctic Circumpolar Current,” Dr Constable said.

“However we found krill much further north – even north of the Southern Antarctic Circumpolar Current Front– which suggests that sea ice is not a primary factor influencing the distribution of adult krill in this region.

“We also encountered a rare super-swarm of Antarctic krill in 3500 m-deep water to the west of the Kerguelen Plateau and north of the Southern Antarctic Circumpolar Current Front, with more than 100 humpback whales feeding on it.”

The encounter was the first time a super-swarm has been properly observed and measured in the Indian Ocean since an observation in the early 1970s by a fishing vessel. Other large swarms have only been observed much nearer the Antarctic continental shelf in the K-Axis area.

The research team now has the first comprehensive ecosystem data set to formulate new and improved theories on krill distribution and movement in the region, and to improve food web reconstruction in ecosystem models.

“Salps are clearly a dominant grazer in the ecosystem. Data on these and different species of krill and zooplankton, and the types of phytoplankton in their diets, will enable a more comprehensive reconstruction of the food web and different energy pathways, which is a major gap in our understanding of Southern Ocean ecosystems,” Dr Constable said.

“We’ll also be able to add our observations of whales, seals, penguins and flying seabirds in the region, to tracking studies of these animals based out of Mawson and Davis, to identify relationships between land-based and at-sea observations.”

The K-Axis voyage on board the Aurora Australis was part of an international study that included the US vessel Roger Revelle, CSIRO’s Investigator (see Volcanic hotspot may fortify ocean life), French ship Marion Dufresne and the Japanese vessels Umitaka Maru and Hakuho Maru – with each ship taking samples and measurements in multiple locations around the Kerguelen Axis.

This comprehensive sampling will enable the development of an observing program that will help measure the current status of the East Antarctic ecosystem and future trends as a result of climate change.

The data from the voyage will be available through the Australian Antarctic Data Centre, the Australian Integrated Marine Observing System, the Southern Ocean Observing System and the Australian Ocean Data Network. The research will also contribute to the Commission for the Conservation of Antarctic Marine Living Resources. Results will be presented at an international conference on status and trends of Southern Ocean ecosystems in Hobart in 2018.

Corporate Communications
Australian Antarctic Division

*The Kerguelen Axis is a line of longitude where the Antarctic Circumpolar Current flows across the Antarctic continental shelf, the deep ocean and subantarctic islands. It runs from the Amery Ice Shelf on the East Antarctic coast, out into the Southern Ocean, between Heard Island and Kerguelen Island.

The voyage was a collaboration between the Australian Antarctic Division, the University of Tasmania’s Institute for Marine and Antarctic Studies, the Australian Research Council-funded Antarctic Gateway Partnership and the ACE CRC. Read more about the research in the previous issue of Australian Antarctic Magazine 29: 2-3, 2015.