The largest animal on earth feeds on one of the smallest one: Whales in the frigid waters of the Southern Ocean feast on krill, crustaceans only a few centimetres long. This extraordinary dinner party was recently witness by Australian scientists on a research expedition to the Kerguelen Islands where a rare ‘super-swarm’ of krill attracted more than 100 humpback whales.

Antarctic krill (Euphausia superba) gather in the trillions in Antarctic waters providing a feast for whales, seals and sea-birds. (Photo: anniekatec.blogspot.com)
Antarctic krill (Euphausia superba) gather in the trillions in Antarctic waters providing a feast for whales, seals and sea-birds. (Photo: anniekatec.blogspot.com)

The Research voyage for the Kerguelen Islands in the Southern Indian Ocean departed Hobart, Tasmania on January 11 (see Polar News Article on January 25 and March 10 2016). On Valentine’s Day (14 February) the scientists encountered a rare super-swarm of Antarctic krill (Euphausia superba) containing trillions of krill. These ‘super-swarms’ can stretch for tens of kilometres and are formed by juvenile krill tightly packed not adults. Swarming is a defensive mechanism, confusing smaller predators that would like to pick out individuals. Chief Investigator, Dr Andrew Constable, said: “This is the first time a super-swarm has been properly observed and measured in the Indian Ocean since initial observations in the early 1970s by a fishing vessel. We were able to use the latest technology to map the swarm to learn about the behaviour and abundance of krill in the region.”

Chief Investigator of the voyage, Dr Andrew Constable examining a salp (a herbivore). Salps are part of the zooplankton. Similar to krill they can form large swarms and are at the base of the food web, but in contrast to krill they are less nutritious. (Photo: Nick Roden)
Chief Investigator of the voyage, Dr Andrew Constable examining a salp (a herbivore). Salps are part of the zooplankton. Similar to krill they can form large swarms and are at the base of the food web, but in contrast to krill they are less nutritious. (Photo: Nick Roden)

”We observed whales including humpbacks and fin whales in many locations where Antarctic krill were in abundance, and also saw Adelie, emperor and king penguins, crabeater and Antarctic fur seals and several species of flying birds, including albatross, petrels and terns,” Dr Constable said.

A humpback is straining water through its baleen after lunging for a mouth full of krill. Like a sieve the baleen plates filter out the krill. (Photo: Robert Pitman, NOAA, Public Domain)
A humpback is straining water through its baleen after lunging for a mouth full of krill. Like a sieve the baleen plates filter out the krill. (Photo: Robert Pitman, NOAA, Public Domain)

Productive swarms of krill were found much further north than scientists had expected and were not restricted to areas to the south of the Antarctic Circumpolar Current. “We also found krill in high densities in seemingly unproductive waters and for the first time, we have comprehensive ecosystem data to formulate new and improved theories on their distribution and movement in this region.” Dr Constable said the voyage also confirmed that the fish-based food chain is much more important, diverse and widespread through areas where Antarctic krill are also found. “Our ability to measure all aspects of the food web at each research station meant that we could see how fish and krill are inextricably linked rather than being separate from one another, which is what many current theories say.”

The productivity of water can be measured by studying its trace elements. Here scientist are preparing the trace metal rosette, which samples the concentrations of iron in the water. Iron acts as a fertilizer for phytoplankton. (Photo: Rob King)
The productivity of water can be measured by studying its trace elements. Here scientist are preparing the trace metal rosette, which samples the concentrations of iron in the water. Iron acts as a fertilizer for phytoplankton. (Photo: Rob King)

Oceanographic observations in two separate locations showed bottom water that is fresher, warmer and less dense than when last sampled in 2006. “The changes in the bottom water are further evidence that rapid change is underway in the Southern Ocean – even in its deepest layers,” Dr Constable said.

The marine science voyage on board the Aurora Australis was part of an international study that includes the US vessel Roger Revelle, 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. Dr Constable said that the comprehensive sampling in the region, combined with the sampling from the French, Japanese and USA vessels will enable the development of an observing program that will help measure status and trends in the East Antarctic ecosystem. Australian Antarctic Division Chief Scientist, Dr Gwen Fenton said the science and analyses will be valuable resources into the future, particularly for the Conservation of Antarctic Marine Living Resources (CCAMLR).  

The data from the voyage will be available through a range of portals, including the Australian Antarctic Data Centre, the Australian Integrated Marine Observing System (IMOS), the Southern Ocean Observing System (SOOS) and the Australian Ocean Data Network (AODN).

Source: Australian Antarctic Division