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Antarctic krill - the key organisme in the Southern Ocean

At 41°S in the southern Atlantic Ocean, on our way to Walvis Bay, Namibia, after weeks in waters with temperatures close to 0°C, we measured surface temperatures of 16°C. The distribution area of our main study organism, the Antarctic krill, is now left behind and we have started analysing our collected data.

Above:
Antarctic krill, Euphausia superba. Photo: Cecilie Broms Årnes

We have mapped the krill distribution along two long sections, southwards along 15°E and northwards along 7.5°E. During the voyage we measured krill abundance and vertical distribution acoustically, as well as with fine-meshed trawls. At the same time we have studied the ecosystem of the krill, from temperatures, salinity and oceanic currents to food- and predator distributions. One main objective of this cruise is to estimate the biomass and numeric abundance of krill in the survey area. Converting acoustic indexes to absolute krill weights and abundances requires information about the acoustic properties of the krill. As described in previous cruise-diaries, we have measured the target strength of krill directly in the ocean by means of submergible echosounders, and indirectly from measurements of density and sound velocity within the krill.


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School of krill photographed at 30 m depth with cameras mounted on the TS-probe (submergible echosounder). Photo: Ronald Pedersen


The figure below shows the distribution of krill from 50°-67° S as measured by the ship echosounder during the southward transect along 15°E. We see that the krill are mainly found shallower water than 100 m and that the highest abundances are found in the south, close to the Antarctic continent. It may also seem that the krill are distributed somewhat shallower in the south, but here we need to correct for diel vertical migrations. The krill in the south were large, with an average bodylength close to 50 mm, while the krill from the central region were smaller. Large krill were also found far north, for instance northeast of Bouvet Island, where the bodylength could reach 63 mm.


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Distribution of krill along 15° E from 50°-67°S, measured by the ships echosounder showing distribution of krill Sa-values (prepared by Peter H. Wiebe)


The krill were primarily observed in dense schools, not unlike herring and capelin in our own waters off Norway. We also noted that the krill were able to maintain within dense schools during the dark nights. Throughout the cruise, we could see shallow schools of krill as red patches on the ocean surface. With distributions reaching the surface, many of these schools will not be visible on the ship echosounders since they are mounted at about 10 m depth. Both the patchy distribution of the krill, and their shallow distribution partly above the echosounders contribute to the uncertainty in estimation of krill abundance.

 

 

Written by


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Wejørn Melle, dr.scient.  Senior scientist at the Institute of Marine Research in Bergen. He studies plankton ecology, and is the cruise leader on the 2 Leg of the AKES cruise.


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Espen Bagøien, dr.scient. Scientist at the Institute of Marine Research in Bergen. He studies zooplankton in the Norwegian Sea.