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Marine Ecology Progress Series

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MEPS 465:69-83 (2012)  -  DOI:

Antarctic krill swarm characteristics in the Southeast Atlantic sector of the Southern Ocean

Bjørn A. Krafft1,*,**, Georg Skaret1,**, Tor Knutsen1, Webjørn Melle1, Thor Klevjer2,3, Henrik Søiland1

1Institute of Marine Research, 5870 Bergen, Norway
2Department of Biology, University of Oslo, 0316 Oslo, Norway
3King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia
**These authors contributed equally

ABSTRACT: Knowledge about swarm dynamics and underlying causes is essential to understand the ecology and distribution of Antarctic krill Euphausia superba. We collected acoustic data and key environmental data continuously across extensive gradients in the little-studied Southeast Atlantic sector of the Southern Ocean. A total of 4791 krill swarms with swarm descriptors including swarm height and length, packing density, swimming depth and inter-swarm distance were extracted. Through multivariate statistics, swarms were categorized into 4 groups. Group 2 swarms were largest (median length 108 m and thickness 18 m), whereas swarms in both Groups 1 and 4 were on average small, but differed markedly in depth distribution (median: 52 m for Group 1 vs. 133 m for Group 4). There was a strong spatial autocorrelation in the occurrence of swarms, and an autologistic regression model found no prediction of swarm occurrence from environmental variables for any of the Groups 1, 2 or 4. Probability of occurrence of Group 3 swarms, however, increased with increasing depth and temperature. Group 3 was the most distinctive swarm group with an order of magnitude higher packing density (median: 226 ind. m−3) than swarms from any of the other groups and about twice the distance to nearest neighbor swarm (median: 493 m). The majority of the krill were present in Group 3 swarms, and the absence of association with hydrographic or topographic concentrating mechanisms strongly suggests that these swarms aggregate through their own locomotion, possibly associated with migration.

KEY WORDS: Euphausia superba · Aggregation · Acoustics · Swarm · Bouvetøya

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Cite this article as: Krafft BA, Skaret G, Knutsen T, Melle W, Klevjer T, Søiland H (2012) Antarctic krill swarm characteristics in the Southeast Atlantic sector of the Southern Ocean. Mar Ecol Prog Ser 465:69-83.

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