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Acoustic determination of the distribution of fish and krill across the Scotia Sea in spring 2006, summer 2008 and autumn 2009

Fielding, Sophie; Watkins, Jonathan; Collins, Martin Anthony; Enderlein, Peter; Venables, Hugh. 2012 Acoustic determination of the distribution of fish and krill across the Scotia Sea in spring 2006, summer 2008 and autumn 2009. Deep Sea Research II, 59-60. 173-188. 10.1016/j.dsr2.2011.08.002

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Abstract/Summary

Multifrequency acoustic backscatter data were examined from transects at eight stations across the Scotia Sea, from the South Orkneys to the north west of South Georgia. These transects were repeated in austral spring (November 2006), summer (December 2007/January 2008) and autumn (March 2009). A dB identification window (Sv120-38) identified two types of scatterers dominating the acoustic backscatter; a positive Sv120-38 indicative of Antarctic krill (Euphausia superba) and a negative Sv120-38 indicative of mesopelagic fish with swim bladders, such as myctophids. Acoustic backscatter attributed to schooling myctophid fish was greater at stations in the north of the Scotia Sea across all seasons and found to increase significantly with increasing mean sea temperature and absolute dynamic height. A schools analysis identified more fish schools at the stations with high fish acoustic backscatter. The number of fish schools increased, were distributed further south, and occurred deeper from spring through to autumn. Krill density, calculated using a target strength model, was similar in spring (27.7 g m−2) and summer (27.2 g m−2), but lower (2.2 g m−2) in autumn 2009. An analysis of swarm characteristics showed that during the spring cruise the high mean krill density resulted from a small number of swarms containing a large number of small krill, particularly in the south of the survey region. In contrast in the summer cruise the high mean krill density resulted from a large number of swarms containing a low number of large krill. Finally, in autumn the low krill density resulted from a small number of swarms containing a small number of large krill. Krill swarms were located at a mean depth of 58 m in spring, 94 m in summer and 123 m in autumn and were generally further apart, deeper, contained fewer krill and smaller in the north of the survey region. Large-scale shifts in the latitudinal and vertical distribution of krill and schooling mesopelagic fish in the Scotia Sea were observed. This seasonal reorganisation in both horizontal and vertical distribution will alter the role of these prey items in the ecosystem.

Item Type: Publication - Article
Digital Object Identifier (DOI): 10.1016/j.dsr2.2011.08.002
Programmes: BAS Programmes > Polar Science for Planet Earth (2009 - ) > Ecosystems
BAS Programmes > Polar Science for Planet Earth (2009 - ) > Polar Oceans
ISSN: 0967-0645
Additional Keywords: Myctophid fish, Schooling behaviour,Swim bladder, Polar waters
Date made live: 21 Feb 2012 16:30
URI: http://nora.nerc.ac.uk/id/eprint/16881

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