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    Occurrence of Antarctic krill (Euphausia superba) concentrations in the vicinity of the South Shetland Islands: relationship to environmental parameters

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    T. Ichii, K. Kayatama, N. Obitsu, H. Ishii and M. Naganobu (Japan)
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    The influences of abiotic and biotic parameters on the occurrence of Antarctic krill (Euphausia superba) concentrations were studied in the waters north of the South Shetland Islands, a major krill fishing ground in the Antarctic, during the 1990/1991 austral summer. From early to mid-summer krill density increased and showed distinct offshore-inshore differences in abundance and maturity stages. In mid-summer, krill density was low in the oceanic zone (8 g m –2, and higher in the slope frontal zone (36 g m –2), being highest along the shelf break (131 gm–2) in the inshore zone. Krill were in the reproductive stage in the oceanic and frontal zones, but non-reproductive in the inshore zone. Water circulation, food and frontal features were considered important environmental factors determining the occurrence of high krill concentrations in these waters. Drifting buoys indicated that the surface water circulation in areas of krill concentrations was characterized by a sluggish current with eddies along the shelf break in the inshore zone and a shear current in the frontal zone, thus enhancing the residence time for krill. Diatom abundance in both the frontal and inshore zones increased from early to mid-summer, improving krill feeding conditions. Gravid females were closely associated with the frontal zone, showing that the latter may also be a favored spawning area because of the higher probability of survival for embryos and larvae, in addition to enhanced opportunities for their transport to nursery grounds by prevailing currents. In the waters north of the South Shetland Islands, krill appeared to be highly adapted for exploitation of the rich food resources of the frontal/inshore zone, taking advantage of oceanographic features that led to their retention in these zones.