OCCURRENCE OF ANTARCTIC KRILL (EUPHAUSIA-SUPERBA) CONCENTRATIONS IN THE VICINITY OF THE SOUTH SHETLAND ISLANDS - RELATIONSHIP TO ENVIRONMENTAL PARAMETERS

The influences of abiotic and biotic parameters on the occurrence of A ntarctic krill (Euphausia superba) concentrations were studied in the waters north of the South Shetland Islands, a major krill fishing grou nd in the Antarctic, during the 1990/1991 austral summer. From early t o mid-summer krill density increased and showed distinct offshore-insh ore differences in abundance and maturity stages. In mid-summer, krill density was low in the oceanic zone (8 gm(-2)) and higher in the slop e frontal zone (36 gm(-2)), being highest along the shelf break (131 g m(-2)) in the inshore zone. Krill were in the reproductive stage in th e oceanic and frontal zones, but non-reproductive in the inshore zone. Water circulation, food and frontal features were considered importan t environmental factors determining the occurrence of high krill conce ntrations in these waters. Drifting buoys indicated that the surface w ater circulation in areas of krill concentrations was characterized by a sluggish current with eddies along the shelf break in the inshore z one and a shear current in the frontal zone, thus enhancing the reside nce time for krill. Diatom abundance in both the frontal and inshore z ones increased from early to mid-summer, improving krill feeding condi tions. Gravid females were closely associated with the frontal zone, s howing 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 p revailing 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 oceanograp hic features that led to their retention in these zones. (C) 1998 Else vier Science Ltd. All rights reserved.