Factors influencing Antarctic krill distribution in the South Shetlands

The influences of biological and physical factors in the environment upon krill (Euphausia superba) distribution were studies in the area north of South Shetland Islands during 1990/91 austral summer. Krill showed a distinct offshore-inshore heterogeneities in abundance and maturity stage in mid-summer the abundance was low in the oceanic zone (8.5 g/m2), while higher in the slope frontal zone (37.3 g/m2), and the highest along the shelf break (135.1 g/m2) in the inshore zone ; krill were reproductive in the oceanic and frontal zones, whereas non-reproductive in the inshore zone. The following factors were considered to be responsible for this characteristic distribution of krill. Diatoms were the main food of krill, and a spatial correlation between distributions of krill and diatoms were observed. Hence, higher diatom biomass may be one of factors forming krill concentrations in the inshore and frontal zones. The water flow was sluggish in the inshore zone (3.2 km/day), while meandering in the oceanic zone (7.9 km/day) and straightforward in the frontal zone (13.8 km/day). Especially, in the inshore zone, convergent complex eddies were generated along the shelf break, where krill were densely aggregated. Hence, the mechanical accumulation may another factor concentrating non-reproductive krill there. The frontal zone was considered to be favorable spawning ground for krill : not only because of the greater depth (which prevents krill embryos from being predated by benthic animals) and of the presence of warm Circumpolar Deep Water (which helps the development of the embryo) ; but also because of the higher chance of larvae's being transported to their nursery ground. Based on above mentioned factors, we further discussed why the change in spatial distribution of krill occurs from early to late summer.