Abundance, sizes and developmental stages of larval krill, Euphausia superba, during winter in ice-covered seas west of the Antarctic Peninsula

Abstract

Larval krill were sampled west of the Antarctic Peninsula during three winter cruises: September 1991, June 1993 and September 1993. Larval abundances were estimated from net catches and compared directly to visual counts (made by a SCUBA diver) of larvae occupying the ice habitat at the same sampling stations. The number of larvae per square meter sampled with nets was more often greater than that observed by the diver, irrespective of the sampling period. However, comparisons of larval abundance within sampling periods were not statistically significant. Larval krill collected by divers were significantly larger than those collected with nets for each of the three cruises. The stage composition of larval krill also depended on the collection method: net-collected samples contained a disproportionately high number of early furcilia larvae in June 1993 (early winter), and a disproportionately low number of early juveniles during September 1991 and 1993 (late winter). These results lead us to suggest that larval/juvenile krill occupy both the water column and sea ice habitat during the austral winter, and that there are often differences in the sizes and developmental stages of the two groups. For larval krill that occupied the sea ice habitat, aggregations were larger and more numerous during late winter than in early winter. In addition, larvae within aggregations occupied structurally complex microhabitats, provided by over-rafted ice floes, more often than they occupied smooth, downward-facing ice surfaces where ice was not over-rafted.