Chapter 2: Distribution, biomass and demography of Antarctic Krill, Euphausia superba

There are increasing concerns over potential long-term changes in krill distribution and abundance, determining whether such changes are related to climate change and/or harvesting and how they might be distinguished from multi-scale variability. With nearly a century of observations on krill distribution, the general patterns of occurrence were determined over 50 years ago, however the last decade has seen an important consolidation of details of seasonal and interannual distributions, although some potentially important krill habitats such as the Bellingshausen, Amundsen and Ross Seas are still poorly sampled. Moreover the unexplored perennial multi-year pack-ice regions still await survey by remote under-ice samplers. Krill consistently occur in the upper 200 m of the ocean but various recent observations show that a substantial proportion of the population may be found below this pelagic zone. To further understand variability and change there has been a focus on the importance of historic data with the compilation of comprehensive databases, re-analysis of large scale synoptic surveys and the generation of smaller scale time series that now span several decades. In particular the compilation of 90 years of net samples has provided a key resource both in showing changes in large-scale abundance over the last 50 years and together with acoustic data in generating modern estimates of overall krill abundance and biomass. Meanwhile, the increasing availability of ocean circulation models together with remotely sensed data, has enabled the exploration of large-scale teleconnections at both the regional (Scotia Sea) and circum-polar scale and at the seasonal to decadal time scales.