Fishery for krill is a major economical activity in the Antarctic Ocean. The increased Norwegian fishery inherently gives Norway a responsibility to contribute to the management of the marine resources in the southern ocean. During the International Polar Year 2008, RV “G.O. Sars” spent 3 months in the Antarctic Ocean to do investigations on euphausiids and other key components of the ecosystem. Major acoustic activities were to acoustically identify krill and estimate specimen size, investigate behaviour, measure target strength in situ, and of course verify the acoustic measurements biologically. Krill were identified from the relative frequency response of a 6-frequency hull-mounted echo-sounder system, and specimen size was estimated acoustically by means of several acoustic scattering models implemented in an optimized framework in the post-processing system LSSS.