OCEANIC CIRCUMPOLAR HABITATS OF ANTARCTIC KRILL

Surveys of Euphausia superba often target localised shelves and ice edges where their growth rates and predation losses are atypically high. Emphasis on these areas has led to the current view that krill require high food concentrations, with a distribution often linked to shelves. For a wider, circumpolar perspective we compiled all available net-based density data on postlarvae: 8137 mainly summer stations from 1926-2004. Unlike Antarctic zooplankton their distribution is highly uneven, with 70% of the total stock concentrated between 0o and 90oW. Within this Atlantic sector, krill are abundant over both shelf and ocean. At the Antarctic Peninsula, by contrast, they are found mainly over the inner shelf whereas in the Indian-Pacific sectors krill prevail in the ocean within 200-300 km of the shelf break. Overall, 87% of the total stock live over deep oceanic water (>2000 m) and krill occupy regions of moderate food (0.5-1.0 mg chl a m-3). Advection models suggest some loss northwards from these regions and into the low chlorophyll belts of the Antarctic Circumpolar Current (ACC). We found possible evidence for a compensating southwards migration, with an increasing proportion of krill found south of the ACC as the season progressed. The retention of krill in moderately productive oceanic habitats is a key factor in their high total production. While growth rates are lower than those over shelves, the ocean provides a refuge from shelf-based predators. The unusual circumpolar distribution of krill thus reflects a balance between advection, migration and top-down and bottom-up processes.