Spring distribution, size composition and behaviour of krill Euphausia superba in the western Weddell Sea
Distribution, size composition and behaviour of Euphausia superba were investigated in the northwestern Weddell Sea (59–63°S, 45–52°W) in October-November 1988 using RMT trawling, SCUBA diving and visual examination of the ice undersurface using a remotely-operated vehicle (ROV). Amounts of krill washed onto the ice during ice-breaking along transects were noted. Juvenile and sub-adult krill were found, often in high numbers, in association with seasonal pack-ice, from the outer marginal ice zone to at least 200 nautical miles [3 50 km] into the closed pack-ice zone. Krill caught with the RMT or observed within or close to the ice usually had full guts. They were frequently seen feeding on ice algae, and seemed to concentrate in pressure zones, melting ice and infiltration layers, ie where ice provided both confining crevices and rich algal growth. During twilight numbers of krill increased in open water close to the ice, though ROV observations at night revealed even greater numbers remaining in ice cavities. Direct observations from deck, by divers and by ROV, confirmed that most of the krill population in the uppermost water layer was confined to ice habitats, though in three out of 20 RMT catches krill reached densities of 0.1 nr3 ie normal summer values. ‘Miniswarms’ forming in early November may indicate seasonal transition of at least part of the krill population from winter ice habitat and grazing on ice-algae, to summer pelagic life and filter-feeding on phytoplankton.