Interactions between the behaviour of an Euphasia superba superswarm, observed in November 1994 in the Ross Sea, and its physical and pelagic environment

A characteristic of Antarctic krill (Euphausia superba) is that they live in dense swarms of millions of individuals and such swarms have significant effect on the physical and pelagic environment. Krill body density is about 3% higher than the surrounding water and krill phytoplankton carbon demand is very high, therefore krill swarms need to move continuously and quickly through the water and their movements influence the gross features of the bio-physical environment together with the spatial distribution of krill-eating predators. An enormous aggregation of E. superba swarms was found on the 22nd and 23rd Nov. 1994 in an area of about 2100 km2, in the central part of the Ross Sea. The mean krill areal density resulted 860 g/m2 distributed from 10 to 120m in depth. The bulk of this aggregation was centered on a gigantic swarm containing roughly 57000 million adult Antarctic krill. Krill density in the same area, sampled on the 28-29th Nov., resulted 30 times lower than five days before and the "super swarm" apparently had scattered Northwestward in small and dense swarms. The paper concentrates on both the gigantic swarm found on the 23rd November and one of the swarms sampled acoustically on the 29th November, on the respective surrounding bio-physical environments and on the predators feeding in the swarm areas. The bio-physical factors that may limit swarm density and duration and the influence of swarms on the surrounding environment and predator spatial distribution are discussed.