Abundance, Distribution and Behaviour of Euphausia superba in the Southern Ocean Between 15° and 30° E During FIBEX

Summary

The abundance, distribution and behaviour of the Antarctic Krill, Euphausia superba, between 15°E and 30°E, and south of 62°S, was studied acoustically during FIBEX (First International BIOMASS Experiment) in February-March 1981. The biomass is estimated at 9.05 × 10⁵ t. The Krill were spread relatively uniformly over the survey area, probably because of an absence of large-scale hydrodynamic features to concentrate them in any part of the region, and were almost invariably found in swarms. The mean biomass and density of the 1304 swarms detected were estimated by multi-channel echo integration at 0.139 t and 31 g m⁻² respectively. Ninety-eight per cent of the swarms were estimated to have weighed < 1 t. Most of the biomass, however, was contained within a few large swarms. From relationships between swarm size and concurrent grazing indices, it is suggested tentatively that the large swarms were migrating or foraging and the small swarms were feeding or digesting. The distributions of swarm depth, biomass and density at night and by day indicate that swarms which did not migrate to the surface at night behaved almost identically to those detected by day. The depth distribution of Krill in the vicinity of 13 day-time stations indicates that Krill tended to be concentrated near the top of the thermocline. It is suggested that this was due to lateral current shear in the boundary region creating favourable conditions for feeding and aggregation. About 35% of the Krill in the vicinity of the stations were found at oxygen levels below reported lethal limits, suggesting that these limits may be too high or that a measure of adaptation to low oxygen concentrations may have occurred.