On the basis of hydroacoustic observations it is shown that migrations of krill during spring are stronger than during summer. Migrations of krill are described by the function: H(t) = A + Bcos((2ᴨt/T + φ ) + C c o s ( 2 ᴨt/T + φ ), where: H is depth of the mass center of krill biomass, A — mean depth of krill occurrence, В — amplitude of migrations with period T! = 24 h, С — amplitude of migrations with period T2 = 12 h, (φ1, φ2 — phases of migration process with T, = 24 and T2 = 12 hours. Parameters of the equation are the following: spring — A = 62.2 m, В = 19.5 m, С = 4.6 m, φ1 = 0.1 h, φ2 = 0 . 1 5 h; summer — A = 75.8 m, В = 0.5 m, С = 3.6 m, φ1 = 1.8 h, φ2 = 6.4 h.
On the basis of acoustically registered cross-sections of krill aggregations, regular, irregular and layer forms were distinguished. Regular forms are most frequently observed during spring and in the day time, while irregular forms are most frequent during summer and night hours. The density histograms made for two hour intervals clearly show the day-night difference, but the seasonal (spring, summer) difference is less pronounced. Mean density of swarm is lowest during the night and reaches a maximum in early morning hours. The mean volume backscattering strength values (Sv) for spring and summer are nearly identical. We suggest that regular forms correspond to foraging swarms and irregular forms to feeding swarms as described by Hamner (1984).
At the ice edge krill undergoes diurnal migrations with the period of 12 hours and amplitude of about 6 meters. The mean depth of krill occurrence is 41 m, shallower then for open waters. In our opinion these migration parameters are characteristic of juvenile adolescent krill dominating at the ice edge.
In the investigated area the overall abundance of krill was small and was increasing with the distance from ice. However, with the data available, it was not possible to decide whether this increase was related to the ice border or was a part of a larger scale phenomenon. The depth distributions as well as the mean values of krill depth were similar to those of open water both in this study and reported in literature.
During SIBEX the acoustically evaluated amount of krill in the Bransfield Strait and Drake Passage was very low with the mean density 3.24 individuals/m2 and 4.29 individuals/m2 accordingly. Any substantial quantities of krill were found North-West from the Elephant Island and North from the King George Island, where the density of krill exceeded 1000 individuals/m2 (about 100 t/nM2]). The total biomass was estimated at 70590 ton in the Bransfield Strait and at 122470 ton in the Drake Passage, which was many times less than during FIBEX 81, especially in the Bransfield Strait.