Variations in vertical distribution and density of krill swarms in the vicinity of South Georgia

Krill abundance and distribution in relation to depth and time of day were investigated by means of a calibrated echo sounder and echo integrator. The study areas were in the Scotia Sea, in the South Georgia area (both on-shelf and off-shelf regions) and a large krill patch close to South Georgia. Comparison of the results from the three areas shows major differences in the diurnal patterns of vertical migration. Mean volume backscattering strength (MVBS) was analysed with respect to size of aggregations. The mean values for day and night were markedly different in the krill patch, the daytime value being higher than expected from concentration effects alone. In the other phases of the study, MVBS and concentration effects appeared to compensate and diurnal variation was less marked