ADCP MEASUREMENTS OF THE DISTRIBUTION AND ABUNDANCE OF EUPHAUSIIDS NEAR THE ANTARCTIC PENINSULA IN WINTER

Euphausiid populations were observed in Gerlache Strait, Antarctic Pen insula region, during austral winter 1992 (12 July-14 Aug) using a hul l-mounted 153-kHz acoustic Doppler current profiler and a multiple ope ning and closing net and environmental sensing system (MOCNESS). Theor etical estimates of target strength based on a straight cylinder model and applied to the numbers and sizes of euphausiids in net samples yi elded estimates of mean volume backscattering strength (MVBS) that com pared favorably with observed values at low abundances. However, net s amples underestimated biomass by almost two orders of magnitude at hig h abundances, a feature we attribute to net avoidance. Other taxonomic groups of macrozooplankton did not appear to contribute significantly to acoustic backscatter. Distributions of euphausiids were similar to those observed in summer; we found no evidence for abundant aggregati ons immediately below surface sea-ice. Euphausia superba dominated the biomass at all stations; individuals of approximately 22 mm were most abundant at 30-70 m depth, whereas 40-mm individuals were most abunda nt at 90-130 m. Thysanoessa macrura was positioned deeper in the water column, from 150 to 250 m, and Euphausia crystallorophias was centere d on 100 m. The first two species were distributed throughout the stud y area, but E. crystallorophias was concentrated in coastal embayments . Total wet weight biomass ranged from 0.009 to 92 g m-3, dry weight r anged from 0.002 to 18 g m-3, and protein ranged from 0.0009 to 9 g m- 3, mostly concentrated in the 30-70 m depth stratum. Over the upper 20 0 m of the water column integrated wet weight biomass estimated from A DCP data was in the range from 29 g m-2 to 92 kg m-2.