ADCP measurements of the distribution and abundance of euphausiids near the Antarctic Peninsula in winter

Abstract

Euphausiid populations were observed in Gerlache Strait, Antarctic Peninsula region, during austral winter 1992 (12 July–14 Aug) using a hull-mounted 153-kHz acoustic Doppler current profiler and a multiple opening and closing net and environmental sensing system (MOCNESS). Theoretical estimates of target strength based on a straight cylinder model and applied to the numbers and sizes of euphausiids in net samples yielded estimates of mean volume backscattering strength (MVBS) that compared favorably with observed values at low abundances. However, net samples underestimated biomass by almost two orders of magnitude at high 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 aggregations 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 abundant at 90–130 m. Thysanoessa macrura was positioned deeper in the water column, from 150 to 250 m, and Euphausia crystallorophias was centered on 100 m. The first two species were distributed throughout the study area, but E. crystallorophias was concentrated in coastal embayments. Total wet weight biomass ranged from 0.0009 to 9 g m −3, dry weight ranged 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 200 m of the water column integrated wet weight biomass estimated from ADCP data was in the range from 29 g m −2 to 92 kg m −2.