Acoustic discrimination of Southern Ocean zooplankton

Acoustic surveys in the vicinity of the sub-Antarctic island of South Georg ia during a period of exceptionally calm weather revealed the existence of a number of horizontally extensive yet vertically discrete scattering layer s in the upper 250 m of the water column. These layers were fished with a L onghurst-Hardy plankton recorder (LHPR) and a multiple-opening 8 m(2) recta ngular mid-water trawl (RMT8). Analysis of catches suggested that each scat tering layer was composed predominantly of a single species (biovolume > 95 %) of either the euphausiids Euphausia frigida or Thysanoessa macrura, the hyperiid amphipod Themisto gaudichaudii, or the eucalaniid copepod Rhincala nus gigas. Instrumentation on the nets allowed their trajectories to be rec onstructed precisely, and thus catch data to be related directly to the cor responding acoustic signals. Discriminant function analysis of differences between mean volume backscattering strength at 38, 120 and 200 kHz separate d echoes originating from each of the dominant scattering layers, and other signals identified as originating from Antarctic krill (Euphausia superba) , with an overall correct classification rate of 77%. Using echo intensity data alone, gathered using hardware commonly employed for fishery acoustics , it is therefore possible to discriminate in situ between several zooplank tonic taxa, taxa which in some instances exhibit similar gross morphologica l characteristics and have overlapping length-frequency distributions. Acou stic signals from the mysid Antarctomysis maxima could also be discriminate d once information on target distribution was considered, highlighting the value of incorporating multiple descriptors of echo characteristics into si gnal identification procedures. The ability to discriminate acoustically be tween zooplankton taxa could be applied to provide improved acoustic estima tes of species abundance, and to enhance field studies of zooplankton ecolo gy, distribution and species interactions. (C) 1998 Elsevier Science Ltd. A ll rights reserved.