EFFECT OF ORIENTATION ON BROAD-BAND ACOUSTIC SCATTERING OF ANTARCTIC KRILL EUPHAUSIA-SUPERBA - IMPLICATIONS FOR INVERTING ZOOPLANKTON SPECTRAL ACOUSTIC SIGNATURES FOR ANGLE OF ORIENTATION

Acoustic scattering experiments involving simultaneous acquisition of broadband echoes and video footage from several Antarctic krill were c arried out to determine the effect of animal orientation on echo spect ral structure. A novel video analysis technique, applied to extract kr ill angle of orientation corresponding to each insonification, reveale d that echo spectra from krill near broadside incidence relative to th e incident acoustic wave exhibited widely spaced, deep nulls, whereas off-broadside echo spectra had a more erratic structure, with several closely spaced nulls of variable depth. The pattern of changes in echo spectra with orientation for the experimentally measured acoustic ret urns was very similar to theoretically predicted patterns based on a d istorted wave Born approximation (DWBA) model. Information contained i n the broadband echo spectra of the krill was exploited to invert the acoustic returns for angle of orientation by applying a newly develope d Covariance Mean Variance Classification (CMVC) approach, using gener ic and animal-specific theoretical and empirical model spaces. The ani mal-specific empirical model space was best able to invert for angle o f orientation. The CMVC inversion technique can be implemented using a generic empirical model space to determine angle of orientation based on broadband echoes from individual zooplankton in the field. (C) 199 8 Acoustical Society of America. [S0001-4966(98)05109-1]