Development and application of multifrequency acoustic techniques to the study of Antarctic krill distribution

Abstract

Multifrequency acoustic techniques were refined and applied in a study of the distribution of Antarctic krill. Acoustic, video, and net data were collected in a continental shelf region west of the Antarctic Peninsula in the falls and winters of 2001 and 2002. Improved parametrization of a theoretical model of krill target strength was achieved through direct measurement of all model parameters. Krill visual acuity was used to determine a threshold level of volume backscattering strength for identifying krill aggregations. Differences in mean volume backscattering strength at 120 and 43 kHz further served to distinguish krill from other sources of scattering. Inversions were performed of mean volume backscattering strength (43, 120, 200, and 420 kHz) in these acoustically identified aggregations to estimate the weighted mean length and density of krill. Inversion results were comparable to net samples for estimated length, but acoustic estimates of krill density exceeded those from nets by one to two orders of magnitude, likely due to avoidance and differences in the volumes sampled by the two systems. Application of these methods to survey data demonstrated strong spatial and temporal variability in the horizontal and vertical distribution of krill aggregations, consistently associated with food availability, temperature, and predators.