Measurements of acoustic volume backscattering in the Indian and Southern Oceans

Abstract

Volume backscattering strengths have been measured at several positions in the Indian Ocean and the Southern Ocean (across the Great Australian Bight). The positions in the Indian Ocean varied from the vicinity of the Equator to a station off the coast of Western Australia near Fremantle. The backscattering strengths have been analysed at frequencies in third-octave steps from 2.5 to 20 kHz. The average daytime scattering strengths at the Equator and in the Bight are similar and range from around - 87 dB re m^-1 at 2.5 kHz to between -75 and -70 dB re m^-1 at 20 kHz. At night, the average scattering strengths in the Bight increase from about -75 dB re m^-1 at 2.5 kHz to about -70 dB re m^-1 at 20 kHz, whereas at the Equator the results increase from about -82 dB re m^-1 at 2.5 kHz to -64 dB re m^-1 at 20 kHz. Deep scattering layers (DSL) were observed both in the Bight and at the Equator. The DSL in the Bight had a resonance frequency of 4 kHz and the average depth of the bottom of the layer was 950 m. From the acoustic scattering strength. it is inferred that the average population density of the fish in the layer is 10^-3 m^-3, and that the average mass of the fishes is around 40 g. The DSL at the Equator had a flat frequency response at frequencies above 10 kHz (there was no peak in the spectrum) and the average depth of the layer was about 500 m. The average abundance of the scatterers in the layer is inferred to be of the order of 5 × 10^-3 m^-3. The backscattering strengths measured in the Great Australian Bight have been compared with predictions based on concurrent net hauls that were conducted to depths of 50 and 100 m. Good agreement occurs only at the higher frequencies and at night-time when most of the organisms are near the surface.