Backscattering of Underwater Sound in Saanich Inlet, British Columbia, Including Observations of Scattering from a Fish School

Abstract

The frequency dependence of acoustic volume backscattering strength has been measured at depths of 100, 200, 300, and 400 ft in Saanich Inlet, British Columbia, Canada, over a 24-h period. The measurements were made for comparison with open-ocean (Pacific) results, since the biology of the inlet has been extensively studied, and it serves to some extent as a reference water volume. In the course of carrying out the above measurements, the backscattering spectral characteristics and strength of a school of young hake, of apparent mean size of 11 in., was obtained. The time series measurements revealed unchanging scattering characteristics at the 300- and 400-ft depths, with broad peaks near 1.2 and 4.8 kHz superimposed on otherwise flat spectra. Greater variability was observed at the 200-ft depth, while the greatest variability was observed near the surface, i.e., at the 100-ft observation depth. The fish school, which was observed to be closely packed (less than a fish length spacing near the surface) and randomly oriented, was shown to have a flat backscattering spectrum between 1 and 9 kHz and an estimated backscattering strength of −47±3 dB.