Predictions of volume scattering strengths from biological trawl data

Abstract

A volume reverberation model which utilizes actual fish distribution data has been developed to permit quantitative predictions of depth-dependent scattering strengths. The model was evaluated using coincident acoustic volume scattering measurements and biological trawl data collected in the Mediterranean Sea. Biological specimens were collected at discrete depths with an Issacs-Kidd midwater trawl. Each fish collected was identified and sized, and its acoustic cross section calculated utilizing regression equations relating fish species, length, and swimbladder volume at frequencies near swimbladder resonance or equations relating fish length to acoustic cross section at frequencies far above resonance. Scattering strengths for each species were then determined, indicating dominant species at each trawl depth for 1/3-octave frequency bands between 2.5 and 20 kHz. The acoustic data were obtained with surface-vented explosive sources and a shallow omnidirectional hydrophone. These data were processed to yield integrated column strength versus depth profiles, which were then numerically differentiated to produce scattering strength versus depth profiles for each frequency band. Comparison of the model predictions to the scattering strength profiles showed good agreement at the higher frequencies, with the agreement decreasing with frequency below 8 kHz. Possible causes of this disagreement are enumerated.