SONAR SYSTEMS AND AQUATIC ORGANISMS - MATCHING EQUIPMENT AND MODEL PARAMETERS

Acoustic technology is an accepted and important component of aquatic research and resource management. Despite the widespread use of echoso unders, few guidelines aid in the choice of appropriate sonar system p arameters for acoustic surveys. Choice of acoustic carrier frequency i s analogous to the choice of spotlight colour used to illuminate a pai nting. Three primary biological factors influence the scattering of so und by aquatic organisms: swimbladder presence, organism length. and o rganism behaviour. We illustrate the influence of these factors on the amplitude of backscattered echoes using a Kirchhoff-ray mode scatteri ng model to quantify fish and zooplankton backscatter as a function of carrier frequency, fish length, and swimbladder aspect. Model results illustrate that echo amplitudes from aquatic organisms are largely de pendent on the presence or absence of a swimbladder. Target strengths generally increase with increasing carrier frequency and organism leng th. Swimbladder angle relative to the incident sound wave affects scat tering amplitudes at all frequencies. Measurements of backscatter from swimbladdered fish are relatively robust when the ratio of fish lengt h to acoustic frequency wavelength ranges between 2 and 10. As fish le ngth to frequency wavelength ratios increase, echo amplitudes become m ore dependent on aspect and peak when the swimbladder is perpendicular to the acoustic wavefront.