Imaging Marine Fauna with a Tritech Gemini 720i Sonar

Multibeam sonar systems are increasingly used to detect, quantify and monitor behaviour of marine fauna. Over ranges of tens to hundreds of metres, animals can be detected as targets. However, at shorter ranges (typically <10 m) and in good conditions, high-frequency (> 1kHz) sonar systems can provide high-quality images earning the term 'acoustic cameras' and have become particularly advantageous for discriminating and counting fish. However, limitations of power and the significant increase in attenuation with frequency limit the achievable range of such acoustic cameras. Systems that operate at frequencies between those of mapping and fisheries sonar (typically <400 kHz) and acoustic cameras (approximate to 1MHz) are often used for short-range navigation and to evaluate underwater structures. While these systems produce images at reduced resolution compared to acoustic cameras, they may also be capable of distinguishing features of marine fauna and do so at greater ranges. This study utilised a Tritech Gemini 720i imaging sonar to produce images from 14 species of fauna at close range. It assessed some simple morphological parameters, such as length and breadth, and highlighted the possibilities of using these to categorise targets. It also provided a coarse description of issues associated with using such a system for monitoring marine animals.