Passive and active acoustic monitoring of mulloway in the Swan River

Abstract

Passive acoustic monitoring is a standard tool to monitor vocal marine fauna. High-frequency multibeam echosounders have developed rapidly in recent years, with the number of applications for detecting and tracking biological targets expanding significantly. In the Swan River, Perth, mulloway (Argyrosomus japonicus) form aggregations each austral summer. Here, a Kongsberg MS1000 scanning sonar and a CMST Underwater Sound Recorder (USR) were deployed onto the riverbed in 12 m of water. The sonar scanned to ranges of 50 and 75 m (angular resolution, 0.45°), taking approximately 120 s for one full 360° scan. The USR sampled at 6 ksps for five of every fifteen minutes. The sonar detected fish travelling slowly (typically <0.5 ms-1) within its range, while the passive recorder detected the development of an evening chorus, starting with individual calling fish. One example target remained within the field of view of the sonar for over an hour, detected 55 times as it moved ≈100 m. Simultaneously, the USR recorded mulloway vocalisations, with received levels approximating those predicted for a mulloway at the range detected by the sonar. This study outlines one of the first successes of matching passive and active acoustic tracking of vocal fish, as a precursor to using sonar techniques to verify estimates of calling numbers of fish from passive acoustic monitoring.