Effect of ship sound on the vocal behavior of dugongs

Abstract

Effects of man-made, low-frequency sounds on the behavior of the dugong are discussed in this paper. We developed a monitoring system of power-driven vessel to assess the impact of man-made noise on dugongs. Ship navigation was monitored by questionnaire for boaters and visual observations from an anchored vessel. We used automatic under water sound monitoring systems for dugongs (AUSOMS-D) to record under water sound and to track ship navigations acoustically. The visual observations were performed for a total of 10 hours and 20 minutes and 72 ships were detected. The acoustic monitoring was conducted for over 81 hours and detected 258 ships. Shortest distance between the visual-observation platform and the power-driven vessels ranged from 18 to 500 m or more. We calculated the monitoring range of the system by comparing the result of the visual observation and the acoustic survey. The system detected 51.4 % of noise-making ships within 500 meters from the observation platform, and 78.1 %, 89.5 %, and 100 % within 300, 200, and 100 meters, respectively. The ship navigation showed bimodal occurrence during 6:00-7:00 and during 15:00-17:00. We could position the sound source of ship sounds and draw the pathway of a ship by using AUSOMS-D. Based on this result, we calculated the position fix accuracy of ship sound, which was 17.1±8.71 m. This study provided information on detailed techniques for tracking the noise-making vessels and will lead to tracking the vocalizing animal, such as the dugong.