Circadian Rhythm of Ambient Noise Off the Southeast Coast of India

Abstract

An ambient noise system consisting of a vertical linear hydrophone array was deployed in the shallow waters off Chennai, southeast coast of India from 1 August to 16 September 2013 to record ambient ocean noise of frequencies up to 10[Formula: see text]kHz. Biological sounds, which are broadband, short duration signals resulting from Terapon theraps, a native species, are a prominent feature of the ocean soundscape. Terapon activity peaks at 8[Formula: see text]pm and 11[Formula: see text]pm, and its presence is not observed after 12 midnight in both the months. In the other period, the ambient noise fluctuation is due to wind and vessel traffic. Hence, the present study focuses on the description of the ambient noise fluctuation over two 12[Formula: see text]h periods, i.e., 12 midnight–12 noon considered as period I, and 12 noon–12 midnight as period II in order to show the circadian rhythm of ambient noise. In this study area, Terapon vocalization reached 25[Formula: see text]dB above the ambient noise level and it dominates the short-term spectra records in the 0.4–4[Formula: see text]kHz range. All Terapon signals had daily patterns of sound production with highest levels of activity after dusk during the study period. The result shows that the circadian rhythm of ambient noise is mainly of biological sound generated by Terapon and it is reported first time in the shallow waters off the southeast coast of India.