Soundscapes in shallow water of the eastern Arabian Sea

The study of soundscapes is an emerging field of research that attempts to provide important baseline information on environmental conditions and biological compositions. In this study, the soundscape in shallow waters of the eastern Arabian Sea is explored through passive acoustic measurements in a time series between October 2015 and January 2016. The noise levels in the spectrum generated from two sources, geophony and biophony, have been observed to vary between diurnal and lunar phases and also between seasons. The noise produced by wind and rain in the frequencies ranging from 0.5 to 5 kHz is found to contribute more intensely to the soundscape during the post-monsoon period. However, during early winter, there is no significant contribution from rain noise. Planktivorous and Terapontidae choruses, the two individual fish choruses, were found to be the sources of biophony in our study. Each chorus displays distinct acoustic characteristics with respective spectral peaks at 0.9 and 1.5 kHz. The choruses exhibit a diurnal pattern and higher noise levels are found during the sunset and lunar phases. Both Terapontidae and Planktivorous choruses exhibit high noise levels every night during the post-monsoon period. However, the same pattern of noise levels is not found between Planktivorous and Terapontidae choruses in early winter as the Terapontidae chorus is absent in that period. In particular, the chorus produced by Planktivorous fish is observed to be associated with the lunar phase, and the noise levels are found to be significantly higher during the new moon and lower during the full moon. This study illustrates that passive acoustics is an effective way of monitoring the soundscape and provides valuable information about the temporal patterns of biological sounds in marine ecosystems.