Marine soundscape and fish chorus in an archipelago ecosystem comprising bio-diverse tropical islands off Goa Coast, India

Abstract

Many biological sources of sound in tropical coastal shallow-water marine environments remain unidentified. Characterizing the soundscape of such environments requires that all sources of biological sound be examined for their distinct patterns and specific frequency ranges. The present study identified soniferous fish in the ecosystem and measured underwater ambient sound in terms of its sound pressure levels (SPL) to quantify and characterize their contribution to the soundscape. Underwater SPLs were measured from 2012 to 2016 at a site near Grande Island Archipelago (15° 18′ N, 73° 41′ E) 18 km off the coast of the state of Goa, which lies along India’s western coast. Acoustic data were recorded using three types of methods for passive acoustic monitoring, namely a hanging hydrophone, five seabed-mounted hydrophones and an autonomous moored vertical hydrophone. Underwater visual census at the site revealed that both species richness and diversity were high indicating the Grande Island Archipelago is a bio-diverse site. The seabed-mounted hydrophones proved to be the best choice for long-term SPL measurements, although the moored arrays were more economical. Ambient sound levels underwater varied markedly over time both within a day and with the season. Long-term SPLs were above 100 dB re 1 μPa over frequencies 62.5–8000 Hz and peaked to 120–130 dB re 1 μPa between frequencies 500 Hz and 1000 Hz, corresponding to the chorus frequencies of soniferous fish at the site. High SPLs and marked temporal variations of the soundscape indicate the abundance, diversity and life activities of fish species and healthy ecological state of the bio-diverse archipelago.