Spatial mapping of the biophony of the fishes living in seagrass meadows

Abstract

Passive acoustics is well suited to assess the diversity and/or activity of marine animals, particularly if cryptic or difficult to observe as in seagrass Posidonia oceanica meadows. The ability to locate the emitters allows not only to detect the presence of a specific sound or specie but also to estimate source levels, the communicative space with respect to ambient noise and anthropogenic impact, the density of vocalizing animals, their movements and potentially home ranges, etc. Here, we located fishes in seagrass meadows (Calvi, France). Two particular sounds dominated the acoustic scene of seagrass meadows: the: 300 Hz drums of Ophidion rochei and the ubiquitous 800Hz /kwa/ sound of unknown origin. The wavelengths of these fish sounds range from 5 m to 2 m. During six 48-hours period, we deployed a squared array of 4 hydrophones (20 x 50 x 50 m). Here, we describe the algorithm for automated fish sound detection and 3D localization of the sources and assess their performances on our real data. Over 1000 fish sounds were detected per night. These detections were used to create localization maps and study the spatial distribution of fish sound production, estimate source levels and the density of the vocalizing fishes within the meadow and their limits.