Three-dimensional mapping of the benthic invertebrates biophony with a compact four-hydrophones array

Abstract

Benthic invertebrates emit broadband transient sounds, that dominate coastal soundscapes and can inform on the environment and its ecological state. Numerous works have studied temporal patterns of Benthic Invertebrate Biophonies (BIB) and linked them to environmental parameters or ecological processes, but there is a lack of knowledge about the spatial variability of BIB. Here, we mapped Benthic Invertebrate Sounds (BIS) in three dimensions using a compact array (2 m × 2 m × 2 m) of four hydrophones within the scope of eco-acoustics studies. Firstly, we developed tools to localize any individual sound emitted by benthic invertebrates. A theoretical calculation of the Cramer Rao Bounds (CRB) consolidated by in situ active emissions from known positions, demonstrated the accuracy of the localization. Secondly, by accumulating all the positions of the BIS recorded during a night, three methods are proposed to map the BIB at different spatial scales. An in situ measurement campaign carried out on a 300 m × 300 m artificial reef system revealed that the marine fauna was binary spatially distributed: rich and numerous on artificial reefs and poor and weak outside the reefs. At small ranges (10 m), the acoustic maps matched the geometric structures of the reefs with an accuracy of 0.5 m. At large scales (100 m and 300 m), the acoustic hot-spots on the maps corresponded to distant reefs, with an accuracy of 7 m and 10 m respectively.This study provides the first maps of the BIB at ecologically relevant scales and shows that they can be used to identify, quantify, follow and track benthic activity hot-spots. In combination with biological and ecological information, passive acoustic monitoring using such maps can be a powerful complementary tool for ecological studies, such as the quantification of grazing activity or the evaluation of the efficiency of marine ecologic restoration programs.