Characterization of sound scattering layers in the Bay of Biscay using broadband acoustics, nets and video.

Sigrid Lehuta,Laurent Berger,Jean-Baptiste Romagnan,Arthur Blanluet,Mathieu Doray,Naig Le Bouffant,Pierre Petitgas

doi:10.1371/journal.pone.0223618

Abstract

Sound scattering layers (SSLs) are observed over a broad range of spatio-temporal scales and geographical areas. SSLs represent a large biomass, likely involved in the biological carbon pump and the structure of marine trophic webs. Yet, the taxonomic composition remains largely unknown for many SSLs. To investigate the challenges of SSL sampling, we performed a survey in a small study area in the Northern Bay of Biscay (France) by combining broadband and narrowband acoustics, net sampling, imagery and video recordings. In order to identify organisms contributing to the observed SSLs, we compared measured frequency spectra to forward predicted spectra derived from biological data. Furthermore, to assess the confidence in SSL characterization, we evaluated uncertainties in modeling, acoustical and biological samplings. Here, we demonstrate for the first time that SSL backscattering intensity in the Bay of Biscay can be dominated in springtime by resonant gas bearing organisms below 100 kHz, namely siphonophores and juvenile fishes and by pteropods at higher frequencies. Thus, we demonstrate the importance of broadband acoustics combined to nets, imagery and video to characterize resonant backscatterers and mixed mesozooplankton assemblages.

Highlights

Sound Scattering Layers (SSLs) are routinely observed with active acoustic devices in a great variety of ecosystems and over wide depth ranges in the global ocean [1,2,3,4].Deep Scattering Layers [5] inhabiting the mesopelagic zone worldwide, are e.g. known to perform daily the largest migrations on earth [6] and their fish component might dominate the world total fishes biomass [4, 7]
The composition of two SSLs detected in the Bay of Biscay in springtime was characterized by jointly analyzing acoustic and groundtruthing data
Their frequency responses from 18 to 100 kHz were dominated by Gas Bearing (GB) siphonophores

Summary

Introduction

Sound Scattering Layers (SSLs) are routinely observed with active acoustic devices in a great variety of ecosystems and over wide depth ranges in the global ocean [1,2,3,4]. Deep Scattering Layers [5] inhabiting the mesopelagic zone worldwide, are e.g. known to perform daily the largest migrations on earth [6] and their fish component might dominate the world total fishes biomass [4, 7]. SSLs might play an important role in the biological carbon pump [8] and in the structure of marine trophic webs [9, 10]. SSLs are generally composed of mixed species assemblages of great diversity in taxa, body size and acoustic backscattering.

Objectives

Methods

Results

Conclusion