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Étude par acoustique large bande de la composition et de la distribution spatiale de couches diffusantes denses dans le golfe de Gascogne

Abstract : Intermediate trophic components of pelagic ecosystem (from mesozooplakton to micronekton) funnel energy and organic matters from primary producers to many commercial species (anchovy, sardines…). Some of these organisms are supposed to be important contributors to the ubiquitous Sound Scattering Layers (SSLs) observed over a broad range of spatio-temporal scales and geographical areas. Yet, the SSLs taxonomic composition remains largely unknown. The aim of this PhD was to describe the composition and repartition of dense SSLs observed in the Bay of Biscay (France) in spring at several spatial scales, using broadband acoustics, nets and videos. In the first chapter, we showed that the echo sounder beam widths had few effect on the SSLs backscatter, suggesting those SSLs could be composed of unidentified small gaseous scatterers. In the second chapter we applied a forward approach to characterize the composition of SSLs sampled in spring 2016 at small scale,by comparing in situ frequency responses to predictions of scatterer models parameterized with biological sampling data. We determined that these SSLs were probably composed of gas-bearing siphonophores, who, together with mesopelagic fish, dominated the frequency spectra at low frequencies (18-150 kHz). The acoustic backscatter at higher frequencies was dominated by a mix of mesozooplankton organisms, including high densities of pteropods. In the third chapter we applied unsupervised classification methods and a supervised discriminant analysis to delineate the spatial distribution of a peculiar surface SSL composed of
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Submitted on : Tuesday, September 8, 2020 - 11:50:14 AM
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Arthur Blanluet. Étude par acoustique large bande de la composition et de la distribution spatiale de couches diffusantes denses dans le golfe de Gascogne. Ecologie, Environnement. Agrocampus Ouest, 2019. Français. ⟨NNT : 2019NSARH110⟩. ⟨tel-02933256⟩

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