Sound Scattering Layers Within and Beyond the Seychelles-Chagos Thermocline Ridge in the Southwest Indian Ocean

Myounghee Kang,Yeon Choi,Jung-Hoon Kang,Minju Kim,Sunghyun Nam,Dong-Jin Kang

doi:10.3389/fmars.2021.769414

Abstract

In global oceans, ubiquitous and persistent sound scattering layers (SL) are frequently detected with echosounders. The southwest Indian Ocean has a unique feature, a region of significant upwelling known as the Seychelles-Chagos Thermocline Ridge (SCTR), which affects sea surface temperature and marine ecosystems. Despite their importance, sound SL within and beyond the SCTR are poorly understood. This study aimed to compare the characteristics of the sound SL within and beyond the SCTR in connection with environmental properties, and dominant zooplankton. To this end, the region north of the 12°S latitude in the survey area was defined as SCTR, and the region south of 12°S was defined as non-SCTR. The results indicated contrasting oceanographic properties based on the depth layers between SCTR and non-SCTR regions. Distribution dynamics of the sound SL differed between the two regions. In particular, the diel vertical migration pattern, acoustic scattering values, metrics, and positional properties of acoustic scatterers showed two distinct features. In addition, the density of zooplankton sampled was higher in SCTR than in the non-SCTR region. This is the first study to present bioacoustic and hydrographic water properties within and beyond the SCTR in the southwest Indian Ocean.

Highlights

Marine biomes account for approximately 71% of the Earth s surface
This study provides some of this information, more details about the ground-truth of scattering layers (SL) organisms at finer scales in the southwest Indian Ocean and the Seychelles-Chagos Thermocline Ridge (SCTR) are needed for a better understanding of the organisms and their surrounding environmental features
This study demonstrated for the first time that distinctly different distributional dynamics of SL organisms within and beyond SCTR are closely associated with oceanographic features

Summary

Introduction

Marine biomes account for approximately 71% of the Earth s surface. One of the most remarkable features of this biome is the presence of ubiquitous and persistent sound scattering layers (SL). The sound SL appear continuous on the echogram from an echosounder, vertically narrow, tens to hundreds of meters, and horizontally extensive for tens to thousands of kilometers (Simmonds and MacLennan, 2005). They consist of two types of layers: the shallow scattering layer, which is found from the sea surface to 200 m deep in the epipelagic zone, and the deep scattering layer, which is detected from 200 to 1,000 m in the mesopelagic zone The SL is composed of broad taxonomic groups and typically ranges in size from 1 to 20 cm (Kloser et al, 2009)

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