Sound scattering layers within and beyond the Seychelles-Chagos Thermocline Ridge (SCTR) in the southwest Indian Ocean

Abstract

In global oceans, ubiquitous and persistent sound scattering layers are frequently detected by 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, ocean-atmosphere coupling, biogeochemistry, and fisheries. Yet, few studies on sound scattering layers within and beyond the SCTR have been conducted. An acoustic survey was conducted from April 20, 2019, to May 15, 2019, at 3‒27°S latitude and 60 and 67°E longitude, using a Simrad EK 80 sounder aboard the RV Isabu. In this study, the region north of 12°S latitude in the survey area is defined as SCTR, and south of 12 ° S latitude is non-SCTR. Acoustic scatterers were categorized based on ranges of ΔMVBS (the difference in mean volume backscattering strength at 18 and 38 kHz). The categorized acoustic scatterers based on time and their metric characteristics were described using the depth layers, such as the epipelagic (0–200 m), upper mesopelagic (200–600 m), and lower mesopelagic (600–1,000 m) layers, in the SCTR and non-SCTR regions. The results showed that diurnal vertical migration was clearly observed at approximately 400–600 m. The acoustic scattering intensity in SCTR was much higher than that of non-SCTR. There were significant differences in most categorized acoustic scatterers between day and night times in the two regions. The profile of acoustic scatterers was significantly low in the epipelagic layer during dawn and day times in both regions. Lower temperature, lower salinity, and lower dissolved oxygen water were observed in SCTR than in the non-SCTR within all depth layers except for the salinity in the lower mesopelagic layer. The acoustic scattering values and environmental properties exhibited strong or weak correlations. Lastly, there was no significant correlation between acoustic scatterer and total dry zooplankton weight.