Interannual Variability in Acoustic Backscatter and Oceanographic Drivers in the Southwestern Indian Ocean

Abstract

Understanding interannual variability in pelagic ecosystems is essential for assessing ecosystem responses to environmental change. This study investigates the interannual differences in acoustic backscatters and oceanographic conditions in the southwestern Indian Ocean. Acoustic data from a 38 kHz echosounder was analyzed alongside water column profiles of temperature, salinity, dissolved oxygen (DO), and fluorescence across three depth layers: epipelagic (0-200 m), upper mesopelagic (200-600 m), and lower mesopelagic (600-800 m), collected during research voyages in 2019 and 2023. The results revealed significantly stronger mean volume backscattering strength (MVBS) in 2023, particularly in the mesopelagic layers, with diel vertical migration (DVM) patterns showing more pronounced vertical displacement. Investigations conducted in both years showed that depth of DVM decreased with increasing latitude, whereas MVBS consistently increased toward the equator, likely driven by upwelling that modified local oceanographic conditions. Although temperature and salinity remained relatively stable in both years, DO concentrations declined significantly in 2023, especially in mesopelagic waters. Lower DO levels were observed concurrently with elevated fluorescence, indicating possible shifts in biological activity and distribution of organisms. Among oceanographic properties, DO showed the strongest correlation with changes in MVBS, suggesting it as a major driver of sound scattering layer dynamics. These findings highlight the sensitivity of midwater acoustic communities to oceanographic physical variability and underscore the importance of long-term, integrated acoustic-oceanographic monitoring to understand ecosystem shifts in response to climate-related changes.