Acoustic characterization of fish and macroplankton communities in the Seychelles-Chagos Thermocline Ridge of the southwest Indian ocean

Abstract

In this study, we describe the dynamics of the sound scattering layers (SSLs), particularly those of fish and macroplankton communities in the epipelagic layer, in the Seychelles-Chagos Thermocline Ridge (SCTR) of the southwest Indian Ocean using hydroacoustic data, net sampling, and oceanographic information. Overall, the acoustic backscattering values of the fish community were considerably higher than those of the macroplankton. Both communities were more densely distributed in the SCTR than in its surrounding region. On the vertical profile, the acoustic peak of the fish community was at 17 m during the day; however, considerably high values of up to 82 m were observed at night. Below 26 m, macroplankton was seldom found, regardless of the time. Hydrographic properties, such as temperature, salinity, dissolved oxygen (DO), and chlorophyll fluorescence, in the SCTR, were similar; however, an area at 5–8°S, 67°E was cooler, saltier, and had slightly lower DO. Weak or moderate positive correlations were observed between acoustic and hydrographic features. Based on all net samples, the most abundant taxon in terms of the total number of samples was found to be krill (Euphausiacea, 81%), followed by lantern fish (Myctophum punctatum, 12%). Understanding the dynamics of SSLs, particularly epipelagic organisms, will help to better clarify the important ecological roles of these organisms and their ability to facilitate vertical incorporation into marine food webs.