Characterization of European sprat acoustic backscatter

Abstract

Abstract In fisheries acoustics, the identification of targets and precise translation of acoustic energy into biological meaningful metrics remains a challenge. Backscattering cross-section, or its logarithmic form, Target Strength (TS, dB re 1 m2), is a key parameter in this process. There are numerous studies on commercially important species, but few studies are available on commercially non-important species, such as the European sprat in the Mediterranean Sea. The application of backscattering models can improve our understanding of the acoustic properties of these species, allowing the manipulation of key parameters affecting the TS. Here we applied several backscattering models, on three-dimensional swimbladder shapes derived from Computer Tomography (CT) scans, of sprat collected during the MEDiterranean International Acoustic Survey (MEDIAS) in 2021. The theoretical TS pattern was compared with empirical TS revealing a good fit at 38 and 70 kHz between 0° (broadside incidence) and − 20° and significant differences at higher frequencies and tilt angles. This study provides estimates of the relative frequency response and broadband backscatter of sprat in the Mediterranean Sea. Moreover, the relationship of TS with fish length at 38 kHz yields a new relevant conversion parameter (b20) of -68.3 dB re 1 m2 for European sprat in the Mediterranean Sea.