Acoustic scattering characteristics of the mesopelagic fish ( Maurolicus japonicus ) in the East sea

Maurolicus japonicus is a small bioluminescent mesopelagic fish widely distributed in the northwestern Pacific, including the waters of Korea, Japan, and China. It is a cold-water species inhabiting temperatures between 5–15 °C and exhibits diel vertical migration. This study investigates the frequency dependent volume backscattering strength (Sv) of M.japonicus and examines its vertical distribution in relation to environmental factors. Acoustic and biological surveys were conducted around Dokdo, East Sea, in March and May 2025 using the R/V TAMGU 23 (1679 GT) operated by the National Institute of Fisheries Science (NIFS). Acoustic data were collected using a SIMRAD EK80 scientific echosounder at 18, 38, 70, 120, and 200 kHz, while hydrographic parameters were measured using a CTD profiler (SBE 19plus). Biological sampling was performed with daytime midwater trawls. Captured specimens were taxonomically identified, and total length and weight were measured. Acoustic data were processed using Echoview (v14). Sv was extracted for each frequency after isolating M. japonicus-dominated scattering layers based on trawl catches and depth overlap. Strong backscatter layers consistently appeared at 100–150 m, coinciding with the thermocline. High Sv values were observed at 18 and 38 kHz, indicating strong acoustic reflectivity. These results highlight the importance of multifrequency acoustics in identifying mesopelagic species and understanding their spatial distribution in thermally stratified environments.