Myounghee Kang ^1,2 Fredrich Simanungkalit ² Dongha Kang ^2* Jihoon Jung ^2* Joohyang Kim ^3* Somang Song ^3* Yeseul Kim ⁴ Dong-Jin Kang ⁴ Sunghyun Nam ^3*

• ¹ Gyeongsang National University, Tongyeong, Republic of Korea
• ² College of Marine Science, Gyeongsang National University, Tongyeong, Republic of Korea
• ³ Seoul National University, Seoul, Republic of Korea
• ⁴ Korea Institute of Ocean Science and Technology (KIOST), Busan, Republic of Korea

Sound scattering layers (SSLs) are vital components of marine ecosystems, yet their morphometric and distributional characteristics remain understudied. This study investigates the SSL core in the southwestern Indian Ocean using a 38 kHz echosounder, focusing on its attributes across latitudinal gradients (20-4°S, 65°E) and three depth layers: epipelagic (0-100 m), intermediate (100-300 m), and mesopelagic (300-600 m). Our findings reveal the highest average acoustic values in the epipelagic layer (-72.9 dB), followed by the mesopelagic layer (-77.8 dB) and the intermediate layer (-82.5 dB). The SSL core was more prominent in northern latitudes (NLS) and showed greater depth variability in southern latitudes (SLS), with vertical profile peaks at 42.2 m and 431.1 m. In the NLS, the SSL core was longer (18,358 m > 6,788 m), thicker (13.8 m > 11.7 m), and more concentrated (aggregation index: 0.0053 m⁻¹ > 0.004 m⁻¹), as well as more dispersed from the center (inertia: 269.9 m² > 112.7 m²). Notably, depth and acoustic fluctuations were more pronounced over circadian cycles in the SLS than in the NLS. Multiple regression analysis revealed that oceanographic factors influencing acoustic values varied by depth, highlighting the complex interactions within SSL ecosystems. These insights enhance our understanding of SSL dynamics and their implications for long-term monitoring and climate influence assessment.