Acoustic evidence of fronts and warm currents as hotspots for biological aggregation in the winter Eastern China shelf Sea

Marine organism hotspots are critical for sustaining ecosystems and marine biological resources. However, remote sensing and station sampling alone cannot fully capture the detailed biological aggregation patterns beneath the sea surface, particularly in dynamic and rapidly shifting shelf-sea environments. The Eastern China Shelf Sea (ECSS) exhibits high productivity during winter, with strong fronts formed by different water masses. The suitability of the overwintering environment (warm and high nutrient) directly influences population recruitment dynamics. We aimed to use the sound scattering layer (SSL) as an indicator of biological hotspots in the ECSS during the winter of 2019/2020. Additionally, Generalized Additive Models (GAMs) were employed to investigate the effects of abiotic factors on SSL metrics. Our findings include: (1) The SSL was primarily located in the frontal and warm-current regions, with greater biological aggregation on the warm side of the front; (2) Diurnal variations in SSL depth were more pronounced in far shore warm water zones; (3) Fortnightly SSL variability, with SSL occurrences centered around the full moon spring tide, and greater acoustically observed biomass during spring tide than neap tide. In contrast to remote sensing, acoustics effectively captures hotspots of unseen underwater organism aggregation. These findings provide acoustic evidence that fronts and warm currents serve as hotspots for biological aggregation, enhancing our understanding of the relationship between aggregation and ocean biogeophysical processes from an underwater acoustic perspective.