Acoustic Monitoring Across the Oceanographic Front Between the Tsugaru Warm and Oyashio Currents

Zhen LU, Daisuke Hasegawa, Takahiro Tanaka, David Mark Spencer, Yuji Okazaki and Takeshi Okunishi, Tohoku National Fisheries Research Institute, Japan Fisheries Research and Education Agency, Shiogama, Japan
Convergence between the Tsugaru Warm Current (TWC) and cold Oyashio Current (OY) results in the formation of highly dynamic and complex secondary oceanographic mechanisms (e.g. eddies and fronts) in the northwest Pacific Ocean. These mechanisms promote high biodiversity and generate hotspots for a range of commercially-important fisheries, but how they affect species distributions over much smaller spatiotemporal scales (1–100 m) remains unclear. With the development of acoustic technology in recent years, echosounders are increasingly being used to conduct high-precision fishery resource assessments and detect small-scale variability in the oceanic thermohaline structure using acoustic backscatter images. In this study, we conducted an acoustic survey along a transect crossing two frontal features in the TWC-OY confluence zone during the R/V Wakataka Maru cruise in July 2019. Our aims were to 1) validate our observations with the Undulating Towed Array (UTA, see poster by Spencer et al.) system that was simultaneously collecting hydrographic information in the top 300 m of the water column and 2) analyze how different fishery species were distributed (specifically krill) with respect to different oceanographic features along the transect. Acoustic backscatter was measured using a quantitative echosounder KFC-3000 (Sonic) with two frequencies, 38 kHz and 120 kHz. The 38kHz echograms were able to detect similar oceanographic features that were obtained from UTA, but these features were not observable from the 120 kHz echograms. Due to the highly dynamic nature of this region, it was difficult to distinguish species distributions from the fine-scale oceanographic structure. Distinguishing between these detections is the current and future focus of this project.