Development of a Custom Bio-acoustic Sonar System for Use in the Southern Ocean

Members of NOAA’s Antarctic Marine Living Resources (AMLR) team have collaborated with ASL Environmental Sciences to field the Acoustic Zooplankton Fish Profiler (AZFP) bio-acoustic sonar to the Southern Ocean. This collaboration is helping to map the distribution and abundance of Antarctic krill (Euphausia superba). The U.S. AMLR Program has transitioned from ship-based to autonomous instruments to address critical questions about krill in support of the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). These questions include understanding trends in biomass, predator prey interactions, and fishery-prey-predator interactions at spatial and temporal scales not sampled efficiently by ships. This study involves using Slocum gliders to mount the AZFP sonars and has involved the development of unique hardware at ASL. The AZFP that has been developed for this mission operates at three center frequencies: 38 kHz, 70 kHz, and 125 kHz. Custom syntactic foam fairings have been developed to assist with ballasting and hydrodynamics of the vehicle. The AZFPs are calibrated at ASL, and additional testing has been performed at NOAA’s Southwest Fisheries Science Center to verify the calibrations. Data from two gliders deployed in austral summer 2019 have provided information on the vertical distribution of krill below 500m (the maximum depth visible from ship-based surveys), changes in the target strength of krill from day to night, and the spatial distribution of krill in relation to predator foraging. Additionally, the deployment of glider based acoustic instruments has allowed the U. S. AMLR Program to continue its long-term monitoring of krill biomass trends in an ecologically important and environmentally changing area of the Southern Ocean.