The U. S. Antarctic Marine Living Resources (U. S. AMLR) program deployed two Teledyne Webb Research Slocum G3 gliders, each equipped with a suite of oceanographic sensors and newly designed Nortek Signature100 single beam wide-band compact echosounder (70 kHz to 120 kHz). We previously reported these krill densities to CCAMLR (EMM2024/53), but noted a large discrepancy between the two gliders, and attributed the differences to calibration errors. Here we present corrected biomass densities after a more rigorous calibration procedure. At 120kHz, differences in biomass density estimates within the BS survey area between gliders ranged from 2.9 to 3.3 g m-2, suggesting that the revised calibration procedure corrected previously reported differences. Revised biomass densities at 120 kHz from both the ALL energy and SWARMS method differed by about 5 to 8 g m-2, at 120 kHz. Biomass density using the ALL energy ranged from 38.22 g m-2 (SD 85.64 g m-2) to 41.67 g m-2 (SD 130.78 g m-2), for AMLR03 and AMLR04, while biomass density using the SWARMS method ranged from 33.91 g m-2 (SD 78.77 g m-2) to 35.01 g m-2 (SD 99.72 g m-2), for AMLR03 and AMLR04, respectively. Gridding the glider data into 0.2 by 0.1 degree cells and estimating the mean biomass density for the entire deployment area, provided estimates that were similar to the survey area method. At 120 kHz, biomass density ranged from 35.67 to 37.4 g m-2 for the ALL energy method while application of the SWARMS method produced biomass density estimates ranging from 32.81 to 33.82 g m-2 for AMLR04 and AMLR03 respectively. Our reanalysis reaffirms the utility of autonomous gliders for conducting acoustic surveys for krill biomass estimation, and argues that estimates from these platforms should comprise an important source of data used to assess krill populations in some areas of the Southern Ocean.
Further, to continue developing our glider-based timeseries of krill biomass density estimates within Bransfield Strait, U.S. AMLR will process the acoustic data from the 2024/25 glider deployment in Bransfield Strait. Additionally, the U. S. AMLR Program has plans to deploy two gliders within Bransfield Strait in the upcoming field season (2025/26) equipped with active acoustic systems and a shadowgraph plankton camera system in early summer. Finally, attached to this document is a preliminary acoustic mooring deployment plan for an 18-month deployment of up to 12 Nortek Signature100 moorings.