Biomass density estimates of Antarctic krill in Bransfield Strait during the 2023/24 austral summer from a new glider-based wideband echosounder

The US Antarctic Marine Living Resources (US 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 used these gliders to estimate biomass densities (g m^-2) of Antarctic krill (Euphausia superba) in the Bransfield Strait during the austral summer of 2023-24. Both gliders were deployed on November 28, 2023 and recovered 61 days later on January 27, 2024. Krill biomass density estimates were calculated from both gliders using two methods, using all echo energy and the other, using schools to delineate krill. Krill length frequencies from the diets of Gentoo penguins at King George Island were used to construct conversion factors. Densities from both methods ranged from 67.61 g m^-2 (SD 181.9 g m^-2 ) to 151.29 g m^-2  (SD 337.26 g m^-2 ). Examination of the number of schools identified between the two gliders suggests a bad calibration on one of the gliders, emphasizing the impact and importance of correct calibrations. We developed a new method to minimize oversampling due to the nature of gliders by creating a grid and bootstrapping the data in the grid. Density estimates were comparable to the other two methods with a low estimate of 75.54 g m^-2 95% CI [51.7, 108.64 g m^-2 ] and a high of 161.95 g m^-2   95% CI [125.08, 213.29 g m^-2 ]. Gliders remain an important tool to continue to provide acoustic biomass estimates of Antarctic krill.