We compare estimates of krill density derived from gliders to those from contemporaneous and previous ship-based surveys. Our comparisons cover several temporal and spatial scales within two strata around the northern Antarctic Peninsula (off Cape Shirreff on the north side of Livingston Island and in the Bransfield Strait). Our objective is to explore the feasibility of using gliders to supplement or replace vessel-based surveys of fishery resources. We deployed two long-duration Slocum G3 gliders manufactured by Teledyne Webb Research (TWR), each equipped with a suite of oceanographic sensors and a three-frequency (38, 67.5, and 125 kHz, each single-beam) Acoustic Zooplankton Fish Profiler. We used the acoustic data collected by these gliders to estimate biomass densities (g·m^-2) of Antarctic krill (Euphausia superba). The two gliders were, respectively, deployed for 82 and 88 days from midDecember 2018 through mid-March 2019. Off Cape Shirreff, glider-based densities estimated from two repeat small-scale surveys during mid-December and January were 110.6 and 55.7 g·m^-2, respectively. In Bransfield Strait, the glider-based estimate of biomass density was 106.7 g·m^-2 during December–January. Contemporaneous ship-based estimates of biomass density, from a multi-ship broad-scale krill survey (Macaulay et al., 2019) restricted to the areas sampled by the gliders, were 84.6 g·m^-2 off Cape Shirreff and 79.7 g·m^-2 in Bransfield Strait during January. We compared two alternative krill-delineation algorithms (dB differencing and SHAPES); differences between biomass densities estimated by applying these algorithms were small and ranged between 4 and 7%. Alternative methods of sampling krill length-frequency distributions (LFDs) (nets or predator diets), which are required to convert acoustic energy to biomass density, also influenced the glider-based results. In Bransfield Strait, net-based estimates of biomass density were 6% less than those based on predator diets. Off Cape Shirreff the biomass density of krill estimated from a net-based LFD was 20% greater than that based on predator diets. Development of a variance estimator for glider-based biomass surveys is ongoing, but our results demonstrate that fisheries surveys using acoustically-equipped gliders are feasible, can provide density estimates to inform management, and may be conducted at lower cost than ship surveys in some cases.

In this paper we present krill density biomass estimates for Subareas 48.1, 48.2, 48.3 and 48.4 and for current fished areas, using post-hoc stratification of krill density estimates from the 2019 International Krill Survey. We highlight the importance of survey strata design for the differing areas, with implications for management. Each level of stratification requires different extrapolations from survey area to stratum area.

WG-ASAM is invited to provide recommendations on improving the repository of acoustic data collected by fishing vessels maintained by the Secretariat and to provide advice on:

• Whether the surveys listed in Annex 1 correspond to what is expected to be included in this repository.
• Whether the metadata provided in Annex 2 meet metadata requirements.
• Responsibility for data submissions and the procedures laid out in this paper.

This working group paper presents ideas and experiences regarding the use of unmanned surface vehicles (USVs) to monitor krill density in a fishing hotspot. While USVs have the advantage of reduced survey costs, reduced risk for personnel and higher data resolution in time and space, they sometimes struggle to follow straight transects and to withstand the rough weather and sea ice conditions of the Southern Ocean. We discuss commercially available technologies and outline strategies for future deployments and developments of USVs, with the goal of acquiring data that can be used to successfully manage the fishery in an adaptive context.