The acoustic data was calibrated, cleaned in Echoview to remove bad data regions, including false bottom echoes, seabed, surface near-field, intermittent noise and attenuated signal (all set to ''no-data''). Data was integrated in 1km distance by 10m depth cells. Further removal of data in water shallower than 1000m was carried out in R version 3.5.1 and the top 1% of high NASC values were manually scrutinised to check for any residual noise bias. Less than 10% of high NASC values were found to be attributable to noise and these were excluded from further analysis. Abundance data were only obtained from valid net hauls where the RMT25 device operated properly during deployment. Standard length measurements were obtained from all undamaged specimens during fieldwork. Abundance data for fish was compiled from the individual on board fish recording data, as length was required to differentiate between gas and non-gas bearing species (specifically for Electrona antarctica).

Biomass estimates are taken from on board catch weight data, where catch weight was recorded to the nearest gram for either total catch (JR15004 and JR16003), or total species catch (JR161, JR177, JR200, JR15004, JR16003). Total species catch for JR161, JR177 and JR200 was calculated by summing wet mass of all species for the catch. Where a catch weight was not recorded for a species, this was estimated by calculating the mean wet mass for each species from the existing combined JR161, JR177 and JR200, and multiplying the species abundance by the mean mass. Where neither wet mass nor abundance was recorded for a species the entire net was removed from the biomass assessment. All weight data were scrutinised for obvious erroneous values as result of measurement or typographic errors prior to compilation of this data set.