Methodology:

Detailed surveys for mesopelagic fish were conducted in the Scotia Sea as part of the British Antarctic Survey's Discovery 2010 programme, as described in Collins et al. (2012). Follow up investigations were carried out using the same methods during JR15004 and JR16003.

An opening and closing 25 m2 rectangular mid-water trawl net (RMT25) was deployed at locations spanning the Scotia Sea and major frontal positions from north of the Antarctic Polar Front (APF) to the sea ice zone (SIZ). Cruise timings: November 2006 (cruise JR161, Austral spring), January 2008 (cruise JR177, Austral summer), March 2009 (cruise JR200, Austral autumn), January-February 2016 (cruise JR15004, Austral summer), and January 2017 (cruise JR16003, Austral summer).

Stratified net hauls were carried that encompassed the main water masses and frontal zones of the region; Polar Front (PF), Southern Scotia Sea (SSS), Mid Scotia Sea (MSS), Western Scotia Sea (WSS), Northern Scotia Sea (NSS), and Georgia Basin (GB), at depths of: 0-200, 200-400, 400-700, and 700-1000 m. Only night sampled net hauls are included in the data set and abundance and proportions restricted to those net samples where the entire water column was sampled.

All fish caught were sorted on-board, identified to the lowest taxonomic level, and measured to the nearest mm using standard length. Wet mass was recorded either for the entire catch or for each species using a motion-compensated balance. Where no wet weight was recorded for a species, this was estimated based on the mean wet mass for a species from the existing data from JR161, JR177 and JR200. Where neither wet mass nor abundance was recorded, the entire net was excluded from the data set.

Myctophid gas presence absence was compiled from a combination of dissection, Computed Tomography and soft tissue X-ray. The presence or absence of gas in Electrona antarctica in relation to fish length are also presented, these fish were sampled opportunistically from RMT25, RMT6 and MOCNESS net during the same cruises.

Data collection:

Acoustic Data collected using a Simrad EK60 echo sounder

Acoustic Data processed using Echoview version 8.0.95.32073

Analysis in R version 3.5.1

Data quality:

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.