Methodology:

Samples were collected during a series of research cruises to South Georgia between 2009 and 2020. Two ships were used: RRS James Clark Ross (9 cruises) and RRS Discovery (1 cruise). Sampling was carried out within a prescribed survey box in waters to the NW of South Georgia. Net sampling efforts took place at eight preset stations along paired transects within this box. It was not always possible to sample every one of these stations on each research cruise.

An RMT8 net (Roe and Shale, 1979) was deployed and hauled in a double oblique fashion between the surface and a maximum depth of 200 m or to within 15 m of seabed. For cruise JR228, the net was deployed with an added RMT1 net (RMT8+1), but only the catches from RMT8 are considered here.

RMT net catch data were recorded per each net deployment, showing all unique taxa and taxon counts, i.e. the number of individuals per each taxon in the total catch. The opening/closing trigger signal in the net-monitor log was used to determine the exact duration of each net opening and net depth. The horizontal and oblique distance and volume filtered through the net was calculated, assuming a ship speed of 2 knots and considering a nominal mouth area of 8 m2. The volume filtered value was divided into the estimated taxon counts to provide volume-specific abundance estimates (individuals m-3). This value was multiplied by the vertical depth range sampled by the net to provide a square-area abundance value (individuals m-2).

Where possible, the total wet weight of each taxon within the catch was recorded using motion-compensated scales. A high-resolution scale was available on certain cruises that was capable of measuring to an accuracy of between 0.01 and 0.05 g. On other cruises, only a lower resolution scale was available with an accuracy of 1 g. On these latter cruises, weights of less than 1 g were assigned as "<1 g". Weights below the level of accuracy for high-resolution scales were assigned either "<0.01 g" or "<0.05 g". To allow an approximation of biomass (g m-3) to be made for every taxon entry, we estimated an individual weight in each of these instances as follows:

<1 g = 0.5 g

<0.01 g = 0.005 g

<0.05 g = 0.0025 g

For a small number of taxon entries, weights were not measured. In those instances, we estimated individual weight by taking the average of all other individual weights measured for that taxon. This was multiplied by the respective number of individuals within the net. Estimated individual weights were required for two taxa, as follows:

Gymnoscopelus = 5.985 g

Cnidaria = 9.195 g

Scientific names from the taxon identifications made during the surveys were verified against the World Register of Marine Species (WoRMS) and the corresponding APHIA ID unique identification code recorded against each taxon entry.

Further columns include the deployment start time (GMT), net type, geospatial coordinates, water depth and estimated volume filtered for each sampling event.

Column headings are described below:

Cruise - Scientific cruise name

Event - Unique number of the sampling activity

Net - Either net 1 or net 2 for an RMT

Event_net - Concatenations of the above

Station_unique_code - Concatenations of the above

Net_type - Either RMT8 or RMT8+1

Survey_season - End year of austral sampling season

Start_of_event - Date and time in GMT

Volume_filtered (m3) - Volume of water filtered by the net

Taxon - Taxonomic name

AphiaID - Unique identification code according to World Register of Marine Species (WoRMS)

Species_code - Code for taxon used by survey team

Number_of_individuals - Total number of individuals in the net

Abund (m-3) - Volume-specific abundance estimates

Abund (m-2) - Square-area abundance values

Data collection:

Rectangular Midwater Trawler (RMT8)

Data quality:

Sources of variance in the dataset

1. As taxonomic analysis was carried out directly on the ship, the taxonomic level to which taxa were identified varied depending on personnel taking part in each cruise.

2. Capturing large quantities of krill or salps made it difficult to be completely thorough in quantifying other taxa in the catch.

3. It was not always possible to sample every station on every cruise, with the outermost stations in particular being difficult to sample in stormier conditions.

4. Wet weights were not always resolved to weights below 1 g depending on the availability of the high-resolution motion-compensated scale.