DOI: 10.5285/e184e81a-e43c-424e-abec-122036ee2cfd
title: Macrozooplankton and nekton vertical distribution and abundance at the sustained observation location P3 in the northern Scotia Sea (Southern Ocean) during November and December 2017

description: Macrozooplankton and nekton were collected with a Rectangular Midwater Trawl 25 (RMT25) over several visits to the sustained observation location P3 (52.70 S, 40.26 W) in the northern Scotia Sea during November and December 2017. The work was carried out as part of the NERC Large Grant, COMICS (Controls on Mesopelagic Interior Carbon) on board the RRS Discovery (cruise DY086). The RMT25 net hauls sampled between 10 and 500 m depth, with the water column divided into 2 depth intervals (10-250 m and 250-500 m). A total of 6 hauls were obtained during 3 separate visits to station P3, each visit comprising a pair of hauls, of which one was carried out in nominal daytime and the other in nominal nighttime. Catches were immediately sorted on board and identified to the lowest taxonomic level feasible. Subsamples of the catches were retained, principally for subsequent biochemical and physiological analyses. In total, 777 fish were caught, belonging to at least 23 species, with catches dominated by the myctophids Krefftichthys anderssoni, Gymnoscopelus braueri, Electrona antarctica and Protomyctophum tenesoni. The water column below 250m was dominated by Bathylagus spp. Temperate myctophid species, such as Protomyctophum parallelum and Protomyctophum andreyeshevi were also caught in small numbers. With regards macrozooplankton, the 250m-500m depth interval was dominated by the jellyfish, Atolla and Periphylla. The tunicate Salpa thompsoni and the euphausiids Euphausia triacantha and Thysanoessa spp. were also relatively abundant. Jellyfish still dominated catches in shallower waters (250m-10m), closely followed by euphausiids and Salpa thompsoni and chaetognaths. Themisto gaudichaudii and Parandania boecki were the most numerous amphipod species caught. Decapods were only caught in the deeper depth interval, both day and night.
description: An RMT25 net was used to sample the mesopelagic fish, squid and macrozooplankton community during the survey DY086 at the sustained observation station P3 (52.70°S, 40.26°W) in the northern Scotia Sea, Southern Ocean. The RMT25 is a midwater trawl with a mouth area of 25 m2 and a variable mesh size, 18 mm reducing to 5 mm towards the cod-end. The RMT25 used in this instance had two separate nets allowing two different depth intervals to be sampled discretely. These intervals were set at 500-250 m and 250-10 m. Samples were collected across three time stations (P3 A, P3 B and P3 C) approximately 10 days apart. At each time station, one deployment was undertaken during the hours of darkness and one in daylight, making a total of 6 deployments over the entire cruise. The RMT25 was operated via a downwire net monitor and was equipped with a flow meter, and temperature and salinity sensors. Each depth interval was sampled for approximately 40 mins while the ship maintained a speed of 2 knots. When retrieved, the total weight of each RMT25 net haul was initially recorded with a calibrated, motion compensated weighing scales before further taxonomic analysis. All fish, squid and macrozooplankton specimens were identified to species level, where possible, and the composite weight and numbers per species recorded. Where necessary, sub-samples of the total catch of a species were counted and then multiplied by the inverse of the sub-sample fraction to determine total numbers in the catch. After sorting, a sub sample of specimens were either frozen for stable isotope and lipid analysis at -80°C or flash-frozen in liquid nitrogen for ETS (details not recorded here). The abundances and weights in the datasheets are those directly recorded after the catch was sorted on board. Subsequent dry weights were calculated based on wet weight to dry weight relationships, either reported in the literature or obtained from dry weight measurements on specimens retained from this cruise or similar cruises in this region (listed in datasheets).
description: 1. An RMT25 trawl (25m2 mouth diameter, 5 mm minimum mesh size) 2. Motion-compensated weighing scales 3. Volumetric flasks 4. Binocular microscope
description: All species were identified according to the taxonomic guides available at time of analysis and the user must be aware that some species names may have since been updated. The entire catch was sorted and all specimens above ~4 mm were recorded at least to a low taxonomic level. The absence of any taxon from the datasheet indicates that it was not present in the catch.