DOI: 10.5285/bfd7eb53-d07a-46e0-a185-54d7c4a829e2
title: Macrozooplankton and nekton vertical distribution and abundance in the Benguela Current region, May-June 2018

description: Macrozooplankton and nekton were collected with a Rectangular Midwater Trawl 25 (RMT25) at locations within the Benguela Current region in May and June 2018. The work was carried out as part of the NERC Large Grant, COMICS (Controls on Mesopelagic Interior Carbon) on board the RRS Discovery (cruise DY090). Depth-discrete samples were collected across four time stations (BS1, BN1-3) between 0-750 m at intervals of 750-500m, 500-250m, 250-125m and 125-10 m. At each time station, two RMT25 hauls were deployed in the hours of darkness and two in daylight, with 16 deployments being undertaken overall. The RMT25 was operated via a downwire net monitor and was equipped with a flow meter, and temperature and salinity sensors. Nets in the deep strata (750-500m and 500-250m) were sampled for approximately 40 mins. and nets in the shallow strata (250-125m, 125-10m) for approximately 20mins. Catches were immediately sorted on board and identified to the lowest taxonomic level feasible. All fishes and subsamples of the other parts of the catch were retained (frozen), principally for subsequent biochemical and physiological analyses. In total, 1917 fish were caught and preserved (not including Cyclothone spp.). Catches were dominated by the myctophids and various other mesopelagic fish species. The water column below 250m was dominated by Bathylagus spp. and genus Melamphidae spp. The most numerous fish overall were the Cyclothone spp. which occurred in large numbers below 500m. In deeper depth intervals (250m-750m), the macrozooplankton component of the RMT25 net catches was mostly dominated by Decapoda and hydromedusae of the genus Atolla spp.. Salps, smaller hydromedusa species and small euphausiids Euphausia hanseni and Nematocelis megalops dominated the shallower depths (10-250m).
description: An RMT25 net was used to sample the mesopelagic fish, squid and macrozooplankton community during the survey DY090 in the Benguela Current region. 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 750-500m, 500-250m in one deployment and 250-125m and 125-10 m in an accompanying second deployment. Samples were collected across four time stations, three at location BN (BN1-3; 18.08°S 11.02°E) and one at location BS (BS1; 21.68°S, 9.54°E). At each time station, one set of deployments was undertaken during the hours of darkness and one in daylight. In total, the dataset comprises the catches of 16 separate deployments. The RMT25 was operated via a downwire net monitor and was equipped with a flow meter, and temperature and salinity sensors. Each depth interval in the deep deployment (between 750 m and 250 m) were sampled for approx. 40 mins while each in the shallow deployment (between 250 m and 10m) were sampled for approx. 20 minutes. The ship maintained a speed of 2 knots during both sets of deployments. When retrieved, the total weight of each RMT25 net catch 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. In cases of high abundance of a particular taxon, sub-samples were extracted, weighed and enumerated 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 recorded directly after the catch was sorted on board.
description: 1. An RMT25 trawl (25m2 mouth diameter, 5 mm minimum mesh size) 2. Motion-compensated weighing scales 3. Volumetric flasks 4. Sorting trays and forceps 5. Binocular microscope
description: All taxa 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 abundance and combined wet weight of all taxa above ~4 mm were recorded, either to species or the next lowest taxonomic level feasible. The absence of any taxon from the datasheet indicates that it was not present in the respective catch.