Description: This file contains data on numerical abundance of early stages of fishes collected in April 2015 across the tropical and equatorial Atlantic. Eleven stations were sampled both day and night, from the oligotrophic region close to the Brazilian coast to the more productive region off the western African coast. Samples were obtained by means of a Multiple Opening/Closing Net and Environmental Sensing System (Wiebe et al., 1985), MOCNESS-1 m2 net, from 800 m to the surface, discriminating 8 layers of the water column. Samples were fixed on board and stored in 5% buffered formalin and seawater for later sorting and identification in the laboratory. These data were obtained within the project: "Migrants and Active Flux In the Atlantic Ocean, financed by the Spanish Ministerio de Ciencia e Innovación CTM2012-39587-C04.

Description: This file contains data on numerical abundance of mesopelagic fishes collected in April 2015 across the tropical and equatorial Atlantic. Twelve stations were visited day and night, from the oligotrophic region close to the Brazilian coast to the more productive region off the western African coast. Hauls were performed with a scientific midwater trawl, the Mesopelagos (designed by Meillat, 2012), which works with a single traction cable. The gear consists of graded-mesh netting starting with 30 mm and ending with 4 mm. At the end of the net, a multi-sampler (designed for the present investigation by one of the co-authors, A. Castellón-CSIC) was installed to obtain samples from 5 different depth layers of the water column. For some stations, where lots of gelatinous organisms, or Sargassum weed occurred, the system could not work and the data presented integrates the water column from 800 m to the surface. The depth of the net was controlled by a SCANMAR system. Samples were identified on board or frozen for a later identification in the laboratory. These data were obtained within the project: "Migrants and Active Flux In the Atlantic Ocean, financed by the Spanish Ministerio de Ciencia e Innovación CTM2012-39587-C04.

Description: This file contains data on numerical abundance of mesopelagic fishes collected in winter (December 2009) and summer (July 2010) in the Balearic Islands region. A relatively small area was repeatedly sampled throughout the day. Samples were obtained by means of a double-warp modified commercial mid-water trawl, with standard pelagic trawl doors (otter boards) and graded-mesh netting to the cod-end (10 mm). Hauls were horizontal and lasted between 30 min and 1 hour, and the depth of the net was controlled by a SCANMAR system. Samples were identified on board or frozen for a later identification in the laboratory. These data were obtained within the project: "Estructura y dinámica del ecosistema bentopelágico de talud en dos zonas oligotróficas del Mediterráneo: una aproximación multidisciplinar y a distintas escalas temporales en las Islas Baleares", financed by the Spanish Ministerio de Ciencia e Innovación CTM2007-65844-C03.

Description: Physical oceanography variables and carbon remineralisation (juveniles/adults of Cyclothone species and Argyropelecus hemigymnus) were analysed during the BATHYPELAGIC cruise (North Atlantic, June 2018). This dataset contains the depth, temperature, and conductivity which were recorded from surface to a maximum depth of 2000 m using a SeaBird SBE 25plus CTD equipped with a Seabird-43 Dissolved Oxygen sensor and a Seapoint Fluorometer. Values of numerical abundance, biomass, specific ETS activity, specific respiraton and respiration flux data analyzed from Northwest Africa (20° N, 20° W) to the South of Iceland are presented. A. hemigymnus specimens were collected using a ''Mesopelagos” net (5 x7 m mouth opening, 58 m total length) equipped with graded-mesh netting (starting with 30 mm and ending with 4 mm) and a multi-sampler for collecting samples from 5 different depth layers. However, Cyclothone specimens were collected using the Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS-1 m²) zooplankton net with a 0.2 mm mesh size and with several nets for collecting samples from 8 different depth layers. The Mesopelagos catches were sorted out and identified on board to the lowest possible taxon, and specimens selected for Electron Transfer System (ETS) analyses were immediately frozen in liquid nitrogen for later analysis in the laboratory. MOCNESS samples were preserved in 5% buffered formalin, and specimens were sorted out later in the laboratory. Stomiiforms respiration in the meso- and bathypelagic zones of the ocean were estimated along the transect. Abundance, biomass, specific ETS activity, specific respiration and respiration are given by layer between e.g. 100 m and 1000 m depth (MOCNESS net, 1900–1600 m, 1600–1300 m, 1300–1000 m, 1000–700 m, 700–400 m, 400–200 m, 200–100 m and 100–0 m; Mesopelagos, 1900–1200 m, 1200–800 m, 800–500 m, 500–200 m and 200–0 m).

Description: Size, weight, body-mass density and swim bladder measurements of Cyclothone species were analysed during the IDEADOS (Mediterranean Sea, July 2010), Bathypelagic (North Atlantic, May-June 2018) and SUMMER (Mediterranean Sea and North Atlantic, September-October 2020), and cruises. Specimens used to analyse size and weight were collected with a Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS-1 m²), fitted with nets of 0.2 mm mesh size. Hauls were carried out between the surface and 700-800 m, in IDEADOS and SUMMER cruises, and up to 1800 m in Bathypelagic survey. Measures of body mass-density were obtained during the SUMMER cruise from specimens collected using the "Mesopelagos" midwater trawl. Finally, measures of swim bladder length, height, width, angle, volume, equivalent spherical radius, and aspect ratio were measured from specimens collected during the Bathypelagic cruise.

Description: Depth, temperature, and conductivity were recorded during the Bathypelagic cruise (from 24th May to 23rd June 2018,) using a SeaBird SBE 25plus CTD, the dissolved oxygen with a Seabird-43 sensor both attached to an oceanographic rosette from surface to a maximum depth of 2000 m, and the fluorometer with a Seapoint sensor in the upper 200 m depth.

Description: A lowerable acoustic wideband autonomous transceiver (WBAT) was deployed during the 2020 SUMMER (Sustainable Management of Mesopelagic Resources) Mediterranean cruise (30 September 2020 to 18 October 2020) aboard the R/V Sarmiento de Gamboa. In total 31 drops were conducted with a depth range from 0 to 500 m to make close up broadband observations of scattering layers. Some of the deployments were done in parallel to ship-based stationary EK80 recordings in fequency modulated mode. There were 4 transducers (with nominal frequencies at 38, 120, 200 and 333 kHz) used with a frequency band covering 35 to 445 kHz. A maximum volume of 100 MB per individual raw file was used, and the recording range was set to 100 m for all frequencies. Transmitted power for 4 transducers from 38 to 333 kHz was set to 225, 125, 75 and 25 W, respectively. Calibration data were recorded in part of the deployments using a 33 mm tungsten sphere. Raw power (W), number of transducer segments and transceiver impedance (Ohm) were stored in raw proprietary Simrad format (.raw). The raw files were recorded according to the UTC-01:00 time zone, the metadata below use UTC+00:00.

Description: The values of carbon remineralisation were measured in juveniles/adults of 5 non-migratory bristlemouth fishes (Cyclothone spp.) and partial migrator (Argyropelecus hemigymnus) sampled during the BATHYPELAGIC cruise (North Atlantic, June 2018). This dataset contains the values of numerical abundance, biomass, specific ETS activity, specific respiraton and respiration flux data analyzed from Northwest Africa (20° N, 20° W) to the South of Iceland. A. hemigymnus specimens were collected using a ''Mesopelagos” net (5 x7 m mouth opening, 58 m total length) equipped with graded-mesh netting (starting with 30 mm and ending with 4 mm) and a multi-sampler for collecting samples from 5 different depth layers (Olivar et al., 2017). However, Cyclothone specimens were collected using the Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS-1 m²) zooplankton net (Wiebe et al., 1985) with a 0.2 mm mesh size and with several nets for collecting samples from 8 different depth layers. The Mesopelagos catches were sorted out and identified on board to the lowest possible taxon, and specimens selected for Electron Transfer System (ETS) analyses were immediately frozen in liquid nitrogen for later analysis in the laboratory. MOCNESS samples were preserved in 5% buffered formalin, and specimens were sorted out later in the laboratory. Stomiiforms respiration in the meso- and bathypelagic zones of the ocean were estimated along the transect. Abundance, biomass, specific ETS activity, specific respiration and respiration are given by layer.

Description: This dataset contains the values of standard length, dry weight and percentage of gas in Cyclothone spp. swim bladder collected during the Bathypelagic cruise (from 24th May to 23rd June 2018) and IDEADOS cruise (from 12th to 29th July 2010). Samples were obtained with MOCNESS-1 m² (with a 0.2 mm mesh size) zooplankton net during day and night hauls carried out from the surface to 700-800 m during IDEADOS cruise, and to 1800 m in the Bathypelagic cruise. Fish for size frequency distribution were preserved in 5% buffered formalin, and those for weight measurements were frozen at 20°C, and then dry weight (DW, ±0.01 g) and standard length (SL, ±0.01 mm) were measured at the land-based laboratory. The percentage of area with fat tissue (%FT) in Cyclothone spp. swim bladders was linearly regressed against standard length (mm). The maximum percentage area with gas (%GAS) was then estimated as 100-%FT.

Description: This dataset contains the values of standard length, wet weight, and body mass-density of Cyclothone spp. measured during the SUMMER cruise (from 30th September to 18th October 2020). Samples were obtained with the "Mesopelagos" midwater trawl during day and night hauls carried out from the surface to 700-800 m depth. The Mesopelagos net consists of graded-mesh netting starting with 30 mm and ending with 4 mm. The specimens collected were immediately identified and measured (standard length, SL). Body-mass densities (ρ) of Cyclothone braueri, C. pseudopallida, C. pygmaea and C. pallida were determined on board via a density-bottle method (Greenlaw, 1977; Mikami et al., 2000).