Description: The vertical distributions of early developmental stages of oceanic fishes were investigated across the tropical and equatorial Atlantic, from oligotrophic waters close to the Brazilian coast to more productive waters close to the Mauritanian Upwelling Region. Stratification of the water column was observed throughout the study region. Fishes were caught with a MOCNESS-1 net with mouth area of 1 m2 at 11 stations. Each station was sampled both during the day and at night within a single 24-h period. The investigation covered both larvae and transforming stages from the surface to 800 m depth. Distribution patterns were analysed, and weighted mean depths for the larvae and transforming stages of each species were calculated for day and night conditions. Forty-seven different species were found. The highest number of species occurred in the three stations south of Cape Verde Islands, characterized by a mixture of South Atlantic Central Water (SACW) and Eastern North Atlantic Central Water (ENACW). There was a marked drop in species richness in the three stations closer to the African upwelling, dominated by ENACW. The highest abundances occurred in the families Myctophidae, Sternoptychidae, Gonostomatidae and Phosichthyidae. Day and night vertical distributions of larvae and transforming stages showed contrasting patterns, both in the depths of the main concentration layers in the water column, and in the diel migration patterns (where these were observed). Larvae generally showed a preference for the upper mixed layer (ca. 0–50 m) and upper thermocline (ca. 50–100 m), except for sternoptychids, which were also abundant in the lower thermocline layer (100–200 m) and even extended into the mesopelagic zone (down to 500 m). Transforming stages showed a more widespread distribution, with main concentrations in the mesopelagic zone (200–800 m). Larvae showed peak concentrations in the more illuminated and zooplankton-rich upper mixed layers during the day and a wider distribution through the upper 100 m during the night. For most species, transforming stages were concentrated in the mesopelagic layers both day and night, although in some species (Diaphus cf. vanhoeffeni and Vinciguerria nimbaria), the transforming stages displayed vertical migration into the upper 100 m at night, in a manner similar to their adult stages.

Description: The ratio of tissue RNA to DNA (R/D) is a widely used index of recent growth and nutritional condition in larval and juvenile fish. To date, however, no standard technique for measuring nucleic acids has been adopted. Because methodological details can affect the estimate of R/D, researchers using different analytical protocols have been unable to compare ratios directly. Here, we report on the results of an international interlaboratory calibration of 4 spectrofluorometric protocols to quantify nucleic acids. Replicate sets of 5 tissue samples and 2 standards (common standards) were supplied to each of 5 researchers for analysis with their own methods and standards. Two approaches were evaluated for mitigating the observed differences in values: 1) the use of common nucleic acid standards and 2) standardizing to a common slope ratio (slope of DNA standard curve/slope of RNA standard curve or mDNA/mRNA). Adopting common standards slightly reduced the variability among protocols but did not overcome the problem. When tissue R/Ds were standardized based on a common mDNA/mRNA slope ratio, the variance attributed to analytical protocol decreased dramatically from 57.1% to 3.4%. We recommend that the ratio of the slopes of the standard curves be provided to facilitate intercomparability of R/D results among laboratories using different spectrofluorometric methods for the analysis of nucleic acids in fish.

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: Mesopelagic fishes experience an extreme body transformation from larvae to adults. DNA barcoding has been useful in establishing phylogenetic relationships among mesopelagic and other deep-sea fishes, however no molecular evidence to confirm the identity of their larvae was available so far. Molecular analyses were conducted to identify the larvae and adults of 18 species of the most abundant and frequent mesopelagic fishes in the western Mediterranean, across 14 genera. For myctophids, 7 of 9 species of the subfamily Lampanyctinae and all species from Myctophinae (6) were analysed. At least 565 nucleotides of the mtDNA CO1 region were sequenced. The 12S rRNA region was selected for C. braueri and C. pygmaea (372 nucleotides). Pairwise nucleotide distances were notably higher inter- than intraspecifically allowing to discern between congeners such as Cyclothone braueri and C. pygmaea, Hygophum benoiti and H. hygomii, Lampanyctus crocodilus and L. pusillus, and Notoscopelus bolini and N. elongatus. The good fit between larvae and adult sequences for each species proved the accuracy of earlier larval descriptions based on morphology and pigmentation characters. A list of the genetic markers used to identify each species with their accession numbers in Genbank is provided here along with their developmental stage, sampling location and depth.

Description: This file contains data on numerical abundance of micronekton organisms, mostly decapods, cephalopods, and cnidarians 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: This file contains data on numerical abundance of early stages of fishes (mostly larvae) 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 0.25 m2 MultiNet HYDRO-BIOS (Weikert and John, 1981) in winter, and a Multiple Opening/Closing Net and Environmental Sensing System (Wiebe et al., 1985), MOCNESS-1 m2, in summer. Hauls were performed both day and night, discriminating different layers and mainly in the epipelagic zone. Samples were fixed on board and stored in a 5% buffered formalin and seawater for later sorting and 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: 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: Mesopelagic organisms play a critical role in marine ecosystems, channelling energy and organic matter across food webs and serving as the primary prey for many open-ocean predators. Nevertheless, trophic pathways involving mesopelagic organisms are poorly understood and their contribution to food web structure remains difficult to assess (St. John et al., 2016). Existing data to assess mesopelagic feeding interactions and energy transfer are scattered in the literature or remain unpublished, making it difficult to locate and use such datasets. As part of the EU funded project SUMMER - Sustainable Management of Mesopelagic Resources H2020-BG-2018-2, GA: 817806) (https://summerh2020.eu/), we created MesopTroph, a georeferenced database of diet, trophic biogeochemical markers, and energy content of mesopelagic organisms and other marine taxa from the Northeast Atlantic and Mediterranean Sea, compiled from 191 published and non-published sources. MesopTroph includes seven datasets: (i) diet compositions from stomach content analysis, (ii) stable isotopes of carbon and nitrogen (δ13C and δ15N), (iii) fatty acid trophic markers (FATM), (iv) major and trace elements, (v) energy density, (vi) estimates of diet proportions, and (vii) trophic positions. The database contains information from 4918 samples, representing 51119 specimens from 499 species or genera, covering a wide range of trophic guilds and taxonomic groups. Metadata provided for each record include the location, dates and method of sample collection, taxonomic ranks (phylum, class, order, family), number and size (or size range) of sampled organisms, method/model used in data analysis, reference and DOI of the original data source. Compiled data were checked for errors, missing information, and to avoid duplicate entries, and scientific names and taxonomy were standardized.