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    During three cruises in the Mid Atlantic Ridge area in 2016 and 2017, we collected samples of mesopelagic species down to a depth of 600?m. We studied fish behaviour and identified and quantified the species composition of the catches using HD videos (60 frames/s). For species identification and the estimation of the number of siphonophores/m³ we analysed the videos frame by frame using the tool VLC media player.

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    This data set has trawl catch weight and biological measurements from the 2nd leg of the Icelandic part of the International Ecosystem Summer Survey in Nordic Seas (IESSNS). The survey area was from the coast, over the shelf edge and into the deep-water basins south and west of Iceland, defined as IESSNS strata 5, 6, and 12, and stratum boundaries (black solid line) are displayed on attached map "IESSNS 2018.png". The survey period is July 19th to August 1st, 2018, and samples were collected at 34 trawl stations (blue symbol on attached map "IESSNS 2018.png"). A specifically designed scientific trawl, called Multpelt832 (ICES, 2013a-b, Valdemarsen et al 2014) with cod-end mesh size of 50 mm, was use at all 34 stations. Trawl depth ranged from surface (trawl headline in surface) to 400 m depth. All trawl catch is identified to lowest possible taxonomic level, total weight of each species recorded and length of 20-50 specimen per species measured. For blue whiting (Micromesistius poutassou) more biological measurements are provided such as weight, sex, maturity, age, and stomach weight. For age determination, the otolith was extracted whole from the blue whiting at sea and read whole under a dissecting microscope on land by an experienced age reader part of the ICES framework. Gonad maturity stage was visually estimated at sea during sampling by an experienced technician. The maturity stages are 1-7, where 1 is immature, 2 is beginning to mature or beginning to recovering from previous spawning event, 3 is maturing, 4 is maturing, 5 is getting fully mature, 6 is spawning, and 7 has finished spawning. In total measurements are provided for 1451 specimen. Each specimen is one row in the data set file. At ten stations (345, 356, 366, 369, 372, 378, 381, 390, 394, 395) Natantia and Scyphozoa specimen could not be identified to species, hence no individual length measurements are provided but total weight of catch per taxon is provided. Information for three pelagic fish species is excluded from the data set and these species are mackerel (Scomber scombrus), herring (Clupea harengus) and lump fish (Cyclopterus lumpus). For detailed information about the survey see the survey report (in English): https://www.hafogvatn.is/is/midlun/utgafa/haf-og-vatnarannsoknir/results-of-the-icelandic-part-of-the-international-ecosystem-summer-survey-in-nordic-seas-iessns-in-2018-on-rv-arni-fridriksson-hv-2019-21. Various information are provide about the trawling operation, weather, position, date and time.The data is provided by the Marine and Freshwater Research Institute (MFRI) in Reykjavik, Iceland.

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    This data set has trawl catch weight and biological measurements from the 2nd leg of the Icelandic part of the International Ecosystem Summer Survey in Nordic Seas (IESSNS) conducted in July 2020. The survey area was from the coast, over the shelf edge and into the deep-water basins south of Iceland and in the southern part of the Irminger Sea, defined as IESSNS strata 6, 11 and 12, see embedded map. The survey period was July 11 – 30, 2020, and samples were collected at 27 trawl stations. A specifically designed scientific trawl, called Multpelt832 (ICES, 2013a-b, Valdemarsen et al 2014) with cod-end mesh size of 50 mm, was used at all 27 stations. Trawl depth ranged from surface (trawl headline in surface) to 550 m depth. All trawl catch was identified to lowest possible taxonomic level, total weight of each species recorded, and length and weight of 50 specimen per species measured, or all if less was caught. For blue whiting (Micromesistius poutassou) more biological measurements are provided such as sex, maturity, age, and stomach weight. For age determination, the otolith was extracted whole from the blue whiting at sea and read whole under a dissecting microscope on land by an experienced age reader part of the ICES framework. Gonad maturity stage was visually estimated at sea during sampling by an experienced technician. The maturity stages are 1-7, where 1 is immature, 2 is beginning to mature or beginning to recovering from previous spawning event, 3 is maturing, 4 is maturing, 5 is getting fully mature, 6 is spawning, and 7 has finished spawning. For the taxa Scyphozoa and Natantia decapods only total catch weight per station is recorded. In total, length measurements are provided for 1383 specimens and weight for 1231 specimen. Each specimen is one row in the dataset file. Information for three pelagic fish species is excluded from the data set and these species are mackerel (Scomber scombrus), herring (Clupea harengus) and lump fish (Cyclopterus lumpus). For detailed information about the survey see the survey report (in English): https://www.hafogvatn.is/is/midlun/utgafa/haf-og-vatnarannsoknir/results-of-the-icelandic-part-of-the-international-ecosystem-summer-survey-in-nordic-seas-iessns-in-2020-on-rv-arni-fridriksson Various information is provided about the trawling operation and weather. The data is provided by the Marine and Freshwater Research Institute (MFRI) in Iceland.

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    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.

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    This data set has trawl catch weight and biological measurements from the 2nd leg of the Icelandic part of the International Ecosystem Summer Survey in Nordic Seas (IESSNS) conducted in July 2019. The survey area was from the coast, over the shelf edge and into the deep-water basins south and west of Iceland, defined as IESSNS strata 5, 6, and 12, and stratum boundaries (black solid line) are displayed on attached map "IESSNS 2019.png". The survey period is July 16 - 28 2019, and samples were collected at 34 trawl stations (blue symbol on attached map). A specifically designed scientific trawl, called Multpelt832 (ICES, 2013a-b, Valdemarsen et al 2014) with cod-end mesh size of 50 mm, was used at all 34 stations. Trawl depth ranged from surface (trawl headline in surface) to 370 m depth. All trawl catch was identified to lowest possible taxonomic level, total weight of each species recorded and length of 20-50 specimen per species measured. For blue whiting (Micromesistius poutassou) more biological measurements are provided such as weight, sex, maturity, age, and stomach weight. For age determination, the otolith was extracted whole from the blue whiting at sea and read whole under a dissecting microscope on land by an experienced age reader part of the ICES framework. Gonad maturity stage was visually estimated at sea during sampling by an experienced technician. The maturity stages are 1-7, where 1 is immature, 2 is beginning to mature or beginning to recovering from previous spawning event, 3 is maturing, 4 is maturing, 5 is getting fully mature, 6 is spawning, and 7 has finished spawning. In total, measurements are provided for 857 specimen. Each specimen is one row in the dataset file. Ten specimen, at eight stations (352, 354, 361, 366, 375, 383, 388, 391) were not length measured but are included and total weight of catch per taxon or species per station is provided. Information for three pelagic fish species is excluded from the data set and these species are mackerel (Scomber scombrus), herring (Clupea harengus) and lump fish (Cyclopterus lumpus). For detailed information about the survey, see the survey report (in English): https://www.hafogvatn.is/is/midlun/utgafa/haf-og-vatnarannsoknir/results-of-the-icelandic-part-of-the-international-ecosystem-summer-survey-in-nordic-seas-iessns-in-2019-on-rv-arni-fridriksson-hv-2019-57. Various information is provided about the trawling operation, weather, position, date and time. The data are provided by the Marine and Freshwater Research Institute (MFRI) in Reykjavik, Iceland.

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    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.

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    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.