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22 pages, 11 figures, 7 tables.-- Data availability statement: Publicly available datasets were analyzed in this study. This data can be found here: Olivar et al. (2022) https://doi.org/10.1594/PANGAEA.947631 and Sarmiento-Lezcano et al. (2023) https://doi.org/10.1594/PANGAEA.95494, Non-migratory bristlemouth fishes (Cyclothone spp.) are the most abundant vertebrates on Earth and play an important role in the biological carbon pump by remineralizing organic carbon in deep ecosystems. Acoustic data and net sampling are often used in combination to estimate fish and zooplankton biomass, but this procedure may be subject to several sources of error when applied to mesopelagic species. For instance, the allocation of echoes to species has often been biased by not considering Cyclothone spp. due to the use of nets targeting larger fish. Furthermore, the acoustic properties of the target organisms must be well understood to convert acoustic density into numerical density. The characteristics of a fish’s swimbladder are the most relevant features necessary to assess its acoustic properties. This study provides information on the swimbladder properties of six Cyclothone species inhabiting the meso- and bathypelagic layers in the North Atlantic Ocean and Mediterranean Sea, including swimbladder location within the body, fat tissue content, morphology, morphometry (only available for C. braueri and C. pseudopallida), and fish body-mass density (only available for C. braueri, C. pseudopallida, C. pallida, and C. pygmaea). The studied species showed a functional physoclistous swimbladder, with well-developed gas glands and rete mirabile and numerous capillaries in the case of the shallower species C. braueri and C. pseudopallida (mainly distributed from 400 to 600 m depth), and a fat-invested swimbladder in species with deeper vertical distribution (C. livida, C. microdon, C. pallida, and C. pygmaea). The fat content in the swimbladder (C. pallida and C. microdon) increased with depth and latitude, reducing the space in the swimbladder that could contain gas. Changes in swimbladder size and volume during growth were analyzed for shallower species, where swimbladder volume and equivalent radius followed negative allometric growth in relation to body length. Finally, values of body-mass density (ρ) and gas content required for neutral buoyancy (VG) were estimated for C. braueri and C. pygmaea collected between 350 and 550 m (ρ = 1.052–1.072 g·cm−3, VG = 2%–4%; ρ = 1.052–1.062 g·cm−3, VG = 3.6%), and for C. pallida and C. pseudopallida sampled in the 450–700 m layer (ρ = 1.052–1.062 g·cm−3, VG = 2.6–3.1%; ρ = 1.052–1.062 g·cm−3, VG = 2.8%–3.25%). Results in this study highlight the change in scattering behavior of Cyclothone species from gas-bearing organisms (those that contain gas in their swimbladder) in the upper mesopelagic zone to the fluid-like scattering (with fat-filled swimbladders) of the deeper and northern individuals. The data presented in this manuscript are important for parametrizing acoustic backscattering models built to estimate the echo of Cyclothone species, although further work is needed, particularly for individuals with partially invested swimbladders with an irregular fat-free shape, This research was funded by projects Bathypelagic (CTM2016-78853-R), Ideados (CTM2007-65844-C02-01-02-03), and Desafío (PID2020-118118RB-I00) from the Spanish Ministry of Science and Technology, “Sustainable Management of Mesopelagic Resources” (SUMMER, 817806) from European Union Horizon 2020 Research and Innovation Programme, and the “South and Tropical Atlantic Climate-Based Marine Ecosystem Prediction for Sustainable Management” (TRIATLAS, 817578) from European Commission. AS-L was supported by a postgraduate grant (BES-2017-082540) from the Spanish Ministry of Science and Innovation, With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)

22 pages, 9 figures, 2 tables, supplementary material https://www.frontiersin.org/articles/10.3389/fmars.2022.1028717/full#supplementary-material.-- Data availability statement: The raw data supporting the conclusions of this article are publicly available through PANGEA platform: Olivar et al. (2022) https://doi.org/10.1594/PANGAEA.947631, and Yang et al. (2021) https://doi.pangaea.de/10.1594/PANGAEA.938688, Mesopelagic fish populations are characterised by high species richness and abundance, and have been identified as important contributors to the active carbon fluxes in the open ocean. We report variability in communities of mesopelagic fish between five zones around the Iberian Peninsula, i.e. Balears and Alboran in the Mediterranean, and Cadiz, Lisboa and Galicia in the Atlantic. Day and night samples were collected from 7 layers of the water column with a midwater trawl fitted with a multisampler. Temperature and salinity regimes were very different on the Mediterranean and Atlantic sides of the peninsula, with much higher values through the entire water column in the Mediterranean, characterized by a strong pycnocline. The highest productivity was observed off Lisboa, where Chlorophyll a concentrations were two orders of magnitude higher than in any other zone. Samples from the western Mediterranean held 22 fish species, while 67 were found in the Atlantic. The lowest diversity and the highest dominance were observed in Balears, and the highest diversity in Cadiz zone. In all zones, but particularly in those in the Mediterranean, mesopelagic populations were dominated by a high number of small fish with low individual biomasses. The species Benthosema glaciale, Cyclothone pygmaea and Ceratoscopelus maderensis were common in the Mediterranean populations, whereas in the Atlantic, Cyclothone microdon/livida, Valenciennellus tripunctulatus, Ceratoscopelus warmingii and Benthosema suborbitale were the most common species. Temperature and salinity (both at surface and in the mesopelagic zone) were the main environmental factors explaining variability in assemblage composition. A persistent (day-night) deep scattering layer was observed using the vessel-based echosounder in all zones, and was comprised primarily of the gonostomatid Cyclothone spp. Night-time echosounder observations of scattering layers near the surface were observed in Balears, Alboran, Cadiz and Lisboa, where night surface net collections indicated that Myctophidae, Stomiidae and Phosichthyidae migration extended to the upper 100 m. Sternoptychids and the gonostomatid Sigmops elongatus seldom reached the upper 100 m in their night vertical migrations. Night stratified hauls of 30 m resolution carried out in the epipelagic zone showed that abundances maxima of migratory fish coincided with the location of the Chlorophyll a maxima, This work was a component of the European Union Project ‘Sustainable Management of Mesopelagic Resources’ (SUMMER, Grant Agreement 817806) as funded by the Horizon 2020 Research and Innovation Programme. [...] AS-L was supported by a postgraduate grant (BES-2017-082540) from the Spanish Ministry of Science and Innovation. We acknowledge the Severo Ochoa Centre of Excellence accreditation (CEX2019-000928-S)

20 pages, 11 figures, 3 tables, supplemental material https://doi.org/10.3389/fmars.2019.00535, Quantification of the actual amount of carbon export to the mesopelagic layer by both zooplankton and micronekton is at present a gap in the knowledge of the biological pump. These organisms perform diel vertical migrations exporting carbon through respiration, excretion, mortality, and egestion during their residence at depth. The role of zooplankton in active flux is nowadays partially assessed. However, micronekton active flux is scarcely known and only a few studies addressed this downward transport. Even less is known about the capacity of both communities to export carbon in the ocean. Here, we show the results of zooplankton and micronekton active flux across a productivity gradient in the tropical and subtropical Atlantic Ocean. Biomass vertical distribution from the surface up to 800 m depth by day and night was studied during April 2015 in a transect from 9°S to 25°N, covering from the quite oligotrophic zone off Brazil to the meso- and eutrophic areas of the equator, Guinea Dome, and the oceanic upwelling off Northwest Africa. Zooplankton and micronekton migrant biomass was estimated from day and night catches at different layers of the water column using MOCNESS-1 (1 m2 mouth area) and Mesopelagos (35 m2) nets, respectively. Respiratory flux was assessed by measuring the enzymatic activity of the electron transfer system (ETS) of organisms at depth. Results showed a close relationship between migrant biomass and respiratory flux in zooplankton and micronekton as expected. Using a rather conservative 50% of efficiency for the net used to capture micronekton, respiratory flux resulted in similar values for both communities. Gravitational (passive) flux measured using sediment traps increased from the oligotrophic toward the meso- and eutrophic zones. Total active flux (including respiration and estimated mortality, excretion, and gut flux) by zooplankton and micronekton accounted for about 25% of total flux (passive plus active) in oligotrophic zones. Total active flux also increased toward meso- and eutrophic zones, reaching about 80% of total flux and being at least twofold higher than passive flux. These results alert about an important underestimation of the ocean biological pump using only passive flux measurements, This work was supported by projects “Migrants and Active Flux in the Atlantic Ocean” (MAFIA, CTM2012-39587-C04), and “Biomass and Active Flux in the Bathypelagic Zone” (Bathypelagic, CTM2016-78853-R) from the Spanish Ministry of Economy and Competitiveness

22 pages, 12 figures, 7 tables, supplementary data https://doi.org/10.1016/j.pocean.2016.12.001, In this investigation we analysed the changes in fish species occurrences and relative abundances across the tropical and equatorial Atlantic, and their vertical distribution patterns in relation to the different environmental scenarios. The study covers a wide region encompassing different water masses, and marked differences in productivity, from an oligotrophic zone close to the Brazilian coast, to a very productive upwelling region close to the Northwest African upwelling. Fishes were collected with a medium-sized midwater trawl (Mesopelagos), complemented by hauls made with a macrozooplankton net (MOCNESS). Species richness in the region was higher than in subtropical, temperate and cold regions. The total number of species and their overall abundance was lower in the stations closer to the Brazilian coast. Abundant species across the entire region were the gonostomatids Cyclothone alba, Cyclothone acclinidens, Cyclothone pallida and Cyclothone pseudopallida, the myctophid Lampanyctus alatus, the sternoptychid Sternoptyx diaphana, and the phosichthyid Vinciguerria nimbaria. The occurrences and abundances of C. parapallida, Lampanyctus nobilis and Lepidophanes guentheri were related to zones where AAIW waters occupied the mesopelagic layers, while other species such as Cyclothone livida and Polyipnus polli increased their abundance when AAIW disappears from their living depths. The presence of Eastern North Atlantic Central Water (ENACW) was associated with the occurrence of several myctophids (Benthosema glaciale, Ceratoscopelus maderensis, Diaphus holti, Diaphus rafinesquii, Hygophum hygomii, Lampanyctus crocodilus, Myctophum punctatum, Symbolophorus veranyi), and the gonostomatid Cyclothone braueri. In spite of the important differences in hydrographic features across the tropical and equatorial Atlantic, all stations showed either the general night migration into the epipelagic layers carried out by myctophids, phosicthyids, and some stomiids, or the presence of the several species of Cyclothone, sternoptychids and melamphaeids that remain in the mesopelagic layers, both day and night. The oxygen minimum zone (OMZ) at mesopelagic depths in the north-eastern sector does not seem to disrupt diel vertical migration. Day-night distributions in our study proved that mesopelagic migratory species are capable of crossing these wide hypoxic layers, and that some species such as Diaphus vanhoeffeni remain in these layers during the day. Other non-migratory fishes (Cyclothone spp. and S. diaphana) proved to be widely tolerant to these low oxygen concentrations, as shown by their high numerical abundances in the OMZ, This research has been funded by the Spanish Ministerio de Economía y Competitividad through project CTM2012-39587-C04-03

17 pages, 9 figures, 8 tables, The mesopelagic fish community of the western Mediterranean was studied during two cruises carried out in December 2009 and July 2010 in the shelf and slope zones around the Balearic Islands. Much of what was previously known about this deep water group of fishes in the Mediterranean Sea came from studies performed using planktonic and small midwater nets. This study was the first attempt to use large pelagic trawls and small nets combined with information about the main sound scattering layers to analyse mesopelagic fish composition, diversity and species assemblages. This community is characterised by a relatively low diversity compared to other oceanic regions of the world, with Myctophiformes and Stomiiformes being the main contributors. Bathymetry and the level of the water column were the most important factors structuring the investigated fish assemblages, and similar vertical patterns were observed for the different species collected during the two study periods. A shelf assemblage composed of a few species of myctophids, with Notoscopelus elongatus being the main contributor, was distinguished. The slope assemblage included both Myctophiformes and Stomiiformes that showed differences in their day–night main location along the water column. In terms of species behaviour, two important groups were detected. The first was non-migrant or weakly migrant species, with the paradigmatic example being the gonostomatid Cyclothone braueri, which occurred at a depth of 400–600 m; this species is partly responsible for the permanent acoustic (38 kHz) response at this depth. The second group, near-surface migrants at night, was represented by most of the juvenile and adult myctophids, exemplified by Ceratoscopelus maderensis, with the exception of just a few of the largest size classes of some species, such as Lampanyctus crocodilus and N. elongatus that remain near the bottom, The research was funded by project CTM2008-04489-C03-01 and 02. A. Bernal acknowledges predoctoral FPI Fellowship support from Spain’s Ministry of Science and Innovation

[EN] Traditionally the horizontal orientation in a ship (heading) has been obtained from a gyrocompass. This instrument is still used on research vessels but has an estimated error of about 2-3 degrees, inducing a systematic error in the cross-track velocity measured by an Acoustic Doppler Current Profiler (ADCP). The three-dimensional positioning system (GPS 3DF) provides an independent heading measurement with accuracy better than 0.1 degree. The Spanish research vessel BIO Hespérides has been operating with this new system since 1996. For the first time on this vessel, the data from this new instrument are used to estimate gyrocompass error. The methodology we use follows the scheme developed by Griffiths (1994), which compares data from the gyrocompass and the GPS system in order to obtain an interpolated error function. In the present work we apply this methodology on mesoscale surveys performed during the observational phase of the OMEGA project, in the Alboran Sea. The heading-dependent gyrocompass error dominated. Errors in gyrocompass heading of 1.4-3.4 degrees have been found, which give a maximum error in measured cross-track ADCP velocity of 24 cm s-1, [ES] Tradicionalmente, la orientación horizontal (heading) de los barcos se ha obtenido a partir de una giroscópica. Este instrumento todavía es utilizado en los barcos de investigación, pero tiene un error asociado de entre 2 y 3 grados, provocando un error sistemático en la velocidad medida por el perfilador de corriente por efecto Doppler (ADCP) perpendicular a la trayectoria del barco. El sistema de posicionamiento tridimensional GPS 3DF permite obtener medidas independientes del heading con una precisión mayor de 0.1 grados. El BIO Hespérides dispone de este nuevo sistema desde 1996. Por primera vez en este barco, los datos de este nuevo instrumento se han utilizado para estimar el error de la giroscópica. La metodología usada está basada en el método de Griffiths (1994), el cuál compara datos del sistema GPS y de la giroscópica a fin de obtener una función representativa del error. En este trabajo aplicamos dicha metodología a una campaña realizada en el mar de Alborán, durante la fase observacional del proyecto OMEGA. El error de la giroscópica dependiente del rumbo dominó. Se detectaron errores comprendidos entre 1.4 y 3.4 grados, que producen un error máximo de 24 cm s -1, en la componente de la velocidad medida por el ADCP perpendicular a la trayectoria del barco, We wish to express our gratitude to all scientists and technical staff who participated in the OMEGA- 1 cruise in the Western Alboran sea, as well as to the BIO Hespérides crew. OMEGA was a project of the Marine Science and Technology (MAST) Programme of the European Commission funded under contract MAS3-CT95-0001. OMEGA-1 was partially funded by the Spanish R+D National Plan (grant AMB95-0901). Special thanks to Pablo Rodríguez for the technical information about the GPS 3DF system