Description: The values of natural abundance of stable isotopes were measured in 13 micronekton fish species sampled during the MAFIA cruise (North Atlantic, April 2015). This dataset contains the values obtained for carbon and nitrogen in bulk tissues, and nitrogen values in amino acids. Length data and the number of individuals analysed for each species are also provided. Mesopelagic fishes were collected using a ''Mesopelagos” net (5x7 m mouth opening, 58 m total lenght) 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). For C:N and stable isotope analyses, individual fish were eviscerated, freeze-dried and weighted. Aliquots of muscular tissue (or whole individuals for species of small size) were analyzed in an elemental analyzer (bulk tissues, Olivar et al., 2019) or a gas chromatograph (derivatized amino acids, Mompeán et al., 2016) coupled to isotope-ratio mass spectrometers.

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.