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Latitudinal transition of mesopelagic larval fish assemblages in the eastern central Atlantic

This study presents a broad-scale view of larval fish species distributions in the eastern Central and North Atlantic and places it in a hydrographic context. Pelagic fish larvae, including metamorphic stages, were sampled to 1000 m depth across a 46◦ latitudinal transect from the equator to the Bay of Biscay. By analysing species composition and relating it to hydrographic parameters, we were able to identify 5 assemblages: Equatorial, Tropical, Subtropical, Temperate-bloom and Temperate-nobloom. We also categorized 4 groups of species by association with specific hydrographic parameters. Tropical species, including Vinciguerria nimbaria, Ceratoscopelus warmingii, and Hygophum macrochir, are associated with high sea surface temperature. Tropical-subtropical species, such as Diogenichthys atlanticus, Benthosema suborbitale and Electrona risso, are associated with high temperature in the upper 200 m, i.e. the epipelagic zone. The Temperate species group, dominated by Benthosema glaciale and Maurolicus muelleri, is associated with high chlorophyll-a in the epipelagic zone, and these species were abundant where the spring bloom occurred. Species with the broadest latitudinal distributions, such as Cyclothone braueri and Argyropelecus hemigymnus, here termed Cosmopolitan species, are associated with high salinity in the epipelagic zone. The Cape Verde Frontal Zone seems to act as a one-way distributional barrier, confining many Tropical larval species to the South Atlantic Central Water mass, while allowing more northerly-distributed species to cross southward. This may be related to a dependence of Tropical species on high nearsurface water temperatures, which decrease sharply at the Cape Verde Frontal Zone. An oxygen minimum zone had no significant effect on larval abundance or species richness, although there were some taxonomic differences. For example, phosichthyids, including Vinciguerria nimbaria, were not found within the oxygen minimum zone, while the family Melamphaidae thrived. Our results suggest that the dominant hydrographic factors that affect the distribution of fish larvae in the eastern Central and North Atlantic vary by latitude and by species. The Cape Verde Frontal Zone and oxygen minimum zone play complex roles, affecting different taxonomic groups in various ways.



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