Vertical structure of mesopelagic fish assemblages in the Cape Verde Frontal Zone

Mesopelagic fish populations are characterised by high species richness and abundance, playing a key role in the active carbon flux in the open ocean. The Cape Verde Frontal Zone (CVFZ), located at the southern boundary of the Canary Current Eastern Boundary Upwelling System, features highly dynamic water mass interactions due to the interplay between the Cape Verde Front and the Mauritanian coastal upwelling. Despite the known ecological importance of mesopelagic fishes in complex oceanographic scenarios such as upwelling zones, frontal systems, and oxygen minimum zones, there are few studies relating them to frontal areas such as the CVZF. Herein, we describe the vertical distribution, composition, and structure of the mesopelagic fish community in the CVFZ. We analysed the diurnal and nocturnal vertical abundance and biomass distribution, the common, dominant, and rare species, and the length-frequency distribution from surface to 1000 m depth during summer 2017. Water temperature, salinity, dissolved oxygen, and chlorophyll- a were measured concurrently. Forty mesopelagic fish species were indentified, with Cyclothone spp. dominating numerically and in biomass, especially within oxygen-depleted layers below 350 m. Abundance and biomass were asymmetrical, reflecting contrasting ecological strategies. Diel vertical migrations were evident, with myctophids ascending to surface layers at night and Cyclothone spp. remaining consistently abundant at depth. Seven assemblages were identified, structured by depth, diel period, and environmental gradients. Epipelagic groups were low-diversity and myctophid-dominated, while mesopelagic assemblages showed numerical dominance of small Cyclothone spp. and biomass driven by rare, large-bodied taxa, with clear shifts in dominant species and length-frequency distributions. Strong temperature and dissolved oxygen gradients, oxygen minimum zones, and the depth chlorophyll- a maximum were identified as key drivers of species turnover and vertical distribution. These results highlight the influence of mesoscale hydrographic structures, frontal dynamics, and water mass interactions on mesopelagic fish communities, providing baseline data for this climate-sensitive region and emphasising the need to account for temporal and vertical variability in community assessments.