Species Composition and Spatial Distribution of Mesopelagic Fish Communities at the Transect along the Canary and Iberian Basins in the East Atlantic Ocean

The composition and structure of deep-sea pelagic fish communities and their relationship with oceanographic factors were analyzed on a transect between 25° N and 43° N (Eastern Atlantic) conducted in summer 2024. Samples were collected from the 0–200 and 0–1000 m water layers during the day and night using a non-closing Samyshev–Aseev–modified Isaacs–Kidd midwater trawl (SA-IKMT). A total of 1,878 juvenile and adult individuals of mesopelagic fish belonging to 100 species from 18 families were recorded in the catches. The families Myctophidae (39 species), Stomiidae (22), Gonostomatidae (11), and Melamphaidae (7) were represented by the largest number of species. Two groups of stations were identified: southern and northern; the boundary between them located along 33°–35° N and was associated with the zone of influence of the Azores Current. Total abundance and species diversity were nearly the same in the catches in the southern and northern parts of transect, but the composition of the dominant species had noticeable differences. The data obtained are consistent with the biogeography of the mesopelagic zone, which distinguishes two ecoregions here. However, the boundary between them located more southwards than indicated earlier. This may be due to both the interannual latitudinal variability of deep-sea ichthyofauna affected by the changes in oceanographic conditions and to the existence of smaller latitudinal subregions within the Central North Atlantic ecoregion.