IMIS

Dynamics of taxonomic and quantitative composition of mesopelagic ichthyocenes on four polygons situated in waters over the central and southern parts of the Mid-Atlantic Ridge and over the southern part of the Walvis Ridge are analyzed depending on the day time, water mass, and various attribution to topographically expressed underwater rises. Abundance and species diversity of mesopelagic fish within sound scattering layers (SSL) when they are near summits of underwater rises demonstrate drastic diurnal changes: at night they attain maximum, while in the daytime the fish in SSL are almost absent over underwater rises. Mesopelagic ichthyocenes attain the highest abundance and diversity over underwater ridges, obviously, in the zones of frontal division of large-scale gyres (polygons two and three). The taxonomic composition of ichthyocenes of SSL is more variable geographically than that of ichthyocenes of the lower mesopelagial.