Cross-front hydrography and fish larval distribution at the Angola–Benguela Frontal Zone

The hydrography and the ichthyoplankton abundance and composition was surveyed along a meridional transect off Angola. It extended from near the equator, cut the western edge of the Angola Dome, and ended off northern Namibia, south of the Angola–Benguela Frontal Zone (ABFZ). The meridional gradient of the ABFZ was much stronger inshore than farther offshore, but both reacted within days to changing wind situations. It exhibited changes in hydrographical morphology, as well as in the distributional limits of surface tropical and subtropical species. In contrast, the faunistic boundaries of midwater species and water masses were locationally more persistent. The cross frontal features were consistent, real and persistent in their meridional gradients, in spite of a Benguela Niño situation. North of the ABFZ, fish larval abundances were six times higher than south of the ABFZ, and three times higher than in the adjacent open ocean. Vertically, larvae occurred from the surface to at least 200 m, but preferentially above 100 m. The maximum concentration of both chlorophyll and fish larvae coincided roughly with the pycnocline and deepened with it at the front. The number of taxa declined southward from 36 species per station at 6°S by about one species per degree of latitude. Gross number of taxa did not differ much across the boundaries of the Angolan Gyre. However, the community structure changed drastically across the front, with maxima of diversity within it. The faunal structures north and south of the ABFZ had a similarity of only 22%. Off Angola, larvae of mesopelagic fish species that are adapted to oxygen deficits dominated.