Environmental drivers of upper mesopelagic fish assemblages in the Benguela Upwelling System

The Benguela upwelling system is one of the most productive marine ecosystems in the world; however, little is known about the community structure of mesopelagic fishes in its northern (nBUS) and southern (sBUS) subsystems. We analyzed around 1900 specimens from 13 stations in the nBUS and 11 stations in the sBUS during the austral summer and found 88 species and 24 families of mesopelagic fishes. Seven distinct assemblages characterized by water mass, oxygen concentration in the surface layer, and chlorophyll concentrations between 50 and 100 m were identified. The stations belonging to the nBUS shelf assemblage were low in oxygen concentration, had a mean abundance of 0.064 ind. 10 m-2, and were represented exclusively by Diaphus dumerilii. The sBUS shelf assemblage had a mean abundance of 15.00 ind. 10 m-2 and Maurolicus walvisensis dominated. Two sBUS offshore assemblages had a mean abundance of 1.12 and 9.48 ind. 10 m-2. The most abundant taxa within the groups were Hygophum hanseni, Cyclothone spp., and D. meadi. Three nBUS offshore clusters had an abundance of 10.21, 11.88, and 14.19 ind. 10 m-2, and D. hudsoni prevailed in 2 groups, while M. walvisensis dominated the third group. Our study provides insight into the environmental factors that drive the composition of mesopelagic fishes on the shelf and in the upper pelagic zone in these highly productive subsystems. Possible future changes in these subsystems, such as an expansion of the oxygen minimum zone, can thus be expected to affect certain mesopelagic taxa that are essential for the food web of the subsystems.