Trophic coupling and environmental constraints shape the distribution of mesopelagic fishes in the northern Benguela Upwelling System

Abstract

We investigated the environmental drivers of mesopelagic fish density and distribution in the northern Benguela region using hydroacoustic data collected by the RV Dr. Fridtjof Nansen in 2017 and 2019. Multifrequency acoustic data (18, 38 and 70 kHz) were analysed using an acoustic classification algorithm to partition backscatter into three functional groups representing mesopelagic fishes, krill and other ‘plankton’. Mesopelagic fish accounted for 48% and 33% of total backscatter in 2017 and 2019, respectively, while krill contributed 41% and 39%, and ‘plankton’ 11% and 28%. Generalised additive models revealed tight spatial coupling between mesopelagic fish and their primary prey (krill), although fish density also increased with increasing fluorescence and ‘plankton’ density, reflecting an association with productive waters. Mesopelagic fish showed a preference for low-oxygen environments and exhibited a clear diel relationship with the oxygen minimum zone, occupying its upper boundary during daytime, likely as a refuge from visually oriented predators. Overall, mesopelagic fish density and distribution in the northern Benguela is primarily structured by prey availability in combination with environmental constraints, including oxygen concentration, temperature, light intensity and bottom depth, which together define key habitat zones.