Community structure and trophic ecology of euphausiids in the Benguela ecosystem

Abstract

Current knowledge of the community structure, life history strategies and trophodynamics of euphausiids in the Benguela ecosystem is synthesized. Three species dominate, Euphausia lucens over the shelf region of the southern Benguela, Euphausia hanseni over the outer shelf off Namibia and Nyctiphanes capensis in the neritic and shelf regions of the northern Benguela. Various models of interactions between biological processes and physical features are considered to explain likely mechanisms whereby these species can remain spatially segregated. All three species breed throughout the year, produce multiple broods and large numbers of eggs per brood and have high turnover rates. These strategies enable them to maintain high production in terms of both somatic growth and reproductive output throughout the year. Biomass of euphausiids in the northern Benguela is about double that of the west coast of the southern Benguela, consistent with similar geographical differences in phytoplankton abundance. Adult euphausiids are opportunistic omnivores and the larval stages are primarily herbivorous. Alternative feeding patterns are reflected in differences in the vertical migratory behaviour of various ontogenetic stages and ambient food type. Factors causing a switch from herbivory to carnivory are considered. Grazing of euphausiids is considered to play a minor role in phytoplankton losses, but euphausiids could exert considerable predatory impact on mesozooplankton and therefore compete directly with pelagic fish. Combined predation by various fish species has an appreciable impact on euphausiid production.