Trophic relationships in the demersal fish community off Namibia

Abstract

The benthic fish community off Namibia (between the Cunene River and Walvis Bay) in 50-650 m of water was studied during three bottom trawl surveys. The community was investigated on the basis of distribution, abundance and diet of 51 species, constituting 95 per cent of the demersal fish biomass. Dietary studies revealed the existence of three major trophic groups, one containing species that prey on pelagic and nektonic organisms, a second dominated by predators that feed on benthic polychaetes and copepods and a third group containing predators of benthic crabs, demersal shrimps and fish. Well differentiated from these groups are a few species that prey mainly on jellyfish and ophiuroids. Because for most of the species, their trophic level changes with growth, they were divided into size classes which are analysed independently. To describe interactions between the size classes, a similarity index combining diet affinity with spatial coincidence was applied. Cluster analysis showed that, for Merluccius capensis, Raja straeleni, R. clavata and Trachurus trachurus, there is a large difference in both geographic distribution and dietary preference between the size classes. For species such as Hoplostethus atlanticus, Austroglossus microlepis and Coelorinchus flabellispinis, the different size classes seem to share the same ecological niche. Variation in the number of species and the parameters mean abundance and niche width are described to enhance knowledge of the general structure of the community