Study of the demersal assemblages of the continental shelf and upper slope off Congo and Gabon, based on the trawl surveys of the RV 'Dr Fridtjof Nansen'

Abstract

The structure of the demersal assemblages (fish, crustaceans and cephalopods) of the continental shelf and upper slope off Congo and Gabon (from about 1 to 5-degrees-S) was studied based on the trawl survey of the RV 'Dr Fridtjof Nansen' in January-February 1989, by means of an ordination technique, Detrended Correspondence Analysis (DCA), implemented by the computer program DECORANA and a classification technique, Two-Way Indicator species Analysis (TWIA), implemented by the computer program TWINSPAN. Major faunal discontinuities were found between the shelf and the slope assemblages and, on the shelf, between the suprathermocline and the subthermocline assemblages. Correlation of DCA Axes 1 and 2 with the environmental variables showed in fact that temperature was most highly correlated with DCA Axis 1. The lack of any correlation with Axis 2 indicates that other factors (e.g. bottom type) also play an important role in explaining the various groups. Highest biomass densities (from bottom trawl catches) were found on the deeper shelf off Gabon and in the intermediate shelf off Congo.