Seasonal and Spatial Patterns of Experimental Trawl Catches in the Southwest Arm of Lake Malawi

Abstract

Experimental demersal trawl samples were collected in the same manner at the same sites on a monthly basis over an annual cycle in the southwest arm (SWA) of Lake Malawi. Catch composition in terms of species representation and mass was compared over time and depth (10, 30, 50, 75, 100 and 125 m). The average catch per unit effort was calculated per species and depth. Haplochromine cichlids dominated the catches at every depth, making up 75 to 92% of the biomass. The remainder was made almost exclusively of catfishes. Despite catching more than 140 species in the trawls, 60 to 80% of the catches consisted of ten or fewer species, including three catfishes. About twenty species accounted for 90 to 95% of the catches at any depth, suggesting that many species are uncommon or rare. Previous authors reported dramatic changes in species composition at 50 m in the southwest arm. Data presented here suggest that this might be due to a change in the nature of the substratum at 50 m. The greatest diversity of species is in shallow waters, but the highest catches in terms of biomass were recorded between 50 and 125 m and peaked at 75 and 100 m. As catches in the deep waters were dominated by fishes with favorable life history characteristics and which are large relative to species in the shallow waters (though the largest species of cichlid occur in the shallow water, the catches are dominated by small species), it is recommended that the possibility for increased exploitation of the SWA deep demersal stocks should be explored by encouraging controlled development of a demersal commercial fishery in the SWA.