Decline of demersal resources in North-West Africa: an analysis of Mauritanian trawl-survey data over the past 25 years

Abstract

Mauritania is characterised by fast-growing fisheries that have developed over the past decades. Since 1982, scientific trawl surveys have been conducted regularly, allowing assessment of the impact of this increasing fishing pressure on exploited species as well as on demersal communities. Based on 55 bottom trawl surveys and using linear model techniques, the annual abundances were estimated for a selection of 24 fish stocks and for the whole demersal biomass. Changes in the demersal community structure were also investigated, using Biomass Trophic Spectra representations. It is shown that the demersal biomass has been reduced by 75% on the Mauritanian continental shelf over the past 25 years, corresponding to a biomass loss of around 20 000t per year. Top predators abundance has been reduced by 8–10-fold and in some case up to 20-fold. The trophic structure has been significantly modified and the mean trophic level of the catchable biomass decreased from >3.7 to <3.5. The results are discussed at the regional scale, taking into account recent studies in Senegal and Guinea in which a similar decline in demersal biomass was observed. This decline was due to severe overexploitation that affected the various groups in succession.

Keywords: biomass decrease, demersal resources, Mauritania, North-West Africa, trawl survey, trophic structure

African Journal of Marine Science 2007, 29(3): 331–345