Trends in the demersal fish community off Namibia from 1983 to 1990 |
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| Abstract: | Demersal fish communities off southern Namibia were analysed on the basis of seven winter and five summer stratified random sampling cruises between 1983 and 1990. The 37 most abundant species selected for analysis represented 78-&ndash96 per cent of the total estimated biomass. The biomass of most of them decreased over the period of study. However, the decreases were only substantial for winter cruises; the fluctuations recorded on the summer cruises appeared to be more closely related to hydrographic conditions. In relatively cool summers the estimated biomasses of seven species (including Merluccius capensis and Lophius vomerinus) were quite similar to, or lower than, the biomasses estimated six months earlier. In contrast, the estimated biomasses of the same seven species during relatively warm summers were distinctly higher than the estimates made the previous winter. In all, 10 species, including the commercially important Austroglossus microlepis, Lophius vomerinus and Merluccius capensis, were more available to bottom trawls in summer than in winter. Multivariate analysis revealed several distinct assemblages separated by latitude and depth, but with seasonal variation. Similarity indices between cruises were high (mean Schoener index around 0,7), indicating that changes in biomass do not affect the structure of the demersal fish community. It is suggested that, although the high levels of fishing effort deployed in the region lowered the total fish biomass, community stability, expressed as absence of change in species assemblages, was significantly high. |
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