Marine Biology 104, 175-182 (1990) Marine ===BiOlOgy © Springer-Verlag 1990 Influence of Benguela upwelling on the structure of demersal fish populations off Namibia J. Mas-Riera, A. Lombarte, A. Gordoa and E. Macpherson Instituto de Ciencias del Mar, Paseo Nacional s/n, E-08003 Barcelona, Spain Abstract The effect of bottom-water dissolved oxygen on the struc- ture of demersal fish communities off south Namibia was analized. Collections were made during two cruises of the zone in winter 1987 and summer 1988, i.e., before and after the intensive upwelling season. Multivariate analysis re- vealed four distinct associations, separated by latitude (ca 27°S) and depth (ca 300 m isobath), indicating that dis- solved oxygen is an important factor affecting the distribu- tion of demersal fish communities in the zone. Faunal diver- sity decreased in areas of low oxygen levels. The seasonal variation in upwelling reflected the extent of fish associa- tion, their composition, and the seasonal geographical dis- placement of their boundaries. and references cited herein). Large areas of bottom-waters with very low levels of dissolved oxygen are one of the most characteristic features of this upwelling (Chapman and Shan- non 1985) affecting not only the distribution and abundance of various species (Bailey et al. 1985, Macpherson and Mas- Riera 1987, Abell6 and Macpherson 1989), but also species activity (Roel and Macpherson 1988). These effects have also been described in other regions (Burd and Brinkhurst 1984, Kramer 1987, McMahon 1988). The aim of this paper was to determine the structure of the demersal fish community in the main upwelling region between Walvis Bay (23 °S) and the mouth of the Orange River (29°30'S), before and after periods of maximum up- welling (October to November); and to analyze the influence of dissolved oxygen levels on zonation in the region. Introduction Research into the structure of fish communities in the south-east Atlantic region has mainly dealt with northern Namibia (Lleonart and Roel 1984, Macpherson and Roel 1987) or South Africa (Roel 1987). The structure and zona- tion of communities in the main upwelling region, and the effect of seasonal variations in upwelling activity are still largely unknown. The coastal region off Namibia is characterized by active upwelling, generated by the intense equator-ward windstress pattern that exists over the Benguela current (Shannon 1985), resulting in some of the world's most productive fish- ing grounds (Crawford et al. 1987). Upwelling intensity is not uniform either in space or in time, being strongest be- tween L/ideritz (26 °S) and the mouth of the Orange River (29°30'S) in spring (October and November; Chapman and Shannon 1985, Shannon 1985). As in other upwelling regions, bottom-waters have a low concentration of oxygen due to high sedimentation rates and incomplete degradation of organic matter (Thiel 1978 Material and methods Material was obtained on two surveys conducted by the freezer-trawler "Chicha Touza" at depths between 50 and 460 m off the southern half of Namibia (23°00'S to 29°30'S) in winter (August to September 1987) and summer (January to February 1988). A bottom trawl with a 20 mm mesh cod-end was used. Trawl duration was 30 min effective tow- ing time, and all trawls were carried out in the daytime - to avoid the effect of diel migrations by certain species e.g. Merluccius paradoxus. In all, 146 trawls were made, 68 on the first survey and 78 on the second, with stations selected on a stratified random-sampling basis. The species composition was determined for each trawl, with data expressed as the number of individuals per nauti- cal mile trawled (1 nautical mile--1 852 m) and the Shan- non-Weaver index of diversity (H) calculated. To smooth the effect of abundance, a log transformation was applied to normalize the density matrix for each species (number of individuals per nautic mile trawled). Species and trawls were classified, hierarchically, for cluster analysis choosing the