Preliminary documentation and assessment of fish diversity in sub-Saharan African estuaries

Lists of indigenous fish species sampled in cool-temperate, warm-temperate, subtropical and tropical estuaries in sub-Saharan Africa were compiled from both published and unpublished data. Comparisons were conducted at the species and family level between the fish assemblages recorded in the different biogeographic regions. Results showed that both species and family diversity declined between tropical and temperate estuaries. Eastern and western tropical estuaries had similar numbers of species and families. The ratio of species to families increased from temperate to tropical systems. In terms of the top five most diverse families, Gobiidae and Mugilidae featured in all the biogeographic regions. Species and family composition comparisons between the different biogeographic regions were undertaken using the Bray-Curtis similarity coefficient. Results indicated that, although family similarities between the different regions were generally high, this was often not the case at the species level. During the late Mesozoic and early Cenozoic, fish families in the eastern and western tropical regions of Africa were able to mix across the northern and southern extremities of the continent. Differentiation of the 'geminate' species on the eastern and western coasts of Africa appears to have commenced at least 12 million years ago, and coincided with the closure of the Tethyan Gateway in the north and the development of the Benguela upwelling system in the south, a situation that has persisted until the present. Fish species sampled in the different biogeographic regions were divided into guilds based upon their life histories and degree of association with estuarine environments. Preliminary results show that marine taxa (marine immigrants and marine stragglers) provided between 68% and 75% of the fish species diversity recorded in sub-Saharan estuaries. Estuarine taxa (estuarine residents and estuarine migrants) accounted for between 10% and 25% of the fish species recorded. Freshwater taxa (freshwater immigrants and freshwater stragglers) constituted < 7% of fish diversity on the east and south coasts of Africa compared to > 21% in the west. The greater representation of freshwater forms in Central/West African estuaries is probably related to the high river flow and lagoonal conditions prevailing in this coastal zone. Estuarine fish communities in Africa are strongly influenced by the prevailing salinity and temperature regimes. Where the salinity/temperature combination exceeds the tolerance of a particular species, fish kills can occur. However, most species tend to avoid estuaries where salinity or temperature regimes are likely to pose a threat to their survival, thus reinforcing the biogeographic regions outlined above.