The African nations possess a variety of lacustrine, riverine, and marine habitats with more than 800 freshwater and marine species. Ten ichthyofauna regions, based largely on present-day drainage systems, have been delineated for Africa (Lowe-McConnell, 1987). These regions are dominated by the Niger, the Nile, the Congo, and the Zambezi River systems; they also include several inland drainage areas associated with lakes (Hlohowskyj et al., 1996). Among the riverine systems, the Congo River (including its major tributaries) contains the most diverse fish fauna, with about 690 species (of which 80% are endemic) (Lowe-McConnell, 1987). Lacustrine systems in Africa (particularly the Rift Valley lakes) contain the most diverse and unique fish assemblages found anywhere in Africa, if not the world. For example, Lake Malawi has more than 240 identified fish species (of which more than 90% are endemic), and another 500 or more species awaiting taxonomic identification (Lewis et al., 1986).
Fish make up a significant part of the food supply in Africa (Hersoug, 1995). FAO (1993) estimates the total fish harvest potential at around 10.5 million tons: 7.8 million in saltwater fisheries and 2.7 million in freshwater fisheries. In a densely populated country such as Nigeria, as much as one-third of the protein supply comes from fish (Hersoug, 1995). Consequently, any fluctuation in the fish stock will impact planning and management. A reduction in fish stocks will have the greatest effect on countries that are heavily dependent on fisheries and cannot diversify easily into other activities; Mauritania, Namibia, and Somalia are examples of African countries in this category (Clarke, 1993).
The productivity of freshwater and sea margins has become stressed mainly by economic activities rather than climate variability. For example, the artificial opening of the sand barrier at the mouth of the Cote d'Ivoire River to clear floating weeds has allowed seawater to enter the lower part of the river and has changed species dominance (IPCC 1996, WG II, Section 16.1.1). On the Nile, the Aswan Dam so thoroughly regulates flows that the delta has become degraded ecologically. Local sardine populations that once thrived and provided food for the region have collapsed with the decline in production that depended on the strong surges of floodwaters and their pulse nutrients. The Sahelian drought is causing increased salinity in the lower parts of Senegalese rivers, but a dam erected near the mouth of the Senegal River to stop the rising salinity and ease severe problems in local agriculture prevents fish migration (Binet et al., 1995).
The vulnerability of fisheries to climate change depends on the nature of the climate change, the nature of the fishery, and its species and habitats. Changes in climatic conditions such as air temperature and precipitation affect fisheries by altering habitat availability or quality. Specifically, fisheries habitats may be affected by changes in water temperature; the timing and duration of extreme temperature conditions; the magnitude and pattern of annual stream flows; surface-water elevations; and the shorelines of lakes, reservoirs, and near-shore marine environments (Carpenter et al., 1992).
Mean annual air temperature is the most important factor in predicting lake fish production across latitudes. Alterations in seasonal climate patterns should change the population distributions in larger lakes. Large-lake fish production could increase about 6% with a 1°C rise in average annual air temperature (Meisner et al., 1987; IPCC 1996, WG II, Section 16.2.1). Warm-water lakes generally have higher productivity than cold-water lakes, and existing warm-water lakes will be in areas with the least change in temperature. It is reasonable to expect higher overall productivity from freshwater systems.
Although changing rainfall patterns and flood regimes may have profound effects on freshwater fish, marine fisheries are likely to be affected more by elevated temperatures (Hernes et al., 1995). The impacts of elevated temperatures could include:
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