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African nations possess a variety of lacustrine, riverine, and marine habitats with more than 800 freshwater and marine species (as noted in IPCC, 1998). GCMs do not provide direct information on water quality and other hydrological parameters that affect fisheries (Hlohowskyj et al., 1996). As such, vulnerability assessments must translate projected atmospheric changes into changes in aquatic environments, making it possible for ecological and biological responses to climate change to be identified and evaluated. Most studies on the potential impacts of climate change on fisheries have been done for temperate-zone fisheries. In these studies, the emphasis has been to evaluate the impacts of changes in the availability of thermal habitat on fishery resources and evaluate the effects of temperature on physiological processes of fish.
Temperature increases may affect lake fisheries, although sensitivity across Africa is likely to vary. For example, Ntiba (1998) used empirically derived models to elucidate the relationship between long-term fishery yield data with climatic index value for Lakes Naivasha and Victoria in Kenya. The mean annual temperature is taken as the climatic index because of data availability and close correlation between air and water surface temperatures. The results indicate that in Lake Victoria, mean annual temperature has greater effect on fish yields than morphoedaphic index (MEI); the opposite is the case in Lake Naivasha. For Lake Naivasha, a rise of as much as 2°C above the current mean annual temperature may not even double the yield from the current 12 kg ha-1 yr-1 (yield regressed to temperature only). The additional effect of MEI will double and triple the yield with a rise of 1.5 and 2°C, respectively. For Lake Victoria, the maximum predicted yield under current climatic conditions is estimated to be 81.8 kg ha-1 yr-1 and is predicted to more than triple to 263.7 kg ha-1 yr-1 with a rise in temperature of 0.5°C. However, natural aquatic ecosystems have a finite carrying capacity at which fishery yields will reach a maximum sustainable yield (MSY) (Russell and Yonge, 1975); it is unlikely that the yield will exceed 84.2 kg ha-1 yr-1 in Lake Victoria, considering that the estimated MSY for the entire lake is 74.0 kg ha-1 yr-1 (Turner, 1996).
On Lake Kariba, Magadza (1996) found that drought years were accompanied by decreased fisheries catch. Hart and Rayner (1994) show that the distribution of copepods on the African continent is temperature-dependent, with species examined showing restricted temperature range preferences. In the laboratory, Magadza (1977) found that the optimum temperature for the reproduction of Moinia dubia is 24°C, with temperatures exceeding 28°C showing reproductive failure. Chifamba (2000) found negative relationships between catch per unit (CPU) effort and temperature in the pelargic Limnothrissa miodon fishery on Lake Kariba, whereas precipitation and river runoff were positively correlated with CPU. These observations indicate possible depression of planktivorous pelargic freshwater fisheries as a result of climate change impacts. Where such fisheries constitute a signifiant protein source, such impact will bear on food security.
A further consideration in the possible impacts of global warming on inland fisheries is the thermal behaviors of inland waters. In tropical areas, a unit change in temperature elicits a greater density change per unit temperature change. Thus, at the higher tropical temperatures indicated by climate models, thermal stratifications are likely to be more stable. In eutrophic lakes, anoxia and amonia intoxication leading to massive fish deaths—as repeatedly witnessed in Lake Chivero—are likely to be significant (Magadza, 1997; Moyo, 1997).
Aquaculture is a food production activity with one of the highest growth rates in the world. Risk management in aquaculture must take into account the level and frequencies of extreme events in assessing available technical options, based on the environment and climatic conditions.
Riverine fisheries will be affected. With a potential warming of 3-5°C projected for the next century, productivity of the Gambia River is estimated to increase by about 13-21% (Jallow et al., 1999). There would be little or no effect on the suitability of the present habitat for some fish and shrimp species. In contrast, warming of more than 3°C will have negative impacts on habitats for catfish, and warming of more than 4°C will reduce the suitability of the present habitat for herring. Shrimp yield is estimated to increase by about 38-54%.
Upwelling of the Canary Current produced by the northeast and southwest tradewinds makes the fishery off the coast of Morocco one of the most productive in the world. Data from the Institut National de Recherche Haleutique du Maroc show that NAO weakens the upwelling, increases temperature, and reduces sardine stocks (Hilmi et al., 1998).
The pelagic fishing industry in the southwest of Africa (based on the Benguela upwelling) contains several migratory species, such as the anchovy (Crawford et al., 1987). These species are an important resource in their own right, but they also are a key element in the food chain of larger fish, seals, and birds. Recruitment in these species is influenced by water temperature, and growth is affected by the state of the upwelling, both of which are linked to global climatic conditions (Shannon et al., 1990; Siegfried et al., 1990). For instance, there is a phenomenon similar to the El Niño effect on the fisheries of Peru that leads to an intrusion of warm, nutrient-poor water from the north in some years, which severely depresses fish yields (Shannon et al., 1990). The net impact of global climate change on southern African fisheries remains unclear because the potential impact on ocean circulation and wind shear in the coastal zone is uncertain. In the event of major reorganization of the circulation of the southern oceans (Hirst, 1999)—which is a possibility at high rates of warming—the impacts most likely would be severe.
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