10.2.2.2. Marine and Freshwater Fisheries
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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