18.104.22.168. Marine and Riverine Fisheries
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).
22.214.171.124.1. Vulnerability of fisheries resources
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
- A shift in centers of production and the composition of fish species as
ecosystems move geographically and change internally. This is in contrast
to freshwater fish species-particularly in small, shallow rivers and lakes-which
will have limited possibilities to adapt to the changes through migration.
- Economic values can be expected to fall until long-term stability is reestablished.
Rapid changes resulting from physical forcing favor smaller, low-priced, opportunistic
species that discharge large numbers of eggs over long periods.
- Where ecosystems shift position, national fisheries will suffer if institutional
mechanisms are not in place that enable fishermen to move within and across
present exclusive economic zone boundaries. Subsistence and other small-scale
fishermen (who dominate in Africa) probably will suffer disproportionally
from such changes (Everett, 1994).