shrink, and also in the dry season when the fish may be trapped in pools
and riverbeds where there is very little aquatic vegetation to give cover
from predators.
Lacustrine tilapia populations include many of the same species found
in the rivers, for example the nilotic species in many. East African lakes.
Some, like the Luapula/Mweru populations of S. macrochir, may make
regular movements between river and lake. But in some long-enough isolated
lakes endemic tilapia species have evolved: for example the Sarotherodon
species flocks of Lake Malawi, and tilapia species in small crater lakes in West
Africa.
As tilapias support the main commercial fisheries in most African lakes
(except Lake Tanganyika) it was predicted that they would become the
dominant fish in the new man-made lakes, such as Lakes Kariba and Kafue
on the Zambezi system, Lake Volta in Ghana, and Lake Kainji on the
Niger. The speed with which the tilapia populations have built up from
indigenous riverine populations has depended on factors such as the draw-
down for hydroelectric needs (which can destroy tilapia nests in shallow
water), and perhaps flow-through/storage ratio, a high flow-through militating
against the buildup of planktonic food sources. Tilapias became predominant
species within two years in Lake Volta and three years in Lake Kariba (Petr
1969), but took longer to build up in Lake Kainji (Lewis 1974).
Ecological Information from Particular Water BodiesThe tilapias most studied in natural and semi-natural communities, and on
which this review is based, are listed in Table 1.
- NILOTIC TILAPIAS FROM RIVERS AND LAKES
Fish communities in the soudanian region include three relatively common
tilapias, Sarotherodon galilaeus and S. niloticus, and Tilapia zillii, plus the
smaller less common S. aureus (syn. S. monodi), which is more abundant in
the Nile delta and Israel. These fish communities also include many much
larger fishes, some of which may compete for resources with tilapias, such as
Labeo and Citharinus which share the bottom detritus used as food by S.
galilaeus in Lake Chad (Lauzanne 1972). Others are large piscivores, such as
Lates niloticus, Hydrocynus species and Gymnarchus niloticus, all known to
include tilapias in their diets. The tilapias are generally restricted to pools,
lagoons and edges of the river and floodplain, places where aquatic vegetation
provides cover. Daget (1954) pointed out that in the Upper Niger S. galilaeus
predominates over sandy bottoms, S. niloticus over muddy ones (as Holden
1963, also found in Sokoto river pools), while T. zillii, an ecologically
tolerant species, lives over rock, sand or mud, in still or running water. A
macrophyte-feeder, T. zillii is generally found near plant stands; riverine S.
galilaeus are detritivores using bottom algae, and S. niloticus takes epiphytic
algae, diatoms or plankton according to the conditions.
In the lacustrine conditions of the huge but shallow Lake Chad, S.
galilaeus is the common species, except in the north and northeast where S.