The original distribution of the African tilapias has been modified by the
deliberate or unplanned introduction of several species outside their distribu-
tion area (within the same basin, or from one basin to the next). Figtares 1
and 2 summarize the main data from the literature (Bardach et al. 1872;
Fryer and Iles 1972; Beadle 1974; Lowe-McConnell 1975; George 1975;
Vincke 1979; Coche 1978; Balarin and Hatton 1979; Moreau 1979a, 1979b),
while Table 2 presents a synthesis of the best known examples of introduc-
tions into natural habitats and into reservoirs.
These transfers had different objectives:
ii.
iii.iv.
v.vi.Stocking natural lakes in which no tilapias occurred (S. alcalicus
grahami into Lake Nakuru, S. spilurus niger and T. zillii into Lake
Naivasha, S. spilurus niger, S. esculentus and S. niloticus into many
lakes in Uganda and Ruwanda; S. macrochir and T. rendalli into Lake
Lusiwashi).
Introduction into a natural habitat to fill an ecological niche not
occupied by any of the tilapias present (T. zillii and S. niloticus into
Lake Victoria and Lake Kyoga) in the hope of increasing fishery
yields.
Introduction into artificial water bodies to develop new fisheries based
on the introduced tilapias (S. mossambicus and S. niloticus into the
reservoir of a southern Tunisian oasis), or to utilize the plankton
production of reservoirs (S. macrochir into Lakes Kariba and McI1-
waine; S. placidus into Lake Kyle; S. mossambicus and S. mortimeri
into the lakes of the Zimbabwe eastern highlands).
Biological control of aquatic vegetation (T. rendalli into Sudanese
irrigation channels and some artificial lakes of Shaba), or for the
control of mosquitos (S. niloticus, see George 1975).
Fish culture in rice fields (Vincke 1979; Khoo and Tan 1980), in
floating cages, and especially in ponds (Bardach et al. 1972; Coche
1978). Although at first confined to these aquaculture systems, the
tilapias have inevitably escaped into natural waters (during draining of
ponds, break-up of dykes, floods). For example, T. rendalli escaped
into the Pangani River in Tanzania (Trewavas 1966) and S. mossam-
bicus into the middle Zarnbezi basin (Jubb 1974).
Involuntary introductions occurring during the deliberate introduction
of another species (S. leucostictus into Lake Naivasha, T. rendalli into
Lakes Victoria and Kyoga), or because of the confusion between
sympatric species which have hardly begun to differentiate (S. nilo-
ticus and S. aureus; S. mossambicus and S. placidus; S. mossambicus
and S. mortimeri, and the Sarotherodon species of the east coast rivers
in general).
The reasons for the success, or lack of it, in these introductions, their
effects on the various ecosystems and the aquacultural aspects will be
discussed in a later section in the light of information on the ecology of the
various species. On the strictly biogeographical level, the older, unrecorded
introductions, or those made in a doubtful manner (lacking presentday