taxonomic and hydrographic knowledge) all represent small catastrophes.
In several cases it is indeed impossible to know if the presence of a given
species is a natural occurrence (which could have far-reaching consequences
in biogeographical and paleographical terms) or simply the results of human
activities.
The following examples may be cited:
i. Natural occurrence or old introduction: S. hornorum in Zanzibar;
S. hunteri in Lake Chala, Kenya (Trewavas 1966).
ii. Endemicity of S. girigan and S. jipe in Lake Jipe (these two species
were introduced into the Pangani River and now occur in the whole
basin, Trewavas 1966).
iii. The biogeographical relevance of the presence of S. mossambicus in
the Thalamakane River, Botswana (Jubb and Gaigher 1971), in small
water bodies in the Narnib desert (Dixon and Blom 1974), and in
two tributaries of the middle Zambezi (Jubb 1974) where this species
was considered to be absent. Another similar case is the presence of S.
macrochir in the middle course of the Buzi River, Mozambique
(Bell-Cross 1973), completely outside the rest of its range.
To add to the confusion, during the fifties many aquaculturists called any
macrophagous substrate-spawning tilapia 'melanopleura', whatever its
geographical origin. Thus T. zillii, T. rendalli, T. tholloni and T. guineensis
have often been spread around in Africa confused under the name "melano-
pleura".
INTRODUCTIONS OUTSIDE AFRICAThe first, probably accidental, introduction of a tilapia outside Africa was
that of S. mossambicus prior to 1939 in Java where a few individuals were
caught in the Serang River (Atz 1954; Riedel 1965). From then on, S.
mossambicus was introduced into various countries (see Riedel 1965). Then
the practice spread to other species, and now tilapias occur in natural waters
throughout the tropics, even in Australia (Queensland). Figure 3, drawn
from data in Balarin and Hatton (1979), Welcomme (1979a, 1979b), Rosen-
thal (1976), Courtenay and Robins (1973), Bardach et al. (1972), FA0
(1977~) and some other authors, shows the worldwide pattern of introduc-
tions of the species of major importance: S. mossambicus, S. niloticus, S.
aureus, S. hornorum, T. rendalli and T. zillii. Other species introduced
outside Africa are dealt with in Table 3.
The objectives of these introductions were, as in the case of Africa, pond
culture (see Bardach et al. 1972; Coche 1978; Vincke 1979), the stocking of
natural habitats, of man-made lakes and water storage reservoirs, of mining
pits, and the control of aquatic vegetation. For twenty years there have also
been introductions to stock cooling waters and geothermally heated waters
in temperate regions, in Japan, the USA and in Europe (e.g. TVA 1978;
Kuroda 1977 ; Mdlard and Philippart 1980).