Pullin (1988) cites many cases where
tilapias have been found in countries to
which they are exotic and were previously
unrecorded. The earliest transfers of
tilapias in North Africa were probably by
the French Foreign Legion from one well
to another (Thys van den Audenaerde
1988). Oreochromis mossambicus was first
reported in Asia in 1938, when two fe-
males and three males of unknown origin
were accidentally discovered in Java
(Schuster 1952).
Other problems are the accuracy of
identification of the original stock and its
possible admixtures of closely related
species. There are often mistakes in the
nomenclature of introduced species, cre-
ating much confusion. The Kafue strain
of 0. macrochir was introduced to fish
culture ponds at Kipopo (Katanga, Zaire)
under the name of andersonii, and 0.
mortimeri to Katanga under the name
"mossambica" (Lowe-McConnell 1988).
As an example of these introductions,
Barbotes spp., endemic to Lake Lanao
(Philippines) have become rare following
the intentional introductions of Clarias
batrachus, Channa striata, 0.
mossambicus, together with the acciden-
tal introduction of Glossogobius giurus.
The native fish were formerly the prin-
cipal sources of livelihood and foodfish for
the local people (Frey 1969).
In Queensland (Australia), tilapias are
considered as pests (Bluhdorn and
Arthington 1990). Negative impact on
indigenous fish can be considered general
for tropical America, where piscivorous
fish were introduced predominantly
(Fernando 1991).
Introductions can produce hybrids with
indigenous species. Introgressive hy-
bridization has occurred in Lake Itasy in
Madagascar, where 0. niloticus crossed
with 0. macrochir has produced the so-
called "tilapia trois-quarts" (Vincke 1971;
Daget and Moreau 1981), and in the
Philippines with 0. mossambicus and 0.
niloticus (Taniguchi et al. 1985; Macaranas
et al. 1986).- niloticus was introduced into Lake
Victoria in a shipment of Tilapia zillii
from Lake Albert, or spread through the
drainage system, having being used in
culture trials carried out at Kajansi
(Uganda) (Lowe-McConnell1988). In the
same lake, T. zillii has largely displaced
the endemic 0. variabilis.
In Lake Titicaca, endemic species of
Orestia suffered a rapid population de-
cline from infection with sporozoan para-
site~ introduced with trout (FAOIUNEP
1981). Introductions of non-native
salmonids have spread diseases into new
habitats, sometimes to the detriment of
native fish (Allendorf and Leary 1988).
Hybridization
Hybridization can be carried out in
breeding programs to obtain heterosis and
required characteristics in the hybrid
progeny, but may also be an unplanned
consequence of the introduction of one
population (or species) in the distribution
area of another. Hybrids between local
and domesticated stocks may affect
neighboring ecosystems if their genes and
parasites spread.
Although hybridization is often used
to increase the amount of genetic varia-
tion in a population, it can be a mode of
genetic diversity (Nelson and Soule 1987).
The effect of loss of gene exchange be-
tween subpopulations is to increase the
variance within groups, decrease the
variance between groups and decrease the
total variance (Crow and Kimura 1970).
However, as intraspecific hybridization
breaks co-adapted gene complexes and
can cause an outbred depression (which
has been often empirically evidenced,
although numerical data are scarce), the
fitness of hybrid generations usually
declines with time.
The change in genotypicvariance from
hybridization between small neighboring