The Biology and Culture of Tilapias

duction (leading to overpopulation of ponds and dwarfing) in the species
transferred from highlands where the reproduction is seasonal to lower and
warmer regions where reproduction becomes continuous (e.g., T. rendalli
from Shaba to the ZaSre basin and S: andersonii from Zambia to Tanzania).
The introduction of tilapias into waters already containing indigenous
tilapias have had rapid and unfortunate consequences on aquatic ecology
(e.g., hybridization, competition for food and breeding sites) and on the
fisheries: for example, in Lake Victoria, hybridization of S. niloticus with
the indigenous S. esculentus, and of T. zillii with T. rendalli and compe-
tition of S. niloticus (but above all of T. zillii) with the indigenous S. varia-
bilis (Fryer 1961a; Welcomme 1967b; kyer and Iles 1972).
Another interesting problem is posed by the failure of the introduction
of S. macrochir into the man-made Lake Kariba, where S. mortimeri is indi-
genous. Based on the existence of a flourishing fishery for S. macrochir in
Lake Mweru, the new lake was stocked with this species, but the source of
supply was not Lake Mweru in the Zake basin but the Kafue River, a Zam-
bezi affluent. S. macrochir never prospered in Lake Kariba, perhaps because
the supply source was wrong, perhaps because the conditions of life in the
lake were very different from the water of origin, or perhaps again because it
was subject to the concurrence of S. mortimeri, which, a little against all
expectation, is very well adapted to withstand the transformation of the
fluviatile environment to a lacustrine one (Balon 1974). However, the S.
macrochir from the Kafue were transplanted successfully into Lake McIlwaine
situated on the Hunyani River, an affluent of the middle Zarnbezi.
Variations in the success of interspecific introductions are apparent
from introductions made simultaneously or successively of tilapias capable
of competing or presenting differential capacities of population resilience
and eco-physiological adaptations to new environmental conditions (degree
of eurycity). Thus S. spilurus niger, originally from Kenya rivers, was not
able to acclimatize in the Koki Lakes in Uganda and in many other lakes at
altitude in this region at a time when, on the contrary, the introduction of
S. niloticus, a very euryoecious species, was crowned with success. S. spilurus
niger, a fluviatile species, was at first very well adapted in Lake Naivasha
until a change of environmental conditions (rise in water level plus cycles of
littoral flooding plus the creation of lacustrine conditions) led to its dis-
appearance and progressive replacement by S. leucostictus, a lacustrine
species which had been accidentally introduced (Siddiqui 1979b). This
accidental introduction also permitted the hybridization of the two species
and the production of apparently all-male offspring (Fryer and Iles 1972,
p. 168). Lamarque et al. (1975) showed an identical phenomenon in Lake
Itasy, Madagascar, where S. macrochir (introduced in 1958) prospered for
several years before disappearing and being replaced by S. niloticus (intro-
duced in 1961-62); the hybridization of the two species prodwed slow-
growing and sometimes deformed individuals.
The introduction and proliferation of macrophyte-feeding tilapias and
especially of T. rendalli has seriously perturbed the ecology of certain
water bodies, for example, Lake Kyle, Zimbabwe (in Jubb and Skelton