The Biology and Culture of Tilapias

In taking into account the eco-ethology of reproduction, it is often possible
to explain subsequently, or to foresee, the differential dynamics of sub-
strate-spawning and mouthbrooding species in natural water bodies subject
to fluctuations (for example changes in water level, development of vegeta-
tion) in man-made lakes or artificial reservoirs, or since the introductions of
species. The rapid falls in level in certain man-made lakes can be catastrophic
for populations of substrate-spawning species (of which the breeding males
and females as well as the eggs and alevins are strongly attached to the nest
and, moreover, often situated in sites close to the shores) but have little or no
effect on mouthbrooding species in which only the males (which stay
billeted on the arenas and always at greater depths than substrate-spawners)
are eventually affected, for the females are capable of moving the alevins
towards deeper water. In Lake Mwadingusha, ex-Katanga, Zaire (Ruwet
1962, 1968) the variations in level affected T. rendalli much more than S.
macrochir, a species which also coped successfully with water level fluctua-
tions in Lake McIlwaine, Zimbabwe (Marshall 1979b).
In Lake Naivasha, Kenya, the development of the (introduced) population
of T. zillii is associated with an evolution of the environment (rise in level
and appearance of a six-year flood cycle) which has permitted the develop-
ment of a fringe of riverine and marginal vegetation favorable for the repro-
duction of this species; this evolution has also favored S. leucostictus to the
detriment of S. spilurus niger, a fluviatile species unable to adapt to the
disappearance of bottoms suitable for its reproduction (Siddiqui 1979a,
1979b).
In Lake Kainji, Nigeria created in 1969, the tilapias are represented by
S. galilaeus, S. niloticus and T. zillii, in the proportion 16:5:1. The pre-
dominance of S. galilaeus is partly explained by its less specialized require-
ments for breeding than those of S. niloticus and T. zillii, which need special
bottoms and vegetation (Ita 1978). One can see, however, evolution from
the original riverine situation in tilapia populations in several other man-
made lakes (Lake Nasser, Latif 1976) characterized by a progressive substitu-
tion of S. galilaeus for S. niloticus, initially the more abundant.
Certain deceptive results in fish culture are explained largely by a modifica-
tion of the frequency of spawning in species transplanted into an environ-
ment climatically different from their original environment. Thus, the good
results obtained with T. rendalli in fish culture in Shaba where the existence
of a cold dry season limits the numbers of spawnings (and in consequence
population of the ponds and dwarfing) are not also found in the Zaire
basin at Yangarnbi, where reproduction is continuous (Gosse 1963). A com-
parable phenomenon is probably responsible for the failure of the transfer of
S. andersonii from Zambia (a single reproduction per year and maturity
at 12 to 15 months old) to Tanzania (maturity at 3 to 5 months old and
excessive breeding) (Lema and Ibrahim 1975).
The relative aggression of the species at intraspecific (intensive mono-
culture) and interspecific (introductions, polyculture) levels and its variations
with sex, size, age and environmental factors such as temperature, must be
taken into consideration by fish culturists. In Taiwan, T. zillii is considered
of little interest because it is too aggressive (Chen 1976); in ponds in Zaire
T. zillii competes with T. rendalli (Gosse 1963) and in Lake Victoria it
competed with, and supplanted, S. variabilis (Table 2).