tolerance limits and the preferences of tilapias for the physico-chemical
properties of their habitats. Here we shall recall some of these data and
complement them with more recent results (post-1976), dealing mainly with
data pertaining to ecological conditions actually encountered in the natural
habitats and some semi-natural habitats (fish ponds). Data obtained under
laboratory conditions are discussed only when this is necessary to define
better the ecological amplitude of certain species.CURRENT VELOCITY AND DEPTHSeveral tilapias are perfectly adapted to fast-flowing rivers with rapids, for
example, T. rheophila endemic to the Konkoure river, Liberia (Thys 1969)
and T. busumana of Lake Bosumtwi, Ghana and the rivers flowing into it,
especially the Ebo river where T. busumana occurs in places where the slope
ranges from 13 to 60760 (Lelek 1968). S. andersonii and T. sparrmanii may
be found in the upper and middle reaches of the Kalomo river (a tributary
of the Zambezi) where the slope has an average of 2.21%0 (min. 0.9760, max.
10960) and 5.73750 (min. 3760, max. 19%0) respectively, but not in the lower
reaches where the slope is very steep (average 6.63760) (Balon 1974, p. 459).
Even for such species well adapted to river life, rapids and falls are hostile
zones which often represent ecological barriers (mechanical obstacles,
excessive current velocities, oxygen and nitrogen supersaturations) which
prevent the mixing of neighboring but different ichthyofaunas.
Quite a number of tilapias, especially those with a wide area of distribution,
may be encountered in both rivers and lakes where they tend to remain in
shallow inshore waters (for reproduction and feeding) and in the pelagic
epilimnion (for their nutrition in the case of plankton-feeders). This depth
limitation of tilapia distribution (see Table 4) can be found even in the
majority of species that are endemic in the African Great Lakes.
According to Caulton and Hill (1973) for S. mossambicus, and Caulton
(1975a, 1975b) for T. rendalli and T. sparrmanii, tilapias should be physiol-
ogically unable to adapt to the increased pressure that goes with increasing
depth. However, the depth distribution of lakedwelling tilapias is also in-
fluenced by temperature and oxygen gradients, as well as by concentration
of dissolved toxic gases such as C02, and especially H2S and NH3. The
temporal dynamics of distributions with regard to habitats are examined
below because it is obvious that these are the result of several factors, some of
which are interacting, for example, the influence of temperature on the DO
and other dissolved gases, on the toxicity of NH3 and H2S, on the innate
abilities to compensate and on the speed of adaptation to depth/pressure; in
S. mossambicus the maximum adaptation depth is 20 m at 30°C but only
7 m at 15°C (Caulton and Hill 1975).
Whether a tilapia is rheophilic (current loving), lirnnophilic, or indifferent
to current velocity, may be readily appreciated from the results of intro-
ductions (deliberate or not) of typically riverine species into lakes, of typically
lacustrine species into rivers, and from the evolution of tilapia populations
after the transformation of habitats with a fast flow to ones with a slow flow