rapids and fast-flowing water and preferred still water and pools. In the same
way, electric fishing in the Shire River, Malawi (Lamarque and Closset 1975)
showed the absence of tilapias (mainly S. mossambicus) from the central
part of the river where the current is rapid, and their concentration along the
sheltered banks provided with vegetation.
The habitat of a species is especially more difficult to characterize as it
differs with size, age and stage of sexual maturity and it varies for each
intraspecific group with the seasons and even the time of day (variations
which lead to daily movements and seasonal cyclical migrations).
a. Influence of size, age and stage of sexual maturity. The size of fish
limits the physical accessibility of certain habitats (depth) in such a way that
the alevins, juveniles and adults distribute themselves in depth zones from
the shore towards open water; this spatial separation favors complementary
exploitation of the feeding niches of different habitats, avoiding specific. competition. The alevins and the juveniles generally have a much larger tem-
perature tolerance range than the adults (16.5 to 39"C, with a preference for
19 to 35" C in the alevins and juveniles of S. mossambicus, compared with a
tolerance range of 19 to 32" C and a preference for 22 to 30" C in the adults,
Bruton and Boltt 1975), which explains why they are able to live throughout
the year in the littoral zone (warmer than open water in summer but colder
in winter) while the adults only stay there during the summer breeding
period.
b. Seasonal changes of habitat. Bruton and Boltt (1975) studied the sea-
sonal changes in the distribution of S. mossambicus of Lake Sibaya. During
the cold season (May to July, 20°C), the juveniles and adults live in the
pelagic zone at a depth of generally less than 12 to 13 m. When the water
warms up (August to September), first the juveniles, then the adults migrate
towards the shallower littoral zone which forms the feeding and breeding
grounds until the end of the hot season. In January, there is a return migra-
tion towards deeper water. Seasonal migrations of the same type, between
littoral and pelagic zones have been observed in S. lidole of Lake Malawi
(Lowe 1953), S. variabilis of Lake Victoria (Fryer (1961a) and S. rnacrochir
of Lake Mweru (Carey 1965).
Changes (seasonal or more irregular) in the spatial distribution of tilapias
are introduced by the variations of salinity in certain saline lakes (T. rendalli
and S. shiranus chilwae of Lake Chilwa ; S. macrochir of Lake Mweru Wantipa)
and by variations in the turbidity of certain river systems (Pienaar 1968).
Whitefield and Blaber (1979) showed equally important displacements of
S. mossambicus populations in estuaries which take into account their open
and closed phases. S. mossambicus are abundant in the lower reaches during
the closed phase (salinity stabilized and current weak) but retreat into the
upper reaches when the estuary is open (strong daily variations in salinity
and faster current).
Finally, it is well known (Lowe-McConnell1975) that important seasonal
changes in habitat are produced in all rivers subject to alternating floods (colo-
nization of the flood plain, partly for reproduction) and dry periods (with-
drawn into the river bed and into the permanent pools of the intermittent
streams). Whyte (1975) has studied the seasonal changes in the spatial distri-