The Biology and Culture of Tilapias

because of its inability to tolerate salinities above 19%0 that T. rendalli never
succeeded in colonizing, by way of estuaries, coastal lagoons and the sea, the
more southern rivers where the temperature regime is still sufficient for its
survival and reproduction. In this T. rendalli differs from S. mossambicus
(Jubb and Jubb 1967, in Beadle 1974).
Other extreme situations with regard to salinity are represented by the
rivers and lakes containing very low mineral contents in which tilapias
occur: S. macrochir, T. sparrmanii and T. rendalli in Lake Mweru (salinity,
O.O23%0 ; C,, = 14 to 35 pS/cm; Beadle 1974), S. variabilis, S. esculentus, T.
zillii and S. niloticus (the latter two species introduced) in Lake Nabugabo
near Lake Victoria, Uganda (salinity, 0.015%0; C2, = 25 pS/cm) and T.
congica in Lake Tumba, Zaire (salinity = 0.016%0; C2,= 24 to 32 pS/cm)
Dubois (1959).
Several species can adapt to a wide range of salinities: T. zillii (0.16 to
44%0), S. mossambicus (0 to 120%0) (Whitfield and Blaber 1979), and S.
amphimelas, endemic in the hypersaline Lake Manyara (salinity 58950;
C20 = 94,000 pS/cm) and in Lake Kitangiri, Tanzania where a normal C20 =
185 pS/cm (Fryer and Iles 1972; Beadle 1974).
The different ranges of tolerance with regard to salinity of different
tilapias often show when natural or man-made changes occur in certain
habitats. Thus the construction of a dam across coastal rivers has isolated
freshwater populations from species which normally prefer brackishwaters:
T. guineensis in Lake Ayeme on the Bia River (Ghana/Ivory Coast border),
and in Lake Mount Coffee on the St. Paul River (Liberia) (Thys 1971a).
The gradual increase of salinity in Lake Qarun (near Cairo, Egypt)
(11%0 in 1920 and 22%0 in 1932) had led to the gradual disappearance of
species that were abundant (S. niloticus, S. aureus) and their replacement by
T. zillii which is much more euryhaline (Fryer and Iles 1972).
In the closed lagoons, lakes and coastal lagoons along the southeast coast
of Africa, the irregular connections with the sea lead to a sequence of high
and low salinities (Whitfield and Blaber 1979). The populations of S. mos-
sambicus occurring in these habitats have thus to tolerate very wide ranges of
salinity (0 to 120%0 in St Lucia Lake), the gradual character of which,
however, allows the populations to adapt. This adaptation may be accom-
panied by a reduction of the biomass (S. mossambicus formed 12.3% of the
catch when the salinity was 10960 but only 1.2% when the salinity was
80% ) or by a migration towards the upper parts of the estuaries where the
salinity remains lower (Whitfield and Blaber 1979).
Studies by these authors demonstrate the absence of S. mossambicus from
most of the estuaries that are permanently open to the sea and which are
characterized by rapid variations in salinity because of the tides. S. mossam-
bicus thus seems incapable of tolerating rapid changes of salinity, but tole-
rates seawater and/or slow changes in salinity very well. However, the
absence of S. mossambicus from the estuarine systems studied by Whitfield
and Blaber (1979) may not be caused by rapidly changing salinities, but
could be linked to other negative factors also operating in these systems:
rapid currents, bad conditions for nest construction, competition with
marine fishes, predation by piscivorous fishes, etc.