The Biology and Culture of Tilapias

Several tilapias are euryhaline, able to live and reproduce at salinities
higher than 30%0. These occur naturally in estuaries and coastal lagoons
along the coast of west Africa (T. guineensis, S. melanotheron) or along the
east coast of Africa (S. mossambicus and the related S. hornorum and S.
placidus). Tilapias also occur in hypersaline habitats such as the Bay of Suez
(42 to 43%0) in the Red Sea where T. zillii is now established (Bayoumi
1969, in Balarin and Hatton 1979), and in the Bardawil Lagoon in Israel (41
to 45%0) where T. zillii and S. aureus occur (Chervinski and Yashouv 1971;
Chervinski and Hering 1973). The high salinity of certain lakes is also tole-
rated: Lake Magadi and Lake Natron (salinity 30 to 40%0) have endemic
species S. alcalicus grahami and S.a. alcalicus (Coe 1966, 1967), and Lake
Manyara, Tanzania (58%0), the endemic species S. amphimelas.
Several species, though less euryhaline, are capable of maintaining popula-
tions in habitats where the salinity reaches 30% : S. niloticus and S. galilaeus
in the Bitter Lakes of Egypt (13 to 29760) (Kirk 1972), S. niloticus, S. aureus
and T. zillii in Lake Qarun, Egypt (11 to 29%0) (kyer and Iles 1972) and
S. shiranus chilwae in Lake Chilwa (12 to 29760). The last species, however,
leaves the lake when the salinity becomes too high:conductivity at 20°C
(C,,) = 5,000 pSJcm (Morgan and Kalk 1970, in Beadle 1974).
Other species are less euryhaline and do not tolerate salinities above 20950,
such as T. sparrmanii, S. andersonii, S. macrochir and T. rendalli. These
species occur essentially in freshwater, although some populations prosper in
saline habitats, notably S. macrochir which (as the result of an introduction)
occurs in the Nemakia mangrove swamp in Madagascar, in places where the
conductivity reaches 350 to 10,000 pS/cm (Lamarque et al.' 1975), as well as
in Lake Mweru Wantipa, Zambia (a natural population) where the salinity
sometimes reaches 7%0 and the electrical conductivity 60,000 pS/cm (Fryer
and Iles 1972). However, S. macrochir occurs in the lake during only part of
the year, when the salinity is lower than the extreme value given above.
During high salinity periods, the populations take refuge in the rivers flowing
into the lakes (Fryer and Iles 1972, in Beadle 1974).
A similar phenomenon may be observed in Lake Chilwa, Malawi, where
salinity fluctuations occur (0.3 to 16.7950 for the 1966 to 1970 period
according to Morgan and Kalk 1970, cited by Beadle 1974). S. shimnus
chilwae leaves the lake and takes refuge in the rivers and coastal pools when
the salinity increases above 5%0 (C,, = 5,000 pS/cm), at salinities which
were unfortunately not recorded precisely.
Also, it is well established that T. rendalli has colonized certain brackish
habitats, Lake Poelela, Mozambique among others, where the salinity reaches
8%0 (Whitfield and Blaber 1976). These authors have demonstrated exper-
imentally that T. rendalli (isosmotic at 10%0), can tolerate a maximum of
19%0, the salinity tolerance being maximal at 20 to 28°C. Temperature and
salinity are considered to be the determining factors for the distribution of
this species. It is interesting, in this context, that T. rendalli has been intro-
duced successfully into rivers and reservoirs south @f the Pongolo River,
South Africa ( > 29"S), which marks the southe* limit of the natural
distribution of this species. Whitfield and Blaber (2#76) suggested that it is