Another factor that affects low temperature tolerance of tilapias is the
thermal history (acclimation) before exposure to low temperature. Chervinski
and Lahav (1976) found that S. aureus acclimated to 28°C for two weeks
began to die at 11°C while those acclimated to 18°C (for two weeks) began
to die only at 9°C.
S. mossambicus is killed between 8 and 10°C (Chimits 1957). The mini-
mum temperature at which the fish ceases to feed is 15.6"C and 100% mor-
tality occurs at 8.3 to 9.4"C (Kelly 1956). Lower temperature tolerance
(5.5"C) was reported for S. mossambicus in Hanoi, Viet Nam (Li et al.
1961). The influence of salinity on the lower temperature tolerance limits
of S. mossambicus was investigated by Allanscn et al. (1971). They found
that S. mossambicus tolerated 11°C in 5% saline water but when in fresh-
water it could not survive at that temperature. It was suggested by Allan-
son et al. (1971) that the ability of S. mossambicus to withstand low tem-
perature is correlated with the maintenance of high plasma sodium and
chloride concentrations.
In contrast to their limited low temperature tolerance, tilapias are very
tolerant to high water temperatures. S. alcalicus grahami, which lives in the
hot springs of Lake Magadi, Kenya, tolerated temperatures up to 42°C (Coe
1966). A similar resistance to high temperatures (up to 42°C) was found in
S. shiranus chilwae (Morgan 1972). Gleastine (1974) reported a low level of
mortality at 41°C for S. aureus.The upper lethal limit for S. niloticus was
also determined to be 42°C (Denzer 1968), while the upper median lethal
temperature was 38.2"C for S. mossambicus (Allanson and Noble 1964).
Experiments conducted by Beamish (1970), using S. niloticus, showed
that the temperature preferendum was 30°C when the fish was acclimated
to temperatures between 15 and 30°C. A lower preferendum of 2B°C was
found when the fish was acclimated to 35'C.
SalinityIt is assumed that tilapias evolved from a marine ancestor and that their
penetration to fresh water is secondary (Myers 1938; Steinitz 1954). This
may account for marked euryhalinity of certain species (Chervinski 1961a).
S. aureus is able to survive direct transfer from freshwater to 60 to 70%
sea water (20.2 to 25.0%0 salinity) and through gradual adaptation is able to
withstand up to 150% sea water (Lotan 1960). The growth and survival of S.
aureus and S. galilaeus in brackishwater have been studied in small-scale
experiments in aquaria, concrete tanks and earth ponds (Chervinski 1961a,
1961b, 1961c, 1966). The growth of S. aureus in seawater ponds investigated
by Chervinski and Yashouv (1971) was found not to differ significantly from
that in freshwater. Good growth of F1 hybrid offspring of the cross S.
niloticus 9 x S. aureus d in brackishwater ponds was found by Fishelson and
Popper (1968) and Loya and Fishelson (1969).
S. mossambicus is euryhaline and grows and reproduces in fresh, brackish
and seawater. According to Vaas and Hofstede (1952) reproduction does not
occur in salinities between 30 and 40%,. However, Hora and Pillay (1962)
found that reproduction occurred in seawater up to a salinity of 35%0. Recent