Experimental studies, aquacultural and field observations (Balarin and
Hatton 1979) demonstrate that many tilapias which are not especially
adapted to warm waters can tolerate temporary temperatures of 35 to
42°C. For T. zillii, mortality begins when the temperature exceeds 395°C
and the upper lethal temperature is 42.5"C (Hauser 1977). For T. rendalli,
there is a sublethal zone of tolerance between 37 and 40°C (Caulton 1977)
and an upper tolerance limit of 40.6 to 41.9"C after adaptation at 24°C
(Morgan 1972). For S. niloticus, there is an upper tolerance limit of 42°C
(Denzer 1968 cited by Aston and Brown 1978). S. niloticus also tolerates
36°C for 6 hours (with peaks of 40°C during brief periods) in industrial
cooling waters in Belgium (Melard and Philippart 1980). Lowe (McConneU)
(1958) reports the capture of an S. niloticus individual from a hot spring
of the Turkwel River, Kenya (40" C).
On the basis of their thermal preferences and their tolerances (both
determined experimentally and from their geographical distributions), three
categories of tilapias emerge:
- Eurythermal species which tolerate a wide range of temperatures-
T. zillii (6.5 to 42.5"C), S. mossambicus and S. niloticus (8 to 42"C),
and also probably S. aureus and S. galilaeus. - Species that are eurythermal but seem less tolerant than 1. to high
temperaturesT. sparrmanii (7°C to ?)-or to low temperatures, S.
spilurus niger (8 to 10°C to ?), T. rendalli (11 to 41°C), S. macrochir
(11°C to ?) and S. alcalicus grahami (10 to 41°C). - More stenothermal species such as T. guineensis (14 to 33"C, Mdlard,
pers. comm.) and S. melanotheron which lives in a temperature range
of 18 to 33°C in West African lagoons (Fagade 1974; Pauly 1975,
1976) and is reported to die at 20°C in aquaria (Sterba 1967, in
Balarin and Hatton 1979).
Within a species, the temperature preferences and tolerances depend on size,
the young fish generally being more tolerant to higher, and often to lower
temperatures than the adults. Thus Bruton and Boltt (1975) reported 16.5
to 39°C as the tolerated range, and 19 to 35°C as preferred range for finger-
lings (less than 15 cm) compared to 19 to 32 and 22 to 30°C respectively, for
adult S. mossambicus in Lake Sibaya, South Africa. These intraspecific
variations in thermal tolerance have a physiological basis (see Whitfield and
Blaber 1976 for T. rendalli) and play an important role in the spatial and
dynamic aspects of the fish's distribution.
Because of their adaptations to specific environmental conditions, different
populations of the same species may display different characteristics with
regard to temperature which can be used when selecting strains for aqua-
culture or introductions.
Finally, the relationships between the thermal resistance of tilapias and
other ecological factors, especially salinity, must be discussed. Thus S.
mossambicus (Allanson et al. 1971) and S. aureus (Chervinski and Lahav
- tolerate low temperatures in saline water (5% seawater) better than in
freshwater. This phenomenon helps to explain how the limit of the range of
S. mossambicus can be so far south (to the estuary of the Kongie River, Port
Alfred, South Africa) where, in spite of the influence of the sea, the winter
temperature drops to 12" C (Allanson et al. 197 1).