The Biology and Culture of Tilapias

related environmental requirements. The principal environmental factor
influencing physiology, i.e., temperature, will be the theme of this paper
which is structured to include some aspects of feeding, metabolism and
growth and to demonstrate the inter-relationships of these functions through
the influence of temperature in the development of a simple energy budget.
Historically, the spread of tilapias from the hot equatorial inland lakes and
rivers of Africa immediately suggests that these fish are well adapted to the
prevailing climatic conditions of these areas. Just how well these fish have
adapted to their environment may be of importance in our attempts to
domesticate these fish. The survival of tilapias is generally limited to waters
with temperatures warmer than 10°C to 12"C, although under exceptional
circumstances (e.g., in areas of increased saline content such as estuaries -
Allanson and Noble 1964), thermal tolerance of lower temperature may be
marginally increased. It is not perhaps the tolerance to temperature that is
important to most wild populations, but rather the effect that temperature,
by its influence on the physiology, may have on growth.

Temperature and Metabolism

It is generally accepted that tilapias cease growing significantly at temper-

atures below about 20°C, but at the same time constant warm temperatures

may not, by themselves, be adequate for optimal growth. Figure 1 illustrates

the distribution of two species when subjected experimentally to a thermal

gradient in a horizontal test tank. In each instance, the fish respond positively

to the warm water, orientate rapidly and swim actively toward water which

is only marginally cooler than their upper lethal temperature tolerance.

Temperature OC

Figure 1. Temperature 'selection' by juvenile a) Tilapia rendalli and b) Sarotherodon nilo-

ticus in a thermal gradient test tank (after C%ulton 1979).

Such an intense behavioral response to temperature clearly indicates

that these fish are strongly thermophilic and in some instances, this response

is so strong that individuals briefly enter water warmer than that in which

they could normally survive. The eccritic temperatures demonstrated by the

species tested (Sarotherodon mossambicus, S. niloticus, S. macrochir and

Tilapia rendalli) varied between 30°C and 36°C. Temperatures of this