The Biology and Culture of Tilapias

showed stunting and poor growth. These fish were generally located close to
the 'eye' of the spring (35" C) at night (response to thermophilia) but during
the day fed in the cooler (28°C) periphery-the only source of food. This
movement is a complete reversal of the diel migrations previously discussed
and it may be expected that as a consequence growth efficiency would be
extremely low. Notwithstanding the possibility of poor food quality or
abundance or possible problems associated with water quality in this envi-
ronment, all indications are that the thermal peculiarities of the system are
themselves responsible to some degree for the poor growth. These fish do
not show any genetic malfunction since on transference to a more 'normal'
habitat, growth resumes a norrrral pattern (Fryer and Iles 1972).
Similarly, an early record by van Someren and Whitehead (1959b), who
reported that the growth rates of S. spilurus niger (T. nip) in ponds was
better in shallow, 30-cm deep ponds than in 60-cm deep ponds, may indicate
another example of growth enhancement due to thermal oscillation. Ponds
of 30-cm deep would be expected to exhibit a larger diel variation in tem-
perature and tilapia that are restricted to such an environment would be
expected to show growth superiority or, at least a better growth efficiency,
if food was not limiting. Thus it may not be surprising that the shallow
ponds (caused by sedimentation) from which van Someren and Whitehead
made the observations showed a good production. My own observations on
pond trials tend to confirm this observation but the elimination of the
numerous variables in trials of this type is extremely difficult.
A number of documented examples from various African lakes show that
tilapias, especially juveniles, have a preference for shallow inshore waters.
Recent examples of this distribution preferenda include the tilapias present
in Lake Sibaya (Bruton and Boltt 1975), Lake Kariba (Donnelly 1969;
Caulton 197713) and Lake George (Burgis et al. 1973; Moriarty et al. 1973;
Gwahaba 1975). In the two former examples food distribution and/or
predator pressure may well influence the inshore distribution of young
tilapias but in the Lake George example food is more abundant in the deeper
areas of the lake yet tilapias are far more abundant in the inshore areas
(Gwahaba 1975).
There are obviously a variety of factors contributing to the distribution of
tilapia in any particular water body, but indications are that a diel thermal
fluctuation may be an important factor to consider. With the exception of
the deep rift valley lakes, natural inland waters in Africa are generally
composed of shallow river floodplains, seasonal pans, lagoons and marshes
as well as large areas of permanent swamp. These areas are sites of tilapia
radiation and, prior to the advent of man-made impoundments, constituted
the major tilapia habitat. All these areas are subjected to daily temperature
fluctuations often in excess of 10°C and thus it stands to reason that the
tilapias would have evolved various physiological functions suited to optimiz-
ing such conditions.
In concluding this presentation, it is clear that more applied physiologicd
work is urgently required in order to determine the functioning of the
various species of tilapia in their natural environment and only after deter-
mining the optimum levels of physiological efficiency can we begin to
optimize on productivity both in natural water fisheries and in pond culture
conditions.