The Biology and Culture of Tilapias

Summary and Conclusions

The characteristic diet of adult tilapias is a mixture of algae, detritus,
bacteria, and, in some cases, macrophytes. Tilapias possess morphological
and physiological adaptations for utilization of this diet. Pharyngeal teeth
break food components into smaller units for easier peristaltic mixing and
increased exposure to digestive enzymes. Gastric acid secreted to an unusually
low pH, frequently below pH 1.5, lyses prokaryotic cell walls and denatures
prokaryotic and eukaryotic cell membranes to expose the cytoplasm therein
to intestinal enzymes. The extreme length of the intestine is essential for
efficient digestion and assimilation of some components of the diet.
Food quality, quantified in terms of mg assimilable protein / kJ assimilable
food energy, is potentially a limiting factor in the growth of tilapias. Studies
of the detritivorous S. mossambicus show that the quality of available food
resources varies widely, and that the fish utilize selectively the resource
that produces maximum growth. Future studies of tilapia that feed on
macrophytes and suspended matter are also likely to reveal selective feeding
for growth maximization.
These trophic abilities of tilapia make them ideally suited for aquaculture.
Since they feed at the base of the food chain, they are energetically very
efficient and ecosystem carrying capacity should be high for these organisms.
Their uncommon ability to exploit prokaryotic algae and bacteria as sources
of protein makes them unusually protein efficient since they do not rely on
invertebrate intermediates. In addition, the ability to utilize prokaryotic
protein may open new doors to cost-efficient aquaculture. A principal
economic obstacle in culture of fish is the cost of protein in culture diets
(Weatherley and Cogger 1977). It is likely that bacteria and blue-green algae
can be grown either directly in culture ponds or in adjacent facilities using
locally available agricultural waste products and low-technology manip-
ulations (Schroeder 1978). The proteinlenergy criterion could be used to
optimize the quality of the food produced. Although systematic evaluation
of this approach has not yet been attempted, what we have learned about
the feeding of tilapias in natural systems suggests that it is a very promising
possibility for the future.

Acknowledgments

I am grateful to Dr. D.J.W. Moriarty for helpful discussion of the manu-

script. Plates 1 and 2 were produced by the Electron Microscopy Unit,

Rhodes University, Grahamstown, South Africa.

Discussion

LOVSHIN: Dr. Bowen, I have one questitn, just a yes or no will do. Did you study the

density of fish per unit area or the predator populations within the two lakes? Was there

some sort of control on the effects of tilapia density between the two lakes, because I

think your results could be affected tremendously by differences in density.

BOWEN: In Lake Sibaya the densities of all fish were very low and there were a lot of

predators for tilapia. In Lake Valencia the tilapias were very dense, but still in very good

condition. (See Caulton, this volume, for further discussion-Editors.)