The Biology and Culture of Tilapias

some east African countries. It seems that there is considerable variation in
the performance of cultured T. zillii since this species can reach a good
marketable size in certain east African countries, while in other countries,
such as Israel, it is considered a pest due to its small size at maturation. The
major production of tilapias in ponds is derived from the three Sarotherodon
species and most of the references in this paper will deal with these.
Tilapias, though valuable pond fish, pose a special problem when grown in
ponds, whether in polyculture or in monoculture. Their early maturation
and prolific "wild" spawning produce such large numbers of small fry as to
cause stunting of the entire tilapia population and often of other species
present in the pond. In some regions it is customary to harvest tilapias a
number of times during the growing season. Each harvest removes the larger
fish, leaving the smaller ones. When this practice is employed with fish that
do not spawn freely in ponds, it usually results in a larger cumulative yield
by providing more food for the remaining fish (Van der Lingen 1959a). With
tilapia, however, this practice usually lengthens the culture period and results
in heavy "wild" spawning. The longer the culture period, the more fry are
produced and the population becomes stunted. The number of market-sized
fish and the yield at each harvest becomes progressively smaller. In commer-
cial farms, which have to supply relatively large quantities of market-sized fish
regularly, such sequential harvesting is not practical and reduces profitability.
Silliman (1975) points to another disadvantage of this practice in that
the remaining fish which spawn in the pond and from which fry are usually
taken for further rearing, are actually selected for slower growth, and this
trait may be inherited by their offspring.
There is no doubt that early and prolific breeding under tropical pond
conditions is perhaps the greatest disadvantage in the use of indigenous
cichlid species for aquaculture in many parts of the world (Okorie 1975;
Pillay 1979). Kis pbvious that Huet (1970) was correct in recommending
the culture of tilapia during a definite period, with complete draining of the
pond between cycles to control populations. Even then, special measures
must be taken to overcome "wild" spawning. The two most common ways
to overcome this problem are: culturing a mixed sex population and harvest-
ing before they attain sexual maturity or culturing an all-male population.
Two main factors should be taken into account when deciding which method
to employ: the size preference of the market and the species used.
If the market accepts small fish (up to 150 to 300 g) and if these can be
obtained before they reach sexual maturity, the first method is usually
preferred since it saves effort, fingerlings and space. If, however, the market
accepts only large fish which reach maturity before marketing, it will be
better to use only male fry and thus avoid "wild" spawning and stunted
populations.
In some cases the market accepts both sizes but prices are higher for the
larger fish. Careful economic calculations should then be made, weighing the
higher income for producing larger fish against the extra costs of obtaining
male fry and wastage of females. The calculations should also take into
account the higher yield expected from an all-male pond as compared to a
mixed sex one, since in most tilapia species the males grow faster than the
females (Fry'er and Iles 1972).