The Biology and Culture of Tilapias

ponds, using a 1:l sex ratio and counting all small fry, never exceeded 1,500
frylfemale. The normal average production, using a 5 mm mesh net, is usually
less than 1,060 frylfemale (Table 6). Therefore, we could only expect to
increase production by about 100% from existing facilities, even if all the
problems cited above were solved.

Discussion

With this information we can attempt a new synthesis of the interrelation-

ships of the various factors influencing intraspecific and interspecific spawn-

ing in tilapias and point out the factors that will have to be studied in order

to overcome the problem of low fry production. The various factors fall into

three main categories: genetical, behavioral and environmental.

There is a very big phenotypical individual variability in the fecundity of
the various tilapias. This variability exists within individuals of the same
species as well as between the different species. Therefore, females whether
from the same or from different species, will not nece arily produce the
same amount of eggs even when crosses occur under id S ntical conditions.
This big variability has probably been the cause of some of the discrepancies
between results of previous experiments. It seems that the smaller the
number of spawners in a spawning pond, the bigger the chances will be of
obtaining inconsistent results. If more fecund genotypes can be selected
for cultured strains then higher yields of fry can be expected from both
intraspecific and interspecific crosses.

The different behavioral patterns that exist between the various species

cannot be altered, although techniques like the removal of the male pre-

maxilla may reduce behavioral incompatibility in some cases. Other solutions

to this problem may be altering the conventional parental sex ratios, espe-

cially by using a higher male to female ratio, or using species with a closer

behavioral pattern.

In normal environmental conditions female tilapias of any species will

always spawn according to their natural individual spawning capacity,

but the number of fertilized eggs and the production of fry seem to be

mainly determined by behavioral factors. The improvement of environ-

mental factors will most probably have a beneficial effect on fry production

from any kind of cross.