The Biology and Culture of Tilapias

Conclusion

It is evident that three methods can be used for the control of tilapia

reproduction: monosex culture, the use of predators and stocking at high

densities. Each method, however, has disadvantages.

The use of stock manipulation techniques, irradiation, chemosterilants

and reproduction inhibitors have shown promise in limiting tilapia reproduc-

tion on an experimental scale, but further research is needed to determine

the practicality of these methods for commercial use.

Discussion

HENDERSON: Dr. Guerrero, do you have any idea of the comparative cost of sex
reversal by hormone treatment compared with that of manual sexing?

GUERRERO: For manual sexing in the Philippines, we pay twenty centavos (US$0.03)
per fish and a worker can sex 2,000 fish per day. The daily wage is up to US$3/day, on
average about $2.60. Sexable size is taken as 10 g or above. Hormone sex-reversal is of
course labor saving, but you can see that our labor costs are low. Moreover, we have to
use alcohol for dissolving the hormone for food incorporation. For every kilogram of feed,
with hormone we use one liter of alcohol. This is quite expensive unless you can recycle
the alcohol. We consider that it costs about 20 Philippine pesos (about US$3) to treat
about 1 million fry. These costs would be only for feed and labor, ignoring the deprecia-
tion cost of facilities. If we sell these sex-reversed fingerlings, we get about 150 Philippine
pesos (which is about US$20) per thousand so we still make a good profit. Sex-reversal
techniques are fine if you have practical experience, but in general, I would not recom-
mend them in a developing country like ours because of the relatively high capital require-
ments and the need for skilled workers. There are no commercial hatcheries usingsex-
reversal in the Philippines at present.

ROBERTS: We use similar methods to yours but with a 35-day treatment period and 30
ppm dose regime. Going back to the anabolic steroid growth promotion effects, we find
that this anabolic effect is so significant and gives the fry such a head start that it is worth
doing quite irrespective of any sex-reversal objectives. Recently, we had indications that
sex-reversed females grew better than normal females and normal males. We are also
studying the residence time of steroids in our fish after cessation of treatment using
radioactively-labelled compounds. They are generally undetectable after 5 days. This is
important as many of the developing countries are borrowing their food and drug legisla-
tion direct from the USA and we have to show that this short-term fry treatment is very
different from caponisation in the poultry industry which uses long-lasting steroid im-
plants. We have had our best sex-reversal results with S. mossambicus and S. spilurus and
our worst with T. zillii.
It may be of interest that the black coloration of species like S. mossambicus does not
produce sales resistance in Africa-unlike in Southeast Asia.

HEPHER: I wonder whether space is a severe limiting factor on the whole sex reversal
operation. If you talk in terms of raising 2,000 fry/m2 then you will need 15 of such
tanks to serve 1 ha of monoculture pond with a normal stocking ratio of about 30,00O/ha.
Therefore, think how many tanks you would need for a whole farm. Won't this be a
bottleneck?