Abstract:

Tilapias, (Oreochromis spp) have become increasingly important as cultured fish
species worldwide. However, the problem of overpopulation in ponds, caused by prolific
breeding, is still a major constraint in the development of tilapia aquaculture in developing
countries. The potential for improved yields through the use of controlled reproduction has
provided an impetus for studies in monosex tilapia production. The current strategy is to
emphasize production of male-only groups because they have faster growth than females.
Production of monosex tilapia has been achieved through the use of androgens to reverse
the sex, hybridization between specific tilapia species, chromosomal manipulation and,
more recently, through the heat treatment of embryos and larvae.
Experiments on breeding and nutrition were conducted in a recirculating, closed
system. Water was pumped through a UV unit, a biofilter and a reservoir where it was
heated before flowing into 32 L glass aquaria. The water temperature at 25±1.0°C was
regulated by an automatic thermostat control.
Fish diet is the largest single expenditure in fish culture operations. Since the world
fish meal production is declining, alternative sources of proteins need to be identified. We
evaluated performance of tilapia on sources of protein which are alternative to high-cost
fish meal. Evaluations were performed on growth rate, feed utilization, digestibility,
survival and general condition of the fish fed graded levels of animal- and plant-based
products.
To evaluate the success of heat treatment in producing sex reversal, one group of

tilapia embryos was kept at 36±1.0°C and control group was maintained al 23±1°C. Both groups were fed diets which had the fish meal component gradually replaced by animal byproducts
meal for 5 months. The heat treatment produced 74% males in comparison to the
control group which had 51% males. With respect to growth, all the heat treated tilapia
performed better than the control during the first two months even at 0% fish meal content
in their diets. Males of each group grew faster than females. Chemical body composition
of the two groups did not differ significantly. We concluded that heat treatment produced
predominantly male offspring and enhanced utilization of animal by-product meal-based
diet.
A set of experiments was performed to evaluate the use of cottonseed meal as a
replacement for fish meal. Good growth performance was obtained by feeding tilapia up to
50% cottonseed meal inclusion. At higher percentages the fish became stressed, had
retarded growth, demonstrated mineral deficiency and showed 11% mortality. These
problems were associated with the presence of gossypol, a toxic compound which
complexed with some of the minerals and accumulated in fish tissues. Reproductive
parameters were not affected by the level of gossypol found in the formulated diets,
although final conclusion can only be made after carrying out histological studies.
Progeny test results performed on the heat-treated males showed that 15% of the
males could have been sexually reversed from genotypic females to phenotypic males.
However, none of those males gave 100 % female progeny, suggesting that the sex in this
strain of tilapia is not determined by simple XY factors. The sex determining system could
have been compromised if the tilapia strain used in tire present study was a hybrid species.
Although molecular markers suggested that it is closely related to Nile tilapia (Oreochroniis

niloticus), it is possible that in the past the Piketon tilapia strain was back-crossed to
species with a homogametic male system, such as Oreochromis aureus.