Selection experiments to increase the male proportion in temperature treated groups of Nile tilapia (Oreochromis niloticus)

Tilapias are one of the most important species in commercial aquaculture world wide. As mixed-sex tilapia stocks are mainly farmed in extensive systems their high fecundity and reproduction rate results in overcrowding of ponds. Therefore all-male stocks are highly desirable. Previous investigations in this species showed a strong influence of temperature on sex determination with an suspected underlying genetic basis. Thus, losses in productivity due to stunting of mixed sex Nile Tilapia stocks could be alleviated by production of temperature sensitive lines to sustainably increase the proportion of males by temperature treatment (36°C/10 days). Starting with 36 families of Oreochromis niloticus from Lake Manzala (Egypt) as base population a two generation selection experiment was conducted to develop high and low temperature sensitive lines. The percentage of males in treatment groups was the selection criteria nd the families showing the largest or lowest surplus in their sex ratios were selected. The results clearly indicated that selection for sex ratio in temperature treated groups is possible. In the second generation of selection a male percentage of 89.7% was obtained in the high line. Downwards selection resulted in a decrease of the male percentage from 65.2%.to 54% in the second generation of selection. No correlated response to selection was observed in the sex ratios of control groups. The results showed that selection for temperature dependent sex ratios is a feasible approach. Thus, it seems to be possible to increase the male proportion in Nile tilapia by means of selection for temperature dependent sex determination.

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