Abstract:The Nile tilapia Oreochromis niloticus is a species of great commercial interest in the world. Although various approaches have been introduced to improve productivity,difference in growth between male and female fish is still the major bottleneck for tilapia cultivation. Therefore,high-frequency or all male stocks are preferred to enhance overall productivity of Nile tilapia. In order to investigate the effect of high-temperature treatment on the sex differentiation in Nile tilapia and screen high and low frequency families of males, 20 families were established for Nile tilapia “Ji-Fu” strain. During the reproductive season of Nile tilapia, the 2~3 year-old Nile tilapia females and males were cultivated in 10 m3 pond. The female mouth was checked to see whether there were fertilized eggs or not every 5~7 days. If there were fertilized eggs, the eggs were taken out, cultivated in a single tank, and named as a family. Fry were counted at 9 d post-fertilization and the every batch of offspring was divided into two groups: control group and high-temperature group. The control group was maintained at a constant 28°C water temperature, and the high-temperature group at 36°C. Treatment durations were 12 d, and at the end of the treatment, the number of fish in each group was recounted.Treated fry were gradually adapted to 28°C, and then all the fry was transferred into tanks in natural water temperature. Progenies were subjected to sexing at ages of 2 to 4 months. Sexing was conducted by observing the morphology of gonadal cell under microscope. The mortality of fry in control group and high-temperature group was not significantly different. Among 20 families, the frequency of males in high-temperature group was significantly higher than that in the control group in 3 families,including F27(88% in treated group; 49% in control group), F37(90% in treated group; 53% in control group), and F41(91% in treated group; 53% in control group). The high-temperature treatment did not significantly increase the male frequency of most families. However, the frequency of males in high-temperature group was significantly lower than that in the control group in 2 families: F8 (43% in treated group; 46% in control group) and (29%in treated group; 38% in control group). These results prove the possibility to modify the sexual ratio of Nile tilapia and provide new methodology to breed new strain with high frequency of males.
