Effect of rearing temperatures on the sex ratios of Oreochromis niloticus populations

A comprehensive experiment was carried out to study the effect of different temperature regimes (18 °C for 20 days, 36 °C for 10 days or 38 °C for 10 days starting on day 10 post fertilization) on sex ratios in two different Nile tilapia populations (Lake Manzala-Egypt, Lake Rudolph-Kenya) and their crosses, as well as to study the repeatability of the obtained results and the effects of mating partners, including temperature-treated males, on temperature responsiveness. In total about 15,000 temperature-treated fish and their corresponding 10,000 fullsib controls were sexed. Results of this study confirmed results from other studies that high temperature treatments could increase the percentage of males in the offspring of Nile tilapia by converting females into functional males (XX neomales). Compared to the 36 °C treatment, a further increase in males was not obtained with a treatment of 38 °C in the majority of the tested progenies in both populations. The low temperature treatment (18 °C) was, in general, not effective in influencing the sex ratios. Significant differences between populations to high temperature responsiveness were observed with regard to the degree and the range of response. In the Lake Manzala population, 66% of the temperature-treated progenies (36 °C) showed sex ratios with more than 80% males, while in the Lake Rudolph population, no temperature-treated progeny (36 °C) showed such a corresponding surplus of males, neither did the crossbreds. Spawns within the Lake Manzala population were repeated and the percentage of males in the 36 °C-treated replicated spawns differed only by 2% to 6%. Temperature-treated progenies derived from genetic males out of temperature-sensitive progenies reflected the sex ratios of the progenies the sires had been taken from and indicated that the sensitivity to temperature treatment is heritable. Data on paternal and maternal half sibs used as experimental fish, showed that the sensitivity of sex ratios to temperature treatment varied between breeding pairs. Both parents, male and female, contributed to the different sex ratios after treatment. It was concluded that the sensitivity of sex determination to temperature treatments is under genetic control and suggests very promising chances for a selection response.