The sex ratio of individual spawns has been studied in only a limited number of strains of Oreochromis niloticus. Although O. niloticus females are thought to be homogametic and males heterogametic, progeny of single pair spawns have not conformed to the expected 50:50 sex ratio inheritance. The variance in O. niloticus sex ratio inheritance may be related to strain differences. Pair spawning and grow-out of the Turkana strain of Nile tilapia (O. niloticus vulcani) is being conducted at the Sagana Fish Culture Farm. This strain originated from a stock isolated in a crater lake on an island in Lake Turkana, Northern Kenya.
In the current study, the sex ratio of each set of progeny with a minimum of 100 fish will be determined though gonadal examination. Spawning hapas were constructed and stocked between three and five times between February and June, 1997. When water temperatures were less than 24°C, females were often killed by males. Bird predation on brood fish also contributed to losses. Of the five spawns obtained, survival of fry was very low, due to predation from insects, and no spawns resulted in fry numbers greater than 30. More spawning hapas will be constructed and protection against predators will be enhanced to obtain improved spawning results. Sex ratio data from each spawn will be analyzed by Chi square to determine whether it differs from the expected 50:50 inheritance. This research is carried out in conjunction with the study "Monosex Tilapia Production Through Androgenesis." For further description of the study see the Global Research section of this report.