Growth performance of male monosex and mixed sex Nile tilapia (Oreochromis niloticus L.) reared in cages, Lake Victoria, Kenya

Growth performance of 17 alpha-methyl testosterone treated monosex and hormone untreated mixed sex tilapia was done for a period of 180 days in cages adjacent to Lwanda Disi beach, Lake Victoria, Kenya. Mixed sex Nile tilapia is known for frequent spawning in culture systems, resulting to fish with varied sizes and stunted growth. To attain male monosex fingerlings, the newly hatched fry were sex reversed by feeding them a diet laced with 17-alpha-methyl testosterone hormone for 21 days. Thereafter, the fry were fed 40% crude protein (CP) diet until stocking into cages. Mixed sex fry of same cohort were fed 40% crude protein starter diet until the time of stocking. A total of 6 cages (2×2×2 m) were randomly stocked with 1000 male monosex and mixed sex fingerlings. The individual mean (± SD) weight at stocking was 9.5 ± 0.06 g and 8.8 ± 0.08 for monosex and mixed sex, respectively. The experimental fish were fed starter diet 40% CP at 10% of body weight for the first two months. From the third month till the end of the experiment, the fish were fed 30 CP diet at 3% of body weight. Measurement of fish total length, body weight and selected water quality parameters was done once a month. There was a significantly higher (P < 0.05) mean body weight gain (BWG), specific growth rate (SGR) and condition factor (CF) for male monosex fish. Conversely, Feed conversion ratio (FCR) was significantly higher (P < 0.05) in mixed sex Nile tilapia. Survival rates (%) and water quality parameters did not differ between the groups. Though the mixed sex Nile tilapia did not reproduce in cages they performed poorly compared to male monosex.