Effects of water temperature on growth and sex ratio of juvenile Nile tilapia Oreochromis niloticus (Linnaeus) reared in geothermal waters in southern Tunisia

Growth of juvenile Nile tilapia, Oreochromis niloticus (Maryut strain) was studied under laboratory conditions. Four thermal regimes (22, 26, 30, and 34 °C) were tested on 480 20-day-old fry.Significant (P<0.05) effects of temperature on growth were observed. Results showed that the final mean weight was significantly higher at 26 and 30 °C than at 22 and 34 °C. Feed conversion ratio (FCR) and daily weight gain (DWG) were better at 26 and 30 °C. At all temperatures, survival rates were not affected. These results suggest that the best growth and feed utilization of O. niloticus juveniles may be higher at 26 and 30 °C.A second experiment was performed to assess the effect of ambient water temperature during the period of sex differentiation on the sex ratio. Results showed that the high-temperature (almost 36.90 °C) treatments yielded a significantly higher proportion of males (64.20–80%) with lower survival rates (60–81%), whereas the sex ratio of progenies reared at temperature below 36 °C never deviated significantly from the balanced sex ratio.This study provided clear evidence that the exposure of progenies to “masculinizing” temperature may significantly decrease the survival rate of fish.