Effect of water temperature on survival, growth and phenotypic sex of mixed (XX-XY) progenies of Nile tilapia Oreochromis niloticus

The ambient temperature during early ontogeny affects the phenotypic sex of various fish species, including the Nile tilapia. Forty-seven groups of Nile tilapia (220 to 350 fish each) from 11 progenies were exposed to different rearing temperatures (20.4-39.0 degrees C) during the first 28 days of exogenous feeding (50-l aquariums, 12L:12D, feeding ad libitum) to model the relationship between ambient temperature and (a) the reversal of phenotypic sex in this species, (b) fish growth and survival rates. The efficacy of the masculinising treatment was estimated with respect to the survival rates in the treatment and control groups (i.e., T degrees of circa 27 degrees C), and to their respective sex ratios, in order to take into account the possibility that mortality affected mainly females (minimum efficacy, E(mi)) or males (maximum efficacy, E(Mi)), depending on ambient temperature, and parental influence. Ranges of masculinising efficacy were given by log-logistic equations ln(E(mi)(1-E(mi))-1) = -27.495+0.757T degrees (R2 = 0.548) and ln(E(Mi) (1-E(Mi))-1) = -36.707+1.050T degrees (R2 = 0.709). The corresponding temperatures giving a masculinising efficacy of 90% were 37.8 degrees C and 39.2 degrees C, for E(Mi) and E(mi), respectively. As a corollary, high percentage male progenies could only be obtained at very high temperatures, close to the upper incipient lethal temperature (UILT) of juvenile tilapia (38.5-39.0 degrees C). During the treatment period, the survival and growth of fish reared at circa 37 degrees C averaged 41.9% and 74.5% of those of fish reared at 27-33 degrees C, resulting in a production loss of over 70%, which was not compensated after the temperature was returned to 27 degrees C. These results suggest that the advantage of producing faster growing males of Nile tilapia at high temperature would hardly compensate the loss of production incurred during the masculinising treatment, unless after the selection of the most thermosensitive strains or breeders.