Effect of rearing water temperature on protandrous sex inversion in cultured Asian Seabass (Lates calcarifer).

Asian Seabass, Lates calcarifer (Bloch, 1790), is a protandrous species cultured for Aquaculture. The cultured Asian Seabass in Australia exhibits precocious sex inversion before 2years of age. This phenomenon highly affects on maintaining a proper broodstock in a hatchery. The effect of temperature on sex inversion inducement in Asian Seabass was thus investigated at five different temperature regimes experienced in Australia. Asian Seabass (14months) grown in fresh water under natural temperature in a commercial farm in Queensland were transported to the research facility at James Cook University, Australia and held in fresh water at 28°C until acclimatized to the experimental conditions. Fish were acclimated to the experimental conditions (30ppt salinity) over the first and final week (22°C, 25°C, 28°C, 31°C and 34°C) of one month acclimatizing period. Fish were fed daily with a commercial pellet (50% protein, 18MJkg(-1)) to satiety. Blood, brain and gonad collected before transfer to the experimental temperature regime in the final week of acclimatization and at the end of the experiment were analysed. Plasma sex steroids level and aromatase activity of brain and gonad were also measured. There was an increase in plasma estradiol levels with increasing temperature from 25°C while no significant difference was observed among all treatment temperatures except at 25°C. However, fish held at 22°C showed higher estradiol level than at 25°C and 28°C. Significantly higher (p<0.05) plasma testosterone levels were detected in fish held at 31°C and 34°C while a reducing trend was observed towards lower temperature regimes. Fish held at 22°C had significantly lower plasma testosterone than all others as well those sampled at the beginning. The plasma 11-ketoTestosterone was at non-detectable levels in all experimental temperatures as shown at the beginning. The average aromatase activity in brain was highest at 28°C among all temperatures, but no significant differences (p>0.05) observed. The Average aromatase activity in gonad was highest at 31°C followed by at 34°C and 28°C. No or very low level of gonad aromatase activity recorded in fish sacrificed prior to treatment. The aromatase activity was greater in brain than in gonad suggesting that the aromatase produced in the brain yet to transfer to the gonad or brain is the first place to response for culture environmental temperature. It is concluded that plasma sex steroids levels and aromatase activity in Asian Seabass have positive response to increasing temperature in culture facilities.