Effect of temperature and salinity on sex inversion in Asian Seabass (Lates calcarifer): relationship with plasma sex steroids concentration and aromatase activity of gonad and brain Saman Athauda 1,2 & Trevor Anderson 1 1 School of Marine Biology & Aquaculture, James Cook University, Townsville, QLD, 4811, Australia 2 Department of Animal Science, Faculty of Agriculture, University of Peradeniya, Peradeniya, Sri Lanka Correspondence: S Athauda, Department of Animal Science, Faculty of Agriculture, University of Peradeniya, Peradeniya, 20400, Sri Lanka. E-mail: sbathauda@pdn.ac.lk Abstract Asian Seabass which shows precocious sex inversion in cultured environment is a major impediment to maintain a proper broodstocks. Seabass grown com- mercially in freshwater were transported to the research facility and held in freshwater at 29°C for 4 weeks and acclimatized to the experimental condi- tions. Fish were daily fed with a commercial pellet (50% protein, 18 MJ kg À1 ) to satiety. Blood, brain and gonad collected before and at the end of the experiment were analysed for sex steroids level and aromatase activity. There was an increase in Plasma E 2 levels with temperature in fish at 34°C whereas no significant difference was observed at 24 and 29°C although the highest plasma T level was detected in fish at 34°C which had significantly lower level than at the beginning, except those held at 24°C in fresh- water. Plasma 11-KT was significantly greater in fish at 24°C compared with 29 or 34°C which had clear opposite to the E 2 . Aromatase activity in brain was higher at 29°C than at either 24 or 34°C, whereas gonadal aromatase recorded highest at 34°C. These findings concluded that high temperature in culture facility induces sex inversion of Asian Seabass. Keywords: Asian Seabass, Lates calcarifer, tem- perature, salinity, sex steroids, aromatase activity, sex inversion Introduction Asian Seabass, Lates calcarifer (Bloch, 1790), also known as barramundi, is an important species for both aquaculture and fisheries. This protandrous hermaphroditic species (Moore 1979; Davis 1985) undergoes sex inversion from male to female. In the wild, the male fish reach first sexual maturity at the age of 3–4 years (Milton & Salini 2008), whereas sex change appears between 6- and 8-year-old (Guiguen, Cauty, Fostier, Fuchs & Jalabert 1994; Allan & Stickney 2000). Obtaining seeds for culture operations from wild broodstock is a practice, which is expensive, highly seasonal, unreliable and conflicts with wild stock resources management (Barlow, Williams & Rimmer 1996) for sustainable aquaculture operations. This leads to development of commercial hatcheries that retain captive broodstock for seed production since the first large scale Asian Seabass farm was estab- lished in 1986 (Schipp 1996). However, the hatchery seed production is limited by the inability of culturists to maintain functional males in the breeding population due to their pre- cocious (fish that mature and change sex at a smaller size than wild fish) sex inversion to female (Schipp 1996; Pankhurst 1998; Allan & Stickney 2000). This phenomenon is often leading to a shortage of suitable male broodstock for hatchery seed production and is a major impediment to the development of a genetic improvement program (Pankhurst 1998; Allan & Stickney 2000) for com- mercial traits of interest such as meat quality, growth rate, feed conversion efficiency in the hatchery Seabass progeny. Water temperature and salinity appear to be the controlling factors (Davis 1985) of reproduction of Asian Seabass compared with European Seabass in © 2012 Blackwell Publishing Ltd 1 Aquaculture Research, 2012, 1–11 doi: 10.1111/are.12018