Early sex change of barramundi Lates calcarifer farmed in Singapore

Barramundi, Lates calcarifer, are protandrous hermaphrodites where individuals first sexually mature as male at 3-5 years of age, before reorganizing gonadal tissue to change to female several years later when they reach around 70 cm in total length (-4-5 kg). However, recent anecdotal evidence suggests that in populations farmed in Singapore, this sex change may be occurring much earlier than previously recorded, thus posing challenges for hatcheries to select and maintain male fish for breeding purposes. This study examined the gonadal sex and maturation status of barramundi from a commercial sea-cage farm located in the southern waters of Singapore and confirmed this precocious sex change phenomenon. Barramundi (n = 173) harvested between 2 and 3 years old were individually measured (mean +/- SE); total length (70.3 +/- 7.3 cm), weighed (4.7 +/- 1.4 kg), and had their gonads dissected for sex identification through histological analyses. Histology showed that only 19 % were still male, 8% were in transitional stages of sex change and 73 % of fish were female. In this precocious farmed population, females as small as 2 kg were found, with the proportion of males in the population decreasing to less than 50 % from 2.5 to 3 kg (55-60 cm). Above 3 kg and 5 kg, 70 % and 90 % of sampled individuals were females, respectively. Comparative farm studies from French Polynesia and in wild Australian stocks indicate that the farmed population in our study is changing sex at body weights approximately half that of what was previously known. The findings of this study are important for local breeders to make decisions of when, and at what size to select candidate L. calcarifer broodstock in Singaporean waters so that the recommended 2:1 male-to-female spawning ratio can be achieved. The exact causes for this early sex change are still unknown and deserve further investigation.