The Asian seabass, Lates calcarifer, is a key food fish species that has been farmed in Singapore for the past 30 years and is now receiving global attention. As production of the species intensifies, so does the need to secure steady and sustainable seedstock. As such, it is critical to understand the reproductive biology of the species. The Asian seabass is a sequential hermaphrodite whereby it first sexually develops as male, and then subsequently sex changes to female (testis to ovaries) a few years later. In Singapore, there has been anecdotal evidence that male-female sex change may be occurring much earlier than what has been recorded in Australia and Southeast Asia. This study looks into establishing a baseline of maturity rates and sex change of Asian seabass farmed in Singapore.
Fish cultured in sea cages of a commercial farm located in the Southern waters of Singapore and aged approximately 2.4 years old were individually measured (70.3 ± 7.3 cm), weighed (4.7 ± 1.4 kg) and had their gonads (n = 176) dissected for histological analyses. Results indicated that 73% were female, 19% were male and 8% were in transitional stages (both sperm and oocytes observed in gonads). In this population, sex change seemed to start after 2 kg, with the proportion of males reduced by half between 2.5 and 3 kg (55~60cm). Above 3 kg, and 5 kg, females represented 70% and 90%, respectively (Figure 1). Comparative farm studies from French Polynesia and in wild Australian stocks indicate that the farmed population in our study is sex changing into female at body weights approximately half that of what was previously known. The findings of this study are important for breeders to tailor decisions of when and at what size to select candidate L. calcarifer broodstock in Singaporean waters so the recommended 2:1 male-to-female spawning ratio can be successfully achieved. The exact causes for early sex change in Singaporean L. calcarifer are still unknown and deserve further investigation.