As a preliminary investigation into the effects of environmental factors on gonadal development using adaptive physiological characteristics, we examined the involvement of photoperiod and water temperature in the ovarian development of the red‐spotted grouper, Epinephelus akaara. During October 2014 (non‐breeding season), the photoperiod and water temperature were adjusted to 12L:12D and 18°C respectively. Every 3 weeks, daylight was increased as follows a 13L:11D and 14L:10D in the photo‐thermal treatment group, and the control group was maintained under natural conditions. After 9 weeks, water temperature was increased to 23°C in the photo‐thermal treatment group. In photo‐thermal treatment group, GSI increased significantly after 9 weeks (1.36 ± 0.19) and 12 weeks (3.31 ± 0.20) compared with the control group. At 12 weeks after photo‐thermal treatment, all fishes matured into ovaries containing late vitellogenic oocytes. The expression of FSHβ and LHβ mRNAs also increased significantly. However, in the control group, the ovarian development was maintained at an immature stage. Based on these results, we demonstrate that exposure to long photoperiod and high water temperature can induce the sexual maturation of immature red‐spotted grouper during the non‐breeding season.

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This study aimed to investigate the correlation between melatonin and the reproductive mechanism of farmed female red‐spotted groupers, Epinephelus akaara. Through the reproductive cycle experiment, the sexual maturity period was determined to be from June to July, following which the spawning season occurred in August. In addition, kiss1, kiss2, and GPR54 mRNA expression levels peaked in January, whereas that of sbGnRH mRNA remained at high levels from January to May and decreased during the sexual maturity and spawning seasons. During the day–night cycle, a nocturnal rhythm of AANAT2 mRNA expression was observed in the brain, and AANAT2 mRNA expression showed a seasonal pattern with increased expression during summer compared with that in winter. A correlation between melatonin and reproduction‐related genes could not be observed via melatonin administration, and sexual maturity was observed to be more induced under long photoperiod conditions compared with short photoperiod conditions. These results indicate that the reproductive mechanism of the farmed female red‐spotted grouper is related to changes in photoperiod and melatonin, and further research is needed to clarify the reproductive mechanism. Our findings provide basic information for identifying the reproductive mechanisms of farmed E. akaara.