Maturation of captive male giant grouper Epinephelus lanceolatus in subtropical waters in Japan

This study aimed to determine the maturation process of captive male giant grouper Epinephelus lanceolatus (GG) in the subtropical waters of the Amami Islands, Japan. The experiment commenced in February 2019 using 134 GG (3 years and 5 months old; mean body weight [BW] = 6.0 kg) raised from hatchery-reared juveniles. Of those, 31 were administered 17α-methyltestosterone (MT) at a concentration of 2 mg/kg BW using cholesterol pellets, and the others were not. We assessed the spermiation response (SpR) of all individuals approximately once a month for over one year, from April 2019 to May 2020. Sperm motility (%SM) was also assessed when sufficient semen samples were obtained. The SpR was observed in 22.3% (23/103) untreated and 93.5% (29/31) of MT-treated GG. Some individuals from both groups showed consistent SpR throughout the year. The mean %SM for MT-untreated and treated GG were 49 and 56%, respectively, with no significant difference between the groups. Histological analysis of the gonads of the untreated GG was conducted on three fish harvested in July 2019. While two transitioned from female to male and contained immature oocytes, another possessed fully functional testis; this suggests that sex reversal in GG involves a direct transition from immature female to male. Our study demonstrated that hatchery-reared male GG mature year-round and could maintain motile spermatozoa of consistent quality in the subtropical waters of the Amami Islands. This result is critical for improving any hybridisation using GG males. This study is the first to report on the reproductive ecology of male GG in subtropical waters.