The reproductive potential of Pacific bluefin tuna (Thunnus orientalis) farmed in sea cages in South Korea

Abstract Identifying environmental effects on the reproduction cycle and sex hormones is essential for ensuring the sustainable productivity of Pacific bluefin tuna (PBT, Thunnus orientalis) aquaculture. In the present study, 70 PBTs, commercially farmed in sea cages near Yokjido Island, South Korea, were investigated from April 2020 to March 2021. Environmental data, including water temperature and photoperiod duration, were collected. The gonadosomatic index (GSI) was calculated, and male and female maturity stages were classified monthly in proportions based on histological analysis of the gonads. Male and female gonadal developmental stages were classified into four major stages. Mature male and female PBTs were observed from May to August and June to August, respectively. Monthly changes in sex steroid hormone levels in PBT plasma were evaluated. The GSI and sex steroid hormones of males and females increased in May, peaked in June, and decreased thereafter. Our results confirmed the reproductive potential of farmed PBT and verified that reproductive maturity is reached in July. This study, to the best of our knowledge, is the first to investigate the reproductive biology of PBT in South Korea, and our results constitute a reference for future research on PBT aquaculture.