Continuous darkness is associated with improved growth performance during a high‐temperature period and sex‐specific modulation of early gonadal development in juvenile yellowtail ( <i>Seriola quinqueradiata</i> )

Selective breeding of yellowtail (Seriola quinqueradiata) is expanding in Japan, increasing the demand for practical reproductive control tools in commercial aquaculture to reduce genetic risks from escapees. Slowing early gonadal development through environmental manipulation may support future sterilization strategies, particularly when combined with other techniques. This study aimed to examine somatic growth and early gonadal development in juvenile yellowtail reared under continuous darkness (0L:24D). Juveniles were reared under 0L:24D or a 12L:12D photoperiod (control) and sampled at Weeks 0, 2, 4, 6, 8 and 10. Growth was recorded at each time point, and gonads were examined by histology and immunohistochemistry to assess gonadal sex differentiation, germ-cell proliferation and onset of meiosis. Water temperature increased similarly in both groups and exceeded 28°C from Week 6 onward. Despite identical feeding rations, fish reared under 0L:24D were significantly larger at Weeks 8 and 10 than controls. Gonadal sex differentiation progressed normally in both groups, and germ-cell proliferative activity did not differ between groups. SYCP3-positive germ cells were not detected at Week 6 under 0L:24D, whereas the proportion of SYCP3-positive germ cells at Weeks 8 to 10 was comparable to that in controls, suggesting a possible transient difference in the initial appearance of SYCP3-positive germ cells. In females, fish reared under 0L:24D showed lower area fraction and diameter of chromatin nucleolus- and perinucleolus-stage (CN/PN) oocytes at Weeks 8 and 10, suggesting reduced development of CN/PN-stage oocytes. These results suggest that, under the present single-tank experimental conditions, rearing under continuous darkness was associated with improved growth performance during the high-temperature period and sex-specific changes in early gonadal development in juvenile yellowtail.