Absence of males is associated with delayed progression of vitellogenesis in yellowtail Seriola quinqueradiata

Abstract Stable hatchery production of yellowtail requires effective brood stock management, particularly control of vitellogenic progression. Although the absence of males impairs oocyte maturation and ovulation in several fish species, its influence on yellowtail reproduction remains unclear. This study compared female-only and mixed-sex groups over two reproductive seasons, referred to as year 1 and year 2, respectively. Oocyte diameter, ovarian histology, and gonadosomatic index were evaluated before human chorionic gonadotropin (HCG) administration, and ovulation was assessed. In both years, maximum oocyte diameter before HCG administration was significantly lower in the female-only group than in the mixed-sex group. The female-only group exhibited individual variation, with some females progressing through vitellogenesis, whereas others showed arrested vitellogenic development. The proportion of ovulated HCG-treated females was significantly lower in the female-only group during year 1 and showed a similar numerical trend in year 2. However, some females with oocyte diameters > 600 μm ovulated under female-only conditions. These results indicate that the absence of males is associated primarily with delayed vitellogenic progression, whereas ovulation can be induced in females that complete vitellogenesis. Therefore, the absence of males should be considered a potential risk factor in brood stock management and may provide a foundation for future maturation control strategies.