In vivo effect of long-acting estradiol implants and recombinant FSH on sex-change in the protandrous barramundi (Lates calcarifer)

As global demand for sustainable and high-quality aquatic protein continues to grow, aquaculture operations are broadening their focus to include species with commercial potential. One such species, the Australian Barramundi (Lates calcarifer), has attracted interest due to its rapid growth and adaptability. However, its reproductive biology presents specific challenges for selective breeding programs. Barramundi are protandrous hermaphrodites, initially maturing as males and undergoing a sex reversal to become females over the course of several years. This delayed transition hampers the establishment of intergenerational breeding cycles and slows genetic improvement efforts. Previously we have shown that hormonal manipulation using sustained-release estradiol implants has been shown to induce earlier feminization, offering a strategy to shorten the time required to produce functional females. In the present study, we investigated whether administration of recombinant follicle-stimulating hormone (rFSH), either alone or in combination with E2 implants, could enhance gonadal development and potentially accelerate sex transition in undifferentiated Barramundi. Fourteen-month-old juveniles were assigned to five treatment groups: E2 implants alone, rFSH injections at two doses, combined E2 + rFSH at two doses, and a control group. Hormonal profiles were assessed by measuring plasma 17β-estradiol (E2) and 11-ketotestosterone (11-KT) levels, and gonadal differentiation was evaluated through histological examination. Our findings confirm that E2 implantation led to elevated estradiol concentrations and reduced 11-KT levels, with histological evidence of ovarian tissue development in treated fish. Notably, combined treatments with E2 and rFSH produced similar feminizing outcomes. In contrast, rFSH alone promoted advanced testicular development but did not initiate full sex reversal. These results reinforce the reliability of E2 implants in accelerating feminization in Barramundi and highlight their value in reducing generation time for breeding programs. Although rFSH did not independently induce sex change, its role in supporting gonadal maturation suggests potential utility as an adjunct in hormone-based reproductive manipulation protocols.