Natural reproductive modulators in aquaculture: endocrine mechanisms, microalgal and plant-derived bioactives, and sustainability perspectives

Synthetic hormonal induction is an established tool in modern aquaculture for overcoming reproductive dysfunction, synchronizing spawning, inducing final gamete maturation, and supporting reliable seed production. Growing interest in complementary reproductive strategies has stimulated research on natural bioactive compounds that may support reproduction through endocrine, nutritional, metabolic, and antioxidant pathways. This review critically evaluates microalgal and plant-derived reproductive bioactives in relation to conventional hormonal induction, with particular emphasis on the strength and directness of mechanistic evidence. Microalgal compounds evaluated include long-chain polyunsaturated fatty acids (LC-PUFAs), carotenoids, sterols, and antioxidant polysaccharides, whereas plant-derived compounds include phytoestrogens, flavonoids, saponins, alkaloids, and terpenoids. Available studies associate selected interventions with measurable reproductive endpoints, including gonadal development, fecundity, gamete quality, fertilization, hatchability, spawning performance, and offspring survival. As one study-specific example, pituitary extract treatment in African catfish produced 89.23% fertilization, 93.31% hatchability, 87.16% survival, and 69.41% fry production; however, such values cannot be generalized across interventions because reproductive studies differ substantially in species, compounds, doses, treatment duration, reproductive stage, and outcome definitions. Microalgal LC-PUFAs and carotenoids are supported primarily by evidence of nutritional, membrane-associated, metabolic, and antioxidant reproductive support, whereas direct endocrine effects remain less consistently demonstrated. Plant-derived bioactives show greater mechanistic heterogeneity, with responses frequently dependent on compound identity, dose, species, sex, and reproductive stage. Current evidence therefore supports natural bioactives mainly as complementary tools for broodstock conditioning, gamete support, and reproductive management rather than universal substitutes for conventional hormonal induction. Major limitations include heterogeneous protocols, inadequate dose standardization, species-specific responses, limited direct endocrine validation, insufficient long-term safety assessment, commercialization constraints, and regulatory uncertainty. Future research should prioritize standardized dose-response experiments, direct hormonal and molecular measurements, chemically characterized preparations, long-term and multigenerational safety assessment, and commercially relevant hatchery trials. Overall, natural reproductive bioactives have considerable potential within integrated reproductive-management strategies, but their practical application should be guided by mechanistic evidence, species-specific validation, and clearly defined reproductive objectives.