Comparative efficacy of regular and slow-release GnRH in inducing spawning in Asian seabass (Lates calcarifer)

This study aimed to identify the most effective hormone treatment for artificial breeding of Asian sea bass (Lates calcarifer) by comparing two delivery methods of GnRH hormone: regular release (R) versus slow-release (SL). SAIB-PC-based SL-GnRH was developed for sustained delivery. The 20 µg/kg R treatment achieved 100 % spawning success (P < 0.05), with 94.4 % fertilization and 95.5 % hatching rates, while 20 µg/kg SL yielded 89.2 % fertilization and 71.4 % buoyant eggs (P < 0.05), indicating prolonged but less synchronized stimulation. Hormonal profiling revealed elevated testosterone and 17β-estradiol (particularly in SL groups) (P < 0.05), with sex-specific cortisol responses (decreased in females, increased in males) (n.s.). Histology confirmed enhanced gametogenesis at 20 µg/kg for both formulations, showing increased mature oocytes and spermatozoa. Metabolic markers (glucose, cholesterol, triglycerides) rose significantly in treated groups (P < 0.05), reflecting reproductive activation. Gene expression analysis demonstrated coordinated gnrh-1 (sharp increase) and kiss2 (gradual rise) transcription patterns, revealing their synergistic regulation of the hypothalamic-pituitary-gonadal axis (P < 0.05). These results establish 20 µg/kg as the optimal dose for both R and SL formulations, with R-GnRH maximizing spawning synchronization and SL-GnRH extending egg production duration (P < 0.05). The study provides the first evidence that SAIB-PC systems can reduce effective GnRH doses by 50–75 % compared to traditional implants while maintaining efficacy. These protocols address critical bottlenecks in seabass aquaculture by improving reproductive outcomes and reducing handling stress, offering practical solutions for hatchery management.