The study tested a new way to give a fish hormone using tiny chitosan particles, which kept the hormone stable and let researchers use lower doses while still boosting sperm production in male fish. Adding a drug called domperidone made the effect even stronger, and the half‑dose worked almost as well as the standard commercial hormone.

Hormone-induced breeding is pivotal in freshwater aquaculture, yet conventional gonadotropin-releasing hormone analogues (GnRH-a) are limited by their rapid degradation and cause stress associated with high-dose injections. Chitosan nanocarriers offer improved hormone stability and delivery. This study evaluated the reproductive efficacy of chitosan nanoconjugated salmon GnRH-a (ChN-sGnRH-a), alone or with domperidone, in male Labeo rohita. Six treatments were tested: Cn (Negative control, nanocarrier control), Cp (Positive control, commercial GnRH-a: Gonopro-FH® at 0.1 ml/kg), T1 (ChN-sGnRH-a at full dose, 0.1 ml/kg), T2 (T1 + domperidone 10 mg/ml), T3 (ChN-sGnRH-a at half dose, 0.05 ml/kg), and T4 (T3 + domperidone 10 mg/ml). T2 resulted in significantly higher (P < 0.05) relative milt volume (8.11 ± 0.47 ml/kg), sperm motility (85.7 ± 1.8 %), and sperm count (3.97 ± 0.05 × 10 ¹⁰/ml) compared to Cp. Serum testosterone (5.2 ± 0.3 ng/ml), 11-ketotestosterone (4.8 ± 0.2 ng/ml), and 17α,20β-dihydroxyprogesterone (2.1 ± 0.1 ng/ml) peaked at 12 h post-injection in T2, sustaining elevated levels over 18 h. T2 also showed significantly (P < 0.05) higher expression of fshβ, lhβ, fshr, and lhr genes, along with advanced testicular histoarchitecture marked by abundant spermatozoa and active leydig/sertoli cells. Principal component analysis revealed that T2, T4 and Cp grouped separately with more than 97 % variance covered by reproductive indices. Notably, T4 (half-dose) was comparable to Cp, suggesting efficacy at reduced hormone dosage. Overall, the findings demonstrate that full-dose ChN-sGnRH-a coadministered with domperidone is superior to commercial GnRH-a, and half-dose nanoconjugates provide equal efficacy, justifying a dose-reduction regime for sustainable, low-stress broodstock management of L. rohita.