Dietary Melatonin Boosts Reproduction and Growth Performance of Ornamental Fish Giant Danio (Devario aequipinnatus): A Transformative Approach for Scrapping Wild-Caught Fish Business

The present global trade of endemic ornamental fishes is heavily relied wild-caught species that concerns long-term sustainability. This study examined the effects of dietary melatonin on the reproductive performance and health of Devario aequipinnatus (giant danio). A basal diet of 35% protein (basal diet as control) was supplemented with four different doses of melatonin (2 mg [M1], 10 mg [M2], 50 mg [M3] and 100 mg [M4] per 100 g of feed) given to experimental groups in triplicate. Fish (average weight: 1.13 ± 0.15 g) was stocked in tanks (n = 10) and fed 5% of body weight twice daily. After 60-day feeding, key reproductive metrics such as the gonadosomatic index (GSI), fecundity, egg diameter and histological changes were analysed along with growth and physiological status. The GSI was observed to be better with the increase in dosage and was higher in M3 (3.05 ± 0.03%) (p  < 0.05). Histological examination revealed the presence of advanced oocyte stages IV and V in M3, while higher (>50 mg) melatonin levels suppressed the GSI. Egg diameter increased with the dosage of melatonin up to 50 mg/100 g (1.18 ± 0.6 mm). Testicular development was most advanced in 50 mg (M3) dose of melatonin with significant higher appearance of stage II spermatids or spermatozoa. In addition, M3 exhibited markedly elevated levels of vitellogenin (VG) (3.38 ± 0.22 nmol/L) in female fish and testosterone (16.4 ± 1.11 nmol/L) in male fish compared to the control. Broken-line regression analysis indicates that the optimal dose for improved growth performance was identified at 63 mg/100 g of diet. Melatonin supplementation significantly increased (p  < 0.05) haematological indices such as haematocrit value, leucocyte count, haemoglobin (Hb) and packed cell volume (PCV) compared to the control, except for the 100 mg group (M4). Although stress markers such as glucose and cortisol were similar to the control, there was a plausible rise in the amount of antioxidant enzyme (p  < 0.05) in the melatonin groups. Overall findings of the study demonstrate the potential of melatonin improving the reproductive and physiological status of endemic ornamental fish for accelerating the captive breeding programme for sustainable trade.