Effects of dietary quality on melatonin in gut, serum and ovary in mediating growth, digestive and ovarian physiology in juvenile carp (Catla catla)

The dietary protein and tryptophan of fish feed have vital roles in the growth performance, stress management, and digestive physiology of fish. However, in this context, the functions of gut melatonin, which depend on the availability of food, the timing of food supply, the frequency of feeds/day, the quality of food, and the growth stages of carp, still need to be clarified. Hence, the present study aimed to investigate the impact of dietary quality on specific growth rate (SGR), residual gut content (RGC), feeding intensity (FI), gut melatonin, gut oxidative stress and its major antioxidants, vital digestive enzymes in juvenile carp, Catla catla (mean body weight: 274 ± 16.63 g). Besides, serum melatonin and ovarian melatonin, gonado-somatic index (GSI), and different growth stages of oocytes were also assessed before the estimation of oxidative stress and major antioxidant enzymes in the ovary. The fish were fed four diets viz. (i) a standard diet (SD/control) (with 34.99 % protein, 14.56 % carbohydrate, 9.84 % oil, and 0.36 % L-tryptophan) (ii) one protein (PRD with 41.02 % protein and 0.46 % L-tryptophan), (iii) two L-tryptophan (TrpRD1 with 0.96 %, and TrpRD2 with 1.36 % tryptophan), -rich diets for 30 days. The SGR was higher only in PRD, but feeding was reduced in PRD and TrpRDs compared to SD-fed carp. However, the PRD and TrpRDs stimulated gut melatonin, digestive enzymes and all the studied antioxidants with reduced oxidative stress by lowering malondialdehyde (MDA). Moreover, a parallel increase in serum melatonin and ovarian melatonin with gut melatonin has been noticed in both the PRD and TrpRDs. All these experimental diets showed pro-gonadal effects by increasing stage-I oocytes and by reducing oxidative stress in the developing ovary in juvenile carp, compared to SD-fed carp. This opens avenues for future research on the role of feed-induced gut melatonin in fish nutrition and reproduction.