The present study was performed with the gut tissue of early juvenile (EJv) (body weight range between 40-70g) and late juvenile (LJv) (body weight range between 200-300g) carp, Catla catla, to address three queries. First, to search for any variations in the levels of gut melatonin, oxidative stress levels, its management, and vital digestive enzymes between the EJv and LJv carps. Second, to understand the response of dietary supply of protein-, L-tryptophan-, carbohydrate-, and oil-rich diet on the levels of melatonin concentration, oxidative stress, antioxidative agents, and activity of major digestive enzymes in the gut. Third, to comprehend the role of gut melatonin in managing oxidative stress and modulating the digestive capacity. Data analysis revealed that gut melatonin concentration is higher in EJv carp and lower in LJv carp, while feeding intensity was the reverse. The L-tryptophan and protein-rich diets exhibited stimulatory but different responses to gut melatonin, amount of residual gut content and feeding intensity between the EJv and LJv carps. Substantial evidence was recorded on the role of gut melatonin in regulating stress, levels of antioxidative agents, and vital digestive enzymes, particularly in protein and microbial digestion. Nonetheless, the current study reports for the first time that the nature of gut melatonin synthesis, feeding intensity, generation of oxidative stress, defense by different antioxidants, and digestive capacity vary in LJv carp from its EJv stage. Further, such variations may also be modulated by the dietary components of fish feeds, particularly by the protein or L-tryptophan enrich fraction.
