The current study investigated the optimal dietary methionine requirements of local and improved strains of Nile tilapia Oreochromis niloticus fed diets containing graded levels of dietary methionine in black soldier fly larvae meal-based diets. Six isonitrogenous (∿35% crude protein), isolipidic (∿11% crude fat) and isocaloric (∿4920 kcal gross energy/kg) diets (D0.36, D0.96, D1.05, D1.15, D1.23 and D1.41) were formulated with graded levels of DL-methionine (0, 0.67, 0.77, 0.87, 0.97 and 1.17%, respectively) at the expense of carboxymethyl cellulose to give analyzed methionine levels of 3.6, 9.6, 10.5, 11.5, and 12.3 g/kg of diet, with a constant cysteine level of 4.2 g/kg. Thirty-five fish of each strain (4.08±0.12 g) were randomly stocked in each 100 L plastic tank in a recirculating aquaculture system and fed to satiation twice daily for 84 days.
The best growth performance, feed utilization, somatic indices, hemato- biochemical, and antioxidant profiles were recorded with a methionine inclusion of 10.5 g/kg in the diet (D1.05) for the local strain and 11.5 g/kg in the diet (D1.15) for the improved strain. Quadratic regression revealed equal optimum dietary methionine requirement for both the local and improved strains at 9.1-9.6 g/kg of diet, with a cysteine level of 4.2 g/kg. There were significant effects of dietary methionine, strain, and their interaction on all analyzed parameters (p < 0.05), except survival rate. These findings suggest that aligning dietary formulations with strain-specific nutritional requirements is necessary for optimal fish performance and aquaculture productivity. Moreover, insect-based diets, when properly balanced with essential amino acids such as methionine, can effectively support optimal performance in Nile tilapia.