This study evaluated yellow mealworm (Tenebrio molitor) larvae meal as an alternative animal-protein ingredient in diets for common carp (Cyprinus carpio) fingerlings. Two diets were compared for 60 days: a conventional fish-meal diet (T1; 32.09% crude protein) and a diet in which the animal-protein component was replaced weight-for-weight with T. molitor meal (T2; 23.88% crude protein). Each treatment was tested in duplicate tanks containing 28 fingerlings per tank, and growth was assessed over three consecutive 20-day phases. The initial mean body weight was 3.21 g. Weight gain differed significantly between treatments (p < 0.05), with the highest total value recorded in T2 (4.87 ± 0.47 g). The T2 diet produced a significantly higher feed conversion ratio (FCR; 1.93 ± 0.27) than T1 (1.51 ± 0.17), indicating lower feed efficiency under the reduced-protein replacement diet. Feed conversion efficiency (FCE) also differed significantly during the trial and was numerically higher in T2 (0.396 ± 0.11). Relative growth rate (RGR) and specific growth rate (SGR) did not differ significantly during the individual feeding phases; however, total RGR was higher in T2 (1.517 ± 0.17; p < 0.05), whereas total SGR was higher in T1 (0.294 ± 0.07%; p < 0.05). These findings indicate that direct weight-for-weight substitution of fish meal with T. molitor larvae meal is biologically feasible for common carp fingerlings, but the lower dietary protein level and higher FCR show that future applications should optimize inclusion level, protein balance, and economic efficiency before practical use.
