Effects of fishmeal substitution with mealworm meal ( <i>Tenebrio molitor</i> ) on the growth performance, flesh quality, feed utilization, digestibility, and digestive system histology of fingerling rainbow trout ( <i>Oncorhynchus mykiss</i> )

Abstract This study examined the effects of fishmeal substitution with mealworm meal (MWM) in the diets of rainbow trout fingerlings ( Oncorhynchus mykiss ) on growth performance, flesh quality, amino acid digestibility, and digestive system histology. Five diets, which were iso‐nitrogenous (average of 46.50% crude protein), iso‐lipidic (average of 19.76% crude lipid), and iso‐energetic (average of 15.74 kJ/g), were formulated and fed to triplicate groups of rainbow trout fingerlings (17.02 ± 1.33 g) for 90 days. Dietary treatments included one control (100% fishmeal) and four experimental diets. The experimental diets included MWM30 (150 g/kg MWM, no addition of methionine or arginine), MWM50 (250 g/kg MWM, 1 g/kg methionine, 1 g/kg arginine), MWM70 (350 g/kg MWM, 2 g/kg methionine, 2 g/kg arginine), and MWM100 (500 g/kg MWM, 4 g/kg methionine, 3 g/kg arginine). The growth performance was optimal up to the MWM70 group, while there was a significant decrease in the MWM100 group for weight gain (70.33 ± 1.13 g) and increased feed conversion ratio (1.04 ± 0.04). There was no significant difference in the proximate composition of MWM on crude protein and lipid levels in whole‐body and fillet samples. Apparent digestibility coefficients were at acceptable levels, although there was a decrease in crude protein, crude lipid, and amino acid digestibility in parallel with increasing MWM levels. Histological analyses showed little adverse effects, with a significant reduction in liver vacuolization in MWM70 and MWM100 groups, and the experimental treatment did not result in severe negative effects on the formation of interstitial tissue, tubules, or glomeruli in the kidney.