| Summary: | Abstract A 14‐week feeding trial was conducted to evaluate the effect of supplemental L‐methionine for total replacement of fish meal (FM) with soybean meal (SBM) on growth and feed utilisation of the stinging catfish, Heteropneustes fossilis. Five diets were formulated, where Diet 1 contained 15% FM and 45% SBM as a source of protein. The other four diets (2–5) contained 0% FM and around 60% SBM (59.5%, 59.0%, 58.5% and 58.0% SBM, respectively) with various percentages (0.5%, 1.0%, 1.5% and 2%) of L‐methionine. The significantly highest mean final weight (g), weight gain (g), percent weight gain (%) and specific growth rate (% day−1) were found in Diet 4 containing 58.5% SBM with 1.5% L‐methionine, and there was no significant difference between Diets 1 and 4. The feed conversion ratio (FCR), protein efficiency ratio (PER) and survival rate did not differ significantly among fish groups. However, the best FCR and higher PER were found in Diet 4. The whole‐body carcass protein was significantly lower in Diets 2 and 5, which contained 0.5% and 2% L‐methionine, respectively, and might be below or above the requirements of stinging catfish. Significantly higher lipid content was found in Diets 2 and 4. Dietary 1.5% supplementation of L‐methionine for total replacement of FM with SBM increased haemoglobin, red blood cells, white blood cells and blood glucose, and 2% supplementation of L‐methionine decreased these values significantly. Moreover, considerable morphological improvement of the intestine was observed in Diet 4. Therefore, the results suggest that Diet 4 containing 58.5% SBM with 1.5% supplemental L‐methionine could completely replace FM without adverse effects on growth, feed utilisation and health condition of stinging catfish, H. fossilis.
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