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Production of freshwater catfish (Mystus numerus Cuv. & Val.) has soared to meet consumer demand, but little is known about the nutritional requirements of the species. This study was conducted to determine the dietary lysine requirement of juvenile Mystus numerus fed semipurified diets containing graded levels of lysine. The control diet contained fish meal, gelatin and crystalline amino acids simulating the amino acid profile of Mystus nemurus muscle protein except for lysine. Seven experimental diets were formulated to contain lysine as 0.7% (2.00%), 1.1% (3.14%), 1.5% (4.29%), 1.9% (5.43%), 2.3% (6.57%), 2.7% (7.71%) and 3.1% (8.86%) of the diet (% of protein). Triplicate groups of fish were fed to satiation for 90 days. Specific growth rate, feed efficiency ratio, protein efficiency ratio, productive protein value and daily net protein retention of fish fed diets containing lysine at 2.00% of protein were significantly lower than fish fed diets containing lysine at higher levels. Plasma ammonia concentrations were similar in all groups of fish. A digestibility trial between fish fed the experimental diet with 7.71% of lysine and a diet with fish meal as the sole source of protein showed that protein digestibility of the experimental diet was significantly higher than that of the fish meal based diet. Non‐linear regression analysis of specific growth rate against dietary lysine levels indicated a lysine requirement of 3.47% of protein for this species.

Skipjack tuna viscera hydrolysate (TVH) was produced by enzymatic hydrolysis using alcalase. A study of the effect of time and enzyme concentration was performed and formed the basis for preparing TVH with the desired degree of hydrolysis (DH). An enzyme to substrate ratio of 5 g kg À1 (TVH A1) and 16.4 g kg À1 (TVH A2) at pH 8.0 with a temperature of 50°C, and hydrolysis time of 60 min were found to be optimal conditions to obtain hydrolysis of 42.9% and 62.9%, respectively. Both TVH A1 and TVH A2 were analysed for chemical and amino acid composition and peptide size. The effectiveness of TVH at different DH as feeding attractants was estimated in a feeding trial at different inclusion levels. Ten experimental diets, including a fish meal-based reference diet, were fed to Asian seabass for 28 days. Fish fed the diet supplemented with 30 g kg À1 of TVH A2, and the reference diet showed the highest feed intake (P < 0.05). The growth performance of fish fed diets supplemented with TVH was slightly higher than those of fish fed the control diet, but the reference group showed a significantly higher growth rate (P < 0.05). These findings suggest that the supplementation of feed with TVH that has been processed under suitable conditions can improve feed palatability and growth performance when utilizing alternative protein sources in Asian seabass diets.

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