Optimum ration size of Indian major carp, Cirrhinus mrigala fingerlings was determined by feeding purified diet (40% crude protein (CP); 3.61 kcal g(-1)) at five ration sizes (2%, 4%, 6%, 8% and 10% of body weight per day) for 6 weeks. Feeding trial was conducted in triplicate. Fishes were randomly stocked at the rate of 20 fish per trough fitted with water flow-through system. The best feed conversion ratio (FCR), specific growth rate (%) and protein efficiency ratio (PER) were evident at the ration size of 4-6% body weight. Second-degree polynomial regression analysis of the FCR, PER, and protein and energy retention data indicated the breakpoints at ration size of 5.16%, 5.24%, 5.52% and 5.42% body weight per day. Carcass composition of fish fed different ration sizes varied significantly. Maximum carcass protein and minimum moisture content were noticed at 4% and 6% ration levels. A linear increase in fat content was evident with increasing ration levels up to 6% body weight. Ash content remained insignificantly different among various ration levels except at 2%, showing the significantly highest value. Water temperature, dissolved oxygen, free carbon dioxide, pH and total alkalinity were recorded regularly during the length of the experiment. No mortality was observed during the feeding trial. Based on the above results, it is recommended that feeding in the range of 5-5.5% body weight per day corresponding to 20 g protein and 181 kcal energy to 22 g protein and 199 kcal energy per kg of the diet per day is optimum for the growth and efficient feed utilization of C. mrigala.
