Dietary histidine requirement of fingerling Catla Catla (Hamilton) based on growth,protein gain,histidine gain,RNA/DNA ratio,haematological indices and carcass composition

To investigate the histidine requirement of fingerling Catla catla (3.65 ± 0.15 cm; 0.65 ± 0.36 g), six casein‐gelatin based diets (33% CP; 13.58 kJ g^?1 DE) containing graded levels of L‐histidine (0.25%, 0.39%, 0.53%, 0.67%, 0.83%, 0.96% of the dry diet) were fed near to satiation thrice a day for 12 weeks. Maximum absolute weight gain (AWG; 8.63 g fish^?1), protein gain (PG; 1.45 g fish^?1), histidine gain (HG, 48.19 mg fish^?1), RNA/DNA ratio (4.15), best feed conversion ratio (FCR; 1.31), highest haemoglobin (Hb, 9.61 g dL^?1), RBCs (2.84 × 10⁶ mm^?3) and haematocrit (Ht, 30.12%) were recorded in fish fed diet containing 0.67% histidine. However, broken‐line regression analysis of AWG, PG, HG, RNA/DNA ratio, FCR, Hb, Ht and RBCs against dietary histidine reflected the histidine requirement at 0.65%, 0.64%, 0.63%, 0.68%, 0.63%, 0.66%, 0.68% and 0.65% dry diet respectively. Carcass protein was found to improve significantly (P < 0.05) from 13.36% to 16.42% with the increase in dietary histidine from 0.25% to 0.67%. Based on regression analysis of AWG, PG, HG, RNA/DNA ratio, FCR, Hb, Ht and RBCs, it is recommended that the diet for fingerling catla should contain histidine in the range of 0.63–0.68% dry diet, equivalent to 1.91–2.06% of the dietary protein for optimum growth, feed utilization, blood profile and carcass composition.