Evaluation of iron methionine and iron sulphate as dietary iron sources for juvenile cobia (Rachycentron canadum)

An 8‐week experiment was designed to determine the optimum dietary iron requirement of juvenile cobia Rachycentron canadum (mean initial weight, 15.89 ± 0.84 g) with iron sulphate (FeSO4•7H2O) and iron methionine (FeMet) as iron sources, using a semi‐purified diet based on casein and white fish meal as the protein sources. The basal diet was supplemented with 0, 30, 60, 120, 240 and 480 mg iron kg−1 dry diet from either FeSO4 or FeMet, respectively. Survival was not significantly affected by the all dietary treatment. Weight gain (WG), feed efficiency (FE), serum catalase activity (SCAT), and haemoglobin were significantly affected by any of the dietary treatments from both of two iron sources. Based on broken‐line regression analysis of WG, FE and SCAT, a minimum requirement for dietary iron was recommended to be 80.5–94.7 mg kg−1 from FeSO4 and 71.3–75.1 mg kg−1 from FeMet. Iron supplement to the basal diet had no significant effect on haematocrit, erythrocyte count, iron concentration in whole body and fillet. Our experiment also showed that the bioavailability of FeMet and FeSO4 to juvenile cobia was similar for WG and FE, and the relative bioavailability of FeMet and FeSO4 to juvenile cobia was 275% for maximum SCAT.