Background. Cobalt (Co) is an essential mineral required in trace quantity in the diet of fish. Although freshwater fish are capable of accumulating adequate quantity of trace minerals from the medium, Co is extremely scarce in the freshwater resources. Therefore, freshwater fish require a supplement of Co in the diet. Since information regarding requirement of Co for growth and its balance in the body of fish is poorly documented there is scope to quantify requirement of cobalt for common carp. Materials and Methods. Two separate trials were made with fingerlings of common carp, Cyprinus carpio: a growth trial made in outdoor cement tanks for 60 days and a digestibility trial made in 15-L glass aquaria in the laboratory for 7 days. Four experimental diets (average crude protein 30.94%) supplemented by four different levels of Co (0.00%, 0.05%, 0.10%, and 1.0%) were formulated and tested in these two trials. Growth was evaluated from gain in weight, specific growth rate, feed conversion ratio, and deposition of protein and lipid in the body of the fish. Apparent protein digestibility (APD) of the diets was evaluated from the proportion of chromium (Cr) and protein in the diet and faeces. Water qualities were checked every week and levels of Co in liver, kidney, gill, gut, and caudal trunk of the fish were determined by atomic absorption spectrophotometer at the end of 60 days in the growth trial. Results. Fingerlings of C. carpio fed 0.1% to 1.0% cobalt-supplemented diet (CSD) showed significantly higher growth than the control diet. Although conversion rate, weight gain and SGR were significantly higher in 0.05% CSD as compared with control diet (0.00% CSD), there was no significant difference in apparent protein digestibility (APD), apparent net protein utilization (ANPU), and deposition of crude protein in the body between fish fed control and 0.05% CSD. Fish fed 1.0% CSD showed significantly higher level of Co in different tissues as compared with other diet groups. Conclusion. It is concluded from the present study that dietary supplement of Co (0.1% to 1.0%) serves as a growth promoter for common carp. Increased growth with no additional tissue burden of Co is achieved at 0.1% dietary Co, while a higher level of supplement (1.0%) may result in increased deposition of Co in different tissues.
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