Reduction in elemental waste loading from commercial carp feeds by manipulating the dietary phosphorus levels

The effect of dietary phosphorus (P) manipulation on environmental loading of P and nitrogen (N) from carp was investigated. Two commercial diets (A and B) were re‐prepared as four experimental diets A, B, AP, and BP, the latter two being topped up to the required available P level for carp. The total P ranged between 1.52 and 1.99% and water‐extractable (or available) P was 0.50, 0.36, 0.62, and 0.63% for A, B, AP, and BP, respectively. A 20% fishmeal diet was chosen as the control and it contained 1.41% total P and 0.67% water‐extractable P. Duplicate groups of carp (size, 8.6 g) were fed the test diets for 12 weeks. The best growth performance was that of fish fed the control diet, followed by the test diets supplemented with P. The highest rates of P absorption and retention of P and N were obtained for the control group followed by the AP, A, BP, and B experimental groups. In the case of waste discharge, the reduction in P loading was marginal, but N loading was markedly lower for P supplemented test diets. The control diet was the least‐loading diet. Thus commercial carp feed formulations could be improved by incorporating sufficient available P in order to reduce discharges of P and N into the water.