Total phosphorus loading onto the water environment from common carp fed commercial diets

The recent status of total phosphorus loading (T-P) from carp culture was e valuated by feeding fingerling carp (average weight 12.4+/-0.4 g, duplicate tanks, 4 weeks at 23-25 degrees C) six commercial diets used in Lake Kasum igaura (Ibaragi Prefecture, Japan) during 1996. The T-P was determined by e mploying three methods based on 1) water extractable phosphorus (P), 2) P a bsorption, and 3) P retention. The total P contents of six diets ranged fro m 1.07 to 2.09% and the absorption rates were 26.9 to 37.3%, being close to the amount of P extracted with deionized water. Growth performances were m arkedly different and generally high in fish fed diets containing 0.6-0.8% available P. The values for T-P between those calculated based on the water extractable and absorption rates of dietary P were nearly identical wherea s the values based on retention were slightly higher. The difference betwee n the total amount of P given to fish and the amount of P retained in fish body can be regarded as T-P onto the environment. Therefore, the rate of P retention should be employed in the practical estimation of T-P from carp c ulture operation. The T-P (based on retention) varied greatly between the d iets and ranged from 9.1 to 18.8 kg/t carp production (13.6 on average) amo ng the diet groups. A high lipid content was observed in the whole body of fish fed diets with low amounts of available P as a typical P deficiency sy mptom.