Dietary potassium requirement of juvenile grass carp (Ctenopharyngodon idella Val.) based on growth and tissue potassium content

A growth trial was conducted to estimate the optimum concentration of dietary potassium (K) for grass carp (Ctenopharyngodon idella). Triplicate groups of grass carp (3.96 +/- 0.06g) were fed diets containing graded levels (0.87, 2.90, 5.37, 7.54, 9.87 and 12.4gkg(-1)) of K for 8weeks. Final body weight, weight gain and feed efficiency and gill Na+-K+ ATPase activity were highest in fish fed with 9.87gkg(-1) dietary K and lowest in fish fed the basal diet (P<0.05). The K contents in whole body and muscle were linearly increased up to the 9.87gkg(-1) dietary K and then levelled off beyond this level, whereas in scales and vertebrae up to the 7.54gkg(-1) dietary K (P<0.05). However, dietary K levels had no significant effect on ash, Ca, P and Mg contents in whole body, scales, vertebrae or muscle. Analysis using polynomial regression of weight gain and gill Na+-K+ ATPase activity and using the broken-line regression of whole body K concentrations indicated that the adequate dietary K concentration for grass carp is about 9.45-9.99gkg(-1) diet.