Available phosphorus requirements of food-size channel catfish ( Ictalurus punctatus) fed practical diets in ponds

A pond feeding experiment was conducted in which Year 2 channel catfish (average initial weight, 61 ± 5 g) were stocked in 400 m 2 earthen ponds (13950 fish ha −1) and fed an all-plant extrusion processed commercial type diet that contained 0.20, 0.27, 0.36, 0.44 and 0.60% available phosphorus from plant ingredients and monosodium phosphate. Availability of phosphorus in the basal diet and monosodium phosphate was determined by digestibility (net absorption) trials. All diets contained an inorganic calcium:available phosphorus ratio of 1:1. Each diet was fed to fish in four replicate ponds once daily to satiation throughout a 140 day growing season. Data for weight gain, feed conversion ratio, dressing percentage, serum phosphorus, serum alkaline phosphatase activity, bone ash, bone phosphorus, bone breaking strength, muscle composition and visceral fat were subjected to regression analysis to determine effects of the dietary levels of available phosphorus on these responses. There was no significant effect ( P > 0.10) of increasing dietary available phosphorus on weight gain, feed conversion, and dressing percentage. Serum phosphorus, bone ash, bone phosphorus, fat, moisture and protein contents of muscle and visceral fat showed significant linear responses ( P < 0.06) and serum alkaline phosphatase activity and bone breaking strength showed significant quadratic responses ( P < 0.04) with increasing concentrations of dietary available phosphorus. Broken-line analysis showed that maximum serum alkaline phosphatase activity and bone breaking strength were obtained at dietary available phosphorus concentrations of 0.25 and 0.31%, respectively. Data from this study indicated that an all-plant, commercial type diet with no phosphorus supplement, containing 0.20% available phosphorus, was sufficient for maximum weight gain by channel catfish grown to marketable size in ponds. Based upon alkaline phosphatase activity and bone strength, 0.3% available phosphorus is recommended for production diets for catfish grown in ponds. Increasing the dietary available phosphorus to higher concentrations appears to reduce muscle and visceral fat in intensively-fed, pond-grown channel catfish.