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 catfis h (average initial weight, 61 +/- 5 g) were stocked in 400 m(2) earthe n ponds (13 950 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% a vailable phosphorus from plant ingredients and monosodium phosphate. A vailability of phosphorus in the basal diet and monosodium phosphate w as determined by digestibility (net absorption) trials. All diets cont ained an inorganic calcium:available phosphorus ratio of 1:1. Each die t was fed to fish in four replicate ponds once daily to satiation thro ughout a 140 day growing season. Data for weight gain, feed conversion ratio, dressing percentage, serum phosphorus, serum alkaline phosphat ase activity, bone ash, bone phosphorus, bone breaking strength: muscl e composition and visceral fat were subjected to regression analysis t o determine effects of the dietary levels of available phosphorus on t hese responses. There was no significant effect (P > 0.10) of increasi ng dietary available phosphorus on weight gain, feed conversion, and d ressing percentage. Serum phosphorus, bone ash, bone phosphorus, fat, moisture and protein contents of muscle and visceral fat showed signif icant linear responses (P<0.06) and serum alkaline phosphatase activit y and bone breaking strength showed significant quadratic responses (P < 0.04) with increasing concentrations of dietary available phosphoru s. Broken-line analysis showed that maximum serum alkaline phosphatase activity and bone breaking strength were obtained at dietary availabl e 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 suff icient for maximum weight gain by channel catfish grown to marketable size in ponds. Based upon alkaline phosphatase activity and bone stren gth, 0.3% available phosphorus is recommended for production diets for catfish grown in ponds. Increasing the dietary available phosphorus t o higher concentrations appears to reduce muscle and visceral fat in i ntensively-fed, pond-grown channel catfish. (C) 1997 Elsevier Science B.V.