Dietary Phosphorus Modifications in Practical Feeds Do Not Affect Waterborne Phosphorus Concentrations and Phytoplankton Abundance in Channel Catfish Ponds

Abstract

We measured concentrations of soluble reactive phosphorus, total phosphorus, and chlorophyll a in 0.04-ha ponds containing channel catfish Ictalurus punctatus during three feeding trials to evaluate the effect of dietary phosphorus modifications on water quality and the potential discharge of phosphorus and organic matter in pond effluents. In experiment 1, a basal diet with 0.20% available phosphorus was compared with diets supplemented with 0.5% or 1.0% dicalcium phosphate to provide 0.27% or 0.35% available phosphorus. In experiment 2, fish were fed diets supplemented with either dicalcium phosphate or defluorinated rock phosphate to contain 0.40% available phosphorus. In experiment 3, fish were fed one of three diets containing 250 or 500 phytase units of phytase per kilogram (0.27% available phosphorus) or 0.75% dicalcium phosphate (0.39% available phosphorus). Husbandry practices in all three experiments were typical of commercial culture conditions. Quantitative and qualitative modifications of dietary phosphorus did not affect waterborne phosphorus concentrations or phytoplankton abundance and, therefore, will not reduce phosphorus or organic matter mass loading in pond effluents. Lack of effectiveness results from high baseline nutrient loading from phosphorus contained in practical feed ingredients combined with high internal phosphorus loading (recycling) within ponds. These factors overwhelm any effect of small changes in external phosphorus loading associated with diet modification. Therefore, the source and level of dietary phosphorus in channel catfish feeds should be based on nutritional and economic considerations rather than potential environmental impact.