Phosphorus utilization in rainbow trout (Oncorhynchus mykiss) fed practical diets and its consequences on effluent phosphorus levels

Excessive dietary phosphorous (P) concentrations in effluents from aquaculture present a major environmental problem. We therefore studied the effect of dietary P and vitamin D-3 on P utilization by rainbow trout-fed practical diets and on P concentrations in the soluble, particulate and settleable components of the effluent from fish tanks. Rainbow trout (average weight: 78 g, initial biomass: 13 kg in 0.7 m(3) tanks) were fed for 11 weeks, practical diets that varied in total P, available P, and vitamin D-3 concentrations. Soluble, particulate (10-200 mum) and settleable (>200 mum) P in the effluent were sampled every 0.5-6 h for 1-3 days in the third and eleventh weeks of the experiment. Trout in all diets more than doubled their weight after I I weeks. Increasing the concentrations of available dietary P from 0.24% to 0.88% modestly enhanced growth rate. Feed conversion ratio (FCR) and biomass gain per gram P consumed decreased as dietary P concentrations increased. Carcass P, daily P gain, and plasma P concentrations were lower in fish fed with low P diets. Soluble P concentrations in the effluent peaked immediately after and again 4-6 h after feeding, and is a linear function of available dietary P. No soluble P would be produced during consumption of diets containing less than 0.22 +/- 0.02% available P. Above this dietary concentration, soluble P would be excreted at 6.9 +/- 0.4 mg/day/kg for each 0.1% increase in available dietary P. Particulate P concentrations in the effluent were independent of dietary P concentrations. Settleable, presumably fecal, P I concentrations tended to increase with dietary P concentrations. In trout fed with low P (0.24% available P, 0.6% total P) diets, 60% of total dietary P were retained by the fish and the remaining 40% were excreted in the effluent as settleable P (20-30%) and particulate or soluble P (10-20%). In trout fed with high P (0.59-0.88% available P; 0.9-1.2% total P) diets, 30-55% of total dietary P was retained by fish, and the remaining 15-25% appeared in the effluent as settleable P, 20-55% as soluble P, and 5-10% as particulate P. Vitamin D-3 did not affect fish growth nor effluent P levels. Physicochemical management of aquaculture effluents should consider the effect of diets on partitioning of effluent P, the peaks of soluble P concentration following feeding, and the contributions of particulate P to total P in the effluent. Increasing our understanding of how dietary P is utilized and is subsequently partitioned in the effluent can contribute significantly towards alleviating this important environmental and industry problem. (C) 2003 Elsevier Science B.V. All rights reserved.