Effect of dietary phosphorus and vitamin D3 on phosphorus levels in effluent from the experimental culture of rainbow trout (Oncorhynchus mykiss)

Excessive phosphorus (P) levels in aquaculture effluents violate federally mandated limits and pose a serious threat to the freshwater environment. In rainbow trout culture, effluent P probably originates as fecal and metabolic waste product because assimilation of dietary P is relatively low. We therefore decreased dietary P and increased dietary vitamin D3 levels, methods that enhance P assimilation in mammals, in purified and semi-purified trout diets, then monitored effluent P. Soluble effluent P reached a peak right after feeding and returned to baseline levels in between feeding times. The peak and average concentrations of soluble P in the effluent were mainly influenced by dietary P. Average P in fecal dry matter also decreased with dietary P. Neither dietary P nor vitamin D3 under the conditions of the experiment had significant effects on whole body P content but P deposition (as a percentage of P intake) decreased with increased dietary P. The dietary combination of low P and high vitamin D3 decreased soluble and fecal P levels in the effluent indicating a strategy whereby effluent P concentrations can be reduced by regulation of P metabolism