Phosphorus fractionation and mobility in the food and faeces of hatchery reared rainbow trout ( Onchorhynchus mykiss)

Abstract

The fractionation of phosphorus in both food and faecal material of rainbow trout has been determined. The total phosphorus (TP) concentration in faeces was 8.25 mg P g −1 dry wt., approximately half that in the food (16.06 mg P g −1 dry wt.). Greater than 80% of the decrease between food and faeces could be accounted for by the uptake of calcium phosphate by the fish. Only 25% of the TP in food was readily labile but this fraction increased to 44% in faecal material. The release of phosphorus from food or faeces was investigated in the laboratory with and without the addition of formaldehyde. When bacterial activity was inhibited, soluble reactive phosphate (SRP) concentrations rapidly increased in the water phase and could account for the labile phosphorus fraction in food or faeces after 24 h of incubation. In the absence of formaldehyde, SRP was rapidly assimilated into bacterial biomass but concentrations of dissolved unreactive phosphate increased. The rate at which labile phosphorus was lost from food increased if the sample was shaken or the food pellets ground to a dust, but decreased slightly if the temperature was lowered. Leaching of SRP from sedimenting food or faecal material was estimated as about 5% of the soluble phosphorus load per tonne of fish produced. The bulk of this came from faecal solids.