PHOSPHORUS FRACTIONATION AND MOBILITY IN THE FOOD AND FECES OF HATCHERY-REARED RAINBOW-TROUT (ONCHORHYNCHUS MYKISS)

The fractionation of phosphorus in both food and faecal material of ra inbow trout has been determined. The total phosphorus (TP) concentrati on in faeces was 8.25 mg P g(-1) dry wt., approximately half that in t he food (16.06 mg P g(-1) dry wt.), Greater than 80% of the decrease b etween 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 in creased in the water phase and could account for the labile phosphorus fraction in food or faeces after 24 h of incubation. In the absence o f 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 slightl y if the temperature was lowered. Leaching of SRP from sedimenting foo d or faecal material was estimated as about 5% of the soluble phosphor us load per tonne of fish produced. The bulk of this came from faecal solids.