VOLUNTARY FEED-INTAKE, NITROGEN AND PHOSPHORUS LOSSES IN RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) FED INCREASING DIETARY LEVELS OF SOY PROTEIN-CONCENTRATE

High energy extruded diets were formulated to contain the same level o f protein supplied either by soy protein concentrate (SPC) or fish mea l. Three experiments were performed in order to measure voluntary feed intake and feed waste, faecal losses and soluble losses of nitrogen a nd phosphorus in rainbow trout (average body weight: 100 g). Voluntary feed intake and growth performance of fish fed with demand feeders we re not different when diets contained 0, 50 or 75 % SPC instead of fis h meal. Total replacement of fish meal by SPC led to a significant dec rease in feed intake and resulted in poor growth. This was partly due to methionine deficiency in the SPC based diet. With the addition of c rystalline DL-methionine in the diets, an improvement of feed intake a nd growth performance was apparent. Protein digestibility was high, re gardless of the protein source. Excretion of ammonia and urea increase d with the level of SPC in the diet. Nitrogen losses decreased when me thionine was added to the diet containing only SPC as a protein source . Availability of phosphorus increased with the level of SPC in the di ets. Daily soluble losses were not affected by the dietary treatments but the pattern of phosphorus excretion after feed intake was modified . The rise in soluble phosphorus in water occurred later when fish wer e fed diets with soy protein whatever the dietary level of soy protein concentrate. (C) Ifremer/Elsevier, Paris.