PARTIAL OR TOTAL REPLACEMENT OF FISH-MEAL BY SOYBEAN PROTEIN ON GROWTH, PROTEIN-UTILIZATION, POTENTIAL ESTROGENIC OR ANTIGENIC EFFECTS, CHOLESTEROLEMIA AND FLESH QUALITY IN RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

Groups of rainbow trout (initial body weight 83 +/- 1 g) were fed diets (crude protein (CP) 46%; gross energy 21 kJ/g DM; crude fat 12%) containing graded levels of either a soyflour (CP 52% DM) or a soy protein concentrate (CP 72% DM) supplemented with L-methionine as partial or total replacement of fish meal protein. A growth trial was conducted over 12 weeks at a water temperature of 18 degrees C. Digestibility of the dietary ingredients and of the complete diets was determined using an indirect method, At the end of the growth trial, in addition to body composition analyses, the following parameters were measured: plasma vitellogenin and cholesterol, antigenic activity (passive haemagglutination) in the soy sources and antisoya antibodies in serum, phytoestrogens in the soy sources and in the bile. Sensory and physical evaluations of fish flesh were also made. Replacement of fish meal with soy protein concentrate (33 to 100% replacement) did not affect growth performance or nutrient utilization. However, replacement of fish meal with soyflour (up to 50%) reduced the growth rate, as did complete replacement with either casein alone, or casein and soyflour in equal amounts. Plasma vitellogenin levels were not significantly affected by the dietary levels of protein from soybean. No antisoya antibodies were detected in the serum of fish fed soy-based diets. Few phytoestrogens were present in the ingredients but only trace amounts were detected in the bile of trout fed 100% soy protein. Plasma cholesterol levels were reduced in fish fed soybean protein in comparison to those fed 100% fish meal. Physical and sensory quality of fish flesh were little affected by dietary treatments.