The effects of increasing dietary levels of amino acid-supplemented soy protein concentrate and constant dietary supplementation of phosphorus on growth, composition and immune responses of juvenile Atlantic salmon ( Salmo salar L.)

Diets with 50 (SPC50), 65 (SPC65) and 80 % (SPC80) substitution of prime fish meal (FM) with soy protein concentrate (SPC) were evaluated against a commercial type control feed with 35 % FM replacement with SPC. Increases in dietary SPC were combined with appropriate increases in methionine, lysine and threonine supplementation, whereas added phosphorus was constant among treatments. Diets were administered to quadruplicate groups of 29 g juvenile Atlantic salmon were exposed to constant light, for 97 days. On Day 63 salmon were subjected to vaccination. Significant weight reductions in SPC65 and SPC80 compared with SPC35 salmon were observed by Day 97. Linear reductions in body cross-sectional ash, Ca/P ratios, and Ca, P, Mn and Zn were observed at Days 63 (prior vaccination) and 97 (34 days post-vaccination), while Mg presented a decrease at Day 63, in salmon fed increasing dietary SPC. Significant reductions in Zn, Ca, P and Ca/P ratios persisted in SPC65 and SPC80 compared with SPC35 salmon at Day 97. Significant haematocrit reductions in SPC50, SPC65 and SPC80 salmon were observed at Days 63, 70 and 97. Enhanced plasma haemolytic activity, increased total IgM, and a rise in thrombocytes were demonstrated in SPC50 and SPC65 salmon on Day 97, while increased lysozyme activity was demonstrated for these groups on Days 63, 70 and 97. Leucocyte and lymphocyte counts revealed enhanced immunostimulation in salmon fed with increasing dietary SPC at Day 97. High SPC inclusion diets did not compromise the immune responses of salmon, while SPC50 diet also supported good growth without compromising elemental concentrations.