Immunological, physiological and pathological responses of rainbow trout (Oncorhynchus mykiss) to increasing dietary concentrations of soybean proteins.

High concentrations of dietary soya were shown to suppress salmonid growth rates and non-specific immune capacity. The immunosuppression became evident at dietary inclusion rates of 60-70% and was coincident with a reduction in weight gains and the appearance of demonstrable pathological changes in the distal intestine. Further increases in soya concentrations to 80-89% caused a progressive decline in specific growth rates and exacerbation of the intestinal pathology. There was no evidence of circulating antibody responses to dietary soybean proteins at any of the rates of inclusion. These observations confirm the findings of other authors that, at concentrations of up to 20-30% inclusion in diets, soybean proteins can provide a partial replacement for fish meal, but at higher concentrations detrimental effects become apparent, not only through reduced weight gains, but also through other physiological and immunological changes.