Growth performance, feed utilization and non-specific immune response of gilthead sea bream (Sparus aurata L.) fed graded levels of a bioprocessed soybean meal

The objective of this study was to evaluate the effects of partial fish meal replacement by a bioprocessed soy product (BS) in juvenile gilthead sea bream over the course of a 3-month trial. A diet with fish meal as the sole protein source (FM) was compared to diets with 20% (BS20), 40% (BS40) and 60% (BS60) of replacement. The effects of diets on growth performance, feed utilization, gut integrity, liver structure and immune status were evaluated. Weight gain, feed efficiency, daily growth index and specific growth rate decreased at the highest level of BS, while feed intake increased. In fish from BS40 and BS60 groups, indices indicating growth in response to protein intake such as protein efficiency ratio and protein productive value were decreased significantly. Body composition did not differ among groups for all parameters analysed. As far as non-specific immune response is concerned, spontaneous respiratory burst activity of blood significantly decreased at BS20 and BS40 groups, while stimulated activity did not indicate any significant change. Serum myeloperoxidase levels and total bacteriolytic activity of complement increased significantly at the highest dietary inclusion of BS, compared to the control group, while serum lysozyme levels did not indicate significant changes. Liver structure was not affected at any level, but some alterations in intestinal morphology were observed in the distal intestine mainly in the BS60 group. Overall, FM replacement by BS affected negatively gilthead sea bream growth performance at the 60% level, protein efficiency from the 40% level and affected some indicators of the immune response.