Air-classified faba bean protein concentrate is efficiently utilized as a dietary protein source by post-smolt Atlantic salmon (Salmo salar)

High-quality sources of protein for the formulation of feeds of carnivorous fish species such as Atlantic salmon are currently being sought. In an earlier screening trial we evaluated for the first time in Atlantic salmon (Salmo salar) the applicability of air-classified faba bean (Vicia faba) protein concentrate (BPC) inclusions in combination with soy protein concentrate (SPC) and fishmeal (FM) using parr as a model. Based on the results in parr in freshwater, the present study tested the hypothesis that BPC can effectively replace SPC and/or FM as a dietary protein source in post-smolt Atlantic salmon in seawater. The trial was designed to test nine different combinations of BPC, SPC and FM around the ideal mixture proportions modelled from data on parr (20% BPC, 46% SPC and 34% FM as percent of the total dietary protein). Somatic, physiological, biochemical and molecular parameters were assessed to determine fish performance. On average, fish grew from ~1.28kg to 2.80kg over the course of the 17week trial. Results demonstrated that a mixture of BPC, SPC and FM (208gkg^-1, 193gkg^-1 and 85gkg^-1, respectively) resulted in the highest weight gain of post-smolts without significantly altering the FCR. Specifically, fish fed the highest inclusion of BPC and lowest level of FM had final weights on average 0.24kg more than those with the lowest inclusion of BPC and highest FM. Improved weight gain was not achieved at the expense of poorer FCR, which was lower in fish fed diets with higher content of BPC and lower FM. The highest level of inclusion of BPC tested herein (~21% of feed) could replace up to 8.5% FM (from 17% to 8.5% of feed) and 10% SPC (from 29% to 19% of feed), resulting in the largest improvement in weight gain. There were no biologically significant or physiologically important effects on nutrient digestibility, enteritis scores or hepatic gene expression of dietary BPC. Statement of relevance: This study is highly relevant to the industry of feed manufacture.