Digestibility of bacterial protein by Atlantic salmon (Salmo salar) is affected by both inclusion level and acclimation time

The impact of acclimation time and inclusion level of a bacterial protein (BP) meal on the digestibility of protein and amino acids to post-smolt Atlantic salmon (Salmo salar) was examined using a factorial experimental design that compared acclimation over five time points (3-days, 10-days, 17-days, 24-days, and 35-days) and five BP inclusion levels (0 g/kg, 100 g/kg, 200 g/kg, 300 g/kg and 400 g/kg). An additional test diet with soy protein concentrate (300 g/kg) was included as a comparison. Each of the diets were prepared from the same basal mash and using the same processing methods. Feed intake was measured daily. The results of the study showed that diet apparent digestibility coefficients (DADC) changed markedly over the time-course of the study, initially being low and variable before stabilizing at higher values from day-17 onwards. However, the effect on temporal variability of DADC values was consistent across all diets. This consistency translated to comparatively stable ingredient apparent digestibility coefficients (IADC) across each of the time points assessed, though there was a significant decline in all IADC values over time. Additionally, the IADC data was less reliable at the 10% inclusion level with higher variability and significantly differed from the other values determined at the higher inclusion levels. Furthermore, there was not always consistency in the IADC values for nitrogen and sum of amino acids, although both are often used as measures of protein. There were also significant differences among the different amino acids in terms of their IADC values. Although there was a trend towards poorer feed intake with the higher inclusion levels of the BP, this was no worse than that observed for the soy protein concentrate. In recognition of the findings of this study we are suggesting that digestibility studies with Atlantic salmon post-smolt should acclimate for a minimum of 14-days prior to faecal collection and that test ingredient inclusion should be 200 g/ kg or greater to obtain reliable data.