Bacterial protein grown on natural gas in diets for Atlantic salmon, Salmo salar, in freshwater

Bacterial protein meal (BPM) produced on natural gas was evaluated with respect to growth, survival, body composition and histopathological indices in Atlantic salmon during the freshwater stage. In the diets, experimentally produced bacterial protein accounted for 0%, 6.25%, 12.5%, 25% and 50% of total dietary amino acids gradually replacing high-quality fish meal. Each diet was fed to three groups of salmon for 1 year from first feeding. In addition, a digestibility experiment was carried out. At 112 days, fish fed the diet containing 6.25% bacterial protein weighed significantly more than fish fed diets containing 12.5% and 25% bacterial protein, while the fish meal control group was intermediate. Growth and survival of fish fed 50% bacterial protein were lower compared to the other groups. Gut to body weight ratio and whole body fat increased with increasing bacterial protein inclusion in a curvilinear manner. From days 113 to 364, growth and survival were not significantly different for fish fed the fish meal control diet and diets with bacterial protein replacement up to 25%. For the highest replacement level, growth was reduced. At day 364, fish fed the diet containing 50% bacterial protein had a significantly higher liver and gut/body weight ratio than the other groups of fish. The histological evaluation showed no systematic difference in tissue morphology between the two dietary groups (6.25% and 50% bacterial protein) examined. The apparent digestibility of nitrogen from the bacterial protein was 78.4%. The digestibility of fat was reduced in a curvilinear manner with increasing inclusion of bacterial protein. In conclusion, the results indicate that bacterial protein can replace up to 25% of the amino acids from high-quality fish meal in a diet for juvenile Atlantic salmon in freshwater, but that both growth and survival were negatively affected at an inclusion rate as high as 50%.