Nutrient utilization in Atlantic salmon (Salmo salar L.) fed increased levels of fish protein hydrolysate during a period of fast growth

The present work was designed to study whether changes in dietary protein quality by means of partial inclusion of fish protein hydrolysate (FPH) would alter fish growth, feed utilization, protein retention and metabolism and fish health in general. FPH was produced after hydrolysing whole minced herring using the industrial enzyme Alcalase®. The dietary protein source, low‐temperature‐dried (LT) fishmeal nitrogen was exchanged with FPH nitrogen at six levels of inclusion ranging from 0 to 300 g kg−1. The experimental diets were fed to post‐smolt (1+) Atlantic salmon (Salmo salar), with mean initial weight of 174 g for a period of 68 days. All diets were iso‐nitrogenous, iso‐energetic and contained the same amount of amino acids. Fish fed medium inclusion of FPH (180–240 g kg−1) showed a tendency to have higher feed intake than fish fed lower and higher levels of FPH inclusions. Significant higher individual specific growth rates were present in fish fed diets with 180 and 240 g kg−1 FPH when compared with those fed 300 g kg−1. Feed conversion ratio increased significantly (R2 = 0.61) and protein efficiency ratio decreased significantly (R2 = 0.59) in fish fed increased levels of FPH. Further, apparent digestibility of crude protein and the amino acids arginine, lysine, methionine and phenylalanine increased significantly with increased dietary inclusion of FPH. Plasma free amino acids, ammonium and urea indicated that FPH amino acids was absorbed earlier and nonsynchronously, and may thus be more prone to be catabolized than in those fish fed the less solubilized protein. FPH inclusion did not have an impact on fish health, as evaluated by haematology and clinical parameters.