Effects of highly purified rapeseed protein isolate as fishmeal alternative on nutrient digestibility and growth performance in diets fed to rainbow trout (Oncorhynchus mykiss)

The effective inclusion of rapeseed protein products as fishmeal alternatives in diets for carnivorous fish is still limited. Previous studies observed restrictions in both nutrient utilization and feed intake. Contents of nitrogen‐free extracts (NfE) and anti‐nutritive substances (ANFs) were made responsible for the latter mentioned limitations. Consequently, a highly purified rapeseed protein isolate with high protein content and low levels of both NfE and ANF was investigated in this study. In the first experiment, digestibility of the rapeseed protein isolate was determined. In the second experiment, the fishmeal portion (190 g kg‐1) of a control diet was gradually replaced by rapeseed protein isolate to 33%, 66% and 100% of digestible protein and energy. Diets were fed twice per day to apparent satiation to triplicate groups of rainbow trout (Oncorhynchus mykiss). After 56 days of feeding, growth performance and health parameters were evaluated. Protein digestibility of the rapeseed protein isolate was 95.2%, and up to 66% of dietary fishmeal could be replaced without significantly affecting growth or health parameters. Total replacement of fishmeal led to significantly reduced feed intake and consequently reduced growth performance.