Nutritional evaluation of rapeseed protein isolate as fish meal substitute for juvenile turbot (Psetta maxima L.) — Impact on growth performance, body composition, nutrient digestibility and blood physiology

The potential of a highly concentrated rapeseed protein isolate (RPI) as partial or total fish meal (FM) alternative in diets for turbot (Psetta maxima L.) was evaluated. In a feeding trial 12 experimental tanks of a saltwater recirculation system were stocked with 15 fish each. Fish were organized in triplicate groups and received isonitrogenous and isoenergetic experimental diets with 0, 33, 66, or 100% (designated as control, RPI 33, RPI 66, RPI 100) of FM protein replaced with RPI. Fish were fed over a period of 56 days until apparent satiation.Feed intake, feed conversion ratio and growth performance did not significantly vary between fish fed the RPI 33 (129 g RPI kg− 1 diet) and the control diet. The RPI 66 (258 g RPI kg− 1 diet) and the RPI 100 (391 g RPI kg− 1 diet) diets caused reduced feed ingestion and feed efficiencies, resulting in lower growth performance. Apparent digestibility coefficient (ADC) of dry matter and protein calculated on the basis of stripped feces revealed significantly reduced values for fish fed with RPI 66 diet compared to control diet, whereas ADC in the RPI 100 group recovered to control value. Protein retention decreased with increasing FM substitution levels. Body composition of fish showed a significant reduction in dry matter and crude protein values of the RPI 66 and RPI 100 group compared to control group. Gross energy content varied between the RPI 33 and RPI 100 group, while crude lipid and ash content were unaffected among the treatment groups, suggesting no limitations in dietary mineral or phosphorus availability. Blood parameters including hematocrit, glucose, triglycerides and cortisol were similar between the treatment groups. Histopathology of liver tissue revealed a slight hypertrophy of hepatocytes in the control group and a severe hypertrophy in fish fed with RPI 33 diet. In line with a reduced condition factor and hepatosomatic index a slight hypotrophy of hepatocytes was observed in fish fed with the RPI 100 diet. Neither inflammatory nor degenerative changes of the mid gut were observed among dietary treatments.In conclusion, 66% (314 g kg− 1) of dietary FM could be replaced by RPI (258 g kg− 1) without affecting physiological parameters of turbot in terms of animal welfare, but growth performance decreased using FM protein replacement levels above 33% (corresponding to 157 g kg− 1).

► Rapeseed protein isolate (RPI) as alternative ingredient in diets for turbot.
► 66% of dietary fish meal was replaced by RPI without effects on health status.
► 33% of dietary fish meal was replaced by RPI without reduced growth performance.