Assessment of undephytinized and dephytinized rapeseed protein concentrate as sources of dietary protein for juvenile rainbow trout ( Oncorhynchus mykiss)

Abstract

This study was undertaken to evaluate three sources of rapeseed protein concentrate (RPC) as partial or total replacements of steam-dried whole herring meal (HM) in a practical diet for juvenile rainbow trout. Groups of 4.2–4.4 g trout held in 10.0–10.3°C well water on a natural photoperiod were fed one of 10 isonitrogenous (43% protein) and isoenergetic (21.6 MJ gross energy/kg) diets to satiation 3 times daily. Each of three test protein sources, viz. undephytinized, untreated control RPC (UDC), undephytinized solvent-treated control RPC (UDSC), and dephytinized RPC (DP), comprised about 19.0, 39.0 and 59.0% respectively of dietary protein by replacement of one-third (L), two-thirds (M) and all (H) of the HM protein in the control diet. Rainbow trout growth rate, feed intake, feed efficiency, protein and gross energy utilization, mortality and health were not compromised when either UDC or DP replaced up to 66% of HM protein in the control diet. Total replacement of HM protein with each of the RPC sources did not depress feed intake, but did significantly reduce growth rate, feed efficiency, and protein and energy utilization. The procedure used to dephytinize RPC lowered RPC protein quality. Whole body concentrations of calcium and phosphorus were inversely related to the dietary levels of UDC and UDSC. Body levels of zinc were significantly depressed in fish fed the diet with UDSC-H. Fish fed diets with DP displayed normal mineral levels. Significant elevation of thyroid follicle epithelial cell heights was found only in those groups fed the diets with UDC-H or UDSC-H. Whole body proximate composition was generally uninfluenced by diet treatment. It is concluded that RPC can comprise 39% of the dietary protein (fish meal only 11%) for rainbow trout without adversely affecting performance. Nutritional strategies that may enable complete replacement of fish meal protein in diets for rainbow trout by RPC are outlined.