The water soluble fraction in fish meal (stickwater) stimulates growth in Atlantic salmon (Salmo salar L.) given high plant protein diets

In this study, increasing levels of stickwater (SW) separated during the fish meal production process were added back to produce 5 experimental fish meals. Five experimental diets were prepared which consisted of 50 g commercial fish meal and 50 g of the different experimental fish meals with graded inclusion levels of SW kg⁻¹ diet. The remaining dietary protein sources consisted of a mixture of plant protein sources (soy protein concentrate; 215 g kg⁻¹ diet, corn gluten; 215 g kg⁻¹ diet; field beans 140 g kg⁻¹ diet; wheat gluten 55 g kg⁻¹ diet) giving 17% of the total dietary protein from marine sources. All the experimental diets were isonitrogenous, isolipidic and isoenergetic. Two more low fish meal diets were prepared containing the low (permeate) or high (retentate) molecular weight fraction of fractionated and ultrafiltrated SW. A control diet containing 300 g commercial fish meal kg⁻¹ diet was prepared. The diets were fed to 8 triplicate groups of salmon (initial body weight 137 g) for 74 days. Fish growth was positively affected by the increasing inclusion level of SW in the low fish meal diets. The low fish meal diets that were supplemented with SW performed the same, both in terms of growth and feed utilization, as the control diet. The growth parameters were also positively correlated to the dietary levels of taurine and hydroxyproline but it is not known if this is because of taurine and hydroxyproline themselves or if taurine and hydroxyproline are indicators for other functional water soluble components present in the marine raw materials. No significant differences were found in feed intake or feed efficiency, nor was there any difference in ADC of protein, lipid, gross energy or phosphorus (P) among the diets containing the experimental fish meals, except for the control diet which had a lower ADC of protein compared to the high plant protein diets. Whole body proximate composition and total and soluble P content of fish were not affected by the inclusion level of SW in the diets. Significant effects were identified in the free amino acid profile in the fillet of fish fed the different experimental diets. The present work gives further evidence for the importance of marine water-soluble fractions and small marine water-soluble components in high plant protein diets for salmon. The present results may be a significant contribution in explaining why marine protein sources are important in feed for carnivorous fish species. By the addition of SW to the diets higher flexibility in the choice of protein sources may be obtained.