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RESEARCH PRODUCT

Optimal diet composition for European whitefish (Coregonus lavaretus): carbohydrate stress and immune parameter responses

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Abstract A feeding trial was conducted on the European whitefish to study the effects of replacing fish meal with fish oil and corn starch on the stress response and immune system parameters. Nine diets with varying levels of fish meal (FM; 38–86%), fish oil (FO; 2–22%) and corn starch (CS; 0–33%), and fixed levels of wheat meal (10%) and vitamin–mineral premix (2%) were formulated and replicates were allocated among 25 tanks following the D-optimality criteria. Fish were fed the extruded diets to satiation for 10 weeks in a flow-through freshwater system at 15 °C. The liver and plasma were sampled at the termination of the trial, and the response surfaces were modeled as Scheffe polynomials specific for mixture designs. Liver glycogen and plasma glucose increased and plasma IgM decreased with increasing CS level. Plasma lysozyme levels decreased with an increase in FM. Plasma cortisol showed a linear response, with the highest values in high-FO, low-CS diets. Mortalities only occurred with high-CS diets. After the feeding trial, the effects of rapid water cooling on stress responses were measured in fish fed either a low-starch diet (66% FM, 22% FO, and 0% CS) or a high-starch diet (53% FM, 2% FO, and 33% CS). Initial plasma samples were collected at 15 °C, whereafter the temperature was reduced to 2 °C in 24 h. Plasma samples were withdrawn, and sampling was repeated after three more days at 2 °C. Cortisol increased from an initial 7 to a peak value of 70 ng ml −1 at the second sampling, and decreased thereafter to 20–30 ng ml −1 . Plasma glucose increased from an initial 3.9 to 4.8 mmol l −1 after 24 h, and further increased to approximately 7.8 mmol l −1 after three more days in cold water. The responses did not differ significantly between fish fed low-starch or high-starch diets. On overall, whitefish showed inadequate adaptation to high dietary carbohydrate levels.