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An experiment was conducted to investigate the effect of dietary mannan oligosaccharide (MOS) supplementation on Atlantic salmon (Salmo salar L.) smolts (~ 47 g) reared in sea cages. The first treatment (control) consisted of fish fed the basal diet and the second treatment (MOS) fish were fed the same basal diet supplemented with 0.4% MOS. In the basal diet, 51.8% of the diet protein was derived from fish meal and 48.2% from vegetable protein (e.g. soy and wheat). After 14 weeks feeding on the experimental diets the results demonstrated that MOS supplementation did not affect growth performance, however, body protein composition was significantly increased. Additionally, liver histochemistry revealed that glycogen deposition in liver tissue increased from 1.80 ± 0.73 AU in the control fed fish to 2.58 ± 0.91 AU in the MOS fed fish. Histology of the anterior intestine demonstrated that MOS supplementation produced a significantly higher absorptive surface of 4.63 ± 0.62 AU compared to the control fed fish of 3.65 ±0.49 AU. The microvilli density was also significantly higher in the anterior intestine in the MOS fed fish,12.02 ± 5.95 AU, when compared to the control fed fish 5.90 ± 1.53 AU. Similar results for the absorptive area and microvilli density were observed in the posterior intestinal region.Microvilli length increased in the posterior intestine from 1.10 ± 0.18 μm in the control group to 1.41 ± 0.19 μm in the MOS fed fish. Furthermore, counts of sea lice attached to fish and total number of fish infected by sea lice were significantly lower in the MOS fed fish. The present study shows that 0.4% MOS supplementation was able to improve intestinal morphology, increase carcass protein content and glycogen deposition in the liver.

The effect of dietary protein to carbohydrate ratios (P/CH, weight percentage) of P34/CH21, P39/CH15 and P44/CH10 was studied in Atlantic salmon (Salmo salar L.) with respect to growth and feed conversion ratio (FCR) in one medium‐scale experiment (Experiment 1) and to slaughter quality in two production scale experiments (Experiments 2A and 2B). The dietary fat was maintained at 290 g kg−1 whereas the protein (fish meal) was exchanged with carbohydrate (wheat). Fish grown from approximately 1 to 4 kg were fed a restricted diet (iso‐energetic on gross energy basis) or to satiation. Nitrogen, fat, starch and energy digestibilities were measured in a separate experiment. There were slight tendencies for lower growth (P = 0.06) and for higher FCRs (P = 0.06) in Experiment 1, and a slight tendency for a lower dress‐out percentage in Experiments 2A and 2B (P = 0.10 and 0.20 respectively) with decreasing P/CH. The P/CH had no effect on the fat concentration of fillets, flesh colour or sexual maturation. The digestibility of starch decreased from 62.1% to 46.1% and the digestibility of energy from 84.9% to 79.5% when P/CH decreased from P44/CH10 to P34/CH21. Growth per unit digestible protein increased with decreasing P/CH. Feeding to satiation improved the growth but the FCR was higher than it was for restricted feeding. The calculated starch load per kg of fish growth increased with decreasing P/CH, whereas the effluent nitrogen decreased.

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