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.

Mucous cell size and distribution were investigated in the skin of five salmon using a novel stereology-based methodology: one (48 cm) fish to test 15 tissue treatment combinations on measures of cell area and density on the dorsolateral region and, using the most suitable treatment, we mapped mucous cell differences between body regions on four (52 cm) salmon, comprising a male and a female on each of two diets. The section site, decalcification, embedding medium and plane of sectioning all impacted significantly on mucous cell size, whereas mucous cell density is more robust. There were highly significant differences in both mucosal density and mean mucous cell size depending on body site: the dorsolateral skin of the four salmon had significantly denser (about 8% of skin area) and larger (mean about 160 μm(2)) mucous cells, whereas the lowest mean density (about 4%) and smallest mean area (115 μm(2)) were found on the head. We found that 100 random measurements may be sufficient to distinguish differences >7 μm(2) in mean mucous cell areas. The results further suggest that salmon exhibit a dynamic repeatable pattern of mucous cell development influenced by sex, diet and possibly strain and season.

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