Muscle lipid storage pattern, composition, and adipocyte distribution in different parts of Atlantic salmon (Salmo salar) fed fish oil and vegetable oil

The effects of dietary oil source on lipid storage were evaluated in harvest size Atlantic salmon fed fish (FO) or vegetable oil (VO) based diets from start-feeding. Final weight and fat content were not significantly different for salmon fed FO and VO diets over a 27 month period. Fish fed these diets were scanned by computerized X-ray tomography (CT) and the whole body scans partitioned into two equal sections according to distance in longitudinal direction. Visible depot fat (area % of soft tissues) and mixed tissue fat levels were significantly higher in the anterior compared to posterior sections, but longitudinal fat distribution was not affected by diet. Differences in the fatty acid (FA) composition of the FO and VO diets were reflected in the tissues (white muscle, red muscle, white muscle myosepta, belly flap and visceral fat) of Atlantic salmon fed these diets. Lipid levels in these tissues were not significantly different for salmon fed FO and VO diets. No obvious differences were observed histologically in the lipid storage patterns of these tissues for salmon fed FO and VO diets. Visceral fat and belly flap tissues were predominated by adipocytes and contained the highest levels of total lipid and triacylglycerol (TAG) per g wet weight for the tissues tested. The red muscle contained higher lipid levels than the white muscle (> 4 fold for salmon fed the same FO or VO diet). Intracellular lipid droplets were observed by light microscopy in oil red O stained frozen sections of the red muscle but not observed to any extent in the white muscle (dorsal, lateral and ventral) for salmon fed FO and VO diets. Observations at higher magnifications, however, revealed small lipid droplets located in close association with mitochondria in the cytoplasm of white muscle cells. High densities of adipocytes were observed in the myosepta of red and white muscle for salmon fed FO and VO diets. White muscle myosepta had a significantly higher total lipid content and proportion of TAG than the white muscle. The belly flap, myosepta and visceral fat, (adipocyte-rich, lipid storage tissues) contained a higher proportion of TAG and monounsaturated FA as well as a lower proportion of polar lipid and n-3 polyunsaturated FA compared to the muscle tissues.