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The increasing use of plant-based ingredients in aquafeeds for Atlantic salmon has led to increasing feed concentrations of plant sterols (phytosterols) combined with decreasing concentrations of cholesterol. Questions have been raised regarding how this may affect performance, health and lipid metabolism in a carnivorous fish species adapted to a high cholesterol diet in nature, and whether phytosterols could be involved in the liver lipid accumulation commonly observed in salmon fed diets with high content of vegetable oils. A feeding trial with 9 diet groups with varying additions of phytosterols and cholesterol was conducted at two temperatures (6 and 12 °C). Fish grew from an initial weight of ~ 250 g to a final weight of ~ 900 g. Overall, the addition of phytosterols and/or cholesterol within the range tested (1353–3147 mg kg− 1 phytosterols, 867–3153 mg kg− 1 cholesterol and phytosterol:cholesterol ratios of 0.7–3.2) exhibited few effects on fish performance, lipid metabolism and health. The main hypothesis regarding phytosterols or phytosterol:cholesterol ratio affecting liver lipid accumulation appears to be false. However, dietary cholesterol did increase muscle pigmentation at both temperatures, while fillet dry matter and hepatosomatic index were also increased by dietary cholesterol at 12 °C. Dietary cholesterol also had some minor effects on the fatty acid composition of liver polar lipids, probably mediated by LXR. In general, more effects were seen from variations in dietary cholesterol compared to phytosterols. The current trial is the first to feed salmon graded levels of phytosterols, cholesterol and their ratio without changing the dietary fatty acid composition, and thereby isolating the effects of dietary sterols.

The aim of the present study was to combine maximum replacement of fish meal and fish oil with plant ingredients in feed for Atlantic salmon, in order to gain a sound and sustainable net fish protein production. The design implied that all known nutrient requirements should be met. Atlantic salmon smolts with an initial weight of 0.3 kg were fed in triplicate either a fully marine control diet or one of three plant based diets through the seawater production phase for 12 months, until final weight of approximately 4 kg. In a maximum plant based diet, 80% of the fish meal was replaced with a mixture of plant protein ingredients and krill meal, while 70% the fish oil was replaced with a mixture of vegetable oils. Two intermediate replacement diets contained either one half of this fish meal replacement level and maximum fish oil replacement, or one half replacement level of fish oil and maximum fish meal replacement. Fish performance was assessed by measuring mortality, feed intake, growth, nutrient digestibility and nutrient utilisation. Specific growth rate was significantly lower in the combined high replacement group compared to the other experimental groups, both for the first 3-month period (12%) and for the complete 12 months (9%) of feeding. The final fish weights were 17% lower in the combined high replacement group and 9% lower in the high plant protein and intermediate vegetable oil group, compared to the marine control and the intermediate plant protein group. Significantly reduced feed intake during the first period and slightly reduced digestibility of 16:0 and starch were identified as possible causes for growth depression, since minor differences in protein or lipid digestibility, feed conversion ratio, and protein and lipid retention were observed. The maximum fish meal and fish oil replacement represented a net production of fish protein, with 2 kg salmon protein produced per kg fish meal protein fed. This being four-fold more efficient usage of fish meal in the 80% plant protein diets compared to the 100% fish raw material diet.

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.