Requirements ofn-3 very long-chain PUFA in Atlantic salmon (Salmo salarL): effects of different dietary levels of EPA and DHA on fish performance and tissue composition and integrity

Abstract

Farmed salmon feeds have changed from purely marine-based diets with high levels of EPA and DHA in the 1990s to the current 70 % plant-based diets with low levels of these fatty acids (FA). The aim of this study was to establish the impacts of low dietary EPA and DHA levels on performance and tissue integrity of Atlantic salmon (Salmo salar). Atlantic salmon (50 g) in seawater were fed fourteen experimental diets, containing five levels (0, 0·5, 1·0, 1·5 and 2·0 %) of EPA, DHA or a 1:1 EPA + DHA plus control close to a commercial diet, to a final weight of 400 g. Lack of EPA and DHA did not influence mortality, but the n-3-deficient group exhibited moderately slower growth than those fed levels above 0·5 %. The heart and brain conserved EPA and DHA levels better than skeletal muscle, liver, skin and intestine. Decreased EPA and DHA favoured deposition of pro-inflammatory 20 : 4n-6 and 20 : 3n-6 FA in membrane phospholipids in all tissues. When DHA was excluded from diets, 18 : 3n-3 and EPA were to a large extent converted to DHA. Liver, skeletal and cardiac muscle morphology was normal in all groups, with the exception of cytoplasm packed with large or foamy vacuoles and sometimes swollen enterocytes of intestine in both deficient and EPA groups. DHA supplementation supported normal intestinal structure, and 2·0 % EPA + DHA alleviated deficiency symptoms. Thus, EPA and DHA dietary requirements cannot be based exclusively on growth; tissue integrity and fish health also need to be considered.Key words: Aquafeed: DHA: EPA: Essential fatty acids: Fat: Phospholipids Continued growth of the Atlantic salmon (Salmo salar) farming industry depends on the availability of sustainable feed ingredients in the world market. For optimal use of ingredients with limited availability, information regarding nutritional requirements is of utmost importance. Fatty acid (FA) composition of salmon diets has changed considerably over the last several decades. Although 90 % of traditional Norwegian salmon diets were composed of marine ingredients in the 1990s, current diets only contain approximately 30 % marine ingredients (1)

Are we what we eat? Changes to the feed fatty acid composition of farmed salmon and its effects through the food chain

“…1C) in the tissues other than muscle are quite stable, although in this instance are at a higher level than that provided in most of the experimental diets. In Atlantic salmon-fed diets with low and deficient levels of n-3 long-chain PUFA, the FA composition of muscle and skin most resembled that of the dietary composition, followed by the intestine and then the heart and liver, whereas the FA composition of the brain appeared very conserved and was the least affected by diet (Bou et al, 2017a). Curiously, despite the high tissue abundance of EPA in fish, ARA still appears to be the preferred precursor for eicosanoids, which are signalling molecules with diverse roles in fish reproduction, the stress response and the immune response (Bell and Sargent, 2003).…”

“…DHA in all tissues and EPA in the liver appear to be relatively spared from β-oxidation compared with other FAs (Torstensen et al, 2004a). This difference between EPA and DHA could reflect their respective biological roles, and the extent to which salmon has a specific requirement for EPA has been questioned (Emery et al, 2016;Bou et al, 2017a). Dietary DHA seems to be selectively retained regardless of the feed concentration, whereas EPA is only retained if the dietary concentration is low (Bell and Waagbø, 2008).…”

“…2), which results in FA production values of >100% [i.e. the increase in the FA content of the whole fish during the trial (final content-initial content) as a % of the amount of FA fed to the fish during the trial] (Sissener et al, 2017;Rosenlund et al, 2016;Glencross et al, 2014;Bou et al, 2017a). FA production values of DHA of up to 200% have been reported in salmon receiving low dietary levels, meaning that twice as much DHA accumulated in the fish as that provided in the feed, clearly showing net in vivo production of this FA .…”

“…Both product inhibition and substrate availability seem to affect the rate of elongation and desaturation of ALA to the longer chain n-3 FAs in Atlantic salmon (Tocher et al, 2003;Tocher et al, 1997). Despite this in vivo production, preformed long-chain n-3 PUFA are required for optimal growth, fish health and tissue integrity during long-term feeding in salmon Sissener et al, 2016b;Bou et al, 2017a). Although survival was high (>99%) when low EPA+DHA (≤1% of the feed) were used in land-based tanks with controlled and optimal conditions , high levels of fish mortality occurred when the same levels were fed to fish in open net pens with fluctuating environmental conditions and stressful handling of the fish (Bou et al, 2017b).…”

Dietary Fish Oil Alters DNA Methylation of Genes Involved in Polyunsaturated Fatty Acid Biosynthesis in Muscle and Liver of Atlantic Salmon (Salmo salar)

“…Consumption of preformed 20:5n‐3 and 22:6n‐3 is obligatory for carnivorous marine fish, although diadromous species, such as Atlantic salmon, are able to produce 20:5n‐3 and 22:6n‐3 from 18:3n‐3 via a pathway (Morais et al, ; Ruyter et al, ; Tocher, ), which is similar to that in rodents (Sprecher et al, ) and humans (Burdge, ). However, Atlantic salmon have limited capacity for 20:5n‐3 and 22:6n‐3 synthesis and, therefore, require at least 10 g 20:5n‐3 + 22:6n‐3 per kg feed depending of life stage to maintain good health and growth (Bou et al, ; Ruyter and Thomassen, ). Diets of farmed salmon have traditionally contained high levels of n‐3 PUFA from fish oil (FO) and fish meal, but while the production of these ingredients has changed little since the 1970s, the demand for marine ingredients containing n‐3 PUFA has been increasing globally (Sprague et al, ; Sprague et al, ; Ytrestoyl et al, ).…”