27It has been known for almost 25 years now that inclusion of intact phospholipids in the diet could 28 improve culture performance of various freshwater and marine fish species. The primary 29 beneficial effect was improved growth in both larvae and early juveniles, but also increased 30 survival rates and decreased incidence of malformation in larvae, and perhaps increased stress 31 resistance. Determination of absolute dietary requirements has been hampered by the use, in 32 different dietary trials, of a wide range of phospholipid preparations that can vary greatly both in 33 phospholipid content and class composition. Larval studies have been compromised further by the 34 need on many occasions to supply phospholipid through enrichment of live feeds with subsequent 35 re-modelling of the phospholipid and fatty acid compositions. Generally, the levels of 36 phospholipid requirement are around 2 -4% of diet for juvenile fish and probably higher in larval 37 fish. The effects were restricted to young fish, as a requirement for dietary phospholipids has not 38 been established for adult fish, although this has been virtually unstudied. As the majority of 39 studies have used crude mixed phospholipid preparations, particularly soybean lecithin, but also 40 other plant phospholipids and egg yolk lecithin, that are enriched in several phospholipids, it has 41 been difficult to elucidate which specific phospholipid classes impart beneficial effects. Based on 42 the few studies where single pure phospholipid species have been used, the rank order for efficacy 43 appears to be phosphatidylcholine > phosphatidylinositol > phosphatidylethanolamine > 44 phosphatidylserine. The efficacy of other phospholipid classes or sphingolipids is not known. The 45 mechanism underpinning the role of the phospholipids in larval and early juvenile fish must also 46 explain their lack of effect in adult fish. The role of phospholipids appears to be independent of 47 fatty acid requirements although the presence of an unsaturated fatty acid at the sn-2 position may 48 be important. Similarly, the phospholipid requirement is not related to the delivery of other 49 essential dietary components such as the bases choline and inositol. Studies also suggested that the 50 phospholipid effect was not due to generally enhanced emulsification and digestion of lipids. 51Rather the evidence led to the hypothesis that early developing stages of fish had impaired ability 52 to transport dietary lipids away from the intestine possibly through limitations in lipoprotein 53 synthesis. The current hypothesis is that the enzymic location of the limitation is actually in 54 phospholipid biosynthesis, perhaps the production of the glycerophosphobase backbone and that 55 dietary supplementation with intact phospholipids in larvae and juvenile fish compensated for this. 56

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Fatty acid compositions of both the polar phospholipids (PLs) and the non‐polar neutral lipids (NLs) in fish tissues are influenced by dietary fatty acids, and tissue fatty acid compositions also change during thermal acclimation. The interaction between these factors in governing fatty acid compositions has been little studied, even though this may have importance when fish are reared in cold water. An experiment was conducted to investigate the effects of temperature (2 vs. 8°C), dietary oil source (fish oil vs. vegetable oils) and feed fat content (21% vs. 34% fat) on tissue fatty acid compositions of Atlantic salmon parr. The fish were held in fresh water under a 12 h light:12 h dark photoperiod until they doubled in body mass (from ca. 19 to 38 g, which took ca. 2 months at 8°C and ca. 6 months at 2°C), and then the muscle, viscera and carcass PLs and NLs were analysed for fatty acid composition. The ratios of unsaturated to saturated fatty acids (UFA:SFA), and the unsaturation indices (UIs, the number of unsaturated bonds per 100 fatty acid molecules) were calculated for each lipid class (PL, NL) within each tissue (muscle, viscera and carcass). Both dietary fatty acids and temperature influenced the compositions of the tissue lipids, with the dietary effects being most pronounced. The fatty acid composition of the feed oils was strongly reflected in the NLs of all three tissues, and also had a significant influence on the fatty acid profiles of the PLs. The effects of temperature were more marked in the PLs than in the NLs. Exposure to the lower temperature resulted in PLs with higher UFA:SFAs, and this is interpreted as a thermal acclimation response that would contribute to the maintenance of membrane fluidity. The PLs of fish given vegetable oils had higher UFA:SFAs than those of salmon provided with feeds containing fish oil, implying that the cell membranes of the former may have had greater fluidity. By contrast, the PLs of salmon fed with fish oil had higher concentrations of n‐3 highly unsaturated fatty acids, and higher UIs, which may have made them more prone to peroxidative damage. This raises the intriguing possibility that the feeding of vegetable oils may have produced fish that were better able to withstand exposure to low temperature as a result of improved membrane fluidity (implied from the higher UFA:SFAs in these fish), while having membrane PLs that were less susceptible to oxidative damage (implied from the lower UIs).

Please cite this article as: Karalazos, V., Bendiksen, E.Å., Bell, J.G., Interactive effects of dietary protein / lipid level and oil source on growth, feed utilisation and nutrient and fatty acid digestibility of Atlantic salmon, Aquaculture (2010), doi: 10.1016/j.aquaculture. 2010.11.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.A C C E P T E D M A N U S C R I P T Regarding the whole body composition, moisture was significantly affected by both factors, the fat content was significantly affected only by the oil source, while significant interactions were shown for the protein content. In conclusion, the results of this study suggest that low protein / high lipid diets can be used with no negative effects on the growth, FCR and chemical composition of Atlantic salmon reared at high water temperatures. Moreover, the replacement of FO with RO can enhance the growth of the fish as well as the nutrient and FA digestibility of the diets.