Search

Thraustochytrids are a group of unicellular marine protists that are noted for their ability to produce high amounts of lipids, especially docosahexaenoic acid (DHA), making them a valuable source for replacing fish oil in aquafeeds. Varying levels of Aurantiochrytrium acetophilum meal (TM) were evaluated as replacement of fish oil in extruded diets for juvenile Atlantic salmon on growth, nutrient apparent digestibility, fatty acid composition in muscle and liver and expression of genes associated with lipid metabolism. Four experimental diets were tested: a control diet without the inclusion of A. acetophilum meal and three experimental diets derived from the control diet, in which fish oil was progressively replaced by the addition of A. acetophilum meal at 2.5% (TM 2.5), 6.5% (TM 6.5) and 12% (TM 12) respectively. After 12 weeks of feeding, the growth, feed conversion, condition factor and survival of the fish were not affected by the inclusion of A. acetophilum in the diet. However, the 12% inclusion significantly decreased the viscerosomatic index of the fish and the digestibility of protein, lipids, ash and energy compared to the other experimental diets. The lipid content and fatty acid profile of the liver and muscle were significantly affected by diet composition. In both tissues, fish fed the TM 12 diet retained the highest levels of DHA. However, it was also observed that EPA concentrations lessened with increasing dietary content of A. acetophilum meal. Finally, the decrease in the expression of Δ5fad in the fish fed with the TM 6.5 and TM 12 diets showed that fish do not require a higher expression of this desaturase for DHA production, due to the high content of this fatty acid in A. acetophilum meal. • A. acetophilum meal can be used as lipids source in Atlantic salmon diets. • Higher dietary inclusions of A. acetophilum meal modify visceral fat reserves in Atlantic salmon. • Substitution of fish oil with 12% A. acetophilum meal affects nutrient digestibility in Atlantic salmon juvenile. • DHA retention in liver and muscle of Atlantic salmon increased with the dietary concentration of A. acetophilum meal. • Diets with high levels of DHA from A. acetophilum meal depressed expression of the Δ5fad. [ABSTRACT FROM AUTHOR]

Recent studies have found that feeding high levels of long chain polyunsaturated fatty acids (LC-PUFA) influenced the steroid biosynthesis pathway of Atlantic salmon on a transcriptomic level, although research observed on a metabolic level is lacking. Dietary lipid and chronic stress have also been suggested to play a role in steroidogenesis. A study was performed on Atlantic salmon post-smolts (184 g) fed with diets of high and low levels of lipid with and without high levels of LC-PUFA under hypoxic (stressed) or normoxic (unstressed) conditions. Liver, head kidney and blood samples were collected after 35 and 116 days to determine short- and long-term effects. On day 35, dissolved oxygen affected the levels of the steroid hormones 11-deoxycortisol, cortisone and testosterone as well as the relative expression of cyp11a involved in steroidogenesis and ud2a2 involved in sex steroid metabolism. On day 116, an interaction between lipid x LC-PUFA x oxygen was found for cyp11a , hsd3b (steroidogenesis) and ud2a2 genes, whereas LC-PUFA and/or oxygen affected the steroid levels of 11-dehydrocorticosterone, corticosterone, cortisol, dehydroepiandrosterone, pregnenolone and testosterone. The observation that both LC-PUFA and oxygen affected cyp11a expression and the levels of pregnenolone, provides evidence for the influence of both parameters on the rate-limiting point of steroid production, which can take >35 days to establish. The 3-way interaction between lipid, LC-PUFA and dissolved oxygen on the expression of steroid genes suggests that the chronic stressor of hypoxia may be over-riding dietary influences of the lipid and LC-PUFA, though the relative proportion of LC-PUFA within the lipid may play a role as well. In terms of fatty acid composition, low dietary lipid and high LC-PUFA increased the levels of total n-6 and n-3 PUFA in the liver, particularly EPA and DHA, and the head kidney to a lesser extent. In terms of immune pathways, hypoxia influenced cytokine gene expression (tgfb, il10), cellular repair gene expression (gr, hsp70 and hsp90) and eicosanoid levels (PGE2 and LTB5) in the head kidney, while dietary lipid and LC-PUFA influenced cytokines (ifnγ and il1β) and eicosanoids (PGE2 and LTB5). These findings demonstrate that feeding different levels of LC-PUFA significantly alters steroidogenesis and innate immune response in Atlantic salmon post-smolts with additional interactions from dietary lipid and hypoxia stress. These results will significantly impact the aquaculture industry since it demonstrates salmon fed high levels of LC-PUFA are likely more resilient to cope with chronic stressors (e.g. prolonged hypoxia) due to lower corticosteroid levels and upregulated immune response. • Effect of dietary lipid, LC-PUFA and oxygen on steroid and immune responses of Atlantic salmon. • Hypoxic stress upregulated expression and levels of steroids and immune factors. • High dietary LC-PUFA downregulated expression and levels of steroids after 116 days. • High dietary lipid and LC-PUFA upregulated cytokine expression and levels of eicosanoids. • Salmon fed higher LC-PUFA are likely more resilient to cope with hypoxic stress. [ABSTRACT FROM AUTHOR]

Abstract: Salmon aquaculture is currently reliant on fish oil as the major source of the very long chain (VLc) n−3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). This report details the first study of an alternative source of EPA from the yeast Yarrowia lipolytica in feed for fish. Atlantic salmon was fed three dietary levels, 10% (Y10), 20% (Y20) and 30% (Y30) of a heat killed dried yeast biomass containing 6% EPA and 20% oil by weight for 95days. The yeast biomass was replacing fish meal, rapeseed oil and wheat meal in the diets. The increase in fish weight from 180 to 400g on the diets containing yeast, was comparable to groups fed control diets with either rapeseed oil (RO) or a mix of rapeseed and fish oil. The apparent digestibility coefficient for energy, protein and lipids, including n−3 fatty acids, decreased with increased inclusion of yeast biomass. Feed conversion ratio was higher, and energy retention lower, in the Y30 group than in the others. Protein retention was the lowest in the Y30 and FO groups followed by Y20. Fillet fatty acid composition reflected the higher ratio of EPA to DHA in the feeds added yeast. However, lower whole body retention of EPA, and higher retention of DPA (docosapentaenoic acid) and DHA, compared to average fatty acid retention, suggest that there had been some conversion of the EPA provided by the yeast into DPA (docosapentaenoic acid) and DHA in fish. [Copyright &y& Elsevier]

Abstract: Recently, we reported that growth of Atlantic salmon was reduced as dietary fishmeal (FM) was lowered from 25% to 5% in dual-substituted feeds compared to a control diet, formulated to represent the current upper levels of substitution of FM and fish oil. In the present study, the effects of different alternative protein blends and binders on growth of salmon fed dual-substituted feeds containing only 11% FM, and with 60% of dietary fish oil replaced by rapeseed oil were investigated. Salmon of initial weight 1.3kg were grown to market size (>3kg) over a period of 19weeks. Salmon fed the diets with reduced FM showed lower final weight, SGR and TGC, associated with reduced feed intake. There was a tendency for increased FCR in fish fed the diets containing reduced FM although this was not significant, and there was no effect on PER. There were no significant effects on digestibility of protein or fat but the two parameters varied reciprocally and there were clear trends of increased protein and lower fat digestibilities in fish fed diets with reduced FM. Although lipid and fatty acid compositions did not vary greatly between diets there were significant effects on fish tissue compositions. Thus, liver lipid was generally reduced in fish fed diets with lower FM, significantly so in two of the four treatments. The proportions of monoenes were significantly lower and those of polyunsaturated fatty acids (PUFA) significantly higher in flesh and liver of fish fed diets with reduced levels of FM. The increased proportions of PUFA were due to increased percentages of 20:4n−6, 20:5n−3, 22:5n−3 and, although not consistently significant, 22:6n−3. The mechanisms for these unexpected effects of diet on tissue lipids and fatty acids are discussed. [Copyright &y& Elsevier]

Abstract: Although the use of fish meal (FM) and fish oil (FO) has been extensive in Atlantic salmon culture, there is a growing need for less reliance on these commodities. Moreover, it is crucial for the aquafeed industry to optimise the use of dietary protein and to improve the protein utilisation in salmon diets. The interactive effects of the dietary protein/lipid level and rapeseed oil (RO) inclusion on growth, feed utilisation, nutrient and fatty acid (FA) digestibility and whole body chemical composition of large Atlantic salmon (Salmo salar L.), reared at summer water temperatures (11.6°C), were investigated in a ten week feeding trial. The fish (initial weight 2053g) were fed six isoenergetic diets in a factorial design containing 350g kg−1/350g, kg−1, 330g kg−1/360g kg−1, 290g kg−1/380g kg−1 of protein/lipid for high protein (HP), medium protein (MP) and low protein (LP) diets, respectively. At all protein/lipid levels the oil source was either FO or RO (60% of the added oil). At the end of the trial the final weights ranged from 3340–3664g and the FCR from 0.99–1.10. The protein level did not affect significantly any of the growth parameters but the oil source had a significant effect on final weight, specific growth rate (SGR) and thermal growth coefficient (TGC), showing improved growth with RO inclusion. This could be explained by the significantly higher lipid digestibility of the fish fed the diets containing RO (86.1 vs. 92.2%) which was probably affected by the diet FA composition; the apparent digestibility coefficient (ADC) of saturated FA, and to a lesser extent of unsaturated FA and especially monoenes, was improved by RO inclusion. The protein ADC was significantly affected by the protein level indicating a higher ADC for the HP diets compared to the LP (80.1 vs. 77.7%, respectively). 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. [Copyright &y& Elsevier]

Abstract: This work reports the interpretation of the spectra from high-resolution 1H MR spectroscopy of extracts, muscle, and whole Atlantic salmon, the latter using a MR image localization technique. It was possible to identify single chemical components, such as hypoxanthine, amino acids, anserine, lactate and some fatty acids, in extracts, whole muscle and whole fish. These techniques may have practical applications for i) the selection of live specimens for breeding, ii) the classification of both live specimens and fillets according to their qualitative and quantitative content of lipids and small molecules, which is of relevance for the nutritional value of fish, and iii) authentication analyses of Atlantic salmon. [Copyright &y& Elsevier]

The use of microbial oil, high in docosahexaenoic acid (DHA), was investigated as a lipid source in diets for farmed Atlantic salmon. Schizochytrium sp. oil is produced from heterotrophic fermentation and it can be a renewable source of long-chain n-3 fatty acids due to its high DHA content. A 16-week feeding trial was conducted on Atlantic salmon (24 g initial weight) in freshwater to test the effects of microbial oil (MO) from Schizochytrium sp. (T18), on fish growth performance and tissue lipid and fatty acid composition. Four experimental diets were tested: a reference diet (20% FO) and a commercial-type control diet (10% FO + 10% vegetable oil), a low MO diet (5% MO) and a high MO diet (10% MO). There were no differences in weight gain (81–98 g), condition factor (1.08), specific growth rate (1.30–1.53%/day), nor feed conversion ratio (0.84–0.90) among all treatments (p > 0.05). Fatty acids in total lipid in the muscle reflected the fatty acid profile in the diet, particularly in salmon fed low and high MO diets, with high muscle storage of DHA and low EPA compared to salmon fed the FO and FO/VO control diets. Overall, Schizochytrium sp. (T18) is a candidate for use in dietary oil blends for juvenile salmon. • Growth performance of salmon (>300%) was not different among treatments. • High levels of DHA and low EPA, were stored in muscle, liver of salmon fed MO diets. • Schizochytrium sp. (T18) can effectively replace fish oil in diets for juvenile salmon. [ABSTRACT FROM AUTHOR]