Low levels of very-long-chain n-3 PUFA in Atlantic salmon (Salmo salar) diet reduce fish robustness under challenging conditions in sea cages

Inclusion of oil from transgenic Camelina sativa in feed effectively supplies EPA and DHA to Atlantic salmon (Salmo salar) grown to market size in seawater pens

Atlantic salmon were fed either a diet reflecting current commercial feeds with added oil supplied by a blend of fish oil and rapeseed oil (COM), or a diet formulated with oil from transgenic Camelina sativa containing 20% EPA + DHA (TCO). Salmon were grown from smolt to market size (>3 kg) in sea pens under semi-commercial conditions. There were no differences in growth, feed efficiency or survival between fish fed the TCO or COM diets at the end of the trial. Levels of EPA + DHA in flesh of salmon fed TCO were significantly higher than in fish fed COM. A 140 g fillet from TCO-fed salmon delivered 2.3 g of EPA + DHA, 67% of the weekly requirement level recommended by many health agencies, and 1.5-fold more than the 1.5 g of EPA + DHA for COM-fed fish. Oil from transgenic Camelina supported growth and improved the nutritional quality of farmed salmon in terms of increased "omega-3" supply for human consumers.

Cloning, tissue specificity and regulation of expression of genes of four key enzymes related to long-chain polyunsaturated fatty acids (LC-PUFA) biosynthesis by ambient salinity during embryogenesis in the marine teleost Siganus guttatus

The rabbitfish Siganus canaliculatus was the first marine teleost reported to possess long-chain polyunsaturated fatty acid (LC-PUFA) biosynthetic ability, and the related molecular mechanisms have been well clarified. Here, we investigated the LC-PUFA biosynthesis capability of the congeneric rabbitfish S. guttatus. First, cDNAs of genes for four key enzymes related to LC-PUFA biosynthesis, namely Δ6Δ5 fatty acyl desaturase (fads2) (1335 bp; 445 aa), Δ4 fads2 (1335 bp; 445 aa), and elongation of very long chain fatty acid proteins (elovl5) (873 bp; 291 aa) and elovl4 (906 bp; 302 aa) were cloned from the liver of S. guttatus. The Δ6Δ5 fads2, Δ4 fads2 and elovl5 genes showed high expression in brain, liver, spleen, gallbladder, and intestine but relatively low expression in eye, whereas the elovl4 gene showed specific and high expression in eye. During embryogenesis, mRNA expression of Δ4 fads2 and elovl4 was detected from 8 h post-fertilization (hpf) and then maintained a high level to 24 hpf, while mRNA expression of Δ6Δ5 fads2 and elovl5 reached a peak at 14 hpf but then declined. In addition, ambient salinity (32 ppt and 20 ppt) exerted some regulatory influence on the expression of the four genes during embryogenesis. The levels of C18 PUFA precursors and, especially, PUFA and DHA of the embryos, decreased from 17 hpf to 24 hpf. These results suggested that S. guttatus, similar to the congeneric S. canaliculatus, would have capability for LC-PUFA biosynthesis, which is still not activated at the fertilized egg stage.

Investigation of the functions of n-3 very-long-chain PUFAs in skin using in vivo Atlantic salmon and in vitro human and fish skin models

The purpose of this study was to investigate the effect of dietary n-3 very-long-chain PUFA (n-3 VLC-PUFA) on the maturation and development of skin tissue in juvenile Atlantic salmon (Salmo salar) in vivo, as well as their effects on skin keratocyte and human skin fibroblast cell migration in vitro. Atlantic salmon were fed different dietary levels of n-3 VLC-PUFA from an initial weight of 6 g to a final weight of 11 g. Changes in skin morphology were analysed at two time points during the experiment, and the effects on skin tissue fatty acid composition were determined. Additionally, in vitro experiments using human dermal fibroblasts and primary Atlantic salmon keratocytes were conducted to investigate the effect of VLC-PUFA on the migration capacity of the cells. The results demonstrated that increased dietary levels of n-3 VLC-PUFA led to an increased epidermis thickness and more rapid scale maturation in Atlantic salmon skin in vivo, leading to a more mature skin morphology, and possibly more robust skin, at an earlier life stage. Additionally, human skin fibroblasts and salmon skin keratocytes supplemented with n-3 VLC-PUFA in vitro showed more rapid migration, indicating potentially beneficial effects of VLC-PUFA in wound healing. In conclusion, VLC-PUFA may have beneficial effects on skin tissue development, function and integrity.

Welfare and performance of ballan wrasse (Labrus bergylta) reared at two different temperatures after a preparatory feeding trial with enhanced dietary eicosapentaenoic acid

Concerns have long been raised about the welfare of ballan wrasse (Labrus bergylta) used for the biological control of sea lice in Atlantic salmon (Salmo salar) aquaculture. This study assessed the effect of increased dietary eicosapentaenoic acid (EPA) levels and initial condition factor (CF) on the subsequent performance and welfare of ballan wrasse farmed in high and low water temperatures. Fish were fed a diet with either commercial or high EPA levels for 3 months at 15°C. Subsequently, fish were tagged with a passive integrated transponder, measured for their CF and divided into two groups consisting of fish from both treatments and reared for 4.5 months at either 15 or 6°C fed a commercial diet. Each fish was categorized as high (≥2.7) or low CF (<2.7) fish based on the calculated average CF of the population. Dietary composition influenced the fatty acid (FA) profile of the stored lipids without affecting the growth or welfare of ballan wrasse. Fish reared at 15°C showed higher growth, more fat and energy reserves and less ash content. Fish reared at 6°C lost weight, using up their body lipids at the end of the temperature trial. Gene expression analyses showed upregulation of the positive growth marker (GHrα) and two genes involved in the synthesis and oxidation of FAs (elovl5, cpt1) and downregulation of the negative growth marker (mstn) in fish reared at 15°C compared to those reared at 6°C. Fish reared at 6°C showed upregulated levels of il-6 compared to those reared at 15°C, suggesting an enhanced immune reaction in response to low temperature. Fish with high CF showed better survival, growth and performance compared to those with low CF. External welfare scoring showed higher prevalence and severity in emaciation, scale loss and the sum index score (of all measured welfare parameters) in fish reared at 6°C compared to those reared at 15°C and better welfare in fish with high CF compared to those with low CF. Histological examination of the skin showed that fish reared at 6°C had decreased epidermal thickness, a lower overall number of mucous cells in the inner and outer epidermis and a different organization of mucous cells compared to fish reared at 15°C, indicating stress in fish reared at 6°C. Overall, low water temperatures had profound effects on the performance and external and internal welfare parameters of ballan wrasse and can be considered a stressor likely affecting the delousing efficacy. These findings support the seasonal use of different cleaner fish species. High CF, but not increased dietary EPA levels, appeared to help fish cope better with low water temperatures and should thus be assessed and considered before deploying them in salmon cages.

Different Dietary Ratios of Camelina Oil to Sandeel Oil Influence the Capacity to Synthesise and Deposit EPA and DHA in Zucker Fa/Fa Rats

Plant-based food provides more ALA (α-linolenic acid) and less EPA (eicosapentaenoic acid) and DHA (docosahexanoic acid) than marine food. Earlier studies indicate that cetoleic acid (22:1n-11) stimulates the n-3 pathway from ALA to EPA and DHA. The present study aimed to investigate the dietary effects of camelina oil (CA) high in ALA and sandeel oil (SA) high in cetoleic acid on the conversion of ALA to EPA and DHA. Male Zucker fa/fa rats were fed a diet of soybean oil (Ctrl) or diets of CA, SA, or a combination of CA and SA. Significantly higher levels of DPA (docosapentaenoic acid) and DHA in blood cells from the CA group compared to the Ctrl indicate an active conversion of ALA to DPA and DHA. Increasing the uptake and deposition of EPA and DHA meant that a trend towards a decrease in the liver gene expression of Elovl5, Fads1, and Fads2 along with an increase in the dietary content of SA was observed. However, 25% of the SA could be exchanged with CA without having a significant effect on EPA, DPA, or DHA in blood cells, indicating that bioactive components in SA, such as cetoleic acid, might counteract the inhibiting effect of the high dietary content of DHA on the n-3 biosynthetic pathway.

Dietary Level of the Omega-3 Fatty Acids EPA and DHA Influence the Flesh Pigmentation in Atlantic Salmon

Atlantic salmon with a start weight of 53 g were fed diets with different levels of EPA and DHA or a diet with 1 : 1 EPA+DHA (0%, 1.0%, and 2.0% of the diet). At 400 g, all fish groups were mixed and equally distributed in new tanks and fed three diets with 0.2%, 1.0%, or 1.7% of EPA+DHA. At 1200 g, the fish were transferred to seawater pens where they were fed the same three diets until they reached a slaughter size of 3.5 kg. The fillet concentration of astaxanthin and its metabolite idoxanthin was analysed before transfer to seawater pens at 1200 g and at slaughter. The fatty acid composition in the fillet was also analysed at the same time points. Salmon fed low levels of EPA and DHA had lower fillet astaxanthin concentration and higher metabolic conversion of astaxanthin to idoxanthin compared to salmon fed higher dietary levels of EPA and/or DHA. DHA had a more positive effect on fillet astaxanthin concentrations than EPA. There were positive correlations between fillet DHA, EPA, sum N-3 fatty acids, and fillet astaxanthin concentration. A negative correlation was found between the concentration of N-6 fatty acids in the fillet and the astaxanthin concentration.

Raman spectroscopy and NIR hyperspectral imaging for in-line estimation of fatty acid features in salmon fillets

Raman spectroscopy was compared with near infrared (NIR) hyperspectral imaging for determination of fat composition (%EPA + DHA) in salmon fillets at short exposure times. Fillets were measured in movement for both methods. Salmon were acquired from several different farming locations in Norway with different feeding regimes, representing a realistic variation of salmon in the market. For Raman, we investigated three manual scanning strategies; i) line scan of loin, ii) line scan of belly and iii) sinusoidal scan of belly at exposure times of 2s and 4s. NIR images were acquired while the fillets moved on a conveyor belt at 40 cm/s, which corresponds to an acquisition time of 1s for a 40 cm long fillet. For NIR images, three different regions of interest (ROI) were investigated including the i) whole fillet, ii) belly segment, and iii) loin segment. For both Raman and NIR measurements, we investigated an untrimmed and trimmed version of the fillets, both relevant for industrial in-line evaluation. For the trimmed fillets, a fat rich deposition layer in the belly was removed. The %EPA + DHA models were validated by cross validation (N = 51) and using an independent test set (N = 20) which was acquired in a different season. Both Raman and NIR showed promising results and high performances in the cross validation, with R2_CV = 0.96 for Raman at 2s exposure and R2_CV = 0.97 for NIR. High performances were obtained also for the test set, but while Raman had low and stable biases for the test set, the biases were high and varied for the NIR measurements. Analysis of variance on the squared test set residuals showed that performance for Raman measurements were significantly higher than NIR at 1% significance level (p = 0.000013) when slope-and-bias errors were not corrected, but not significant when residuals were slope-and-bias corrected (p = 0.28). This indicated that NIR was more sensitive to matrix effects. For Raman, signal-to-noise ratio was the main limitation and there were indications that Raman was close to a critical sample exposure time at the 2s signal accumulation.

Increasing dietary levels of the n-3 long-chain PUFA, EPA and DHA, improves the growth, welfare, robustness and fillet quality of Atlantic salmon in sea cages

The present study evaluated the effects of increasing the dietary levels of EPA and DHA in Atlantic salmon (Salmo salar) reared in sea cages, in terms of growth performance, welfare, robustness and overall quality. Fish with an average starting weight of 275 g were fed one of four different diets containing 10, 13, 16 and 35 g/kg of EPA and DHA (designated as 1·0, 1·3, 1·6 and 3·5 % EPA and DHA) until they reached approximately 5 kg. The 3·5 % EPA and DHA diet showed a significantly beneficial effect on growth performance and fillet quality compared with all other diets, particularly the 1 % EPA and DHA diet. Fish fed the diet containing 3·5 % EPA and DHA showed 400-600 g higher final weights, improved internal organ health scores and external welfare indicators, better fillet quality in terms of higher visual colour score and lower occurrence of dark spots and higher EPA and DHA content in tissues at the end of the feeding trial. Moreover, fish fed the 3·5 % EPA and DHA diet showed lower mortality during a naturally occurring cardiomyopathy syndrome outbreak, although this did not reach statistical significance. Altogether, our findings emphasise the importance of dietary EPA and DHA to maintain good growth, robustness, welfare and fillet quality of Atlantic salmon reared in sea cages.

Long-term feeding of Atlantic salmon with varying levels of dietary EPA + DHA alters the mineral status but does not affect the stress responses after mechanical delousing stress

Atlantic salmon were fed diets containing graded levels of EPA + DHA (1·0, 1·3, 1·6 and 3·5 % in the diet) and one diet with 1·3 % of EPA + DHA with reduced total fat content. Fish were reared in sea cages from about 275 g until harvest size (about 5 kg) and were subjected to delousing procedure (about 2·5 kg), with sampling pre-, 1 h and 24 h post-stress. Delousing stress affected plasma cortisol and hepatic mRNA expression of genes involved in oxidative stress and immune response, but with no dietary effects. Increasing EPA + DHA levels in the diet increased the trace mineral levels in plasma and liver during mechanical delousing stress period and whole body at harvest size. The liver Se, Zn, Fe, Cu, and Mn and plasma Se levels were increased in fish fed a diet high in EPA + DHA (3·5 %) upon delousing stress. Furthermore, increased dietary EPA + DHA caused a significant increase in mRNA expression of hepcidin antimicrobial peptide (HAMP), which is concurrent with downregulated transferrin receptor (TFR) expression levels. High dietary EPA + DHA also significantly increased the whole-body Zn, Se, and Mn levels at harvest size fish. Additionally, the plasma and whole-body Zn status increased, respectively, during stress and at harvest size in fish fed reduced-fat diet with less EPA + DHA. As the dietary upper limits of Zn and Se are legally added to the feeds and play important roles in maintaining fish health, knowledge on how the dietary fatty acid composition and lipid level affect body stores of these minerals is crucial for the aquaculture industry.

Low Omega-3 Levels in the Diet Disturbs Intestinal Barrier and Transporting Functions of Atlantic Salmon Freshwater and Seawater Smolts

Due to a limited access to marine raw materials from capture fisheries, Atlantic salmon feeds are currently based on mainly plant ingredients (75%) while only 25% come from traditional marine ingredients including marine fish meal and fish oil. Thus, current feeds contain less of the essential omega-3 fatty acids. The aim of the study was to assess the impact of different omega-3 levels in fish feed on intestinal barrier and transporting functions of Atlantic salmon freshwater and seawater smolts. Atlantic salmon were fed three levels of omega-3 (2, 1 and 0.5%) and fish performance was followed through smoltification and the subsequent seawater acclimation. Intestinal barrier and transporting functions were assessed using Ussing chamber methodology and combined with transcript analysis of tight junction related proteins and ion transporters. A linear decrease in growth was observed with decreasing omega-3 levels. Low (0.5%) inclusion of omega-3 impaired the barrier function of the proximal intestine compared to 2% inclusion. Further, low levels of omega-3 decrease the transepithelial electrical potential across the epithelium indicating disturbed ion transport. It can be concluded that low dietary levels of omega-3 impair somatic growth and intestinal function of Atlantic salmon.

LatitudeTM Oil as a Sustainable Alternative to Dietary Fish Oil in Rainbow Trout (Oncorhynchus mykiss): Effects on Filet Fatty Acid Profiles, Intestinal Histology, and Plasma Biochemistry

The aim of this study was to evaluate the effects of Latitude™ oil (transgenic canola) fed to rainbow trout, Oncorhynchus mykiss, for 52 weeks on growth performance, non-specific immune responses, histology, and filet omega-3 fatty acid content. Latitude™ oil (LO) has high lipid digestibility (93%), and contains omega-3 fatty acids eicosapentaenoic acid (EPA, C20:5n-3), docosapentaenoic acid (DPA, C22:5n-3), and docosahexaenoic acid (DHA, C22:6n-3). Three isonitrogenous (49%), isolipidic (20%) and isocaloric (24.2 MJ kg−1) diets differing by lipid source (0, 8, or 16% LO, replacing fish oil and poultry fat) were fed over an entire production cycle beginning with 19 g juvenile fish. At the end of the 52-week feeding trial, final body weight, weight gain and specific growth rate of fish fed 8% LO (LO-8) and 16% LO (LO-16) diets were significantly higher than those fed the 0% LO (LO-0) diet (P &lt; 0.05). Phagocytic respiratory burst in fish fed the LO-16 diet was significantly higher than those fish fed the other 2 diets (P &lt; 0.05). There were no differences in superoxide dismutase, catalase and lysozyme. Histological examination of the distal intestine indicated reduced inflammation in fish fed the LO-8 diet but not the LO-0 and LO-16 diets. Filet DHA content of fish fed the LO-8 and LO-16 diets were similar to those of fish fed the LO-0 diet. As these diets had lower DHA content, this suggests dietary EPA and DPA from LO was converted to DHA and deposited in the filet. This is supported by increased expression of genes involved in fatty acid elongation, desaturation and beta oxidation in both liver and muscle of fish fed LO (P &lt; 0.05). Total EPA+DHA content of the edible filet ranged between 1,079–1,241 mg 100 g−1 across treatments, each providing the recommended daily intake for human consumption (500–1,000 mg day−1). Overall, this study demonstrated that LO fed over an entire production period is a highly digestible lipid source suitable and sustainable for meeting the fatty acid requirements of rainbow trout, as well as consumer expectations for filet omega-3 fatty acid content.

A Piece of the Puzzle-Possible Mechanisms for Why Low Dietary EPA and DHA Cause Hepatic Lipid Accumulation in Atlantic Salmon (Salmo salar)

The present study aimed at elucidating the effects of graded levels of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) on the hepatic metabolic health of Atlantic salmon reared in sea cages. Diets containing 10, 13, 16 and 35 g/kg EPA + DHA (designated diets 1.0, 1.3, 1.6 and 3.5, respectively) were fed in triplicate through a full production cycle from an average starting weight of 275 g to slaughter size (~5 kg). Feeding low dietary EPA + DHA altered the hepatic energy metabolism, evidenced by reductions in tricarboxylic acid cycle intermediates originating from β-oxidation, which was compensated by elevated activity in alternative energy pathways (pentose phosphate pathway, branched chain amino acid catabolism and creatine metabolism). Increases in various acylcarnitines in the liver supported this and indicates issues with lipid metabolism (mitochondrial β-oxidation). Problems using lipids for energy in the lower EPA + DHA groups line up well with observed increases in liver lipids in these fish. It also aligns with the growth data, where fish fed the highest EPA + DHA grew better than the other groups. The study showed that diets 1.0 and 1.3 were insufficient for maintaining good liver metabolic health. However, diet 3.5 was significantly better than diet 1.6, indicating that diet 1.6 might also be suboptimal.

Modulation of hepatic miRNA expression in Atlantic salmon (Salmo salar) by family background and dietary fatty acid composition

This study finds significant differences in hepatic fatty acid composition between four groups of Atlantic salmon (Salmo salar) consisting of offspring from families selected for high and low capacities to express the delta 6 desaturase isomer b and fed diets with 10% or 75% fish oil. The results demonstrated that hepatic lipid metabolism was affected by experimental conditions (diet/family). The fatty acid composition in the four groups mirrored the differences in dietary composition, but it was also associated with the family groups. Small RNA sequencing followed by RT-qPCR identified 12 differentially expressed microRNAs (DE miRNAs), with expression associated with family groups (miR-146 family members, miR-200b, miR-214, miR-221, miR-125, miR-135, miR-137, miR_nov_1), diets (miR-203, miR-462) or both conditions. All the conserved DE miRNAs have been reported as associated with lipid metabolism in other vertebrates. In silico predictions revealed 37 lipid metabolism pathway genes, including desaturases, transcription factors and key enzymes in the synthesis pathways as putative targets (e.g., srebp-1 and 2, Δ6fad_b and c, hmdh, elovl4 and 5b, cdc42). RT-qPCR analysis of selected target genes showed expression changes that were associated with diet and with family groups (d5fad, d6fad_a, srebp-1). There was a reciprocal difference in the abundance of ssa-miR-203a-3p and srebp-1 in one group comparison, whereas other predicted targets did not reveal any evidence of being negatively regulated by degradation. More experimental studies are needed to validate and fully understand the predicted interactions and how the DE miRNAs may participate in the regulation of hepatic lipid metabolism.

Microalgae Oil as an Effective Alternative Source of EPA and DHA for Gilthead Seabream (Sparus aurata) Aquaculture

Microalgal oils (AOs) emerged recently as an alternative to fish oil and to nutritionally poorer vegetable oils for fish species. In this trial, two experimental diets containing fish oil (negative control: 2.1%; positive control: 13.8%) and two diets incorporating AO at 3.5 and 0.7% were fed to grow out gilthead seabream (Sparus aurata) of 64.5 g initial body weight. After 110 days of experimental feeding, performance (final body weight mean = 147 g) and survival (>99%) were similar across treatments. The highest eicosapentaenoic acid (EPA) + docosahexaenoic acid (DHA) content in positive control (PC) and 3.5 AO feeds (3.11 and 2.18% of diet, respectively) resulted in the highest EPA + DHA deposition in the fillets (18.40 and 12.36 g/100 g fatty acid, respectively), which entirely reflected the dietary fatty acid profile. Feed and fillets from fish fed the AO diets had lower levels of dioxins and polychlorinated biphenyls (PCBs). Moreover, sensory quality of AO fillets scored equally to the PC fish. Collectively, these findings offer a more resilient means for sustaining the future growth of seabream aquaculture, whilst maintaining the nutritional value of the resulting seafood. The data supports the addition of seabream to the list of aquaculture species where microalgal oil can be used as an ingredient to fulfil their challenging nutritional demands.

Effects of Varying Dietary Docosahexaenoic, Eicosapentaenoic, Linoleic, and α-Linolenic Acid Levels on Fatty Acid Composition of Phospholipids and Neutral Lipids in the Liver of Atlantic Salmon, Salmo salar

Fish oil, the most abundant natural source of docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), is a limited resource; however, terrestrial oils are used as an alternative in fish nutrition. The liver of Atlantic salmon is able to synthesize these two long-chain n-3 polyunsaturated fatty acids (n-3LC-PUFAs) from α-linolenic acid (ALA), but the dietary levels of EPA + DHA and the ratios of linoleic acid (LNA) to ALA may affect its abilities. Feeding Atlantic salmon four experimental diets containing EPA + DHA at 0.3 and 1.0% of dietary levels accompanied with high and low LNA/ALA ratios showed that low LNA/ALA ratios increased the proportions of EPA + DHA in phospholipids (PLs) and neutral lipids (NLs). The pattern of PL-to-NL ratios of n-3 LC-PUFA proportions matched the saw tooth pattern of LNA/ALA ratios in diets. Overall, when fish oil is removed from salmon diets, the dietary LNA/ALA ratio must be reduced to stimulate biosynthesis of n-3 LC-PUFAs in the liver.

Increasing dietary n-6 fatty acids while keeping n-3 fatty acids stable decreases EPA in polar lipids of farmed Atlantic salmon (Salmo salar)

There is an increased use of vegetable oils containing n-6 fatty acids (FA) in aquafeeds, and several trials indicate that there might be an increased requirement of EPA and DHA for Atlantic salmon when they are fed higher dietary n-6 FA. With a limited supply of EPA and DHA for production of aquafeeds, it is important to know how to efficiently use these FA to maintain growth and health of the fish. In the present trial, three diets containing equal amounts of n-3 FA (about 7·7 % of total FA) and different n-6:n-3 FA ratios (about 1, 2 and 6), as well as one diet with n-6:n-3 FA ratio at about 1 but twice as much n-3 FA, were fed to Atlantic salmon. Despite constant dietary n-3, increasing dietary n-6 led to significantly reduced n-3 in tissue polar lipids. Interestingly, EPA was significantly reduced while DHA was not. Maintaining a stable n-3 content in the polar lipids when increasing dietary n-6 FA was only obtained by simultaneously increasing the dietary n-3 content and with this maintaining the same n-6:n-3 FA ratio. Polar lipid n-6 FA in tissues thus primarily reflected the dietary n-6:n-3 FA ratio and not the absolute dietary n-6 FA content. Neutral lipids, on the other hand, reflected the dietary absolute levels of both n-3 and n-6 FA. This study indicates that a better use of dietary EPA is achieved by keeping the dietary n-6:n-3 FA ratio low.

Increasing temperature and productivity change biomass, trophic pyramids and community-level omega-3 fatty acid content in subarctic lake food webs

Climate change in the Arctic is outpacing the global average and land-use is intensifying due to exploitation of previously inaccessible or unprofitable natural resources. A comprehensive understanding of how the joint effects of changing climate and productivity modify lake food web structure, biomass, trophic pyramid shape and abundance of physiologically essential biomolecules (omega-3 fatty acids) in the biotic community is lacking. We conducted a space-for-time study in 20 subarctic lakes spanning a climatic (+3.2°C and precipitation: +30%) and chemical (dissolved organic carbon: +10 mg/L, total phosphorus: +45 µg/L and total nitrogen: +1,000 µg/L) gradient to test how temperature and productivity jointly affect the structure, biomass and community fatty acid content (eicosapentaenoic acid [EPA] and docosahexaenoic acid [DHA]) of whole food webs. Increasing temperature and productivity shifted lake communities towards dominance of warmer, murky-water-adapted taxa, with a general increase in the biomass of primary producers, and secondary and tertiary consumers, while primary invertebrate consumers did not show equally clear trends. This process altered various trophic pyramid structures towards an hour glass shape in the warmest and most productive lakes. Increasing temperature and productivity had negative fatty acid content trends (mg EPA + DHA/g dry weight) in primary producers and primary consumers, but not in secondary nor tertiary fish consumers. The massive biomass increment of fish led to increasing areal fatty acid content (kg EPA + DHA/ha) towards increasingly warmer, more productive lakes, but there were no significant trends in other trophic levels. Increasing temperature and productivity are shifting subarctic lake communities towards systems characterized by increasing dominance of cyanobacteria and cyprinid fish, although decreasing quality in terms of EPA + DHA content was observed only in phytoplankton, zooplankton and profundal benthos.

Dietary inclusion of Antarctic krill meal during the finishing feed period improves health and fillet quality of Atlantic salmon (Salmo salar L.)

There is an urgent need to find alternative feed resources that can further substitute fishmeal in Atlantic salmon diets without compromising health and food quality, in particular during the finishing feeding period when the feed demand is highest and flesh quality effects are most significant. This study investigates efficacy of substituting a isoprotein (35 %) and isolipid (35 %) low fishmeal diet (FM, 15 %) with Antarctic krill meal (KM, 12 %) during 3 months with growing finishing 2·3 kg salmon (quadruplicate sea cages/diet). Final body weight (3·9 (se 0·04) kg) was similar in the dietary groups, but the KM group had more voluminous body shape, leaner hearts and improved fillet integrity, firmness and colour. Ectopic epithelial cells and focal Ca deposits in intestine were only detected in the FM group. Transcriptome profiling by microarray of livers showed dietary effects on several immune genes, and a panel of structural genes were up-regulated in the KM group, including cadherin and connexin. Up-regulation of genes encoding myosin heavy chain proteins was the main finding in skeletal muscle. Morphology examination by scanning electron microscopy and secondary structure by Fourier transform IR spectroscopy revealed more ordered and stable collagen architecture of the KM group. NEFA composition of skeletal muscle indicated altered metabolism of n-3, n-6 and SFA of the KM group. The results demonstrated that improved health and meat quality in Atlantic salmon fed krill meal were associated with up-regulation of immune genes, proteins defining muscle properties and genes involved in cell contacts and adhesion, altered fatty acid metabolism and fat deposition, and improved gut health and collagen structure.

DHA Modulates Immune Response and Mitochondrial Function of Atlantic Salmon Adipocytes after LPS Treatment

Adipocytes play a central role in overall energy homeostasis and are important contributors to the immune system. Fatty acids (FAs) act as signaling molecules capable to modulate adipocyte metabolism and functions. To identify the effects of two commonly used FAs in Atlantic salmon diets, primary adipocytes were cultured in the presence of oleic (OA) or docosahexaenoic (DHA) acid. DHA decreased adipocyte lipid droplet number and area compared to OA. The increase in lipid load in OA treated adipocytes was paralleled by an increase in iNOS activity and mitochondrial SOD2-GFP activity, which was probably directed to counteract increase in oxidative stress. Under lipopolysaccharide (LPS)-induced inflammation, DHA had a greater anti-inflammatory effect than OA, as evidenced by the higher SOD2 activity and the transcriptional regulation of antioxidant enzymes and pro- and anti-inflammatory markers. In addition, DHA maintained a healthy mitochondrial structure under induced inflammation while OA led to elongated mitochondria with a thin thread like structures in adipocytes exposed to LPS. Overall, DHA possess anti-inflammatory properties and protects Atlantic salmon against oxidative stress and limits lipid deposition. Furthermore, DHA plays a key role in protecting mitochondria shape and function.

Liver Transcriptome Profiling Reveals That Dietary DHA and EPA Levels Influence Suites of Genes Involved in Metabolism, Redox Homeostasis, and Immune Function in Atlantic Salmon (Salmo salar)

The optimal dietary requirement of omega-3 long-chain polyunsaturated fatty acids (ω3 LC-PUFA), namely docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), for Atlantic salmon that promotes growth and health warrants careful investigation. We used 44K microarrays to study the influence of increasing levels of dietary DHA + EPA (0, 1.0, and 1.4% of the diet, as formulated) in the presence of high linoleic acid (LA) on Atlantic salmon growth and liver transcriptome. After a 14-week feeding trial, Atlantic salmon fed diet ω3LC0 (i.e. 0% of DHA + EPA) showed significantly lower final weight and weight gain, and higher feed conversion ratio compared with ω3LC1.0 and ω3LC1.4 diet groups. The microarray experiment identified 55 and 77 differentially expressed probes (Rank Products analyses; PFP < 10%) in salmon fed diets ω3LC1.4 and ω3LC1.0 compared with those fed diet ω3LC0, respectively. The comparison between ω3LC1.4 and ω3LC1.0 revealed 134 differentially expressed probes. The microarray results were confirmed by qPCR analyses of 22 microarray-identified transcripts. Several key genes involved in fatty acid metabolism including LC-PUFA synthesis were upregulated in fish fed ω3LC0 compared with both other groups. Hierarchical clustering and linear regression analyses of liver qPCR and fatty acid composition data demonstrated significant correlations. In the current study, 1.0% ω3 LC-PUFA seemed to be the minimum requirement for Atlantic salmon based on growth performance; however, multivariate statistical analyses (PERMANOVA and SIMPER) showed that fish fed ω3LC1.0 and ω3LC1.4 diets had similar hepatic fatty acid profiles but marked differences in the transcript expression of biomarker genes involved in redox homeostasis (mgst1), immune responses (mxb, igmb, irf3, lect2a, srk2, and lyz2), and LC-PUFA synthesis (srebp1, fadsd5, and elovl2). This research has provided new insights into dietary requirement of DHA and EPA and their impact on physiologically important pathways in addition to lipid metabolism in Atlantic salmon.

Effect of fatty fish or nut consumption on concentrations of persistent organic pollutants in overweight or obese men and women: A randomized controlled clinical trial

BACKGROUND AND AIMS

While excess energy intake and physical inactivity constitute the obvious causes of body fat accumulation, persistent organic pollutants (POPs) are novel factors that have been linked to cardiometabolic disorders. Major sources of POPs are animal fats including fatty fish. Given the putative protective effects of fish on cardiovascular disease, we explored whether high consumption of fatty fish increased serum concentrations of POPs.

METHODS AND RESULTS

Men and women aged 35-70 years with body mass index between 25 and 38 kg/m² and at least 1 cardiometabolic component were randomized to high intakes of fatty fish (mostly farmed salmon, ∼630 g/week; n = 45), high intakes of nuts (∼200 g/week; n = 42) or a control group following their usual diet but restricting fatty fish and nuts for 6 months (n = 44). Concentrations of 15 POPs (5 organochlorinated compounds, 2 dioxin-like polychlorinated biphenyls and 8 non-dioxin-like polychlorinated biphenyls) and cardiometabolic risk factors were measured at baseline and end of the study. Results showed that changes in concentrations of individual and classes of POPs did not differ between the dietary groups and controls (p > 0.05). Among cardiometabolic risk factors HDL-cholesterol increased in the fatty fish group compared to controls (+0.10 mmol/L, CI (0.05-0.20); p = 0.005) while no changes were observed in the group consuming nuts.

CONCLUSION

Fatty fish consumption for 6 months did not increase the serum concentrations of POPs in individuals with overweight or obesity and metabolic risk. While this finding appears reassuring regarding short-term intakes of farmed salmon, long term variations in POPs in adipose stores require further study.

Interaction between dietary fatty acids and genotype on immune response in Atlantic salmon (Salmo salar) after vaccination: A transcriptome study

A pivotal matter to aquaculture is the sourcing of sustainable resources as ingredients to aquafeeds. Levels of plant delivered oils as source of fatty acids (FA) in aquafeeds have reached around 70% resulting in reduced levels of long-chain omega-3 polyunsaturated fatty acids (LC n-3 PUFA), such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), in salmon fillet composition. EPA and DHA can modulate inflammation and immune response, so it is crucial to understand how fish immune response is affected by low LC n-3 PUFA diet and if this diet can have a detrimental effect on vaccine response. Atlantic salmon (Salmo salar) can produce EPA/DHA from α-linolenic acid (ALA) and this endogenous capacity can be explored to develop families with higher tolerance to low LC n-3 PUFA diets. Here we analyze innate and adaptive immune response in Atlantic salmon to a commercial vaccine after being fed low levels of EPA and DHA, and we also compare three strains of salmon selected by their endogenous capacity of synthesizing LC- n-3 PUFA. A total of 2,890 differentially expressed genes (DEGs) were identified (p-value adjusted < 0.1) when comparing vaccinated fish against control non-vaccinated. Gene ontology (GO) and KEGG analysis with 442 up/downregulated genes revealed that most DEGs were both related to immune response as well as part of important immune related pathways, as "Toll-like receptor" and "Cytokine-Cytokine interaction". Adaptive response was also addressed by measuring antigen specific IgM, and titers were significantly higher than in the pre-immune fish at 62 days post-immunization. However, diet and strain had no/little effect on vaccine-specific IgM or innate immune responses. Atlantic salmon therefore display robustness in its response to vaccination even when feed low levels of LC n-3 PUFA.

The long-chain monounsaturated cetoleic acid improves the efficiency of the n-3 fatty acid metabolic pathway in Atlantic salmon and human HepG2 cells

The present study aimed to determine if the long-chain MUFA cetoleic acid (22 : 1n-11) can improve the capacity to synthesise the health-promoting n-3 fatty acids EPA and DHA in human and fish models. Human hepatocytes (HepG2) and salmon primary hepatocytes were first enriched with cetoleic acid, and thereafter their capacities to convert radio-labelled 18 : 3n-3 (α-linolenic acid, ALA) to EPA and DHA were measured. Increased endogenous levels of cetoleic acid led to increased production of radio-labelled EPA + DHA in HepG2 by 40 % and EPA in salmon hepatocytes by 12 %. In order to verify if dietary intake of a fish oil rich in cetoleic acid would have the same beneficial effects on the n-3 fatty acid metabolic pathway in vivo as found in vitro, Atlantic salmon were fed four diets supplemented with either sardine oil low in cetoleic acid or herring oil high in cetoleic acid at two inclusion levels (Low or High). The diets were balanced for EPA + DHA content within the Low and within the High groups. The salmon were fed these diets from 110 to 242 g. The level of EPA + DHA in liver and whole-body retention of docosapentaenoic acid and EPA + DHA relative to what was eaten, increased with increased dietary cetoleic acid levels. Thus, it is concluded that cetoleic acid stimulated the synthesis of EPA and DHA from ALA in human HepG2 and of EPA in salmon hepatocytes in vitro and increased whole-body retention of EPA + DHA in salmon by 15 % points after dietary intake of cetoleic acid.

Impact of Reduced Dietary Levels of Eicosapentaenoic Acid and Docosahexaenoic Acid on the Composition of Skin Membrane Lipids in Atlantic Salmon ( Salmo salar L.)

Membrane lipids, including sphingolipids and glycerol-phospholipids, are essential in maintaining the skin's barrier function in mammals, but their composition in fish skin and their response to diets have not been evaluated. This study investigated the impacts of reducing dietary eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) on membrane lipids in the skin of Atlantic salmon through a 26 week feeding regime supplying different levels (0-2.0% of dry mass) of EPA/DHA. Ceramide, glucosylceramide, sphingomyelin, sphingosine, and sphinganine in salmon skin were analyzed for the first time. Higher concentrations of glucosylceramide and sphingomyelin and higher ratios of glucosylceramide/ceramide and sphingomyelin/ceramide were detected in the deficient group, indicating interruptions in sphingolipidomics. Changes in the glycerol-phospholipid profile in fish skin caused by reducing dietary EPA and DHA were observed. There were no dietary impacts on epidermal thickness and mucus-cell density, but the changes in the phospholipid profile suggest that low dietary EPA and DHA may interrupt the barrier function of fish skin.

Dietary resveratrol impairs body weight gain due to reduction of feed intake without affecting fatty acid composition in Atlantic salmon

Recent studies suggest that the use of vegetable oils at expense of fish oil in aquaculture feeds might have potential negative effects on fish redox homeostasis and adiposity. Resveratrol (RESV) is a lipid-soluble phytoalexin present in fruits and vegetables with proven in vivo antioxidant function in animals. The present study aims to assess the potential use of RESV in Atlantic salmon feeds. To this end, post-smolt salmons with an initial BW of 148±3 g were fed four experimental diets for 15 weeks. A diet low in fish oil served as a control and was supplemented with 0, 0.5, 1.5 and 2.5 g/kg of RESV, respectively. The effect of the experimental diets on animal performance, tissue fatty acid composition, and the expression of genes encoding proteins involved in antioxidant signalling, lipid peroxidation, and metabolism were studied. Resveratrol significantly reduced feed intake and final BW of the salmon. Feeding RESV did not affect the sum of saturated and monounsaturated fatty acids or total lipids in the fillet. While the content of total polyunsaturated fatty acids was not affected, the percentages of some fatty acids in the liver and fillet were changed by RESV. Furthermore, in liver, the relative expression of glutathione peroxidase 4b, nuclear factor-like 2, and arachidonate 5-lipoxygenase remained unchanged across treatment groups. In conclusion, the negative impact of dietary RESV on FI and hence reduction of the BW discourages its inclusion in low fish oil diets for Atlantic salmon.

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

'Are we what we eat?' Yes and no. Although dietary fat affects body fat, there are many modifying mechanisms. In Atlantic salmon, there is a high level of retention of the n-3 fatty acid (FA) docosahexaenoic acid (DHA, 22:6n-3) relative to the dietary content, whereas saturated FAs never seem to increase above a specified level, which is probably an adaptation to low and fluctuating body temperature. Net production of eicosapentaenoic acid (EPA, 20:5n-3) and especially DHA occurs in salmon when dietary levels are low; however, this synthesis is not sufficient to maintain EPA and DHA at similar tissue levels to those of a traditional fish oil-fed farmed salmon. The commercial diets of farmed salmon have changed over the past 15 years towards a more plant-based diet owing to the limited availability of the marine ingredients fish meal and fish oil, resulting in decreased EPA and DHA and increased n-6 FAs. Salmon is part of the human diet, leading to the question 'Are we what the salmon eats?' Dietary intervention studies using salmon have shown positive effects on FA profiles and health biomarkers in humans; however, most of these studies used salmon that were fed high levels of marine ingredients. Only a few human intervention studies and mouse trials have explored the effects of the changing feed composition of farmed salmon. In conclusion, when evaluating feed ingredients for farmed fish, effects throughout the food chain on fish health, fillet composition and human health need to be considered.