Fatty acid metabolism (desaturation, elongation and beta-oxidation) in rainbow trout fed fish oil- or linseed oil-based diets

Fish oil replacement with different vegetable oils in Onychostoma macrolepis: Effects on fatty acid metabolism based on whole-body fatty acid balance method and genes expression

To evaluate the fatty acid (FA) metabolism status and possibility as a DHA source of farmed Onychostoma macrolepis, a total of 168 fish (2.03 ± 0.23 g) were fed four diets supplemented with fish oil (FO), linseed oil (LO), soybean oil (SO), and a mixture of LO and SO oil (MO), respectively, for 70 days. Body FA compositions were modified reflecting dietary FAs. Comparing liver and intestine fatty acids with fish fed four diets, the content of ARA in fish fed SO was significantly higher than others (P < 0.05), but showed no difference in muscle. The tissue FA profile showed that the FO-fed group successfully deposited DHA, while the LO-fed group converted ALA to DHA effectively, as well as the liver and intestine EPA was notably highest in the FO group, whereas no difference between the FO and LO group in the muscle. The FA results showed that the DHA contents in the muscle of Onychostoma macrolepis are at a medium-high level compared with several other fish species with the highest aquaculture yield. Correspondingly, in the fish fed diet with LO, SO, and MO, the genes of most FA biosynthesis, transportation, and transcriptional regulation factors were increased in the liver and muscle, but no significant difference was observed in the gene expression of Elovl4b, FATP1, and FABP10 in the muscle. In addition, the enzyme activity involved in PUFA metabolism was higher in fish fed vegetable oil-based diets, corroborating the results of the gene expression. Increased in vivo elongase and desaturase (Δ5, Δ6, and Δ9) activities were recorded in fish fed fish oil-devoid diets, which resulted in the appearance of products associated with elongase and desaturase activities in fish. Besides, as the specific n-3 PUFA synthesis substrate, the dietary supplementation of ALA not only retains most of the nutrition value but also ensures the muscular texture, such as fiber diameter and density. It is concluded that farmed O. macrolepis owns strong n-3 LC-PUFA biosynthetic capacity and high DHA contents so it can be a good DHA source for the population.

A Pretty Kettle of Fish: A Review on the Current Challenges in Mediterranean Teleost Reproduction

The Mediterranean region is facing several environmental changes and pollution issues. Teleosts are particularly sensitive to these challenges due to their intricate reproductive biology and reliance on specific environmental cues for successful reproduction. Wild populations struggle with the triad of climate change, environmental contamination, and overfishing, which can deeply affect reproductive success and population dynamics. In farmed species, abiotic factors affecting reproduction are easier to control, whereas finding alternatives to conventional diets for farmed teleosts is crucial for enhancing broodstock health, reproductive success, and the sustainability of the aquaculture sector. Addressing these challenges involves ongoing research into formulating specialized diets, optimizing feeding strategies, and developing alternative and sustainable feed ingredients. To achieve a deeper comprehension of these challenges, studies employing model species have emerged as pivotal tools. These models offer advantages in understanding reproductive mechanisms due to their well-defined physiology, genetic tractability, and ease of manipulation. Yet, while providing invaluable insights, their applicability to diverse species remains constrained by inherent variations across taxa and oversimplification of complex environmental interactions, thus limiting the extrapolation of the scientific findings. Bridging these gaps necessitates multidisciplinary approaches, emphasizing conservation efforts for wild species and tailored nutritional strategies for aquaculture, thereby fostering sustainable teleost reproduction in the Mediterranean.

Combined Replacement of Fishmeal and Fish Oil by Poultry Byproduct Meal and Mixed Oil: Effects on the Growth Performance, Body Composition, and Muscle Quality of Tiger Puffer

This study aimed to evaluate the effects of combined replacement of fishmeal (FM) and fish oil (FO) with poultry byproduct meal (PBM) and mixed oil (MO, poultry oil: coconut oil = 1 : 1) on growth performance, body composition and muscle quality of tiger puffer (Takifugu rubripes). Fish with an average initial body weight of 14.29 g were selected for the feeding experiment. FM accounting for 0%, 5%, and 10% of the diet was replaced by PBM. For each grade of FM replacement, 5% FO or MO was used as added oil. The six experimental diets were designated as FO-FM, MO-FM, FO-5PBM, MO-5PBM, FO-10PBM, and MO-10PBM, respectively. Each treatment was performed in triplicate with 30 fish per replicate. The feeding period was 45 days. There was no significant difference in growth performance among the groups. Dietary supplementation of both PBM and MO had marginal effects on whole-fish proximate composition, except that dietary MO supplementation significantly increased the liver moisture content. In serum, there were no significant differences in contents of triglyceride, total cholesterol, total bile acid, and protein carbonyl among groups, but the malondialdehyde content was reduced by MO. The fatty acid composition in fish mirrored those in the diets, but the omega-3 sparing effects of saturated and monounsaturated fatty acid in MO can still be observed. Dietary PBM and MO had marginal effects on free amino acid composition and texture of fish muscle, but exerted complicated effects on the muscle volatile flavor compound composition. In conclusion, combined fishmeal (10% of the diet) and fish oil (5% of the diet) replacement with poultry byproduct and mixed oil (poultry oil + coconut oil) had no adverse effects on the growth performance and body proximate composition of farmed tiger puffer. However, these replacements changed the muscle flavor compound profile.

Dietary methylmercury and fatty acids affect the lipid metabolism of adipose tissue and liver in rainbow trout

Methylmercury (MeHg) is a pervasive environmental contaminant in aquatic ecosystems that can reach elevated concentrations in fish of high trophic levels, such as salmonids. The present study aims at investigating the individual and combined impacts of dietary MeHg and fatty acids on lipid metabolism in juvenile rainbow trout (Oncorhynchus mykiss) with a focus on two key organs, adipose tissue and liver. MeHg and fatty acids are both known to act on energy homeostasis although little is known about their interplay on lipid metabolism in fish. Fish were fed diets enriched in linoleic acid (LA, 18:2 n-6), α-linolenic acid (ALA, 18:3 n-3), eicosapentaenoic acid (EPA, 20:5 n-3) or docosahexaenoic acid (DHA, 22:6 n-3) for ten weeks, with the addition of MeHg to the diets during the last six weeks (0, 2.4 or 5.5 mg MeHg/kg dry matter). LA and ALA are polyunsaturated fatty acids (PUFA) typical of plant-derived oils whereas EPA and DHA are n-3 long chain PUFA largely found in fish oil, all used in feed formulation in aquaculture. The results showed that the LA-enriched diet induced a higher whole-body lipid content compared to the three other diets. On the contrary, the addition of MeHg led to a significant reduction of the whole-body lipid content, regardless of the diet. Interestingly, the adipocytes were larger both in presence of LA, compared to EPA and DHA, or MeHg, indicating a lipogenic effect of these two compounds. No effect was, however, observed on lipid accumulation per gram of adipose tissue. The fatty acid composition of adipose tissue and liver was significantly modified by the dietary lipids, reflecting both the fatty acid composition of the diets and the high bioconversion capacity of the rainbow trout. Exposure to MeHg selectively led to a release of n-6 PUFA from the hepatic membranes of fish fed the LA-enriched diet, showing a disruption of the pathways using n-6 PUFA. This study highlights the significant impact of MeHg exposure and dietary fatty acids on lipid metabolism in fish. Further investigation is needed to elucidate the underlying mechanisms and to explore the potential involvement of other organs.

Future climate change-related decreases in food quality may affect juvenile Chinook salmon growth and survival

Global climate change is projected to raise global temperatures by 3.3-5.7 °C by 2100, resulting in changes in species composition, abundance, and nutritional quality of organisms at the base of the marine food web. Predicted increases in prey availability and reductions in prey nutritional quality under climate warming in certain marine systems are expected to impact higher trophic levels, such as fish and humans. There is limited knowledge of the interplay between food quantity and quality under warming, specifically when food availability is high, but quality is low. Here, we conducted an experiment assessing the effects of food quality (fatty acid composition and ratios) on juvenile Chinook salmon's (Oncorhynchus tshawytscha) body and nutritional condition, specifically focusing on RNA:DNA ratio, Fulton's K, growth, mortality and their fatty acid composition. Experimental diets represented three different climate change scenarios with 1) a present-day diet (Euphausia pacifica), 2) a control diet (commercial aquaculture diet), and 3) a predicted Intergovernmental Panel on Climate Change (IPCC) worst-case scenario diet with low essential fatty acid concentrations (IPCC SSP5-8.5). We tested how growth rates, RNA:DNA ratio, Fulton's K index, fatty acid composition and mortality rates in juvenile Chinook salmon compared across diet treatments. Fatty acids were incorporated into the salmon muscle at varying rates but, on average, reflected dietary concentrations. High dietary concentrations of DHA, EPA and high DHA:EPA ratios, under the control and present-day diets, increased fish growth and condition. In contrast, low concentrations of DHA and EPA and low DHA:EPA ratios in the diets under climate change scenario were not compensated for by increased food quantity. This result highlights the importance of considering food quality when assessing fish response to changing ocean conditions.

Feeding Strategy to Use Beef Tallow and Modify Farmed Tiger Puffer Fatty Acid Composition

A 12-week feeding experiment was conducted to evaluate the effects of replacing fish oil (FO) with beef tallow (BT) on the fatty acid composition of farmed tiger puffer (Takifugu rubripes). Two replacement strategies were used: a standard Graded Dietary Replacement of FO with BT (GDR strategy) and Alternate Feeding between FO- and BT-based Diets (AFD strategy). The positive and negative control diets were formulated with 6% FO (FO-C group) or BT (BT-C group) as the sole added lipid source. In the GDR strategy, three experimental diets were formulated, with 25, 50 and 75% of the added FO in the FO-C diet replaced with BT, named 25BT, 50BT and 75BT, respectively. In the AFD strategy, alternated feeding patterns between the FO-C and BT-C diet-namely, 1, 2 and 3 weeks with BT-C followed by 1 week feeding with FO-C (1BT-1FO, 2BT-1FO and 3BT-1FO, respectively)-were applied. Each diet or feeding strategy was assigned to triplicate tanks. The results showed that dietary BT inclusion reduced the contents of long-chain polyunsaturated fatty acids (LC-PUFA) in both the muscle and liver (edible tissues for this species) of the experimental fish, and the liver displayed a more drastic decrease than the muscle. The LC-PUFA content linearly decreased with the decreasing dietary FO levels in the GDR strategy. However, in the AFD strategy, a linear relationship was not observed between the LC-PUFA content and the FO feeding duration. The 3BT-1FO treatment resulted in higher LC-PUFA content than 2BT-1FO. When comparing the two strategies with the same final FO administration level-namely, 50BT vs. 1BT-1FO, and in particular, 75BT vs. 3BT-1FO-the AFD strategy resulted in higher LC-PUFA contents in both the muscle and liver than the GDR strategy. In conclusion, when FO was replaced with BT in the diets, alternate feeding between FO- and BT-based diets resulted in a higher LC-PUFA content than the standard direct replacement. Three weeks of feeding with BT-C followed by one week of feeding with FO-C appeared to be a good alternate feeding pattern. This study provided a promising strategy of FO-sparing in fish farming when the LC-PUFA contents were maintained as high as possible.

Assessment of Dietary Supplementation of Lactobacillus rhamnosus Probiotic on Growth Performance and Disease Resistance in Oreochromis niloticus

Probiotics play a significant role in aquaculture by improving the growth, health, and survival rate of fish against pathogenic organisms. In the present study, we have evaluated the effects of a Lactobacillus rhamnosus (L. rhamnosus) probiotic on growth performance and disease resistance in Oreochromis niloticus (O. niloticus) fingerlings. Four different concentrations of L. rhamnosus (T1: 0.5 × 10¹⁰, T2: 1 × 10¹⁰, T3: 1.5 × 10¹⁰, and T4: 2 × 10¹⁰ CFU/kg feed) were administered to fish over a period of three months. L. rhamnosus treated fish revealed a high growth increment as compared to the control, and the values of macromolecules (amino acids, fatty acids, and carbohydrates) varied significantly among the treated and control groups. Levels of thyroid hormones were noted to be high in the probiotic-treated groups. A challenge assay was performed with Aeromonas hydrophila (A. hydrophila). The optimum calculated concentration of probiotics from the growth assay (1.5 × 10¹⁰ CFU/kg feed) was used for the challenge assay. Fish were divided into four groups as follows: control (Con), probiotic-treated (PL), infected (I), and infected + probiotic-treated (I + PL) groups. Significant variations in hematological parameters were observed among control and treated groups. Histopathological changes were recorded in infected fish, while the infected + probiotic-treated group showed less deformations indicating the positive effect of the probiotic supplementation. The survival rate of fish was also better in the probiotic-treated group. Based on these findings, we conclude that probiotic supplementation enhances the growth and improves immunity of O. niloticus. Therefore, we propose that probiotics can be used as promising feed supplements for promoting fish production and disease resistance in aquaculture.

Broiler meat fatty acids composition, lipid metabolism, and oxidative stability parameters as affected by cranberry leaves and walnut meal supplemented diets

A randomized complete block with a 2 × 3 factorial arrangement was used to design a nutrition experiment conducted for the evaluation of the relation between walnut meal (WM-6% inclusion rate) and cranberry leaves (CL-1% and 2% inclusion rate) supplements and their effects on tissue lipid profile, lipid metabolism indices and oxidative stability of meat. Semi-intensive system conditions were simulated for 240 Ross 308 broilers and the animals were reared on permanent shave litter in boxes of 3 m2 (40 broilers / each group, housed in a single box). The current study results showed that the diets enriched in linolenic acid (LNA) (WM diets) led to broilers meat enriched in LNA, but the synthesis of long-chain omega-3 polyunsaturated fatty acids (LC n-3 PUFA) was stimulated when the diets were supplemented with a natural antioxidants source (CL diets). The CL diet also exhibited the most powerful effect in counteracting the oxidative processes of meat.

Sex difference in fatty acid composition of six marine teleosts

This study investigated the sex difference in fatty acid (FA) composition of six wild marine fish species, namely, Cleisthenes herzensteini, Platichthys bicoloratus, Pseudosciaena polyactics, Platycephalus indicus, Alosa sapidissima and Scomberomorus niphonius. The coefficient of distance value between sexes (D_sex ) and multi-variate similarity of percentages analysis (SIMPER) revealed universal existence of sex difference in FA composition, particularly in gonad, intestine and liver. Nonetheless, this sex difference was highly dependent on fish species. In general, DHA, 18:1n-9, 16:1n-7, 16:0 and EPA appeared to be the TOP FAs differentially abundant between sexes.

Fish Oil Replacement with Poultry Oil in the Diet of Tiger Puffer (Takifugu rubripes): Effects on Growth Performance, Body Composition, and Lipid Metabolism

Booming fish farming results in relative shortage of fish oil (FO), making it urgent to explore alternative lipid sources. This study comprehensively investigated the efficacy of FO replacement with poultry oil (PO) in diets of tiger puffer (average initial body weight, 12.28 g). An 8-week feeding trial was conducted with experimental diets, in which graded levels (0, 25, 50, 75, and 100%, named FO-C, 25PO, 50PO, 75PO, and 100PO, respectively) of FO were replaced with PO. The feeding trial was conducted in a flow-through seawater system. Each diet was fed to triplicate tanks. The results showed that FO replacement with PO did not significantly affect the growth performance of tiger puffer. FO replacement with PO at 50-100% even slightly increased the growth. PO feeding also had marginal effects on fish body composition, except that it increased the liver moisture content. Dietary PO tended to decrease the serum cholesterol and malondialdehyde content but increase the bile acid content. Increasing levels of dietary PO linearly upregulated the hepatic mRNA expression of the cholesterol biosynthesis enzyme, 3-hydroxy-3-methylglutaryl-CoA reductase, whereas high levels of dietary PO significantly upregulated the expression of the critical regulatory enzyme of bile acid biosynthesis, cholesterol 7-alpha-hydroxylase. In conclusion, poultry oil is a good substitution for fish oil in the diets of tiger puffer. Poultry oil could replace 100% added fish oil in the diet of tiger puffer, without adverse effects on growth and body composition.

Insulin can up-regulate LC-PUFA biosynthesis with the involvement of Srebp-1c and stimulatory protein 1 (Sp1) in marine teleost Siganus canaliculatus

The rabbitfish Siganus canaliculatus is the first marine teleost found to have the biosynthetic ability of long-chain polyunsaturated fatty acids (LC-PUFA) from C₁₈ precursors catalyzed by fatty acyl desaturases (Δ6/Δ5 Fads, Δ4 Fads) and elongases of very long chain fatty acids (Elovls). Previously, we predicted the existence of insulin (INS) response elements (IREs) including nuclear factor Y (NF-Y) and sterol regulatory element (SRE) in the core promoter region of rabbitfish Δ6/Δ5 fads and Δ4 fads. To clarify the potential regulatory effect and mechanism of INS in LC-PUFA biosynthesis, INS responding region was identified at -456 bp to + 51 bp of Δ6/Δ5 fads core promoter, but not in Δ4 fads promoter. Moreover, a unique stimulatory protein 1 (Sp1) element was predicted in the INS responding region of Δ6/Δ5 fads. Subsequently, SRE, NF-Y and Sp1 elements were proved as IREs in Δ6/Δ5 fads promoter. The up-regulation of INS on gene expression of Srebp-1c, Sp1, Δ6/Δ5 fads and elovl5 as well as the LC-PUFA biosynthesis was further demonstrated in S. canaliculatus hepatocyte line (SCHL) cells, but no influence was detected on Δ4 fads. Besides, inhibitors of transcription factors Srebp-1c (Fatostatin, PF-429242) and Sp1 (Mithramycin) could inhibit the gene expression of Srebp-1c, Δ6/Δ5 fads and elovl5, and abolish the up-regulation of INS on these genes' expression and LC-PUFA biosynthesis. These results indicated that INS could up-regulate LC-PUFA biosynthesis with the involvement of Srebp-1c and Sp1 in rabbitfish S. canaliculatus, which is the first report in teleost.

Environmental adaptation in fish induced changes in the regulatory region of fatty acid elongase gene, elovl5, involved in long-chain polyunsaturated fatty acid biosynthesis

Fish are the main source of long-chain polyunsaturated fatty acids (LC-PUFA) for human consumption. In the process of evolution via natural selection, adaptation to distinct environments has likely driven changes in the endogenous capacity for LC-PUFA biosynthesis between marine and freshwater fishes. However, the molecular mechanisms underlying adaptive changes in this metabolic pathway are poorly understood. Here, we compared the transcriptional regulation of elongation of very long chain fatty acids protein 5 (Elovl5), which is one of the critical enzymes in LC-PUFA biosynthesis pathway, in marine large yellow croaker (Larimichthys crocea) and freshwater rainbow trout (Oncorhynchus mykiss). Comparative transcriptomic and absolute mRNA quantification analyses revealed that the expression of elovl5 in rainbow trout was markedly higher than that in large yellow croaker. Correspondingly, the number of chromatin accessible areas in the regulatory region of elovl5 in rainbow trout was higher than in large yellow croaker, which revealed that chromatin accessibility in the regulatory region of elovl5 in rainbow trout was higher. Furthermore, the differences in sequence and activity of the elovl5 promoter were observed between rainbow trout and large yellow croaker, and transcription factors including CCAAT/enhancer-binding protein β (CEBPβ), GATA binding protein 3 (GATA3) and upstream stimulatory factor 2 (USF2) displayed different regulatory roles on elovl5 expression between the two species. We propose that changes in the gene regulatory region driven by natural selection likely play a key role in differences in elovl5 expression and the activity of Elovl5, which may influence the LC-PUFA biosynthesis capacities of rainbow trout and large yellow croaker. These findings may also provide opportunities to improve the quality of aquatic products and, consequently, human health.

Insect (black soldier fly larvae) oil as a potential substitute for fish or soy oil in the fish meal-based diet of juvenile rainbow trout (Oncorhynchus mykiss)

Alternative sources of fish oil (FO) are one of the major problems in aquaculture; therefore, the goal of the present study was to examine insect (black soldier fly larvae) oil (BSLO) as a potential replacer of fish/soy oil in juvenile rainbow trout (initial average weight of 32 ± 0.15 g) feed. Four diets were formulated wherein FO (control diet) was completely replaced with either soybean oil (SO) or BSLO, and an additional BSLO-based diet supplemented with 1.5% bile acid (BSLO + BA) were fed to the fish for 10 weeks. Growth performance of the BSLO fed group was similar (P > 0.05) to that of the FO and SO fed groups, however, the fish fed BSLO + BA diet registered the lowest growth (P < 0.05). Oil sources did not (P > 0.05) affect the major nutrient content of whole-body, however, the fatty acid composition of the muscle and liver was influenced (P < 0.05), with the highest 14:0, 16:0, and total saturated fatty acid detected in BSLO or BSLO + BA fed trout compared to the others (P < 0.001). No significant differences were observed in eicosapentaenoic acid + docosahexaenoic acid (EPA + DHA) or total n-3 polyunsaturated fatty acid (PUFA) content in muscle among the groups, whereas, the highest EPA:DHA and n-3:n-6 ratios were detected in the FO group. Gene expression for fatty acid binding protein (fabp), fatty acid synthase (fas), and Δ5 desaturase in the liver was lower in FO (P < 0.05), while BSLO + BA registered the highest Δ6 expression (P = 0.006). Supplementation of BA in the BSLO diet increased superoxide dismutase (SOD) and catalase (CAT) activities compared to the other groups (P < 0.05). In conclusion, BSLO could serve as a substitute for FO and SO in rainbow trout diet without negatively impacting growth performance, whole-body composition and nutrient retention, and modulate the expression of fatty acid metabolism-related genes in rainbow trout.

Genetic architecture and genomic selection of fatty acid composition predicted by Raman spectroscopy in rainbow trout

Background

In response to major challenges regarding the supply and sustainability of marine ingredients in aquafeeds, the aquaculture industry has made a large-scale shift toward plant-based substitutions for fish oil and fish meal. But, this also led to lower levels of healthful n−3 long-chain polyunsaturated fatty acids (PUFAs)—especially eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids—in flesh. One potential solution is to select fish with better abilities to retain or synthesise PUFAs, to increase the efficiency of aquaculture and promote the production of healthier fish products. To this end, we aimed i) to estimate the genetic variability in fatty acid (FA) composition in visceral fat quantified by Raman spectroscopy, with respect to both individual FAs and groups under a feeding regime with limited n-3 PUFAs; ii) to study the genetic and phenotypic correlations between FAs and processing yields- and fat-related traits; iii) to detect QTLs associated with FA composition and identify candidate genes; and iv) to assess the efficiency of genomic selection compared to pedigree-based BLUP selection.

Results

Proportions of the various FAs in fish were indirectly estimated using Raman scattering spectroscopy. Fish were genotyped using the 57 K SNP Axiom™ Trout Genotyping Array. Following quality control, the final analysis contained 29,652 SNPs from 1382 fish. Heritability estimates for traits ranged from 0.03 ± 0.03 (n-3 PUFAs) to 0.24 ± 0.05 (n-6 PUFAs), confirming the potential for genomic selection. n-3 PUFAs are positively correlated to a decrease in fat deposition in the fillet and in the viscera but negatively correlated to body weight. This highlights the potential interest to combine selection on FA and against fat deposition to improve nutritional merit of aquaculture products. Several QTLs were identified for FA composition, containing multiple candidate genes with indirect links to FA metabolism. In particular, one region on Omy1 was associated with n-6 PUFAs, monounsaturated FAs, linoleic acid, and EPA, while a region on Omy7 had effects on n-6 PUFAs, EPA, and linoleic acid. When we compared the effectiveness of breeding programmes based on genomic selection (using a reference population of 1000 individuals related to selection candidates) or on pedigree-based selection, we found that the former yielded increases in selection accuracy of 12 to 120% depending on the FA trait.

Conclusion

This study reveals the polygenic genetic architecture for FA composition in rainbow trout and confirms that genomic selection has potential to improve EPA and DHA proportions in aquaculture species.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08062-7.

Influence of Varying Dietary ω6 to ω3 Fatty Acid Ratios on the Hepatic Transcriptome, and Association with Phenotypic Traits (Growth, Somatic Indices, and Tissue Lipid Composition), in Atlantic Salmon (Salmo salar)

Simple Summary

Plant oils are routinely used in fish feeds as a fish oil replacement. However, these terrestrial alternatives typically contain high levels of ω6 fatty acids (FA) and, thus, high ω6 to ω3 (ω6:ω3) FA ratios, which influence farmed fish and their consumers. The ω6:ω3 ratio is known to affect many biological processes (e.g., inflammation, FA metabolism) and human diseases; however, its impacts on fish physiology and the underlying molecular mechanisms are less well understood. In this study, we used 44 K microarrays to examine which genes and molecular pathways are altered by variation in dietary ω6:ω3 in Atlantic salmon. Our microarray study showed that several genes related to immune response, lipid metabolism, cell proliferation, and translation were differentially expressed between the two extreme ω6:ω3 dietary treatments. We also revealed that the PPARα activation-related transcript helz2 is a potential novel molecular biomarker of tissue variation in ω6:ω3. Further, correlation analyses illustrated the relationships between liver transcript expression and tissue (liver, muscle) lipid composition, and other phenotypic traits in salmon fed low levels of fish oil. This nutrigenomic study enhanced the current understanding of Atlantic salmon gene expression response to varying dietary ω6:ω3.

Abstract

The importance of dietary omega-6 to omega-3 (ω6:ω3) fatty acid (FA) ratios for human health has been extensively examined. However, its impact on fish physiology, and the underlying molecular mechanisms, are less well understood. This study investigated the influence of plant-based diets (12-week exposure) with varying ω6:ω3 (0.4–2.7) on the hepatic transcriptome of Atlantic salmon. Using 44 K microarray analysis, genes involved in immune and inflammatory response (lect2a, itgb5, helz2a, p43), lipid metabolism (helz2a), cell proliferation (htra1b), control of muscle and neuronal development (mef2d) and translation (eif2a, eif4b1, p43) were identified; these were differentially expressed between the two extreme ω6:ω3 dietary treatments (high ω6 vs. high ω3) at week 12. Eight out of 10 microarray-identified transcripts showed an agreement in the direction of expression fold-change between the microarray and qPCR studies. The PPARα activation-related transcript helz2a was confirmed by qPCR to be down-regulated by high ω6 diet compared with high ω3 diet. The transcript expression of two helz2 paralogues was positively correlated with ω3, and negatively with ω6 FA in both liver and muscle, thus indicating their potential as biomarkers of tissue ω6:ω3 variation. Mef2d expression in liver was suppressed in the high ω6 compared to the balanced diet (ω6:ω3 of 2.7 and 0.9, respectively) fed fish, and showed negative correlations with ω6:ω3 in both tissues. The hepatic expression of two lect2 paralogues was negatively correlated with viscerosomatic index, while htra1b correlated negatively with salmon weight gain and condition factor. Finally, p43 and eif2a were positively correlated with liver Σω3, while these transcripts and eif4b2 showed negative correlations with 18:2ω6 in the liver. This suggested that some aspects of protein synthesis were influenced by dietary ω6:ω3. In summary, this nutrigenomic study identified hepatic transcripts responsive to dietary variation in ω6:ω3, and relationships of transcript expression with tissue (liver, muscle) lipid composition and other phenotypic traits.

Camelina Oil Supplementation Improves Bone Parameters in Ovariectomized Rats

The aim of the present study was to determine the effect of administration of Camelina sativa oil (CO) as a source of polyunsaturated fatty acids (PUFA) on bone parameters in ovariectomized rats (OVX). Overall, 40 10-week-old healthy female Wistar rats were divided into 4 groups with 10 animals in each. Rats in the control group (SHO) were subjected to a sham operation, whereas experimental rats (OVX) were ovariectomized. After a 7-day recovery period, the SHO the rats received orally 1 mL of physiological saline for the next 6 weeks. The OVX rats received orally 1 mL of physiological saline (OVX-PhS), 5 g/kg BW (OVX-CO5), or 9 g/kg BW (OVX-CO9) of camelina oil. The use of camelina oil had a significant effect on body weight, lean mass, and fat mass. The camelina oil administration suppressed the decrease in the values of some densitometric, tomographic, and mechanical parameters of femur caused by estrogen deficiency. The CO treatment increased significantly the serum level of osteocalcin and decreased the serum level of C-terminal telopeptide of type I collagen in the OVX rats. In conclusion, camelina oil exerts a positive osteotropic effect by inhibiting ovariectomy-induced adverse changes in bones. Camelina oil supplementation can be used as an efficient method for improving bone health in a disturbed state. However, further research must be carried out on other animal species supplemented with the oil.

Effects of High Levels of Dietary Linseed Oil on the Growth Performance, Antioxidant Capacity, Hepatic Lipid Metabolism, and Expression of Inflammatory Genes in Large Yellow Croaker (Larimichthys crocea)

A growth experiment was conducted to evaluate the effects of dietary fish oil (FO) replaced by linseed oil (LO) on the growth performance, antioxidant capacity, hepatic lipid metabolism, and expression of inflammatory genes in large yellow croaker (Larimichthys crocea). Fish (initial weight: 15.88 ± 0.14 g) were fed four experimental diets with 0% (the control), 33.3%, 66.7%, and 100% of FO replaced by LO. Each diet was randomly attributed to triplicate seawater floating cages (1.0 × 1.0 × 2.0 m) with 60 fish in each cage. Results showed that the growth performance of fish fed the diet with 100% LO was markedly decreased compared with the control group (P < 0.05), while no remarkable difference was observed in the growth performance of fish fed diets within 66.7% LO (P > 0.05). The percentage of 18:3n-3 was the highest in the liver and muscle of fish fed the diet with 100% LO among the four treatments. When dietary FO was entirely replaced by LO, fish had a markedly higher total cholesterol, total triglyceride, low-density lipoprotein cholesterol content, and alanine transaminase activity in the serum than the control group (P < 0.05). The concentration of malondialdehyde was markedly higher, while the activity of catalase was markedly lower in fish fed the diet with 100% LO than the control group (P < 0.05). When dietary FO was entirely replaced by LO, hepatic lipid content, transcriptional levels of fatp1 and cd36, and CD36 protein expression were significantly higher, while transcriptional level of cpt-1 and CPT-1 protein expression were significantly lower than the control group (P < 0.05). As for the gene expression of cytokines, fish fed the diet with 100% LO had markedly higher transcriptional levels of il-1β, tnfα, and il-6 than the control group (P < 0.05). In conclusion, the substitution of 66.7% FO with LO had no significant effects on the growth performance of fish, while 100% LO decreased the growth performance and increased the inflammation and hepatic lipid content of fish. The increase of hepatic lipid content was probably due to the increased fatty acid uptake and decreased fatty acid oxidation in fish.

Effects of different dietary oils on egg quality and reproductive performance in rainbow trout Oncorhynchus mykiss

The study was conducted to evaluate effects of different dietary oils on egg quality and reproductive performance in rainbow trout. Broodfish (≈ 870 g) were fed four iso-nitrogenous and iso-lipidic diets differing in lipid sources: fish oil (FO), linseed oil (LO) and sesame oil (SO) as well as a commercial trout diet (CD) for about 5 months prior to spawning. Growth performance did not differ among the trout in the treatment groups. Mean diameter, volume and weight of eggs did not differ among the dietary treatments. Absolute fecundity, relative fecundity and gonadosomatic index were not affected by dietary treatment. A sub-set of eggs from females fed the experimental diets were fertilized to assess the reproductive performance of broodfish. When diets were fed, devoid of fish oil, fertilization rates were 89.2 ± 5.8 and 92.1 ± 4.9 %, eyeing rates were 87.3 ± 5.3 and 84.1 ± 4.4 % and hatching rates were 81.2 ± 4.3 and 78.3 ± 3.4 % in LO and SO fed fish, respectively. Fatty acid content of the eggs from broodstocks with a different nutritional history was affected by the dietary lipid sources. Eicosapentaenoic acid (EPA), arachidonic acid (ARA), and docosahexaenoic acid (DHA) concentrations in females fed vegetable oil based diets were greater than the dietary concentrations. Overall, results from the present study indicate there can be inclusion of LO or SO as dietary lipid sources without compromising egg quality and reproductive performance. Furthermore, there is efficient bioconversion of 18C fatty acids to 20-22 C fatty acids in rainbow trout.

Influence of different oil sources on growth, disease resistance, immune response and immune-related gene expression on the hybrid grouper (♀ Epinephelus fuscoguttatus × ♂ E. lanceolatu), to Vibrio parahaemolyticus challenge

The aim of this study was to investigate the effects of feeding alternative dietary oils to hybrid grouper fish (♀Epinephelus fuscoguttatus × ♂E. lanceolatu) on their growth, histological morphology of hepatocytes, disease resistance, immune response, and expression of immune-related genes. Seven experimental fish meal-based isonitrogenous and isolipidic diets were formulated containing 5% fish oil (FO; acting as controls) and various vegetable oils (VOs): corn oil (CO), sunflower oil (SO), tea oil (TO), olive oil (OO), rice oil (RO), and mixed oil (MO); comprising equal amounts of these oils). Each diet was fed to triplicate groups of 40 fish (initial mean body weight ± standard error = 15.09 ± 0.01 g) for eight weeks. The results show that 1) alternative dietary oils had no significant effects on weight gain rate, specific growth rate, protein efficiency ratio, and survival rate compared with controls (P > 0.05). The weight gain rate (WGR) and specific growth rate (SGR) of the SO group were lower than in the CO and OO groups. 2) These were no differences in morphological indexes among groups; except for the CO group, in which the condition factor and hepatosomatic index were lower than those in other groups. 3) Compared with controls, the whole-body moisture and crude protein contents in the VO groups were higher, while their crude lipid contents were lower. 4) The fatty acid contents in liver and muscle were affected by lipid type, and the contents of eicosapentaenoic acid and docosahexaenoic acid in liver and muscle in the VO groups were markedly lower than in controls. 5) Compared with control group, VO groups damaged the histological morphology of hepatocytes. 6) After a challenge with the Vibrio parahaemolyticus bacterium, there were no differences in mortality among groups. However, VO enhanced the activity of non-specific immune enzymes while down-regulating the expression of Nrf2 and inducing the expression of pro-inflammatory factors (IL1β, TNFα, TLR22, and MyD88) in the kidney. It can be concluded that dietary VO substitution does not affect the growth of fish but damaged the histological morphology of hepatocytes and induced the expression of pro-inflammatory factors in tissues. Finally, OO and CO were recommended as the appropriate lipid replacement for FO.

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

Adequate dietary supply of eicosapentaenoic acid (20:5n-3) and docosahexaenoic acid (22:6n-3) is required to maintain health and growth of Atlantic salmon (Salmo salar). However, salmon can also convert α-linolenic acid (18:3n-3) into eicosapentaenoic acid (20:5n-3) and docosahexaenoic acid (22:6n-3) by sequential desaturation and elongation reactions, which can be modified by 20:5n-3 and 22:6n-3 intake. In mammals, dietary 20:5n-3 + 22:6n-3 intake can modify Fads2 expression (Δ6 desaturase) via altered DNA methylation of its promoter. Decreasing dietary fish oil (FO) has been shown to increase Δ5fad expression in salmon liver. However, it is not known whether this is associated with changes in the DNA methylation of genes involved in polyunsaturated fatty acid synthesis. To address this, we investigated whether changing the proportions of dietary FO and vegetable oil altered the DNA methylation of Δ6fad_b, Δ5fad, Elovl2, and Elovl5_b promoters in liver and muscle from Atlantic salmon and whether any changes were associated with mRNA expression. Higher dietary FO content increased the proportions of 20:5n-3 and 22:6n-3 and decreased Δ6fad_b mRNA expression in liver, but there was no effect on Δ5fad, Elovl2, and Elovl5_b expression. There were significant differences between liver and skeletal muscle in the methylation of individual CpG loci in all four genes studied. Methylation of individual Δ6fad_b CpG loci was negatively related to its expression and to proportions of 20:5n-3 and 22:6n-3 in the liver. These findings suggest variations in dietary FO can induce gene-, CpG locus-, and tissue-related changes in DNA methylation in salmon.

Linseed oil can decrease liver fat deposition and improve antioxidant ability of juvenile largemouth bass, Micropterus salmoides

A feeding trial was conducted to evaluate the effect of linseed oil (LO) on growth, plasma biochemistry, hepatic metabolism enzymes, and antioxidant capacity of juvenile largemouth bass, Micropterus salmoides. Four isonitrogenous (crude protein, 45%) and isoenergetic (gross energy, 18 MJ/kg) diets were formulated by replacing 0 (the control), 33.3%, 66.7%, and 100% of fish oil with linseed oil. Each diet was fed to three replicate groups of fish (initial body weight, 22.02 ± 0.61 g) for 8 weeks. The results indicated that fish fed diet with 100% LO substitution level had lower weight gain (WG), specific growth rate (SGR), and protein efficiency ratio (PER) than the other groups (P < 0.05), while feed conversion ratio (FCR) was higher compared to the other groups (P < 0.05). Feed intake (FI) and hepatosomatic index (HSI) of 66.7% LO substitution level were significantly lower than the control groups (P < 0.05). Glycogen, lipid, and non-esterified fatty acid content in the liver decreased significantly with increasing dietary LO levels (P < 0.05). Moreover, the replacement of fish oil (FO) with LO could significantly reduce the content of triglyceride (TG) and total cholesterol (TC) and the activity of alanine amiotransferase (ALT) in plasma of M. salmoides (P < 0.05). There were significant differences in hepatic metabolism enzymes in fish fed diets with different dietary LO levels. Adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK) and peroxisome proliferator-activated receptor (PPAR-α) activities in liver significantly increased with increasing dietary LO level (P < 0.05). In addition, phosphoenolpyruvate carboxykinase (PEPCK) and fructose-1,6-bisphosphatase (FBPase) activities in the liver significantly increased with decreasing dietary LO level (P < 0.05). Both the lowest superoxide dismutase (SOD) and catalase (CAT) activities in the liver were recorded in the control group (P < 0.05). Moreover, nitric oxide content, glutathione peroxidase (GPx), and inducible nitric oxide synthase (iNOS) activities in the liver significantly increased with increasing dietary LO level, while malondialdehyde (MDA) content significantly reduced. These findings demonstrated that LO can improve liver function and antioxidant ability of M. salmoides. In addition, replacing partial FO with LO cannot affect growth performance, but all substitutions inhibit growth performance of M. salmoides.

Influence of dietary linseed oil as substitution of fish oil on whole fish fatty acid composition, lipid metabolism and oxidative status of juvenile Manchurian trout, Brachymystax lenok

In this study, juvenile Manchurian trout, Brachymystax lenok (initial weight: 6.43 ± 0.02 g, mean ± SE) were received for nine weeks with five types of diets prepared by gradually replacing the proportion of fish oil (FO) with linseed oil (LO) from 0% (LO0) to 25% (LO25), 50% (LO50), 75% (LO75), and 100% (LO100). The eicosapentaenoic (EPA) and docosahexaenoic (DHA) composition decreased with increasing inclusion level of LO (P < 0.05). With increasing LO inclusion level, triglyceride (TAG) content of serum increased significantly, however, there was a decrease in high-density lipoprotein cholesterol (HDL) (P < 0.05). LO substitution of FO up-regulated the gene expression level of lipid metabolism-related genes Fatty Acid Desaturases 6 (FAD6), Acetyl-Coa Carboxylase (ACCα), Sterol Regulatory Element Binding Protein 1 (SREBP-1), and Sterol O- Acyl Transferase 2 (SOAT2), and down-regulated the gene expression level of Peroxisome Proliferator-Activated Receptor a (PPARα) (P < 0.05). The SOD activities of both serum and liver in LO100 were significantly lower than in LO25 (P < 0.05). The CAT activity of the liver in LO100 was significantly lower than in LO0 and LO25 (P < 0.05). This study indicates that the Manchurian trout may have the ability to synthesize LC-PUFAs from ALA, and an appropriate LO in substitution of FO (<75%) could improve both the lipid metabolism and the oxidation resistance.

Dietary n-3 PUFA content as a modulator of the femur properties in growing pigs

The relationships between both dietary and empty body fatty acid composition and the morphometry, densitometry, geometry and biomechanical properties of the femur of growing pigs were analysed. A total of thirty-two pigs aged 115 d were divided into four groups (n 8 per group). The pigs were fed either a control diet (group C) or a diet supplemented with linseed oil (rich in α-linolenic acid (C18 : 3n-3), group L), fish oil (rich in EPA (C20 : 5n-3) and DHA (C22 : 6n-3), group F) and beef tallow (rich in SFA, group T). The diets differed in n-3 PUFA contents (0·63-18·52 g/kg) and n-6:n-3 PUFA ratios (0·91-14·51). At 165 d of age, the pigs were slaughtered and the fatty acids in the empty body were determined. Moreover, the left femur was dissected. The cortical wall thickness, cross-sectional area, cortical index, bone mineral content, bone mineral density, maximum elastic strength and maximum strength were lower (P<0·05) in the femurs of pigs from groups C and T than in those from groups F and L. Significant positive correlations were found between the densitometry, geometry and biomechanical properties of the femur and both dietary and empty body n-3 PUFA content, whereas significant negative correlations were observed between the same properties and both dietary and empty body n-6:n-3 PUFA ratio. The results of the present study suggest that in growing pigs α-linolenic acid has a similar positive effect on bone health to that of EPA and DHA.

Screening dietary biochanin A, daidzein, equol and genistein for their potential to increase DHA biosynthesis in rainbow trout (Oncorhynchus mykiss)

Plant oil utilization in aquafeeds is still the most practical option, although it decreases the content of the nutritionally highly valuable omega-3 fatty acids eicosapentaenoic acid (20:5n-3, EPA) and docosahexaenoic acid (22:6n-3, DHA) in fish. Phytoestrogens and their metabolites are putatively able to affect genes encoding proteins centrally involved in the biosynthesis of EPA and DHA due to their estrogenic potential. Thus, the aim of the study was to screen the potential of the phytoestrogens to stimulate the biosynthesis of EPA and DHA in rainbow trout (Oncorhynchus mykiss). Additionally, the potential effects on growth performance, nutrient composition and hepatic lipid metabolism in rainbow trout were investigated. For that, a vegetable oil based diet served as a control diet (C) and was supplemented with 15 g/kg dry matter of biochanin A (BA), daidzein (DA), genistein (G) and equol (EQ), respectively. These five diets were fed to rainbow trout (initial body weight 83.3 ± 0.4 g) for 52 days. Growth performance and nutrient composition of whole body homogenates were not affected by the dietary treatments. Furthermore, feeding EQ to rainbow trout significantly increased DHA levels by +8% in whole body homogenates compared to samples of fish fed the diet C. A tendency towards increased DHA levels in whole body homogenates was found for fish fed the diet G. Fish fed diets BA and DA lacked these effects. Moreover, EQ and G fed fish showed significantly decreased hepatic mRNA steady state levels for fatty acyl desaturase 2a (delta-6) (fads2a(d6)). In contrast, carnitine palmitoyl transferases 1 (cpt1) hepatic mRNA steady state levels and hepatic Fads2a(d6) protein contents were not affected by the dietary treatment. In conclusion, when combined with dietary vegetable oils, equol and genistein seem to stimulate the biosynthesis of DHA and thereby increase tissue DHA levels in rainbow trout, however, only to a moderate extent.

Dietary Buglossoides arvensis Oil as a Potential Candidate to Substitute Fish Oil in Rainbow Trout Diets

The utilization of vegetable oils in salmonid diets substantially decreased the body content of omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA), and thus the product quality for human consumption. Therefore, new ingredients for aquaculture feeds are needed that maximize the deposition of health-promoting n-3 LC-PUFA. This study investigated Buglossoides arvensis (Ahiflower) oil, a plant oil rich in alpha-linolenic acid (18:3n-3, ALA) and stearidonic acid (18:4n-3, SDA), as a source of n-3 fatty acids in rainbow trout (Oncorhynchus mykiss) nutrition. Rainbow trout (87.4 ± 0.6 g) were fed for 56 days. The oils of the control diet (FV) were substituted by Ahiflower oil at 33%, 66%, and 100% (A33, A66, A100). Dietary Ahiflower oil increased the final body weights of fish. mRNA steady state levels of fatty acyl desaturase 2a (delta-6) (fads2a(d6)) and 2b (delta-5) (fads2b(d5)) as well as carnitine palmitoyl transferase 1 a (cpt1a) were not altered by dietary treatments. In contrast, cpt1c mRNA steady state levels were significantly downregulated in samples of fish fed A66 and A100. Significantly higher eicosapentaenoic acid (20:5n-3, EPA) and docosahexaenoic acid (22:6n-3, DHA) levels were found in the liver and significantly higher EPA levels in the fillet of rainbow trout of A66 and A100 compared to FV. The content of DHA in fillets of fish fed Ahiflower oil was not significantly different to fish fed FV. Thus, high dietary amounts of Ahiflower oil can compensate for reduced dietary EPA and DHA levels.

Genetic effects of fatty acid composition in muscle of Atlantic salmon

BACKGROUND

The replacement of fish oil (FO) and fishmeal with plant ingredients in the diet of farmed Atlantic salmon has resulted in reduced levels of the health-promoting long-chain polyunsaturated omega-3 fatty acids (n-3 LC-PUFA) eicosapentaenoic (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3) in their filets. Previous studies showed the potential of selective breeding to increase n-3 LC-PUFA levels in salmon tissues, but knowledge on the genetic parameters for individual muscle fatty acids (FA) and their relationships with other traits is still lacking. Thus, we estimated genetic parameters for muscle content of individual FA, and their relationships with lipid deposition traits, muscle pigmentation, sea lice and pancreas disease in slaughter-sized Atlantic salmon. Our aim was to evaluate the selection potential for increased n-3 LC-PUFA content and provide insight into FA metabolism in Atlantic salmon muscle.

RESULTS

Among the n-3 PUFA, proportional contents of alpha-linolenic acid (ALA; 18:3n-3) and DHA had the highest heritability (0.26) and EPA the lowest (0.09). Genetic correlations of EPA and DHA proportions with muscle fat differed considerably, 0.60 and 0.01, respectively. The genetic correlation of DHA proportion with visceral fat was positive and high (0.61), whereas that of EPA proportion with lice density was negative. FA that are in close proximity along the bioconversion pathway showed positive correlations with each other, whereas the start (ALA) and end-point (DHA) of the pathway were negatively correlated (- 0.28), indicating active bioconversion of ALA to DHA in the muscle of fish fed high FO-diet.

CONCLUSIONS

Since contents of individual FA in salmon muscle show additive genetic variation, changing FA composition by selective breeding is possible. Taken together, our results show that the heritabilities of individual n-3 LC-PUFA and their genetic correlations with other traits vary, which indicates that they play different roles in muscle lipid metabolism, and that proportional muscle contents of EPA and DHA are linked to body fat deposition. Thus, different selection strategies can be applied in order to increase the content of healthy omega-3 FAin the salmon muscle. We recommend selection for the proportion of EPA + DHA in the muscle because they are both essential FA and because such selection has no clear detrimental effects on other traits.

Characterization of lipid metabolism genes and the influence of fatty acid supplementation in the hepatic lipid metabolism of dusky grouper (Epinephelus marginatus)

Dusky grouper is an important commercial fish species in many countries, but some factors such as overfishing has significantly reduced their natural stocks. Aquaculture emerges as a unique way to conserve this species, but very little biological information is available, limiting the production of this endangered species. To understand and generate more knowledge about this species, liver transcriptome sequencing and de novo assembly was performed for E. marginatus by Next Generation Sequencing (NGS). Sequences obtained were used as a tool to validate the presence of key genes relevant to lipid metabolism, and their expression was quantified by qPCR. Moreover, we investigated the influence of supplementing different dietary fatty acids on hepatic lipid metabolism. The results showed that the different fatty acids added to the diet dramatically changed the gene expression of some key enzymes associated with lipid metabolism as well as hepatic fatty acid profiles. Elongase 5 gene expression was shown to influence intermediate hepatic fatty acid elongation in all experimental groups. Hepatic triglycerides reflected the diet composition more than hepatic phospholipids, and were characterized mainly by the high percentage of 18:3n3 in animals fed with a linseed oil rich diet. Results for the saturated and monounsaturated fatty acids suggest a self-regulatory potential for retention and oxidation processes in liver, since in general the tissues did not directly reflect these fatty acid diet compositions. These results indicated that genes involved in lipid metabolism pathways might be potential biomarkers to assess lipid requirements in the formulated diet for this species.

Long-term dietary replacement of fishmeal and fish oil in diets for rainbow trout (Oncorhynchus mykiss): Effects on growth, whole body fatty acids and intestinal and hepatic gene expression

The effects of replacing fishmeal and fish oil with a plant-based diet were studied in juvenile (10g) and ongrowing (250-350g) rainbow trout from first-feeding. Feed-related differences in the intestinal and hepatic transcriptome were examined in juveniles after 7 months of feeding at 7°C. Based on microarray results obtained for juveniles, the expression of selected genes related to lipid, cholesterol and energy metabolisms, was assessed by RT-qPCR in ongrowing trout after 6 additional months of feeding at 17°C. Plasma glucose and cholesterol, lipid content and fatty acid profile of whole body were analyzed at both stages. After 7 months at 7°C, all juveniles reached the same body weight (10g), while at 13 months ongrowing fish fed the totally plant-based diet exhibited lower body weight (234 vs 330-337g). Body lipid content was higher in juveniles fed the totally plant-based diet (13.2 vs 9.4–9.9%), and plasma cholesterol was about 2-times lower in trout fed the plant-based diets at both stages. Fatty acid profile mirrored that of the respective diet, with low proportions of long-chain n-3 polyunsaturated fatty acids in fish fed plant-based diets. Genes involved in protein catabolism, carbohydrate metabolism and trafficking were down-regulated in the intestines of juveniles fed the plant-based diets. This was not true for ongrowing fish. Genes involved in lipid and cholesterol metabolisms were up-regulated in the livers of fish fed plant-based diets for both stages. In this study, feeding trout a totally plant-based diet from first-feeding affect a relatively low proportion of metabolism-related genes. In the longer term, when fish were reared at a higher temperature, only some of these changes were maintained (i.e. up-regulation of lipid/cholesterol metabolism). Although the plant-based diets tested in this study had no major deficiencies, small adjustments in the feed-formula are needed to further optimize growth performance while sparing marine resources.

Effect of Dietary Fish Oil Replacement with Plant Oils on Growth Performance and Gene Expression in Juvenile Rainbow Trout (Oncorhynchus mykiss)

An eight-week feeding trial was conducted to evaluate the effects of total (100%) replacement of dietary fish oil with alternative lipid sources in juvenile rainbow trout. Six iso-nitrogenous and iso-lipidic experimental diets were formulated: CO (14%) – cod liver oil; SSO (14%) – safflower seed oil; SBO (14%) – soybean oil; LO (14%) – linseed oil; SBO (7%) + LO (7%) – a blend of soybean oil and linseed oil; and SSO (7%) + LO (7%) – a blend of safflower seed oil and linseed oil. Growth performance [specific growth rate (SGR), weight gain (WG), food conversion ratio (FCR) and survival rate (SR)], growth hormones [growth hormone (GH-I), insulin-like growth factor-I (IGF-I) and insulin-like growth factor-II (IGF-II)], immune transforming growth factor-β (TGF-β) and antioxidant [superoxide dismutase (SOD), glutathione peroxidase (GPx), glutathione reductase (GR), catalase (CAT) and glutathione S-transferase (GST)] response, and heat shock protein 70 (HSP70) mRNA levels were determined in muscle and liver. Our data indicated that final weight, weight gain, FCR and SGR showed significant difference among the six dietary treatments (P<0.05) while there were no significant differences in survival rate between the rainbow trout from supplement fed groups and control group. HSP70 mRNA level expression in muscle was higher in fish fed SSO (P<0.05) while highest level in liver was obtained from fish fed SBO compared to the other treatments (P<0.05). There were no significant differences among treatments for TGF-β mRNA expression level in muscle and liver. In conclusion, growth performance and expression levels of growth hormones, antioxidants, HSP70, except TGF-β were affected by five separate lipid sources. In addition, LO positively increased growth performance of juvenile rainbow trout by means of preventing oxidative stress and HSP70 and, enhanced expression of growth hormone related gene.

Successful selection of rainbow trout (Oncorhynchus mykiss) on their ability to grow with a diet completely devoid of fishmeal and fish oil, and correlated changes in nutritional traits

In the context of limited marine resources, the exponential growth of aquaculture requires the substitution of fish oil and fishmeal, the traditional components of fish feeds by terrestrial plant ingredients. High levels of such substitution are known to negatively impact fish performance such as growth and survival in rainbow trout (Oncorhynchus mykiss) as in other salmonids. In this respect, genetic selection is a key enabler for improving those performances and hence for the further sustainable development of aquaculture. We selected a rainbow trout line over three generations for its ability to survive and grow on a 100% plant-based diet devoid of both fish oil and fishmeal (V diet) from the very first meal. In the present study, we compared the control line and the selected line after 3 generations of selection, both fed either the V diet or a marine resources-based diet (M diet). The objective of the study was to assess the efficiency of selection and the consequences on various correlated nutritional traits: feed intake, feed efficiency, digestibility, composition of whole fish, nutrient retention and fatty acid (FA) profile. We demonstrated that the genetic variability present in our rainbow trout population can be selected to improve survival and growth. The major result of the study is that after only three generations of selection, selected fish fed the V diet grew at the same rate as the control line fed the M diet, whilst the relative reduction of body weight was 36.8% before the selection. This enhanced performance on the V diet seems to be mostly linked to a higher feed intake for the selected fish.

Effects of dietary lipids on the hepatopancreas transcriptome of Chinese mitten crab (Eriocheir sinensis)

Fish oil supplies worldwide have declined sharply over the years. To reduce the use of fish oil in aquaculture, many studies have explored the effects of fish oil substitutions on aquatic animals. To illustrate the effects of dietary lipids on Chinese mitten crab and to improve the use of vegetable oils in the diet of the crabs, 60 male juvenile Chinese mitten crabs were fed one of five diets for 116 days: fish oil (FO), soybean oil (SO), linseed oil (LO), FO + SO (1:1, FSO), and FO + LO (1:1, FLO). Changes in the crab hepatopancreas transcriptome were analyzed using RNA sequencing. There were a total 55,167 unigenes obtained from the transcriptome, of which the expression of 3030 was significantly altered in the FLO vs. FO groups, but the expression of only 412 unigenes was altered in the FSO vs. FO groups. The diets significantly altered the expression of many enzymes involved in lipid metabolism, such as pancreatic lipase, long-chain acyl-CoA synthetases, carnitine palmitoyltransferase I, acetyl-CoA carboxylase, fatty acid synthase, and fatty acyl Δ9-desaturase. The dietary lipids also affected the Toll-like receptor and Janus activated kinase-signal transducers and activators of transcription signaling pathways. Our results indicate that substituting fish oil with vegetable oils in the diet of Chinese mitten crabs might decrease the digestion and absorption of dietary lipids, fatty acids biosynthesis, and immunologic viral defense, and increase β-oxidation by altering the expression of the relevant genes. Our results lay the foundation for further understanding of lipid nutrition in Chinese mitten crab.

Effects of dietary fat on the saturated and monounsaturated fatty acid metabolism in growing pigs

The effect of dietary fats differing in fatty acid (FA) composition on the metabolism of saturated FA (SFA) and monounsaturated FA (MUFA) in growing pigs was investigated. The deposition of FA in the body and the fate of individual dietary FA were assessed after slaughter. Gilts with an initial body weight (BW) of 60 kg were used as experimental animals. Six pigs were slaughtered at 60 kg BW, while further 18 pigs received three isoenergetic and isonitrogen experimental diets containing linseed oil, rapeseed oil or beef tallow at 50 g/kg diet until they reached 105 kg (six pigs per group). The chemical composition and the content of FA in the whole body were determined and compared across groups. Regardless of dietary treatment, the whole body contained similar amounts of protein, fat and total FA. The total accumulation (percentage of net intake and de novo production) of SFA and MUFA was similar in all groups, but the processes of elongation and desaturation of SFA and MUFA depended upon the type of FA added to the diet. A high dietary content and intake of MUFA inhibits desaturation compared to SFA- and PUFA-rich diets, whereas a high SFA content and intake lowers elongation rate. The increasing net intake of total SFA and MUFA was associated with a lower total de novo production of these FA in the whole body of pigs.

A n-3 PUFA depletion applied to rainbow trout fry (Oncorhynchus mykiss) does not modulate its subsequent lipid bioconversion capacity

Nutritional strategies are currently developed to produce farmed fish rich in n-3 long-chain PUFA (LC-PUFA) whilst replacing fish oil by plant-derived oils in aquafeeds. The optimisation of such strategies requires a thorough understanding of fish lipid metabolism and its nutritional modulation. The present study evaluated the fatty acid bioconversion capacity of rainbow trout (Oncorhynchus mykiss) fry previously depleted in n-3 PUFA through a 60-d pre-experimental feeding period with a sunflower oil-based diet (SO) followed by a 36-d experimental period during which fish were fed either a linseed oil-based diet (LO) (this treatment being called SO/LO) or a fish oil-based diet (FO) (this treatment being called SO/FO). These treatments were compared with fish continuously fed on SO, LO or FO for 96 d. At the end of the 36-d experimental period, SO/LO and SO/FO fish recovered >80 % of the n-3 LC-PUFA reported for LO and FO fish, respectively. Fish fed on LO showed high apparent in vivo elongation and desaturation activities along the n-3 biosynthesis pathway. However, at the end of the experimental period, no impact of the fish n-3 PUFA depletion was observed on apparent in vivo elongation and desaturation activities of SO/LO fish as compared with LO fish. In contrast, the fish n-3 PUFA depletion negatively modulated the n-6 PUFA bioconversion capacity of fish in terms of reduced apparent in vivo elongation and desaturation activities. The effects were similar after 10 or 36 d of the experimental period, indicating the absence of short-term effects.

Regulation of the Omega-3 Fatty Acid Biosynthetic Pathway in Atlantic Salmon Hepatocytes

Limited availability of the n-3 fatty acids EPA and DHA have led to an interest in better understanding of the n-3 biosynthetic pathway and its regulation. The biosynthesis of alpha-linolenic acid to EPA and DHA involves several complex reaction steps including desaturation-, elongation- and peroxisomal beta-oxidation enzymes. The aims of the present experiments were to gain more knowledge on how this biosynthesis is regulated over time by different doses and fatty acid combinations. Hepatocytes isolated from salmon were incubated with various levels and combinations of oleic acid, EPA and DHA. Oleic acid led to a higher expression of the Δ6 fatty acid desaturase (fad) genes Δ6fad_a, Δ6fad_b, Δ6fad_c and the elongase genes elovl2 compared with cells cultured in medium enriched with DHA. Further, the study showed rhythmic variations in expression over time. Levels were reached where a further increase in specific fatty acids given to the cells not stimulated the conversion further. The gene expression of Δ6fad_a_and Δ6fad_b responded similar to fatty acid treatment, suggesting a co-regulation of these genes, whereas Δ5fad and Δ6fad_c showed a different regulation pattern. EPA and DHA induced different gene expression patterns, especially of Δ6fad_a. Addition of radiolabelled alpha-linolenic acid to the hepatocytes confirmed a higher degree of elongation and desaturation in cells treated with oleic acid compared to cells treated with DHA. This study suggests a complex regulation of the conversion process of n-3 fatty acids. Several factors, such as that the various gene copies are differently regulated, the gene expression show rhythmic variations and gene expression only affected to a certain level, determines when you get the maximum conversion of the beneficial n-3 fatty acids.

Dietary fish oil replacement by linseed oil: Effect on growth, nutrient utilization, tissue fatty acid composition and desaturase gene expression in silver barb (Puntius gonionotus) fingerlings

Silver barb (Puntius gonionotus) is considered a promising medium carp species for freshwater aquaculture in Asia. This study in silver barb was carried out to evaluate the effects of total or partial substitution of dietary fish oil (FO) with linseed oil (LO) on growth, nutrient utilization, whole-body composition, muscle and liver fatty acid composition. Fish (12.1±0.4g of initial body weight) were fed for 60days with five experimental iso-proteinous, iso-lipidic and iso-caloric diets in which FO (control diet) was replaced by 33.3%, 50%, 66.7% and 100% LO. Final weight, weight gain, percent weight gain, SGR decreased linearly (p<0.001) with increasing LO levels in the diets. Dietary LO substitution levels did not significantly (p>0.05) affect the feed conversion ratio (FCR), protein efficiency ratio (PER) and whole body proximate composition. Furthermore, enhanced level of LO increased α-linolenic acid (ALA; 18:3n3) and linoleic acid (LA; 18:2n6) and decreased eicosapentaenoic acid (EPA; 20:5n3) and docosahexaenoic acid (DHA; 22:6n3) in muscle and liver. To understand the molecular mechanism of long chain-polyunsaturated fatty acid (LC-PUFA) biosynthesis, we cloned and characterized the fatty acyl Δ6 desaturase (Δ6 fad) cDNA and investigated its expression in various organs/tissues following replacement of FO with LO in the diet. The full-length Δ6 fad cDNA was 2056bp encoding 444 amino acids and was widely expressed in various organs/tissues. Replacement of FO with LO increased the expression of Δ6 fad mRNA in liver, muscle and intestine but no significant difference was found in the brain.

Temperature Increase Negatively Affects the Fatty Acid Bioconversion Capacity of Rainbow Trout (Oncorhynchus mykiss) Fed a Linseed Oil-Based Diet

Aquaculture is meant to provide fish rich in omega-3 long chain polyunsaturated fatty acids (n-3 LC-PUFA). This objective must be reached despite (1) the necessity to replace the finite and limited fish oil in feed production and (2) the increased temperature of the supply water induced by the global warming. The objective of the present paper was to determine to what extent increased water temperature influences the fatty acid bioconversion capacity of rainbow trout (Oncorhynchus mykiss) fed a plant-derived diet. Fish were fed two diets formulated with fish oil (FO) or linseed oil (LO) as only added lipid source at the optimal water temperature of 15°C or at the increased water temperature of 19°C for 60 days. We observed that a temperature increase close to the upper limit of the species temperature tolerance range negatively affected the feed efficiency of rainbow trout fed LO despite a higher feed intake. The negative impact of increased water temperature on fatty acid bioconversion capacity appeared also to be quite clear considering the reduced expression of fatty acid desaturase 2 in liver and intestine and the reduced Δ6 desaturase enzymatic activity in intestinal microsomes. The present results also highlighted a negative impact of increased temperature on the apparent in vivo enzymatic activity of Δ5 and Δ6 desaturases of fish fed LO. Interestingly, this last parameter appeared less affected than those mentioned above. This study highlights that the increased temperature that rainbow trout may face due to global warming could reduce their fatty acid bioconversion capacity. The unavoidable replacement of finite fish oil by more sustainable, readily available and economically viable alternative lipid sources in aquaculture feeds should take this undeniable environmental issue on aquaculture productivity into account.

Preliminary Validation of a High Docosahexaenoic Acid (DHA) and α-Linolenic Acid (ALA) Dietary Oil Blend: Tissue Fatty Acid Composition and Liver Proteome Response in Atlantic Salmon (Salmo salar) Smolts

Marine oils are important to human nutrition as the major source of docosahexaenoic acid (DHA), a key omega-3 long-chain (≥C₂₀) polyunsaturated fatty acid (n-3 LC-PUFA) that is low or lacking in terrestrial plant or animal oils. The inclusion of fish oil as main source of n-3 LC-PUFA in aquafeeds is mostly limited by the increasing price and decreasing availability. Fish oil replacement with cheaper terrestrial plant and animal oils has considerably reduced the content of n-3 LC-PUFA in flesh of farmed Atlantic salmon. Novel DHA-enriched oils with high alpha-linolenic acid (ALA) content will be available from transgenic oilseeds plants in the near future as an alternative for dietary fish oil replacement in aquafeeds. As a preliminary validation, we formulated an oil blend (TOFX) with high DHA and ALA content using tuna oil (TO) high in DHA and the flaxseed oil (FX) high in ALA, and assessed its ability to achieve fish oil-like n-3 LC-PUFA tissue composition in Atlantic salmon smolts. We applied proteomics as an exploratory approach to understand the effects of nutritional changes on the fish liver. Comparisons were made between fish fed a fish oil-based diet (FO) and a commercial-like oil blend diet (fish oil + poultry oil, FOPO) over 89 days. Growth and feed efficiency ratio were lower on the TOFX diet. Fish muscle concentration of n-3 LC-PUFA was significantly higher for TOFX than for FOPO fish, but not higher than for FO fish, while retention efficiency of n-3 LC-PUFA was promoted by TOFX relative to FO. Proteomics analysis revealed an oxidative stress response indicative of the main adaptive physiological mechanism in TOFX fish. While specific dietary fatty acid concentrations and balances and antioxidant supplementation may need further attention, the use of an oil with a high content of DHA and ALA can enhance tissue deposition of n-3 LC-PUFA in relation to a commercially used oil blend.

Long-term feeding a plant-based diet devoid of marine ingredients strongly affects certain key metabolic enzymes in the rainbow trout liver

Incorporation of a plant blend in the diet can affect growth parameters and metabolism in carnivorous fish. We studied for the first time the long-term (1 year) metabolic response of rainbow trout fed from first feeding with a plant-based diet totally devoid of marine ingredients. Hepatic enzymes were analyzed at enzymatic and molecular levels, at 3, 8 and 24 h after the last meal to study both the short-term effects of the last meal and long-term effects of the diet. The results were compared with those of fish fed a control diet of fish meal and fish oil. Growth, feed intake, feed efficiency and protein retention were lower in the group fed the plant-based diet. Glucokinase and pyruvate kinase activity were lower in the livers of trout fed the plant-based diet which the proportion of starch was lower than in the control diet. Glutamate dehydrogenase was induced by the plant-based diet, suggesting an imbalance of amino acids and a possible link with the lower protein retention observed. Gene expression of delta 6 desaturase was higher in fish fed the plant-based diet, probably linked to a high dietary level of linolenic acid and the absence of long-chain polyunsaturated fatty acids in vegetable oils. Hydroxymethylglutaryl-CoA synthase expression was also induced by plant-based diet because of the low rate of cholesterol in the diet. Changes in regulation mechanisms already identified through short-term nutritional experiments (<12 weeks) suggest that metabolic responses are implemented at short term and remain in the long term.

Nutritional regulation of long-chain PUFA biosynthetic genes in rainbow trout (Oncorhynchus mykiss)

Most studies on dietary vegetable oil in rainbow trout (Oncorhynchus mykiss) have been conducted on a background of dietary EPA (20 : 5n-3) and DHA (22 : 6n-3) contained in the fishmeal used as a protein source in aquaculture feed. If dietary EPA and DHA repress their endogenous synthesis from α-linolenic acid (ALA, 18 : 3n-3), then the potential of ALA-containing vegetable oils to maintain tissue EPA and DHA has been underestimated. We examined the effect of individual dietary n-3 PUFA on the expression of the biosynthetic genes required for metabolism of ALA to DHA in rainbow trout. A total of 720 juvenile rainbow trout were allocated to twenty-four experimental tanks and assigned one of eight diets. The effect of dietary ALA, EPA or DHA, in isolation or in combination, on hepatic expression of fatty acyl desaturase (FADS)2a(Δ6), FADS2b(Δ5), elongation of very long-chain fatty acid (ELOVL)5 and ELOVL2 was examined after 3 weeks of dietary intervention. The effect of these diets on liver and muscle phospholipid PUFA composition was also examined. The expression levels of FADS2a(Δ6), ELOVL5 and ELOVL2 were highest when diets were high in ALA, with no added EPA or DHA. Under these conditions ALA was readily converted to tissue DHA. Dietary DHA had the largest and most consistent effect in down-regulating the gene expression of all four genes. The ELOVL5 expression was the least responsive of the four genes to dietary n-3 PUFA changes. These findings should be considered when optimising aquaculture feeds containing vegetable oils and/or fish oil or fishmeal to achieve maximum DHA synthesis.