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BackgroundWelfare challenges in salmon farming highlights the need to improve understanding of the fish’s response to its environment and rearing operations. This can be achieved by monitoring physiological responses such as heart rate (HR) for individual fish. Existing solutions for heart rate monitoring are typically based on Electrocardiography (ECG) which is sensitive to placement and electrode orientation. These factors are difficult to control and affects the reliability of the principle, prompting the desire to find an alternative to ECG for heart rate monitoring in fish. This study was aimed at adapting an optical photoplethysmography (PPG) sensor for this purpose. An embedded sensor unit measuring both PPG and ECG was developed and tested using anesthetized Atlantic salmon in a series of in-vivo experiments. HR was derived from PPG and compared to the ECG baseline to evaluate its efficacy in estimating heart rate.ResultsThe results show that PPG HR was estimated with an accuracy of 0.7 ± 1.0% for 660 nm and 1.1 ± 1.2% for 880 nm wavelengths, respectively, relative to the ECG HR baseline. The results also indicate that PPG should be measured in the anterior part of the peritoneal cavity in the direction of the heart.ConclusionA PPG/ECG module was successfully adapted to measure both ECG and PPG in-vivo for anesthetized Atlantic salmon. Using ECG as baseline, PPG analysis results show that that HR can be accurately estimated from PPG. Thus, PPG has the potential to become an alternative to ECG HR measurements in fish.

Interspecific hybridisation may improve the farm performance of sterile triploid salmonids via heterosis (i.e. hybrid vigour). We assessed growth over the final 293 days in seawater, and harvest quality, in diploid and triploid Atlantic salmon (Salmo salar) × brown trout (Salmo trutta) hybrids compared to diploid and triploid Atlantic salmon. We measured vertebral deformities, cataracts, flesh colour, gut mass, and body shape at harvest. In triploids, hybridisation had no effect on harvest size, vertebral deformities, cataracts, or body shape, but did improve fillet colouration (Mean digital SalmoFan™ score [95% CI]: 24.6 [24.4–24.9] and 26.0 [25.7–26.2] for triploid salmon and triploid hybrids, respectively) and lower relative gut size (34% lower). Compared to diploid salmon, triploid salmon were significantly heavier at harvest, triploid hybrids tended to be heavier (Post-hoc, least square means, p = 0.08), whereas diploid hybrids were 83% lighter (Mean mass [g] at harvest [95% CI]: 2676 [2470–2898], 3395 [3134–3679], 462 [401–534], and 3086 [2832–3363] for diploid salmon, triploid salmon, diploid hybrids, and triploid hybrids, respectively). However, both triploid groups had a significantly higher incidence of fish with one or more deformed vertebra (Mean % [95% CI]: 23 [14–35], 60 [47–71], 38 [20–60], and 44 [31–57] % in diploid salmon, triploid salmon, diploid hybrids, and triploid hybrids, respectively), more severe cataracts (Mean cataract score [95% CI]: 3.0 [2.7–3.3], 3.5 [3.2–3.8], 2.2 [1.7–2.6], 3.6 [3.3–4.0] for diploid salmon, triploid salmon, diploid hybrids, and triploid hybrids, respectively), and a smaller relative gut size (21% smaller) compared to diploid counterparts. In conclusion, triploid hybrids have no growth advantage over triploid salmon and suffer from similar welfare issues while only benefiting from increased fillet colour.

Nephrocalcinosis is a common disorder in nurseries in Norway (Klykken, Reed, et al., 2022) and was reported as one of the main welfare challenges in farmed salmon by The Norwegian Fish Health Report of 2019 (Sommerset et al., 2020). Nephrocalcinosis is described as deposits of minerals within the kidneys (Bruno, 1996), that can disturb kidney function, which in turn can have dramatic consequences on fish performance and survival. The aetiology is most likely related to environmental factors, and suboptimal water quality has been indicated in several studies as the main risk factor (Fivelstad et al., 1999; Fivelstad et al., 2003; Khan et al., 2018; Fivelstad et al., 2015; Good et al., 2010; Lewisch et al., 2013; Chen et al., 2001). Newly conducted research suggested that osmoregulatory stress may be the trigger for nephrocalcinosis (Boissonnot et al., 2022). Regardless of the severity of the condition, fish rarely present external signs, and it is thus challenging to monitor its prevalence and development. Present diagnostic methods require euthanasia as they consist of visually scoring the accumulation of deposits and the severity of lesions. Macroscopic assessments of necropsied fish are often imprecise, since small deposits are rarely visible to the naked eye, and histopathology is therefore considered as the best existing diagnostic method (Klykken, Boissonnot, et al., 2022). Research on, and monitoring of, nephrocalcinosis has been greatly hampered by the lack of non-invasive methods of assessing the presence and severity of this condition, as it is not possible to follow the development of the disease in single individuals, and as the number of sampled fish is limited due to ethical reasons. Radiology has previously been used for assessing vertebrae deformities in Atlantic salmon (Drábiková et al., 2021; Holm et al., 2020), based on the classification scheme developed by Witten et al. (2009), and there has been a rapid development of the technology (Ou et al., 2021) including portable systems, which allow efficient in situ diagnosis. Nephrocalcinosis in Atlantic salmon is mainly identified as amorphous carbonate apatite, a calciumdominated mineral (Klykken, Reed, et al., 2022), and it has previously been demonstrated that this mineral composition is suitable for xray detection (Smith & Lehr, 1966). Radiology is non-invasive and can be performed on anaesthetized fish, enabling assessment of nephrocalcinosis without euthanazing the fish. A non-invasive method for assessing nephrocalcinosis would allow for monitoring of the condition over time on an individual level and would be ethically and economically preferable. We have therefore explored radiology as a possible tool to detect and evaluate the severity of nephrocalcinosis, comparing it with histological scoring.

Summary Propionate and propionyl-CoA accumulation have been associated with the development of mitochondrial dysfunction. In this study, we show that propionate induces intestinal damage in zebrafish when fed a high-fat diet (HFD). The intestinal damage was associated with oxidative stress owing to compromised superoxide dismutase 2 (Sod2) activity. Global lysine propionylation analysis of the intestinal samples showed that Sod2 was propionylated at lysine 132 (K132), and further biochemical assays demonstrated that K132 propionylation suppressed Sod2 activity. In addition, sirtuin 3 (Sirt3) played an important role in regulating Sod2 activity via modulating de-propionylation. Finally, we revealed that intestinal oxidative stress resulting from Sod2 propionylation contributed to compositional change of gut microbiota. Collectively, our results in this study show that there is a link between Sod2 propionylation and oxidative stress in zebrafish intestines and highlight the potential mechanism of intestinal problems associated with high propionate levels., Graphical abstract, Highlights • Propionate supplementation in high-fat diet induces intestinal damage • Propionate induces oxidative stress via Sod2 propionylation at 132 lysine site • Increased Sod2 propionylation is associated with reduced expression of Sirt3 • Intestinal oxidative stress alters gut microbiota composition, Gastroenterology; Molecular physiology; Cell biology

The increasing replacement of fish oil (FO) by vegetable oils (VO) in aquafeeds for marine carnivorous species may lead to modifications of the dietary arachidonic (ARA), eicosapentaenoic (EPA) and docosahexaenoic (DHA) levels and ratios that may compromise the fish immune status. Therefore, this study aimed to assess how different dietary ratios of ARA, EPA, and DHA modulate the immune response of gilthead sea bream juveniles. For that purpose, four isoproteic and isolipidic plant-based diets were formulated to include different dietary Long-Chain-Polyunsaturated Fatty Acids (LC-PUFA) ratios (%DM): Diet A (2.0% ARA: 0.2% EPA: 0.1% DHA); Diet B (1.0% ARA: 0.4% EPA: 0.4% DHA); Diet C (0% ARA: 0.6% EPA: 0.6% DHA); Diet D (0% ARA: 0.3% EPA: 1.5% DHA). Fish were fed the experimental diets for 56 days and then the haematological profile, humoral innate immune parameters, and expression of immune-related genes in the gut were determined. Our results showed that fish fed diet B exhibited an increase in the monocyte numbers, alternative complement activity, and bactericidal activity against Photobacterium damselae compared to the values found in fish fed other diets. Contrarily, peroxidase, antiproteases, and proteases activity and nitric oxide levels were not affected by the dietary treatments. Immune-related gene expression in the distal intestine did not show differences between dietary treatments. Overall, a diet with a balanced n-6 (ARA) and n-3 (EPA+DHA) LC-PUFA ratio seemed to better modulate the fish innate immune response, thus better preparing the fish innate immune system against potential immunological insults.

The present work assessed the effects of dietary ratios of essential fatty acids, arachidonic (ARA), eicosapentaenoic (EPA) and docosahexaenoic acid (DHA), on liver and intestine oxidative status, intestinal histomorphology and gut microbiota of gilthead sea bream. Four isoproteic and isolipidic plant-based diets were formulated containing a vegetable oil blend as the main lipid source. Diets were supplemented with ARA/EPA/DHA levels (%DM) equivalent to: 2%:0.2%:0.1% (Diet A); 1.0%:0.4%:0.4% (Diet B); 0%:0.6%:0.6% (Diet C); 0%:0.3%:1.5% (Diet D) and tested in triplicate groups for 56 days. Lipid peroxidation was higher in fish fed diets C and D while no differences were reported between diets regarding total, oxidized, and reduced glutathione, and oxidative stress index. Glutathione reductase was higher in fish fed diet A than diets C and D. No histological alterations were observed in the distal intestine. Lower microbiota diversity was observed in intestinal mucosa of fish fed diet C than A, while diets C and D enabled the proliferation of health-promoting bacteria from Bacteroidetes phylum (Asinibacterium sp.) and the absence of pathogenic species like Edwardsiella tarda. Overall, results suggest that a balance between dietary ARA/EPA + DHA promotes gilthead sea bream juveniles’ health however higher dietary content of n-3 LC-PUFA might limited the presence of microbial pathogens in intestinal mucosa.

Plant proteins are widely used for fish meal replacement in aquafeeds, but anti-nutritional factors in plant protein reduce fish growth performance and impair fish health. The present work aimed to study the effects of improving fish meal replacement percentage with ultra-micro ground mixed plant proteins (uPP) on growth, gut and liver health of common carp. Carps were fed with a practical basal diet with partial fish meal replacement by plant proteins or the basal diet supplemented with 2.5 % or 5% uPP for 16-week. Results indicated that uPP addition did not affect growth and survival of common carp at a supplementation level up to 5% (p > 0.05). However, 5% uPP up-regulated the intestinal expression of inflammation related genes (p < 0.05) and reduced HIF-1α expression (p < 0.05). Moreover, dietary 5% uPP increased serum ALT (p = 0.06) and AST level (p < 0.05) and up-regulated liver expressions of inflammation related genes (p < 0.05). The Simpson diversity index of gut microbiota was lower in 5% uPP group compared to control (p < 0.05). The relative abundance of Fusobacteria and Cetobacterium was lower (p < 0.05), while Proteobacteria including Shewanella and Citrobacter was higher in the 5% uPP group compared to control (p < 0.05). In contrast, 2.5 % uPP did not increase inflammatory and injury parameters in fish intestine and liver, but rather improved the expression of occludin and defensin in the intestine compared with control (p < 0.05). Moreover, no significant differences were found in gut microbiota between 2.5 % uPP group and control. Together, our study suggests that low-level uPP addition can be adopted to further improve fish meal replacement, while dietary 5% uPP impairs gut and liver health of common carp and negatively affects intestinal microbiota.

Astaxanthin (Ax), the main carotenoid responsible for the distinct red flesh color in salmonids (Oncorhynchus, Salvelinus, Salmo, and Parahucho), is added to the diet of farmed fish at a substantial cost. Despite the great economical value for the salmon industry, the key molecular mechanisms involved in the regulation of muscle coloration are poorly understood. Chinook salmon (Oncorhynchus tshawytscha) represent an ideal model to study flesh coloration because they exhibit a distinct color polymorphism responsible for two color morphs, white and red flesh pigmented fish. This study was designed to identify the molecular basis for the development of red and white coloration of fish reared under the same experimental conditions and to better understand the absorption mechanism of Ax in salmonids. Pyloric caeca, liver, and muscle of both groups (n = 6 each) were selected as the most likely critical target organs to be involved respectively in the intestinal uptake, metabolism, and retention of Ax. Difference in the transcriptome profile of each tissue using next-generation sequencing technology was conducted. Ten KEGG pathways were significantly enriched for differentially expressed genes between red and white salmon pylorus tissue, while none for the transcriptome profile in the other two tissues. Differential expressed gene (DE) analyses showed that there were relatively few differences in muscle (31 DE genes, p

The intestinal mucosal barrier plays a critical role in the maintenance of host health. In farmed teleost fish, the intestinal epithelium is challenged by a number of factors, leading to damage of the intestinal mucosal barrier. The pathogenesis of intestinal mucosal barrier damage in most farmed teleost fish has been associated with cell death, mainly including apoptosis, pyroptosis, necroptosis as well as secondary injury by cell lysis. However, the mechanisms behind intestinal mucosal barrier damage are not well studied. Here we summarized the factors causing damage of intestinal mucosal barrier in fish. Further we discussed the intracellular and interstitial signaling cascades, with special emphasis on cell death that potentially accelerates the progression of intestinal injury. Deeper understanding of the mechanisms of intestinal mucosal barrier damage may facilitate the development of potential intervention strategies for intestinal diseases in farmed teleost fish.

Salmon aquaculture is in great need of good quality balanced protein and lipid sources, particularly marine omega-3 (n-3) long-chain polyunsaturated fatty acids (LC-PUFA), to sustain a further development of the industry. One possibility is to harvest mesopelagic marine layers. Therefore, the current project analysed mesopelagic hauls from three cruises (November 2015 to October 2016) collected from the inner fjord systems around Bergen and in open-waters off Tromsø and Ålesund, Norway. Jellyfish, krill, shrimps and small amounts of the mesopelagic fish, Maurolicus muelleri and Benthosema glaciale, dominated the mixed mesopelagic hauls. Lipid content ranged between 35-40% of dry matter with two samples from autumn being 21 and 13%, with the latter haul being almost exclusively krill. In contrast, M. muelleri and B. glaciale had lipid contents of around 54 and 47% respectively. Overall, lipid was a relatively good source of marine n-3 LC-PUFA, EPA and DHA, being in the range of 15–20% of fatty acids which increased in lean samples. However, many of the trawl hauls contained wax esters (7 out of 9 hauls), equivalent to 40% or more of the lipid, with B. glaciale containing almost 90% wax esters of lipid. This presents a challenge if used in salmon diets, as their utilisation is limited. Protein contents ranged between 45-50%, increasing in lean samples. The essential amino acid content was well above the requirements for Atlantic salmon (Salmo salar) with B. glaciale generally containing higher levels compared to M. muelleri. Leucine, lysine and valine levels were particularly high. Hauls from open-waters contained mixtures of amphipods resulting in cadmium levels exceeding the maximum allowable level in feedstuffs. Arsenic levels were high or borderline. Reducing crustacean mix in hauls appear to be the only option to reduce these levels, whereas mesopelagic fish contained low levels of all heavy metals. In summary, the mesopelagic layer contains protein and lipid sources that could supply raw materials to the salmon aquaculture industry. However, high levels of wax esters, cadmium and arsenic needs to be addressed. This is an open access article distributed under the terms of the Creative Commons CC-BY license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. permits unrestricted use, distribution, and reproduction in any medium, provided the original work is proper

Domestication of Salmo salar has imposed strong selection for production traits since the 1970s. The domestication has also imposed a radical shift in diet. Whereas wild salmon eats invertebrates, crustaceans and fish, the dietary lipids in domestic feed has since 1990 gradually shifted from fish oil (FO) to vegetable oil (VO), causing a decrease intake of long-chain polyunsaturated fatty acids (LC-PUFA). We tested the hypothesis that this shift has induced domestication-specific features of lipid metabolism in a 96-day feeding trial of domesticated and wild salmon fed diets based on FO, VO or phospholipids (PL). We addressed this by sampling tissues central in fat uptake (pyloric caeca) and processing (liver) and quantifying RNA expression and fatty acid composition. Domesticated salmon grew faster than wild salmon, with higher gene expression in glucose and lipid metabolism pathways. The promoters of differentially expressed genes were enriched for transcription factors involved in circadian clock regulation. Domesticated salmon had lower expression of cry2 and nr1d1, genes involved in negative regulation of circadian rhythm, with possible implications for the diurnal cycle of feed ingestion and basal metabolic rate. Only wild salmon showed a significant impact on growth of VO versus PL or FO feed, whereas domesticated but not wild salmon upregulated key LC-PUFA synthesis genes fads2d5 and fads2d6a in response to VO diet. Domesticated salmon had higher LC-PUFA but lower 18:3n-3 and 18:2n-6 in liver when fed VO, suggesting that domesticated salmon can better compensate for dietary shortage of LC-PUFA compared to wild salmon.

In salmon farming, the use of sterile triploids (3N) can mitigate the problem of escapees interbreeding with wild salmon. However, triploid salmon appear less tolerant to high water temperatures and low oxygen levels compared to diploids (2N). To investigate how the thermal performance and physiology of large (2.5 kg) triploid Atlantic salmon Salmo salar L. differs from those of diploids, both ploidies were subjected to water temperatures between 3 and 18°C. The fish were exposed to reduced oxygen saturations (O2 sat, 70%), termed hypoxia, at 6 and 18°C. Triploids fed more than diploids between 3 and 9°C and at similar levels at 12°C. At 15°C, the feed intake significantly dropped in both ploidies, although more in triploids. During hypoxia, feed intake was higher in triploids at 6°C and equal to diploids at 18°C. The overall feed conversion ratio was similar between ploidies. Muscle energy phosphates were generally lower in triploids than diploids, while muscle glucose, blood haemoglobin and haematocrit were lower in triploids than diploids at ≥12°C. Plasma lactate levels tended to be higher in triploids and increased with increasing temperature and at hypoxia in both ploidies. Plasma cortisol increased in both ploidies at high temperatures and was highest in triploids under hypoxic conditions at 18°C. Triploids had a higher cataract score at the start of the experiment and developed more cataracts throughout the experiment. The present findings show that large triploid Atlantic salmon perform better at colder water temperatures compared to diploids and differ in parts of their physiological expression at increasing and high temperature. © The authors 2018. Open Access under Creative Commons by Attribution Licence. Use, distribution and reproduction are unrestricted. Authors and original publication must be credited.

There is considerable interest in new sources of omega‐3 long‐chain (here defined as fatty acids ≥C20) polyunsaturated fatty acids (LC‐PUFA), specifically eicosapentaenoic acid (EPA; 20:5n‐3) and docosahexaenoic acid (DHA; 22:6n‐3), commonly known as omega‐3 fish oils, to supplement the limited supplies of oceanic fish oil (Tocher, 2015). These alternative sources include extraction of other diverse marine organisms (e.g. krill, plankton etc.), algal fermentation and the genetic engineering of microbes such as yeasts. Another approach is the synthesis of omega‐3 fish oils in transgenic plants (reviewed in Napier et al., 2015), and this short article will discuss the recent results obtained by two major industry collaborations. The objective of engineering plants to accumulate EPA and DHA has long been recognized as worthy (summarized in Domergue et al., 2005). Genes for this biosynthetic pathway (Figure 1) were characterized by the early 2000s, but efficient transgenic reconstitution proved more challenging. Varying levels of EPA and DHA have been reported in both Arabidopsis and Camelina by different groups (comprehensively discussed in Napier et al., 2015). However, significant commercial effort has been also focussed on canola (Brassica napus) as the host for this transgenic pathway, since genetic and agronomic resources are well‐established for this crop, with also significant grower acceptance of GM canola in N. America. © 2018 The Authors. Plant Biotechnology Journal published by Society for Experimental Biology and The Association of Applied Biologists and John Wiley & Sons Ltd. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Calanus copepods are keystone species in marine ecosystems, mainly due to their high lipid content, which is a nutritious food source for e.g. juvenile fish. Accumulated lipids are catabolized to meet energy requirements during dormancy (diapause), which occurs during the last copepodite stage (C5). The current knowledge of lipid degradation pathways during diapause termination is limited. We characterized changes in lipid fullness and generated transcriptional profiles in C5s during termination of diapause and progression towards adulthood. Lipid fullness of C5s declined linearly during developmental progression, but more β-oxidation genes were upregulated in early C5s compared to late C5s and adults. We identified four possible master regulators of energy metabolism, which all were generally upregulated in early C5s, compared to late C5s and adults. We discovered that one of two enzymes in the carnitine shuttle is absent from the calanoid copepod lineage. Based on the geographical location of the sampling site, the field-samples were initially presumed to consist of C. finmarchicus. However, the identification of C. glacialis in some samples underlines the need for performing molecular analyses to reliably identify Calanus species. Our findings contributes to a better understanding of molecular events occurring during diapause and diapause termination in calanoid copepods. © The Author(s) 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/)

Calanus finmarchicus and Calanus glacialis are keystone zooplankton species in North Atlantic and Arctic marine ecosystems because they form a link in the trophic transfer of nutritious lipids from phytoplankton to predators on higher trophic levels. These calanoid copepods spend several months of the year in deep waters in a dormant state called diapause, after which they emerge in surface waters to feed and reproduce during the spring phytoplankton bloom. Disruption of diapause timing could have dramatic consequences for marine ecosystems. In the present study, Calanus C5 copepodites were collected in a Norwegian fjord during diapause and were subsequently experimentally exposed to the water-soluble fraction of a naphthenic North Sea crude oil during diapause termination. The copepods were sampled repeatedly while progressing toward adulthood and were analyzed for utilization of lipid stores and for differential expression of genes involved in lipid metabolism. Our results indicate that water-soluble fraction exposure led to a temporary pause in lipid catabolism, suggested by (i) slower utilization of lipid stores in water-soluble fraction-exposed C5 copepodites and (ii) more genes in the β-oxidation pathway being downregulated in water-soluble fraction-exposed C5 copepodites than in the control C5 copepodites. Because lipid content and/or composition may be an important trigger for termination of diapause, our results imply that the timing of diapause termination and subsequent migration to the surface may be delayed if copepods are exposed to oil pollution during diapause or diapause termination. This delay could have detrimental effects on ecosystem dynamics.

Atlantic salmon can synthesize polyunsaturated fatty acids (PUFAs), such as eicosapentaenoic acid (20:5n-3), arachidonic acid (20:4n-6) and docosahexaenoic acid (22:6n-3) via activities of very long chain fatty acyl elongases (Elovls) and fatty acyl desaturases (Fads), albeit to a limited degree. Understanding molecular mechanisms of PUFA biosynthesis and regulation is a pre-requisite for sustainable use of vegetable oils in aquafeeds as current sources of fish oils are unable to meet increasing demands for omega-3 PUFAs. By generating CRISPR-mediated elovl2 partial knockout (KO), we have shown that elovl2 is crucial for multi-tissue synthesis of 22:6n-3 in vivo and that endogenously synthesized PUFAs are important for transcriptional regulation of lipogenic genes in Atlantic salmon. The elovl2-KOs showed reduced levels of 22:6n-3 and accumulation of 20:5n-3 and docosapentaenoic acid (22:5n-3) in the liver, brain and white muscle, suggesting inhibition of elongation. Additionally, elovl2-KO salmon showed accumulation of 20:4n-6 in brain and white muscle. The impaired synthesis of 22:6n-3 induced hepatic expression of sterol regulatory element binding protein-1 (srebp-1), fatty acid synthase-b, Δ6fad-a, Δ5fad and elovl5. Our study demonstrates key roles of elovl2 at two penultimate steps of PUFA synthesis in vivo and suggests Srebp-1 as a main regulator of endogenous PUFA synthesis in Atlantic salmon.

Atlantic salmon (Salmo salar) possess enzymes required for the endogenous biosynthesis ofn-3 long-chain PUFA (LC-PUFA), EPA and DHA, fromα-linolenic acid (ALA). Linoleic acid (LA) competes with ALA for LC-PUFA biosynthesis enzymes leading to the production ofn-6 LC-PUFA, including arachidonic acid (ARA). We aimed to quantify the endogenous production of EPA and DHA from ALA in salmon fed from first feeding on diets that contain no EPA and DHA and to determine the influence of dietary LA and ALA:LA ratio on LC-PUFA production. Salmon were fed from first feeding for 22 weeks with three diets formulated with linseed and sunflower oils to provide ALA:LA ratios of approximately 3:1, 1:1 and 1:3. Endogenous production ofn-3 LC-PUFA was 5·9, 4·4 and 2·8 mg per g fish and that ofn-6 LC-PUFA was 0·2, 0·5 and 1·4 mg per g fish in salmon fed diets with ALA:LA ratios of 3:1, 1:1 and 1:3, respectively. The ratio ofn-3:n-6 LC-PUFA production decreased from 27·4 to 2·0, and DHA:EPA ratio increased and EPA:ARA and DHA:ARA ratios decreased, as dietary ALA:LA ratio decreased. In conclusion, with a dietary ALA:LA ratio of 1, salmon fry/parr produced about 28 μgn-3 LC-PUFA per g fish per d, with a DHA:EPA ratio of 3·4. Production ofn-3 LC-PUFA exceeded that ofn-6 LC-PUFA by almost 9-fold. Reducing the dietary ALA:LA ratio reducedn-3 LC-PUFA production and EPA:ARA and DHA:ARA ratios but increasedn-6 LC-PUFA production and DHA:EPA ratio.

The in vivo functions of Atlantic salmon fatty acyl desaturases (fads2), Δ6fads2-a, Δ6fads2-b, Δ6fads2-c and Δ5fads2 in long chain polyunsaturated fatty acid (LC-PUFA) synthesis in salmon and fish in general remains to be elucidated. Here, we investigate in vivo functions and in vivo functional redundancy of salmon fads2 using two CRISPR-mediated partial knockout salmon, Δ6abc/5Mt with mutations in Δ6fads2-a, Δ6fads2-b, Δ6fads2-c and Δ5fads2, and Δ6bcMt with mutations in Δ6fads2-b and Δ6fads2-c. F0 fish displaying high degree of gene editing (50–100%) were fed low LC-PUFA and high LC-PUFA diets, the former containing reduced levels of eicosapentaenoic (20:5n-3) and docosahexaenoic (22:6n-3) acids but higher content of linoleic (18:2n-6) and alpha-linolenic (18:3n-3) acids, and the latter containing high levels of 20:5n-3 and 22:6n-3 but reduced compositions of 18:2n-6 and 18:3n-3. The Δ6abc/5Mt showed reduced 22:6n-3 levels and accumulated Δ6-desaturation substrates (18:2n-6, 18:3n-3) and Δ5-desaturation substrate (20:4n-3), demonstrating impaired 22:6n-3 synthesis compared to wildtypes (WT). Δ6bcMt showed no effect on Δ6-desaturation compared to WT, suggesting Δ6 Fads2-a as having the predominant Δ6-desaturation activity in salmon, at least in the tissues analyzed. Both Δ6abc/5Mt and Δ6bcMt demonstrated significant accumulation of Δ8-desaturation substrates (20:2n-6, 20:3n-3) when fed low LC-PUFA diet. Additionally, Δ6abc/5Mt demonstrated significant upregulation of the lipogenic transcription regulator, sterol regulatory element binding protein-1 (srebp-1) in liver and pyloric caeca under reduced dietary LC-PUFA. Our data suggest a combined effect of endogenous LC-PUFA synthesis and dietary LC-PUFA levels on srebp-1 expression which ultimately affects LC-PUFA synthesis in salmon. Our data also suggest Δ8-desaturation activities for salmon Δ6 Fads2 enzymes.

Increased swimming speed of Atlantic salmon is generally considered an improvement to welfare under aquaculture settings, as group structure is improved and agonistic behaviour reduced. As such, establishing fish farms in exposed areas with fast water current velocities should be favourable. However, at some locations, velocities exceed what is known as preferable for salmonids, and this may compromise fish welfare. In this study, behaviour and fin erosion were observed on post-smolt salmon stocked at 39 kg m−3 in raceways at 3 water current velocities: fast (1.5 body lengths [BL] s−1), moderate (0.8 BL s−1) and slow (0.2 BL s−1). Movements that affect group structure and interactions between individuals varied by up to 20-fold between velocities. A behavioural change occurred directly after velocities were set. Severe fin erosion decreased over time in all groups, but new injuries increased almost 3-fold in the faster-velocity group. Our results suggest that moderate velocity is ideal from a welfare perspective. At slow velocity, higher frequency of structural movements and between-individual interactions could be stressful for the fish. At faster velocity, the fish have to focus on swimming, which could increase unintentional collisions with obstacles and other individuals and result in new fin erosion. Our results suggest that management of water currents may be an effective way of controlling behaviour and may thereby improve welfare. © The authors 2016. Open Access under Creative Commons by Attribution Licence. Use, distribution and reproduction are unrestricted. Authors and original publication must be credited.

In salmon farming, the scarcity of fish oil has driven a shift towards the use of plant-based oil from vegetable or seed, leading to fish feed low in long-chain PUFA (LC-PUFA) and cholesterol. Atlantic salmon has the capacity to synthesise both LC-PUFA and cholesterol, but little is known about the regulation of synthesis and how it varies throughout salmon life span. Here, we present a systemic view of lipid metabolism pathways based on lipid analyses and transcriptomic data from salmon fed contrasting diets of plant or fish oil from first feeding. We analysed four tissues (stomach, pyloric caeca, hindgut and liver) at three life stages (initial feeding 0·16 g, 2·5 g fingerlings and 10 g juveniles). The strongest response to diets higher in plant oil was seen in pyloric caeca of fingerlings, with up-regulation of thirty genes in pathways for cholesterol uptake, transport and biosynthesis. In juveniles, only eleven genes showed differential expression in pyloric caeca. This indicates a higher requirement of dietary cholesterol in fingerlings, which could result in a more sensitive response to plant oil. The LC-PUFA elongation and desaturation pathway was down-regulated in pyloric caeca, probably regulated bysrebp1genes. In liver, cholesterol metabolism and elongation and desaturation genes were both higher on plant oil. Stomach and hindgut were not notably affected by dietary treatment. Plant oil also had a higher impact on fatty acid composition of fingerlings compared with juveniles, suggesting that fingerlings have less metabolic regulatory control when primed with plant oil diet compared with juveniles.

To tailor the farming environment to a fish species, we should understand the species-specific responses to stimuli, including the degree of adaption and learning. Groups of gilthead sea bream were given a delay Pavlovian conditioning regime using a conditioning stimulus (CS) of light flashes signalling arrival of food. Controls were exposed to light flashes unrelated to feeding. Fish in both treatments showed an initial fear response of moving away from the CS combined with reduced swimming speed. In subsequent trials, the Control fish largely habituated the fleeing response but sustained to respond by reducing the swimming speed. The Conditioning fish also stopped to escape from the CS, but opposed to the Control group they gradually increased their swimming speed in response to the CS. In addition, the number of fish in the feeding/CS area increased and became similar to basal level after around 16 trials. A small and variable proportion of the fish displayed black vertical bands on their body and territorial behaviour, and a social hierarchy could interfere with the processes of habituation and conditioning. The swimming speed of the fish increased with number of dark individuals, but this was not found during the CS and the light stimulus thus seemed to overrule the effect of territorial behaviour. The persistent negative response to light flashes in the Control suggests that fish seemingly adapted to repetitive stressors are still in a state of alertness. The change in the response to light shows the potential for rewarding aversive stimuli to reduce stress. LOCKED until 29.8.2018 due to copyright restrictions. This is the peer reviewed version of the following article: [Habituation and conditioning in gilthead sea bream (Sparus aurata): Effects of aversive stimuli, reward and social hierarchies], which has been published in final form at [http://onlinelibrary.wiley.com/doi/10.1111/are.13463/abstract]. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.

A common-garden experiment was carried out to compare two genetically distinct strains of Atlantic salmon (Salmo salar) fed diets with either high (CHO) or low (NoCHO) digestible carbohydrate (starch). Twenty salmon from either a commercial farmed strain (F) or a land-locked population (G) were placed in two tanks (10 fish of each population in each tank) and fed either CHO or NoCHO feeds. At the end of the experiment fish were fasted for 8 h, euthanized and blood and liver collected. Both diet and population had an effect on circulating glucose levels with G showing hypoglycaemia and dietary starch increasing this parameter. In contrast, G showed increased plasma triacylglycerol levels regardless of dietary treatment suggesting faster conversion of glucose to triacylglycerol. This different ability to metabolize dietary starch among strains was also reflected at a molecular (gene) level as most of the metabolic pathways evaluated were mainly affected by the factor population rather than by diet. The data are promising and suggest different regulatory capacities toward starch utilization between land-locked salmon and the farmed stock. Further analyses are necessary in order to fully characterize the capacity of land-locked salmon to utilize dietary carbohydrate. © 2018 Betancor, Olsen, Marandel, Skulstad, Madaro, Tocher and Panserat. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

Facing a bottleneck in the growth of aquaculture, and a gap in the supply and demand of the highly beneficial n-3 long-chain PUFA (LC-PUFA), sustainable alternatives to traditional marine-based feeds are required. Therefore, in the present trial, a novel oil obtained from a genetically engineered oilseed crop, Camelina sativa, that supplied over 25 % n-3 LC-PUFA was tested as a sole dietary-added lipid source in Atlantic salmon (Salmo salar) feed. Three groups of fish were fed three experimental diets for 12 weeks with the same basal composition and containing 20 % added oil supplied by either a blend of fish oil and rapeseed oil (1:3) (COM) reflecting current commercial formulations, wild-type Camelina oil (WCO) or the novel transgenic Camelina oil (TCO). There were no negative effects on the growth, survival rate or health of the fish. The whole fish and flesh n-3 LC-PUFA levels were highest in fish fed TCO, with levels more than 2-fold higher compared with those of fish fed the COM and WCO diets, respectively. Diet TCO had no negative impacts on the evaluated immune and physiological parameters of head kidney monocytes. The transcriptomic responses of liver and mid-intestine showed only mild effects on metabolism genes. Overall, the results clearly indicated that the oil from transgenic Camelina was highly efficient in supplying n-3 LC-PUFA providing levels double that obtained with a current commercial standard, and similar to those a decade ago before substantial dietary fishmeal and oil replacement. This article has been accepted for publication in British Journal of Nutrition . This version is free to view and download for private research and study only. Not for re-distribution, re-sale or use in derivative works. © The Authors 2018

BackgroundIt has been suggested that the high phospholipid (PL) requirement in Atlantic salmon (Salmo salar) fry is due to insufficient intestinal de-novo synthesis causing low lipoprotein (LP) production and reduced transport capacity of dietary lipids. However, there has not been performed any in-depth ontological analysis of intestinal PL and LP synthesis with development of salmon. Therefore in this paper we used RNA-seq technology to test the hypothesis that the high PL requirement in salmon fry was associated with undeveloped PL synthesis and LP formation pathways in intestine. There was a special focus on the understanding homologous genes, especially from salmonid-specific fourth vertebrate whole-genome duplication (Ss4R), contribution to salmonid specific features of regulation of PL metabolic pathways. The study was performed in stomach, pyloric caeca and hindgut at 0.16g (1 day before first-feeding), 2.5g and 10g of salmon.ResultsIn general, we found an up-regulation of de-novo phosphatidylcholine (PtdCho) synthesis, phosphatidylethanolamine (PtdEtn) and LP formation pathways in pyloric caeca of salmon between 0.16g and 10g. Thirteen genes in these pathways were highly (qqConclusionsAn increasing capacity for PL synthesis and LP formation was confirmed in pyloric caeca. The up-regulation of the de-novo PtdCho pathway confirms that the salmon fry have increasing requirement for dietary PtdCho compared to adult. The similar expressions between Ss4R homologous genes suggest that the functional divergence of these genes was incomplete compared to homologs derived from other whole genome duplication. The results of the present study have provided new information on the molecular mechanisms of phospholipid synthesis and lipoprotein formation in fish.

Several major market operators argue that the current level of knowledge about quality is too scant to justify a switch to a large-scale production of triploid salmon. The aim of the present study was, therefore, to elucidate how rearing conditions (5, 10 and 15 °C) affect the flesh quality of triploid Atlantic salmon (Salmo salar L., 1.6 ± 0.3 kg). As a reference, diploid salmon kept under equal conditions and with equal genetics were used. The main design discriminant was the holding temperature; increased temperature gave increased blood lactate, rigor index (Ir), drip loss (DL), content of astaxanthin and intensity of redness, but reduced muscle pH, cathepsin activity and fillet lightness. Salmon kept at 10 °C grew the fastest. It is concluded that ploidy gave less variation than temperature. Triploids were characterized by lower blood haematocrit (Hct) and Ir, higher DL and collagenase activity, and on average, paler and less yellowish fillets. © 2017. This is the authors’ accepted and refereed manuscript to the article. Locked until 4.1.2018 due to copyright restrictions. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/