Brown seaweeds such as sugar kelp (Saccharina latissima) are rich in bioactive compounds with potential health benefits for fish. This study evaluated the effects of dietary fermented sugar kelp (FSK) on mid-intestinal histology, antioxidant biomarkers, and immune response in post-smolts Atlantic salmon (Salmo salar). A total of 975 fish (204 ± 37 g) were allocated across 15 tanks and fed one of five isonitrogenous (≈45% crude protein) diets containing 0-4% FSK for 10 weeks. Plasma, liver, muscle, and mid-intestine samples were analyzed for histological and biochemical parameters at the end of the trial. FSK inclusion up to 4% had no adverse effects on mid-intestinal morphology or signs of inflammation. Liver antioxidant capacity was modulated, with significantly higher glutathione (GSH) concentrations and lower malondialdehyde (MDA) levels in FSK 4%-fed groups compared to the control. However, no changes were observed in muscle or intestinal antioxidant levels. Immune-related parameters were also modulated: plasma lysozyme activity doubled at 1% and 3% FSK, and plasma bactericidal activity more than doubled at 3% FSK compared to the control group. Antiprotease activity increased dose-dependently, although plasma peroxidase activity and IgM levels declined slightly. Overall, dietary FSK up to 4% does not alter mid-intestine morphology or induce inflammation. Additionally, it shows potential functional effects in fish, including the modulation of hepatic antioxidant capacity and innate-immune mechanisms, particularly antibacterial effects. These findings support further investigation into the functional use of seaweed-derived ingredients in aquafeeds. The study design and main findings are provided in a graphical abstract.

The growing demand for sustainable aquaculture feeds has increased interest in novel marine ingredients. Blue mussels represent a locally available resource in Norway with potential for inclusion in Atlantic salmon diets. This study aimed to assess the effect of processing on nutrient bioavailability by combining in vitro hydrolysis and in vivo digestibility assays of blue mussel products using different processing methods and thermal treatments. Sixteen blue mussel products were produced using slurry and silage processing combined with different heat treatments. An initial in vitro screening was conducted, followed by validation in an Atlantic salmon digestibility trial. Nutritional composition, buffering capacity, and protein hydrolysis were used to select four representative products. Processing method and intensity significantly influenced nutrient composition and in vitro amino acid release (P < 0.05). Silage products exhibited higher ash content and lower true protein levels, primarily due to residual shell material and non-protein nitrogen fractions. Mild thermal treatment of slurry maintained amino acid release, whereas more intensive heating reduced protein hydrolysis. The in vivo trial showed a similar pattern, with gently processed slurry exhibiting higher protein and amino acid digestibility than more intensively treated products, while the silage solid fraction showed lower protein digestibility but relatively high lipid digestibility. Overall, the combined in vitro–in vivo approach demonstrated that moderate thermal processing preserves protein quality and nutrient bioavailability, whereas excessive heating impairs nutrient utilization. These findings demonstrate the value of sequential in vitro screening and in vivo validation for optimizing blue mussel processing for Atlantic salmon feeds.

More Atlantic salmon (Salmo salar) post-smolt production is occurring at stable temperatures of 10-17 degrees C today, compared to the fluctuating ambient conditions used in the past. This increases growth, but also triggers unwanted sexual maturation. The latter can be avoided by using triploid females as they show no gonadal development during their life. However, triploids are believed to have a lower temperature optimum and suffer more from cataracts than diploids. Here, we assess the performance of mixed sex sibling circa. 260 g diploid and triploid post-smolts reared under one of eight temperatures from 3.0 to 20.5 degrees C (in increments of 2.5 degrees C) for 140 days in seawater and on continuous light. We assessed growth, sexual maturation, ocular cataracts, liver and heart size, and blood parameters associated with osmoregulation, anaerobic metabolism, and oxygen delivery. Cataract severity increased with temperature > 15.5 degrees C in diploids and > 5.5 degrees C in triploids, with the triploids at 20.5 degrees C being euthanised due to their severity. In male and female diploids, and male triploids, the prevalence of puberty increased with temperature. There were no pubertal diploids <= 5.5 degrees C, but all were pubertal >= 18 degrees C. In diploid females, the prevalence of puberty was 0 % <= 13 degrees C, but reached 76 % at 20.5 degrees C. Triploidy reduced the prevalence of male puberty to 75 % at 18 degrees C, whereas no triploid females entered puberty. In both ploidies, growth over the entire trial period was greatest at 10.5 degrees C, in which immature fish reached an average weight of circa. 1.2 kg. In general, triploids were significantly heavier than diploids >= 10.5 degrees C due to the higher levels of male maturation in diploids, but there was no ploidy effect on growth at any temperature in immature female fish. Triploids had larger livers (by 8 %) irrespective of temperature, whereas they tended to have smaller hearts <= 15 degrees C (1-7 %), but heavier hearts >= 18 degrees C (1-8 %), compared to diploids. Irrespective of temperature, plasma lactate was generally 17 % higher, but plasma chloride 1.5 % lower, in triploids. However, other ploidy effects, such as reduced haematocrit and haemoglobin (by up to circa. 30 %) in triploids, were time and temperature dependent. In conclusion, i) 10.5 degrees C was the optimal temperature for growth, ii) stable high temperatures induce puberty in post-smolts, and iii) triploids showed similar growth to diploids, lower levels of sexual maturation, but generally had a higher prevalence of cataracts.

Diet-based smolt production and land-based post-smolt rearing are emerging strategies in Atlantic salmon aquaculture. However, their impact on fish performance and welfare over a full production cycle is poorly understood. To address this, we exposed 0+ Atlantic salmon parr (similar to 50 g) initially kept on continuous light to two different photoperiods at 12 degrees C: either a 'square wave' regime with 6 weeks of short day (12 h light / 12 h dark) followed by a return to continuous light (hereafter LD-LL), or continuous light throughout (hereafter LL). When LD-LL returned to continuous light, half of the LD-LL tanks and half of the LL tanks were provided a salt (S) enriched diet (5.0 % NaCl, 0.5 % CaCl2) in the expectation that it will aid the seawater transition, establishing four different freshwater (FW) regimes: LD-LL, LL, LD-LLS and LLS. FW rearing ended 4 weeks after initiation of the different dietary regimes (October). Then, fish from each FW regime (similar to 100-130 g) were equally distributed between three different post-smolt scenarios in a common garden design: In tanks at 12 degrees C under LL with 15 ppt or 34 ppt, or directly into sea-cages. This created 12 groups (x4 FW regimes and x3 post-smolt scenarios). Five months later (March), the post-smolts in tanks on 15 and 34 ppt were transferred into sea-cages (at similar to 1 kg). The experiment ended 7 months later (September, at about 3.5 kg). Response parameters were weight and length, mortality, and the prevalence of sexual maturation, vertebral deformities, and melanized focal changes (MFC) in the fillet at harvest. FW diet or photoperiod had no effect on mortality, but mortality was higher in those fish that had been moved directly to sea-cages versus those that remained on land for longer. FW diet did not affect growth, but the size at transfer to sea-cage was greater in LL (similar to 130 g) than LD-LL smolts (similar to 100 g). Post-smolt rearing on land in 34 ppt stimulated growth to harvest relative to 15 ppt, however, otherwise there was no effect of FW photoperiod or post-smolt scenario on final weights. Maturity levels were higher following the two landbased regimes (male: 53-68 %, female: 6-20 %) compared to those moved directly to sea-cages as smolts (male: 10-25 %, female: 0 %). LD-LL caused a general increase in male maturation compared to LL, while the S diet increased male maturation in fish directly transferred to sea-cages. MFC prevalence was higher in fish reared on land in 34 ppt (similar to 9.5 %) relative to those reared on 15 ppt (similar to 3.1 %), and was also higher in immature (similar to 6.7 %) fish compared to maturing fish (similar to 3.3 %). All groups had an equal and relatively high level of vertebral deformities (>= 1 def. Vertebra similar to 44 %, >10 def. Vertebrae similar to 23 % and > 20 def. Vertebrae similar to 9 %), mostly 'cross stitch vertebrae'. Harvest yield (total biomass produced excluding mature fish) was higher (similar to 1.8 fold) in groups transferred directly to sea-cages as smolts versus those that remained on-land during early post-smolt rearing. The study suggests that post-smolt rearing on land to 1 kg can jeopardize fish welfare by stimulating a high level of maturation and give reduced harvest yield when compared to transferring smolts directly to a sea-cage.

Atlantic salmon is an important aquaculture species that has fascinated naturalists for centuries, resulting in its biology being widely characterized. Certain details about the early development and the inheritance of meristic variation in the post-cranial axial skeleton are, however, largely unexplored. The present study gives a detailed description of the sequence of formation of the post-cranial axial skeleton based on whole-mount staining and used radiology to investigate the inheritance of meristic variation in isogenic hybrid all-male families of Atlantic salmon (~4 kg). Eight different families were created by crossing two homozygous double haploid XX females (dam A, B) with four different double haploid homozygous YY super males (sires a to d). In the caudal fin complex, the first bone to form is hypural 1 and its associated lepidotrichia followed by a bidirectional formation of new bones. In the dorsal and anal fins, development starts in the cranial part, and new bones form bidirectionally towards the head and tail fin. The neural and haemal arches start to form at segment 43, and further development is bidirectional. The first parapophysis form in the caudal part of the abdomen followed by a unidirectional completion cranially. The first ribs form at segment 3 and new ribs develop unidirectional caudally. Chordacentra formation starts at segment 24 followed by formation of chordacentrum number 58 (caudal-most vertebra). New chordacentrae form bidirectionally from segment 24 in parallel with the formation of chordacentrum number 57. The first epineuralia form at segment 1 followed by a unidirectional completion caudally until segment 30. The first supraneuralia to develop is number 10 closely followed by number 1, then new supraneurals form bidirectionally from number 10. Analysis of the inheritance on the post-cranial axial skeletal bones showed a strong maternal effect on total vertebrae centra and tail fin lepidotrichia counts. For these skeletal counts, dam A produced offspring with modes of 58 and 45 respectively, while dam B produced offspring with modes of 59 and 42. The higher number of total vertebrae centra produced by dam B was associated with additional abdominal and/or transitional vertebrae. The completion of formation in different post-cranial axial skeletal parts are either bi- or unidirectional, and the initiation of formation is site specific for each skeletal part with some inter-part similarities. Further, the present results may suggest that there has been a maternally driven selection for more abdominal vertebrae associated with a higher number of total vertebrae, and more tail fin lepidotrichia associated with a lower number of total vertebrae. These changing meristic counts may impact on important fitness-related traits, such as fecundity and swimming ability, making the present findings relevant for both ecological and aquaculture sciences.

Current procedures to establish vertebral column regionalization (e.g., histology) in fish are time consuming and difficult to apply. The aim of this study was to develop a more rapid and accurate radiology-based method for Atlantic salmon (Salmo salar). A detailed analysis of 90 animals (4 kg) led to the establishment of region-specific radiographic hallmarks. To elucidate its transferability to other salmonid species, radiography was carried out in brown trout (Salmo trutta), Arctic char (Salvelinus alpinus), rainbow trout (Oncorhynchus mykiss), pink salmon (Oncorhynchus gorbuscha), and Chinook salmon (Oncorhynchus tshawytscha). This method was also evaluated for whole ungutted fish. The vertebral column of Atlantic salmon can be subdivided into five regions (R1-R5) based on anatomy: postcranial (R1, V1, and V2), abdominal (R2, V3-V26), transitional (R3, V27-V36), caudal (R4, V37-V53), and ural (R5, V54-V59). The following specific radiographic hallmarks allow the identification of regions: (i) lack of ribs in R1, (ii) modified parapophysis of the first vertebra of R3, (iii) prominent hemal spine of the first vertebra of R4, and (iv) the separated hemal spine of the most cranial pre-ural vertebra of R5. These hallmarks were all transferable to the other salmonid species assessed. The results include a further description of various region-specific characteristics in Atlantic salmon. The method was found applicable for sedated/whole ungutted fish, verifying it as quick and easy compared to other regionalization methods. The regions defined by radiology in this study agree with the vertebral column regions recently defined for Chinook salmon (O. tshawytscha). Thus, and considering the results of this study on various salmonid species, the currently developed regionalization protocol can be generally used for salmonids.

Background: Aquaculture aims to reduce the environmental and climate footprints of feed production. Consequently, low trophic marine (LTM) resources such as blue mussels and kelp are potential candidates to be used as ingredients in salmon feed. It is relevant to study potential undesirables associated with their use, as well as assessing food safety by investigating their transfer from feed-to-fish. The marine biota is well known to contain relatively high levels of arsenic (As), which may be present in different organic forms depending on marine biota type and trophic position. Thus, it is important to not only obtain data on the concentrations of As, but also on the As species present in the raw materials, feed and farmed salmon when being fed novel LTM feed resources. Methods: Atlantic salmon were fed experimental diets for 70 days. A total of nine diets were prepared: four diets containing up to 4 % fermented kelp, three diets containing up to 11 % blue mussel silage, and one diet containing 12 % blue mussel meal, in addition to a standard reference diet containing 25 % fish meal. Concentrations of As and As species in feeds, faeces, liver and fillet of Atlantic salmon were determined by inductively coupled plasma mass spectrometry (ICP-MS) and high-performance liquid chromatography coupled to ICP-MS (HPLC-ICP-MS), respectively. Results: The use of kelp or blue mussel-based feed ingredients increased the concentration of total As, but maximum level as defined in Directive 2002/32 EC and amendments was not exceeded. The concentrations found in the experimental feeds ranged from 3.4 mg kg-1 to 4.6 mg kg-1 ww. Arsenic speciation in the feed varied based on the ingredient, with arsenobetaine dominating in all feed samples (36-60 % of the total As), while arsenosugars (5.2-8.9 % of the total As) were abundant in kelp-included feed. The intestinal uptake of total As ranged from 67 % to 83 %, but retention in fillet only ranged from 2 % to 22 % and in liver from 0.3 % to 0.6 %, depending on the marine source used. Fish fed feeds containing blue mussel showed higher intestinal uptake of total As when compared with fish fed feeds containing fermented kelp. Fish fed fermented kelp-based feeds had higher retained concentrations of total As when comparing with fish fed feeds containing blue mussel. Despite relatively high intestinal uptake of total As, inorganic and organic As, the retained concentrations of As did not reflect the same trend. Conclusion: Although the use of LTM feed ingredients increased the level of total As in this feeds, salmon reared on these diets did not show increased total As levels. The well-known toxic inorganic As forms were not detected in salmon muscle reared on LTM diets, and the non-toxic organic AsB was the dominant As species that was

Using low trophic marine resources such as sugar kelp ( Saccharina latissimi ) is of great interest to increase the circular food production in the ocean. Sugar kelp does, however, contain high levels of carbohydrates and iodine and does not have considerable levels of protein and lipids, which may make it less suitable as a feeding ingredient. A 10-week feeding trial was done to investigate the effect of graded dietary inclusion levels of fermented sugar kelp (FSK), on growth performance, digestibility, retention of nutrients, and mineral composition in postsmolt Atlantic salmon ( Salmo salar L.). The experimental diets were made to simulate a standard grower feed for salmon postsmolts in SW with ∼63% plant-based ingredients vs ∼34% marine ingredients and increasing concentrations of FSK between 0% (control feed) and 4% of the diet. During the feeding trial, the weight gain and specific growth rate (SGR) decreased linearly with increasing dietary FSK levels, where the SGR was slightly reduced from 1.2% for the fish given the control feed to 1.1% in the fish given feeds containing 3% and 4% FSK. This resulted in a lower weight gain of up to 9% in the fish given 4% FSK compared to the control. Feed intake and feed conversion ratio were, however, similar in all diet groups, and FSK inclusion did not influence the digestibility of macronutrients or minerals, except for lipid. The reduced growth is likely related to a lower digestible energy level in the diets, and the retention of both lipids and energy was affected by FSK inclusion. Inclusion of FSK also influenced iodine availability and retention, as well as increasing iodine status in whole body and muscle in a dose-dependent manner until reaching a plateau, which corresponds to 124 mg I kg −1 WW (135 mg I kg −1 DW), at 3% FSK inclusion.

Dietary electrolyte balance is the equilibrium of monovalent cations and anions that influence the acid-base balance of the feed (dEB = Na + K - Cl, mEq kg(-1)). Dietary electrolytes/minerals can influence the physiological changes during smoltification in Atlantic salmon. In this context, we aimed to study if the dEB of the freshwater feeds can be used to pre-adapt the hypoosmotic functionality and the associated effects on mineral metabolism. The dEB of commercial freshwater Atlantic salmon feeds in Norway varied from -9 to 400 mEq kg(-1) feed. Three experimental feeds were formulated to study incremental levels of dEB reflecting the low (L-dEB, -50 to 0), median (M-dEB, 200-250) and high (H-dEB, 350-400). Triplicate groups of Atlantic salmon parr (36 g) were fed one of the three feeds for 8 weeks in freshwater at 12 degrees C. The fish were transferred to full strength seawater in indoor tanks and fed a commercial diet for 6 weeks. Growth was not differentially affected by dEB levels, neither in the freshwater phase nor in the seawater. Plasma electrolytes (Na+ and Cl-) and gill mRNA expression of sodium potassium ATPase (NKA alb, seawater isoform) were significantly lower in L-dEB fed fish. In the intestine, carbonate precipitates 24 h after seawater transfer was higher in fish fed both L-dEB and H-dEB feeds compared to the M-dEB fed fish. Whole body and plasma mineral levels were significantly affected by dEB levels in freshwater feeds. Interestingly, the carryover effect of dEB in freshwater feeds was significant after 6 weeks in seawater for plasma and whole-body Zn status, with the H-dEB fed fish showing significantly increased body Zn status compared to L-dEB and M-dEB fed fish. The study revealed that mineral metabolism and intestinal response to seawater transfer can be pre-adapted by modulating the electrolyte and/or mineral balance in freshwater feeds in Atlantic salmon. Further, dEB did not affect long term development of cataract or vertebral deformities.

The hypothesis of the present study was that increased growth in spring, stimulated by increasing temperature and daylength, leads to oxidative stress in Atlantic salmon with accumulation of oxidation products in the tissues and increased utilization of antioxidants. The drop in fillet pigmentation and astaxanthin, often observed in spring by the industry, could be explained by oxidative stress. Furthermore, oxidative stress may cause pro-duction related diseases such as development of cataracts and melanin spots in the fillet. We sampled Atlantic salmon from two cages in a commercial scale experiment in Northern Norway (67 degrees N), every month from April until August and then every second month until December (510 +/- 160-3060 +/- 510 g, mean weight +/- std). The specific growth rate (SGR) increased with increasing temperature until midsummer and decreased thereafter. We found that vitamin E in the fillet and vitamin C in the liver were depleted in the spring and were restored in the autumn, even though the dietary concentrations were stable. Astaxanthin concentration in the muscle was constant during the spring and summer and increased in the autumn, concomitant with an increase in astax-anthin supplementation. Cataract increased from zero in May until July, when 90% of the fish were affected. The glutathione based redox-potential in the lenses became more reduced from June, indicating a protective mechanism against oxidative stress and cataract. The number of fish with melanin spots was high in June and decreased in August and October, but the size and intensity of the remaining spots increased in the same period. The change in vitamin C and E concentrations, cataract and glutathione metabolism during spring and early summer, indicate that the fish became oxidized in this period, while malon-di-aldehyde (MDA) and astaxanthin concentrations did not support the hypothesis. There are too few data to draw conclusions on possible effects of oxidative stress on melanin spots.

Sterile triploid Atlantic salmon (Salmo salar) show inconsistent seawater grow-out, but the reason why remains unclear. The purpose of this study was to determine the salinity optima of triploid post-smolts. Diploids and triploids were assessed for smoltification status during an underyearling smolt regime before being transferred to one of four different salinities, 0, 11, 23 and 35 ppt at 12 degrees C and under 24 h continuous light for 83 days. During this period, fish growth, plasma biochemistry, and production traits (vertebral deformities, ocular cataracts, sexual maturation) were monitored. Molecular biomarkers in the gill (nka alpha 1a, nka alpha 1b, nkcc1a) suggested triploids reached peak smolt earlier than diploids and began the desmoltification process before the start of the salinity treatments, however this was not reflected in gill Na+/K+-ATPase enzyme activity. At the initiation of the salinity treatments triploids were significantly larger than diploids (mean weight g +/- SE: 71 +/- 0.7 and 87.2 +/- 0.8 for diploids and triploids, respectively) and there was a ploidy effect on post-smolt growth, with body weight showing a clearer positive trend with salinity in diploids (0 < 11 = 23 = 35 ppt) than in triploids (0 < 11 < 35 = 23 ppt) (final mean weight g +/- SE: 255.2 +/- 7.4, 303.9 +/- 9, 313.9 +/- 9 and 342.4 +/- 12 for diploids and 322.9 +/- 9.7, 361.7 +/- 10.7, 425.9 +/- 12.1, 415.2 +/- 12.2 for triploids at 0, 11, 23, and 35 ppt, respectively). Plasma Na+ and Cl- increased, but plasma pH decreased, with increasing salinity in both ploidy. However, ploidy only had transient effects on plasma biochemistry depending on the salinity treatment. There was no ploidy effect on vertebral deformities (21% of both ploidy had one or more deformed vertebra). In contrast, triploids had a significantly higher prevalence of ocular cataracts (84 vs 98% in diploids and triploids, respectively) with a higher mean cataract score (mean +/- SE: 1.93 +/- 0.1 and 2.78 +/- 0.1 for diploids and triploids, respectively), but a significantly lower prevalence of pubertal male post-smolts (15 vs 2% in diploids and triploids, respectively). Salinity treatment had no effect on vertebral deformities, cataracts, or post-smolt sexual maturation. In summary, there was a ploidy mismatch for smoltification biomarkers in the gill and salinity had a strong effect on postsmolt growth, but the effects were ploidy dependent.

Atlantic salmon fed low fish meal feeds supplemented with zinc (Zn) were studied in two feeding trials. In trial I, Atlantic salmon parr were fed six graded Zn levels (40 to 249 mg kg(-1) as ZnSO4) for 8 weeks in freshwater followed by a 4-week seawater phase. In trial II, Atlantic salmon post-smolt were fed for 10 weeks in SW with 10 dietary Zn levels (45 to 280 mg kg(-1)), either as ZnSO4 or Zn-glycinate. Growth was unaffected by dietary Zn in both trials. Dietary Zn affected concentration of Na+ and K+ ions in plasma, branchial and intestinal expression of sodium potassium ATPase, tissue and body Zn status, and cataracts. Seawater transfer significantly reduced apparent availability, body and tissue levels of Zn due to increased endogenous Zn loss. Atlantic salmon postsmolt in seawater improved body and tissue Zn status with increasing dietary Zn levels, irrespective of the Zn source. Body or tissue saturation of Zn occurred at dietary Zn levels between 137 and 156 mg kg(-1) with smolts in freshwater and 181 to 218 mg kg(-1) in SW post-smolts. Dietary Zn levels below 180 mg kg(-1) in low fish meal feeds compromised the Zn status and welfare of Atlantic salmon in seawater.

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) x 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 (TM) 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.

To avoid negative environmental impacts of escapees and potential inter-breeding with wild populations, the Atlantic salmon farming industry has and continues to extensively test triploid fish that are sterile. However, they often show differences in performance, physiology, behavior and morphology compared to diploid fish, with increased prevalence of vertebral deformities and ocular cataracts as two of the most severe disorders. Here, we investigated the mechanisms behind the higher prevalence of cataracts in triploid salmon, by comparing the transcriptional patterns in lenses of diploid and triploid Atlantic salmon, with and without cataracts. We assembled and characterized the Atlantic salmon lens transcriptome and used RNA-seq to search for the molecular basis for cataract development in triploid fish. Transcriptional screening showed only modest differences in lens mRNA levels in diploid and triploid fish, with few uniquely expressed genes. In total, there were 165 differentially expressed genes (DEGs) between the cataractous diploid and triploid lens. Of these, most were expressed at lower levels in triploid fish. Differential expression was observed for genes encoding proteins with known function in the retina (phototransduction) and proteins associated with repair and compensation mechanisms. The results suggest a higher susceptibility to oxidative stress in triploid lenses, and that mechanisms connected to the ability to handle damaged proteins are differentially affected in cataractous lenses from diploid and triploid salmon.

The European Food Safety Authority (EFSA) published a risk assessment of erucic acid (22:1n-9) in 2016, establishing a Tolerable Daily Intake (TDI) for humans of 7 mg kg-1 body weight per day. This report largely excluded the contribution of erucic acid from fish and seafood, due to this fatty acid often not being reported separately in seafood. The Institute of Marine Research (IMR) in Norway analyzes erucic acid and has accumulated extensive data from analyses of fish feeds, farmed and wild fish, and seafood products. Our data show that rapeseed oil (low erucic acid varieties) and fish oil are the main sources of erucic acid in feed for farmed fish. Erucic acid content increases with total fat content, both in farmed and wild fish, and it is particularly high in fish liver, fish oil, and oily fish, such as mackerel. We show that the current TDI could be exceeded with a 200 g meal of mackerel, as at the maximum concentration analyzed, such a meal would contribute 143% to the TDI of a 60 kg person. These data cover a current knowledge gap in the scientific literature regarding the content of erucic acid in fish and seafood.

Fish species show distinct differences in their muscular concentrations of imidazoles and free amino acids (FAA). This study was conducted to investigate whether metabolic response to mildly elevated water temperature (MEWT) relates to species-dependent muscular concentrations of imidazoles and FAA. Thirteen carp and 17 Nile tilapia, housed one per aquarium, were randomly assigned to either acclimation (25°C) or MEWT (30°C) for 14 days. Main muscular concentrations were histidine (HIS; P<0.001) in carp versus N-α-acetylhistidine (NAH; P<0.001) and taurine (TAU; P=0.001) in tilapia. Although the sum of imidazole (HIS+NAH) and TAU in muscle remained constant over species and temperatures (P>0.05), (NAH+HIS)/TAU ratio was markedly higher in carp versus tilapia, and decreased with MEWT only in carp (P<0.05). Many of the muscular FAA concentrations were higher in carp than in tilapia (P<0.05). Plasma acylcarnitine profile suggested a higher use of AA and fatty acids in carp metabolism (P<0.05). On the contrary, the concentration of 3-hydroxyisovalerylcarnitine, a sink of leucine catabolism, (P=0.009) pointed to avoidance of leucine use in tilapia metabolism. Despite a further increase of plasma longer-chain acylcarnitines in tilapia at MEWT (P=0.009), their corresponding beta-oxidation products (3-hydroxy-longer-chain acylcarnitines) remained constant. Together with higher plasma non-esterified fatty acids (NEFA) in carp (P=0.001), the latter shows that carp, being a fatter fish, more readily mobilises fat than tilapia at MEWT, which coincides with more intensive muscular mobilization of imidazoles. This study demonstrates that fish species differ in their metabolic response to MEWT, which is associated with species-dependent changes in muscle imidazole to taurine ratio.

Global warming may alter the bioavailability of contaminants in aquatic environments. In this work, mercury (Hg2+) toxicity was studied in cells obtained from Atlantic salmon smolt kept at 15 °C (optimal growth temperature) for 3 months or at a stepwise increase to 20 °C (temperature-stress) during 3 months prior to cell harvest to evaluate whether acclimation temperature affects Hg toxicity. To examine possible altered dietary requirements in warmer seas, one group of fish following the stepwise temperature regimes was fed a diet spiked with antioxidants. Atlantic salmon hepatocytes were exposed in vitro to 0, 1.0, or 100 μM Hg2+ for 48 h. Cytotoxicity, determined as electrical impedance changes with the xCELLigence system, and transcriptional responses, determined with RT-qPCR, were assessed as measures of toxicity. The results showed that inorganic Hg at a concentration up to 100 μM is not cytotoxic to Atlantic salmon hepatocytes. Significance and directional responses of the 18 evaluated target genes suggest that both Hg and temperature stress affected the transcription of genes encoding proteins involved in the protection against ROS-generated oxidative stress. Both stressors also affected the transcription of genes linked to lipid metabolism. Spiking the diet with antioxidants resulted in higher concentrations of Se and vitamin C and reduced concentration of Hg in the liver in vivo, but no interactions were seen between the dietary supplementation of antioxidants and Hg toxicity in vitro. In conclusion, no evidence was found suggesting that inorganic Hg is more toxic in cells harvested from temperature-stressed fish.

A comparative experiment with Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) postsmolts was conducted over 35days to provide insight into how growth, respiration, energy metabolism and the growth hormone (GH) and insulin-like growth factor 1 (IGF-1) system are regulated at elevated sea temperatures. Rainbow trout grew better than Atlantic salmon, and did not show reduced growth at 19°C. Rainbow trout kept at 19°C had increased blood hemoglobin concentration compared to rainbow trout kept at 13°C, while salmon did not show the same hemoglobin response due to increased temperature. Both species showed reduced length growth and decreased muscle glycogen stores at 19°C. Circulating IGF-1 concentration was higher in rainbow trout than in Atlantic salmon, but was not affected by temperature in either species. Plasma IGF-binding protein 1b (IGFBP-1b) concentration was reduced in Atlantic salmon reared at 19°C after 15days but increased in rainbow trout at 19 °C after 35days. The igfbp1b mRNA level in liver showed a positive correlation to plasma concentrations of glucose and IGFBP-1b, suggesting involvement of this binding protein in carbohydrate metabolism at 19°C. At this temperature muscle igfbp1a mRNA was down-regulated in both species. The muscle expression of this binding protein correlated negatively with muscle igf1 and length growth. The plasma IGFBP-1b concentration and igfbp1b and igfbp1a expression suggests reduced muscle igf1 signaling at elevated temperature leading to glucose allostasis, and that time course is species specific due to higher thermal tolerance in rainbow trout.