Post Smolts Research Articles

Differential Transcriptomic Profile of Piscirickettsia salmonis LF-89 and EM-90 During an In Vivo Spatial Separation Co-Culture in Atlantic Salmon

Salmonid rickettsial septicemia (SRS) is a critical sanitary problem in the Chilean aquaculture industry since it induces the highest mortality rate in salmonids among all infectious diseases. Piscirickettsia salmonis, a facultative intracellular bacterium, is the biological agent of SRS. In Chile, two genogroups of P. salmonis, designated as LF-89 and EM-90, have been identified. Previous studies suggested that their cohabitation triggers the expression of virulence effectors, which may be related to a higher pathogenicity in salmonids during co-infection with both P. salmonis genogroups. Therefore, we aimed to evaluate if the physical contact between two isolates from LF-89 and EM-90 is necessary to activate this effect. Through a spatially separated in vivo co-culture inside Atlantic salmon (Salmo salar) post smolts and RNA-seq analysis, we compared the differentially expressed genes (DEGs) with previous results from an in vivo mixed co-culture. The results showed that although the LF-89-like isolate and the EM-90-like isolate had a similar DEG profile under both co-culture conditions, important virulence factors observed during the mixed co-cultures (i.e., flagellar-related genes, CydD, and NCS2) were absent in the spatially separated co-cultures. Hence, the synergistic effect linked to increased pathogenicity to the host may be driven by the physical co-localization and contact between the P. salmonis LF-89-like and EM-90-like isolates.

Full-fat black soldier fly larvae meal and yellow mealworm meal: Impact on feed protein quality, growth and nutrient utilization of Atlantic salmon (Salmo salar) post smolts

The shift towards insect-based alternatives in aquafeeds stems from the need to reduce the carbon footprint associated with the ingredients used in salmon feeds. In the present study, we describe the effects of full-fat insect meal incorporation on amino acid derivatives in feeds and the growth performance and nutrient utilization of Atlantic salmon post-smolts fed insect meals. A 74-day growth trial was conducted employing 520 fish (143 ± 12.8 g); these fish were fed either a control diet (CO) that contained 20 % fishmeal, 20 % soy protein concentrate (SPC), 14 % wheat gluten, 19 % plant raw materials, 13 % fish oil and 6 % rapeseed oil, or one of four insect meal diets, prepared with either 5 % and 10 % black soldier fly larvae meal (BS5, BS10) or 15 % and 30 % mealworm meal (MW15, MW30), by substituting mainly SPC, wheat gluten and rapeseed oil. The inclusion levels of BS and MW were determined based on their distinct chemical compositions, specifically their lipid content. As regards the results, HPLC-based high-throughput data on amino acid derivatives revealed the following: carboxymethyllysine was significantly higher in the MW30 feed than in BS5, BS10 and MW15 feeds; carboxyethyllysine was higher in the MW15 and MW30 feeds compared to the others; furosine was lower in the MW15 feed compared to the CO, BS5 and MW30 feeds. Regarding the effect of the insect meals on Atlantic salmon post-smolts, their inclusion had no significant effect on feed conversion ratio, health indicators and the retention efficiency of macronutrients. Apparent digestibility coefficients (ADC) of dry matter, lipid, ash and energy were not significantly different. However, protein digestibility in the 30 % MW group (80.4 %) was significantly lower (P ≤ 0.05) compared to the control and BS5 group (84.5 %). Furthermore, the ADC of protein was found to decrease linearly with increasing levels of insect meals, with MW showing a significant relationship (adjusted r2 = 0.77, p = 0.014). Overall, our study highlights the efficacy of 5 and 10 % BS and 15 % MW as ingredients in the feeds of Atlantic salmon. Other focal points included the undesirable amino acid derivatives and a reduction in protein digestibility associated with 30 % of MW in the diet of Atlantic salmon. Hence, future investigations should focus on the effects of feed processing conditions on the protein quality of mealworm before suggesting higher inclusion levels to the feed industry.

Genetic parameters and genotype-by-environment interaction estimates for growth and feed efficiency related traits in Chinook salmon, Oncorhynchus tshawytscha, reared under low and moderate flow regimes

BackgroundA genotype-by-environment (G × E) interaction is defined as genotypes responding differently to different environments. In salmonids, G × E interactions can occur in different rearing conditions, including changes in salinity or temperature. However, water flow, an important variable that can influence metabolism, has yet to be considered for potential G × E interactions, although water flows differ across production stages. The salmonid industry is now manipulating flow in tanks to improve welfare and production performance, and expanding sea pen farming offshore, where flow dynamics are substantially greater. Therefore, there is a need to test whether G × E interactions occur under low and higher flow regimes to determine if industry should consider modifying their performance evaluation and selection criteria to account for different flow environments. Here, we used genotype-by-sequencing to create a genomic-relationship matrix of 37 Chinook salmon, Oncorhynchus tshawytscha, families to assess possible G × E interactions for production performance under two flow environments: a low flow regime (0.3 body lengths per second; bl s−1) and a moderate flow regime (0.8 bl s−1).ResultsGenetic correlations for the same production performance trait between flow regimes suggest there is minimal evidence of a G × E interaction between the low and moderate flow regimes tested in this study, for Chinook salmon reared from 82.9 ± 16.8 g (x¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\overline{\ ext{x}}}$$\\end{document} ± s.d.) to 583.2 ± 117.1 g (x¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\overline{\ ext{x}}}$$\\end{document} ± s.d.). Estimates of genetic and phenotypic correlations between traits did not reveal any unfavorable trait correlations for size- (weight and condition factor) and growth-related traits, regardless of the flow regime, but did suggest measuring feed intake would be the preferred approach to improve feed efficiency because of the strong correlations between feed intake and feed efficiency, consistent with previous studies.ConclusionThis new information suggests that Chinook salmon families do not need to be selected separately for performance across different flow regimes. However, further studies are needed to confirm this across a wider range of fish sizes and flows. This information is key for breeding programs to determine if separate evaluation groups are required for different flow regimes that are used for production (e.g., hatchery, post smolt recirculating aquaculture system, or offshore).

Statistical modelling of voluntary feed intake in individual Atlantic salmon (Salmo salar L.)

Precision feeding aims to provide the correct amount of feed to farmed animals for optimal growth and performance and to avoid feed waste. However, knowledge underlying the meal-to-meal variability in voluntary feed intake of farmed species is still limited. This study examined the relationship between meals, feed deprivation time and the feed (pellets) consumed by Atlantic salmon post smolts. The data was collected from individual fish handfed to satiety without social interaction in three independent short-term (6-12 days) experiments. The fixed variables of our model (feed deprivation time (i.e., time between meals), number of pellets provided, day, previous meal size, and fish growth) explained most of the feed intake (number of pellets ingested) (R2 0.68). Results show that fish ingested more pellets over the course of the trials as they grew, resulting in a positive correlation between feed intake and fish growth (final minus initial fish weight). The time between meals and prior meal size (the number of pellets ingested in the previous meal) significantly affected feed intake in the following meal. Our results suggest that it is possible to optimise meal size by considering the size of the previous meal and the time since it was given.

Effects of a DNA and multivalent oil-adjuvanted vaccines against pancreas disease in Atlantic salmon (Salmo salar) challenged with salmonid alphavirus subtype 3

Salmonid alphavirus (SAV) causes pancreas disease (PD) in Atlantic salmon (Salmo salar). In seawater-farmed salmonids in the southern part of Norway SAV subtype 3 (SAV3) is dominating. PD continues to cause significant economic and fish health concerns in this region despite years of extensive use of oil-adjuvanted vaccines (OAVs) containing inactivated whole virus (IWV) antigens. In the current study, three commercially available PD vaccines were tested. Group A got a DNA vaccine (DNAV) injected intramuscularly (i.m.) plus an OAV without a PD component injected intraperitoneally (i.p.). Groups B and C got different OAV IWV vaccines injected i.p., respectively. The control group was i.p. injected with saline. Approximately 12 weeks after vaccination, the post smolt groups were challenged in seawater with SAV3 by cohabitation. Samples were collected pre-challenge, and at 19, 54 and 83 days post-challenge (dpc). There were no differences in growth or visible intraperitoneal side effects between the immunized groups prior to challenge. Fish in group A had significantly higher SAV3 neutralizing antibody titers than the other groups pre-challenge and significantly lower SAV3 viremia levels than the control group at 19 dpc. Fish in group A had significantly more weight gain than the other groups measured at 54 and 83 dpc. Prevalence and severity of heart necrosis at 19 dpc and loss of exocrine pancreas tissue at 54 and 83 dpc were significantly lower in groups A and B compared to group C and controls. The cumulative mortality in the control group during the challenge period was 10.5%. Group A experienced the lowest mortality (6.4%) albeit not statistically different from the controls. The results suggest that DNAV may reduce the clinical and economic impact of PD by improved protection against SAV3-induced changes in pancreas tissue and growth impairment.

Utilization of Nannochloropsis oceanica in plant-based feeds by Atlantic salmon (Salmo salar)

The phototropic microalga Nannochloropsis oceanica is a promising feed ingredient with the potential to provide nutrients including polyunsaturated fatty acids in aquafeeds. Complex and rigid cell walls limit the nutrient utilization of microalgae by Atlantic salmon. Here, we report results from two studies-a laboratory study on post-smolts and a farm trial on consumer-ready fish-that were conducted to understand the efficacy of both treated and untreated whole biomass.In the laboratory study, we investigated if extrusion can be used as a feasible pre-treatment method to enhance digestibility, growth, feed utilization and health of the fish. Here, we employed post smolt Atlantic salmon with an initial average weight of 141.8 ± 28.2 g and they were fed one of the below mentioned experimental feeds in five replicate tanks for 84 days. Four low-fishmeal feeds were formulated; a plant-based control feed without the microalga (CTRL), two feeds containing 7.5 (NE7.5) and 15% (NE15) of the pre-extruded microalga and one feed containing 15% of the un-extruded microalga (NN15). In the farm study, fish of average weight 1.83 ± 0.01 kg were reared in pens for 197 days. In this trial, two experimental feeds were used, a control low-fishmeal feed (CT) and a feed containing 7.5% un-extruded microalga (NW).In the laboratory study, all alga-supplemented feeds lowered weight gain, SGR and TGC compared to the control feed. The FCR and FI did not differ between the CTRL and NE7.5 groups but the parameters were significantly poor in fish fed feeds with the highest incorporation of the alga. Likewise, in the farm study feed conversion was not significantly affected by the inclusion of the microalga. The retention of lipid in the post-smolts showed a linear decrease with the incorporation of the microalga in the feed, and the protein retention was significantly reduced only at the highest incorporation level. The content of sum PUFA and EPA + DHA in fish fed microalgae were numerically and significantly higher in the farm and laboratory studies, respectively. The apparent digestibility coefficient (ADC) of protein and lipid in the post-smolts showed an inverse relation to the incorporation level of the microalga. The digestible lipid and energy retention efficiencies improved (p < 0.05) in fish fed pre-extruded microalgae. Expression of antioxidant and immune genes suggested that the microalga did not impart any negative effects on the post-smolts. The farm study indicated the ability of the alga to plausibly shape the intestinal micromorphology and affect the nutrient absorption as well as peristalsis.Based on the laboratory study, we can state that the differences between the pre-treated and untreated microalga diets were only minor suggesting that inclusion level of the alga in plant-based diets had a more significant effect than the pre-treatment, at least at 15% incorporation level. The main conclusion from both the studies is that Nannochloropsis oceanica in the feeds of Atlantic salmon can facilitate a slight increase in deposition of n-3 fatty acids, EPA and DHA.

Tailoring freshwater diets towards boosted immunity and pancreas disease infection robustness in Atlantic salmon post smolts

The aim of the current study was to investigate how freshwater diets impact on immunity in Atlantic salmon smolts in freshwater, during transfer to seawater and in post smolts during the seawater stage with and without pancreas disease (PD) infection. Three specific freshwater diets were prepared: (i) A diet similar in composition to commercial salmon freshwater diets (Standard diet); (ii) A diet composed of vegetable oils (rapeseed, palm and linseed oils) mimicking the fat composition in aquatic insects – the natural diet of wild salmon in freshwater (Fatty acid diet); (iii) A diet enriched with possible immune modulating amino acids including dl-methionine, l-lysine, l-threonine and taurine (Amino acid diet). After seawater transfer, all fish were fed the same commercial diet. Head kidneys were extracted, and their leukocytes isolated from smolts right before transfer to seawater, from post smolts one and six weeks after transfer to seawater, and from post smolts in seawater after 8 weeks of ongoing PD infection. In addition, to provoke bacterial or virus induced inflammation in vitro, the individual leukocyte suspension from all fish were stimulated by lipopolysaccharide (LPS) or polyinosinic acid: polycytidylic acid (PIC).The transfer of smolts from fresh-to seawater changed the transcription of several types of genes. Particularly in isolates from fish fed the Standard or Fatty acid diet in freshwater, overall gene transcription (IL-1β, CD83, INF-γ, cox2, cd36, MGAT2, catalase) declined. However, the Amino acid diet stimulated the LPS induced gene transcription of IL-1β, CD83, Cox2, and INF-γ at this stage. In freshwater smolts, PIC stimulated leukocytes showed higher transcription level of Mx and viperin in the Fatty acid and Amino acid diet groups compared to the Standard diet group. In seawater post smolts, Mx and viperin responded similarly to PIC challenge in all diet groups. Furthermore, leukocytes isolated from PD infected fish, continued responding to PIC, regardless of freshwater diet.

Sexual maturation in Atlantic salmon male parr may be triggered both in late summer and early spring under standard farming conditions

Male Atlantic salmon (Salmo salar) display different sexual strategies, maturing either as parr during the freshwater phase (as sneaky spawners), or as post smolts following one or several years at sea. First sexual maturation (puberty) occurs at different times depending on environmental and genetic factors. To improve our knowledge on the timing (age and season) of first sexual maturation in Atlantic salmon male parr, we investigated pubertal activation in second generation farmed salmon from the Norwegian river Figgjo, reared under natural conditions of photoperiod and water temperature. Histological analysis, in combination with morphometric measurements, plasma androgen levels and pituitary gonadotropin gene expression analysis revealed that, as previously reported, some male parr initiated early sexual maturation in spring at one year of age. Interestingly, some male parr were observed to initiate sexual maturation already in autumn, six months after hatching (under-yearlings), much earlier than reported in previous studies. One-year old maturing males showed a low induction in gonadotropin levels, while under-yearling maturing males displayed a significant increase in fshb transcripts as compared to immature/early-maturing fish. Plasma testosterone, detectable also in immature/early-maturing males, increased constantly during testes development, while 11-ketotestosterone, undetectable in immature/early-maturing and early maturing males, increased during more advanced stages of maturation. A mild feminization of the testes (ovotestes) was detected in a subset of samples. This study brings new knowledge on the little investigated field of sexually maturing under-yearlings in Atlantic salmon. This is also the first study comparing the physiology of under-yearling vs one-year old maturing male parr, thus bringing new insights to the remarkable plasticity of Atlantic salmon puberty.

What is the limit of sustained swimming in Atlantic salmon post smolts?

A trend in Atlantic salmon aquaculture is to establish new production sites that are susceptible to extreme weather conditions; however, strong and persistent water currents may compromise fish welfare. Defining acceptable current conditions necessitates an assessment of sustained swimming abilities that are fuelled solely by aerobic metabolism and do not result in fatigue. In this study, the limit of sustained swimming was quantified with regards to both speed and time in Atlantic salmon of ~700 g and ~39 cm at 12°C by testing fish in groups of 10 in a large swim tunnel respirometer. First, critical swimming speed (Ucrit) (107 cm s-1) and minimum cost of transport (66 cm s-1) were measured. Sustained swimming trials at constant speeds were then performed based on a percentage of the group mean Ucrit (80, 85, 90, 95, 100 or 105%). Fish were forced to swim until they fatigued or until 72 h had passed. Surprisingly, most fish were able to sustain 80 and 85% Ucrit for 72 consecutive hours. However, at the highest speeds, fatigue was reached within the first 2 h. By categorizing fatigue times of individual fish into &lt;2, 2-72 or &gt;72 h, significant differences in relative swimming speeds were found that corresponded to 2.7, 2.5 and 2.2 body lengths s-1, respectively. These results document impressive sustained swimming capacities in farmed Atlantic salmon and add important temporal insights about ambient current limits with regards to fish welfare at exposed aquaculture sites.

Sea Lice (Lepeophtheirus salmonis) Infestation Reduces the Ability of Peripheral Blood Monocytic Cells (PBMCs) to Respond to and Control Replication of Salmonid Alphavirus in Atlantic Salmon (Salmo salar L.).

Here we have studied the impact of lice (Lepeophtheirus salmonis) infestation of donor fish on the ability of isolated peripheral blood monocytes (PBMCs) to control the replication of salmonid alphavirus (SAV) ex vivo. PBMCs were collected by Percoll gradients at eight and nine weeks post copepodid infestation of Atlantic salmon post smolt. Uninfested fish were controls. PBMCs were then infected ex vivo with SAV (subtype 3), and samples were collected for analysis at two, four, and six days post virus infection. Virus titer in the supernatant was assayed in CHH-1 cells, and in addition, the relative expression of the virus structural protein E2 and selected host antiviral genes, IRF9, ISG15, Mx, and IFIT5, were assayed using real-time PCR. Significantly higher virus replication was detected in cells collected from lice-infested fish compared to controls. Higher virus titer coincided with an inability to upregulate the expression of different immune genes, IFIT5, IRF9, and Mx. These findings point towards compromised ability of PBMCs from lice-infested fish to control virus replication, and, to our knowledge, is the first report showing the direct effect of lice infestation on the interplay between viruses and immune cells. There is a possible impact on the dynamic spread of viral diseases in the aquatic environment.

Dietary selenium required to achieve body homeostasis and attenuate pro-inflammatory responses in Atlantic salmon post-smolt exceeds the present EU legal limit

Selenium (Se) supplementation either as inorganic or organic form was evaluated in Atlantic salmon post-smolt in vivo and in vitro. The basal diet was formulated to be low in fish meal and contain 0.24 mg Se kg−1; six other diets with Se inclusion of 0.15, 0.4, 0.7 or 1.1 mg kg−1 as sodium selenite (SS) and 0.15 or 0.4 mg kg−1 as L-selenomethionine (SM) were formulated from the basal diet. The diets were fed to Atlantic salmon post-smolt (mean initial weight, 216 ± 27 g) in triplicate groups (35 fish tank−1) and reared in flow-through seawater (33 ppt) at 10–12 °C for 9 weeks. At the end of the feeding trial, whole fish and tissues were sampled for in vivo assessment; whereas, liver cells and head kidney leukocytes (HKL) were isolated and their primary cultures used for in vitro assessment following exposure to hydrogen peroxide (H2O2), lipopolysaccharide (LPS) or poly I:C (PIC). Growth, feed intake, feed conversion ratio, specific growth rate, hepato-somatic index, proximate composition and mineral concentration of the whole fish (except for Se) were unaffected by dietary Se (p > .05). Hematocrit was significantly higher in fish fed the 0.4 mg Se supplemented feeds, irrespective of the Se source (p = .02). The Se concentration in whole body, liver, muscle, plasma, kidney and liver/kidney Se ratio increased with increasing dietary Se concentration (p < .0001). Level of oxidised glutathione (GSSG) in liver and head kidney followed a quadratic function (p < .05) indicating that the concentrations were lower at intermediate SS supplementation of 0.4 and 0.65 mg Se kg−1 (total Se, 0.65 and 0.87 mg Se kg−1). Impact of Se sources on glutathione redox status was similar. Slope-ratio analysis revealed SM to be more efficient than SS in improving apparent availability, whole body or tissues Se status, Se retention and reducing Se loss to the environment. In vitro, the mRNA expression of p38mapk and aif, in liver cells were affected by the impact of dietary Se, but not by the treatment of H2O2 (p < .05). In the HKL, the LPS and PIC induced pro-inflamatory action of il-1β, cox2, nfkβ and viperin were attenuated by SM supplementation, but not by SS (p < .001). Dietary Se supplementation required to the basal diet containing 0.24 mg Se kg−1 was 0.41 mg Se as SS (total 0.65 mg kg−1) or 0.17 mg Se as SM (total 0.41 mg kg−1) based on body Se homeostasis or tissue Se status. SM inclusion at 0.4 mg kg−1 diet (total, 0.65 mg kg−1) attenuated LPS or PIC induced pro-inflammatory responses in vitro. Overall, Se requirement of Atlantic salmon post smolt was 0.27 mg kg−1 diet, on availale basis. Dietary Se level required to maintain body Se homeostasis and improved health status of Atlantic salmon fed plant-based diets (0.65 mg kg−1 diet) exceed the existing EU maximum limit of 0.5 mg Se kg−1 diet. SM as the Se source in salmon feeds has the potential to improve salmon health and reduce Se emissions from Norwegan salmon farming by 60 to 70%.

Is it advantageous for Atlantic salmon to be triploid at lower temperatures?

Marine organisms living at low temperatures tend to have larger genomes and larger cells which suggest that these traits can be beneficial in colder environments. In fish, triploidy (three complete sets of chromosomes) can be induced experimentally following fertilization, which provides a model system to investigate the hypothesis that larger cells and genomes offers a physiological advantage at low temperatures. We tested this hypothesis by measuring metabolic rates and swimming performance of diploid and triploid Atlantic salmon (Salmo salar) post smolts acclimated to 3 or 10.5°C. At 10.5°C, triploids had significantly lower maximum metabolic rates which resulted in a lower aerobic scope compared to diploids. In addition, triploids initiated ram ventilation at lower swimming speeds, providing further evidence of a reduced capacity to meet oxygen demands during strenuous activity at 10.5°C. However, at 3°C, metabolic rates and critical swimming speeds were similar between both ploidies, and as expected substantially lower than at 10.5 °C. Therefore, triploidy in colder environments did not provide any advantage over diploidy in terms of metabolic rate traits or swimming performance in Atlantic salmon. We therefore conclude that traits, other than aerobic scope and swimming performance, contribute to the trend for increased cell and genome size in marine ectotherms living in cold environments.

Approaches to improve utilization of Nannochloropsis oceanica in plant-based feeds for Atlantic salmon

Rigid cell walls of microalgae lower the digestibility and nutrient bioavailability in carnivorous fish. Extrusion is a thermo-mechanical process and a scalable technology that may break cell walls and improves nutrient utilization. It can be hypothesized that certain feed additives may further improve microalgae nutrient digestibility and feed utilization by fish. The aim of the current study was to investigate i) the effect of incorporation of 10% pre-extruded Nannochloropsis oceanica on nutrient digestibility, growth and feed utilization of Atlantic salmon post smolts, and ii) the ability of feed additives in improving the feed utilization. Four low fish meal feeds were formulated; a control without the microalga N. oceanica (CO), a feed containing 10% of the pre-extruded microalga (NC), and two feeds containing 10% of the pre-extruded microalga and supplemented with either 0.06% Digestarom® (ND) or 1% ZEOFeed (NZ). Fish (initial average weight of 227.3 ± 3.4 g) in 5 replicate tanks of each of the study groups were fed one of the experimental feeds for 68 days. The apparent digestibility of dry matter in the NC and NZ groups were significantly higher compared to the control group (CO). The digestibility of lipid was significantly lower, and digestibility of ash was higher in the alga-fed groups (NC, ND and NZ) compared to the control group (CO). The incorporation of 10% pre-extruded N. oceanica in plant-based commercial-like feeds did not affect the growth, feed utilization and whole body proximate composition of salmon. No effects of the feed additives were observed on growth, feed utilization and histomorphology of distal intestine of salmon, but cell proliferation (PCNA) was higher for fish fed the alga alone as well as the alga-ZEOfeed combination. There were no differences in polyunsaturated fatty acids in whole body of fish fed the different feeds. It is noteworthy that whole-body EPA + DHA levels of fish fed the algae feeds were maintained at the same levels as fish fed the control feed that contained 50% more fish meal and 10% more fish oil.

Hypothalamic agrp and pomc mRNA Responses to Gastrointestinal Fullness and Fasting in Atlantic Salmon (Salmo salar, L.).

The orexigenic agouti-related protein (AgRP) and the anorexigenic pro-opiomelanocortin (POMC) are crucial players in the control of feed intake in vertebrates, yet their role in teleosts has not been fully established. Triplicate groups of Atlantic salmon (Salmo salar) post smolts were subjected to (1) fasting for 3 days (fast) and (2) normal feeding (fed), resulting in a significant (p < 0.05) upregulation of hypothalamic agrp1 transcripts levels in the fast group. Moreover, the mRNA abundance of agrp1 was significantly (p < 0.05) correlated with the stomach dry weight content. Corresponding inverse patterns were observed for pomca2, albeit not statistically significant. No significant differences were found for the other paralogues, agrp2 and pomca1 and b, between fed and fast groups. The significant correlation between stomach fullness and agrp1 mRNA expression suggests a possible link between the stomach filling/distension and satiety signals. Our study indicates that hypothalamic agrp1 acts as an orexigenic signal in Atlantic salmon.

Effects of different photoperiod regimes on the smoltification and seawater adaptation of seawater-farmed rainbow trout (Oncorhynchus mykiss): Insights from Na+, K+–ATPase activity and transcription of osmoregulation and growth regulation genes

Photoperiod is thought to be the main zeitgeber that induces smoltification in salmonids. However, its effects on the smoltification of rainbow trout (Oncorhynchus mykiss) are not fully understood and no published data documents the effects of the photoperiod regime currently used commercially, continuous light (LL). The present study compared the effect of four different photoperiod regimes (i.e. advanced phase photoperiod (APP), delayed phase photoperiod (DPP), LL and simulated natural photoperiod (SNP)) on the smoltification and growth of juvenile rainbow trout during their freshwater phase of winter-spring and the following summer post smolt phase. Smoltification was evaluated by monitoring gill Na+,K+–ATPase (NKA) activity and transcription of NKA α-subunit isoforms 1a and 1b, and Na+,K+,2Cl‐cotransporter 1a. Growth was measured as specific growth rate of both length and weight, and through molecular growth proxies such as the levels of circulating insulin-like growth factor 1 (IGF-I) in plasma and transcription of igf-I, igf binding protein 1b (igfbp1b), growth hormone receptor 1 (ghr1) and cathepsin L (ctsl) in the liver. Results indicate that APP induces a longer smolt window and higher levels of plasma IGF-I in both freshwater and seawater (two months post transfer), while DPP led to a shorter smolt window, lower plasma IGF-I levels in freshwater and seawater, an earlier decrease in liver igf-I and ctsl transcription in freshwater (as seen by modelling over time) and lower specific growth rate in freshwater. The transcription analysis of osmoregulatory genes complemented NKA activity and allowed for the detection of a transient response to light and of differences between the osmoregulatory capacity of parr and desmolted fish. Furthermore, an upregulation of the liver transcription of igf-I, ghr1 and ctsl was found in all treatments during the smolt window, which corresponded to the periods with highest growth. Finally, both plasma IGF-I and liver igf-I in seawater were found to be significantly correlated to fish growth in seawater. However, our data did not show that plasma IGF-I prior to seawater transfer could be used as a reliable predictor of growth in seawater. Overall, and especially when compared with other salmonid species, photoperiod seems to be a weaker inducer of smoltification in rainbow trout, according to the parameters that were tested, suggesting that other environmental cues might be more important drivers of this process.

Genome wide association analysis for bacterial kidney disease resistance in a commercial North American Atlantic salmon (Salmo salar) population using a 50 K SNP panel

Renibacterium salmoninarum is the causative agent of bacterial kidney disease (BKD), an important disease of commercial Atlantic salmon causing substantial economic loss. Genetic improvement programs have been developed for many aquaculture species for a variety of economically important traits, and selective breeding has been proposed for low to moderate heritability disease traits to increase disease resistance. The aim of this study was to perform a genome wide association (GWA) analysis to identify the genomic architecture of BKD resistance and detect SNP markers associated with BKD resistance in a commercial population of Atlantic salmon held in seawater. We experimentally challenged 652 post smolts from 63 families with R. salmoninarum through intraperitoneal injection. A binary trait, overall challenge survival, and a quantitative trait, day of death, were recorded for each challenged individual. DNA extraction and 50 K SNP genotyping was performed for 576 individuals. Fish and loci that did not pass quality control and fish with >10% European ancestry were removed leaving 507 post smolts genotyped for 44,527 SNPs. GWA for survival and time to death was conducted using a mixed model approach using the relationship matrix to account for population and family structure and a Cox proportional hazards model, respectively. A single marker on Ssa08 was significantly associated with survival at the chromosome wide level. One marker located on Ssa04 was significantly associated with time to death at the chromosome wide level. The two significant SNPs explained a small percentage of the phenotypic variance and were located on different chromosomes, 4.0% (Ssa08) and 5.3% (Ssa04), indicating a polygenic trait.

Microalgae and organic minerals enhance lipid retention efficiency and fillet quality in Atlantic salmon (Salmo salar L.)

Pure spray dried DHA rich microalgae biomass (Schizochytrium sp.) was used to replace fish oil as a source of long chain n-3 polyunsaturated fatty acids (n-3 LC-PUFA) in medium (150gkg−1; MF) and low fish (100gkg−1; LF) meal diets for Atlantic salmon post smolt supplemented with either inorganic or organic minerals (OM: Zn, Cu, Mn, Fe and Se). The diets were balanced for total saturated fatty acids (SFA), sum DHA+EPA and n-3/n-6 ratio and had similar protein and energy digestibility and high pellet technical quality. Lipid digestibility was above 90% in all diets, nevertheless 2.3% lower in the diet containing 50gkg−1 microalgae, compared to the control, mainly due to the lower digestibility of SFA in the microalgae rich diets. The experimental fish grew from 0.4kg to 1.1kg with no significant differences in growth rate (TGC 3.7–3.8) or feed conversion ratio (FCR 0.7) among the dietary treatments. The retention efficiency of EPA+DHA in salmon body was significantly improved in the fish fed diets containing increasing levels of microalgae, and thus lower EPA/DHA ratios. Moreover, liver lipid levels were decreased and dress-out percentage increased by increasing microalgae dietary supplementation level. The fillet levels of SFA and DHA+EPA were similar for all treatments. Improved fillet quality in terms of lower gaping scores was observed with increasing dietary inclusion level of Schizochytrium sp. and even more pronounced for salmon fed organic minerals. No significant effects on fish blood plasma chemistry and whole body mineral content were observed. Statement of relevanceAquaculture is in urgent need of adequate volumes of new LCn-3PUFA sources, alternative to fish oil. The current work documents the feasibility of using practically relevant levels of heterotrophic microalgae as LCn-3PUFA source in the diet of Atlantic salmon in terms of extruded feed production feasibility, general fish performance, fish fillet product quality (nutritional and technical), nutrient retention efficiency and blood chemistry.

Immune parameters in the intestine of wild and reared unvaccinated and vaccinated Atlantic salmon (Salmo salar L.)

Forming a barrier to the outside world, the gut mucosa faces the challenge of absorbing nutrients and fluids while initiating immune reactions towards potential pathogens. As a continuation to our previous publication focusing on the regional intestinal morphology in wild caught post smolt and spawning Atlantic salmon, we here investigate selected immune parameters and compare wild, reared unvaccinated and vaccinated post smolts.We observed highest transcript levels for most immune-related genes in vaccinated post smolts followed by reared unvaccinated and finally wild post smolts, indicating that farming conditions like commercial feed and vaccination might contribute to a more alerted immune system in the gut. In all groups, higher levels of immune transcripts were observed in the second segment of mid-intestine and in the posterior segment. In the life stages and conditions investigated here, we found no indication of a previously suggested population of intestinal T cells expressing MHC class II nor RAG1 expression.

Effects of cortisol on the intestinal mucosal immune response during cohabitant challenge with IPNV in Atlantic salmon (Salmo salar).

Infectious pancreatic necrosis virus (IPNV) causes high incidence of disease in salmonids during the first period after SW transfer. During this period as well as during periods of stress, cortisol levels increase and indications of a relationship between IPNV susceptibility and cortisol have been suggested. The intestine is an entry route and a target tissue for IPNV displaying severe enteritis and sloughing of the mucosa in infected fish. The mechanisms behind effects of the virus on the intestinal tissue and the impact of cortisol on the effect remain unclear. In the present study, Atlantic salmon post smolts treated with or without slow release cortisol implants were subjected to a cohabitant IPNV challenge. Analysis of genes and proteins related to the innate and acquired immune responses against virus was performed 6 days post-challenge using qPCR and immunohistochemistry. An increased mRNA expression of anti-viral cytokine interferon type I was observed in the proximal intestine and head kidney as a response to the viral challenge and this effect was suppressed by cortisol. No effect was seen in the distal intestine. T-cell marker CD3 as well as MHC-I in both intestinal regions and in the head kidney was down regulated at the mRNA level. Number of CD8α lymphocytes decreased in the proximal intestine in response to cortisol. On the other hand, mRNA expression of Mx and IL-1β increased in the proximal intestine and head kidney in IPNV challenged fish in the presence of cortisol suggesting that the immune activation shifts in timing and response pathway during simulated stress. The present study clearly demonstrates that IPNV infection results in a differentiated epithelial immune response in the different intestinal regions of the Atlantic salmon. It also reveals that the epithelial immune response differs from the systemic, but that both are modulated by the stress hormone cortisol.

Response to M Krkošek, C W Revie, B Finstad and C D Todd's comment on Jackson et al . ‘Impact of L epeophtheirus salmonis infestations on migrating Atlantic salmon, S almo salar L., smolts at eight locations in Ireland with an analysis of lice‐induced marine mortality’

Response to M Krkošek, C W Revie, B Finstad and C D Todd's comment on Jackson <i>et al</i> . ‘Impact of <i> <scp>L</scp> epeophtheirus salmonis </i> infestations on migrating Atlantic salmon, <i> <scp>S</scp> almo salar </i> L., smolts at eight locations in Ireland with an analysis of lice‐induced marine mortality’