Dietary soya saponins increase gut permeability and play a key role in the onset of soyabean-induced enteritis in Atlantic salmon ( Salmo salar L.)

Saponins are naturally occurring amphiphilic molecules and have been associated with many biological activities. The aim of the present study was to investigate whether soya saponins trigger the onset of soyabean-induced enteritis in Atlantic salmon (Salmo salar L.), and to examine if dietary soya saponins increase the epithelial permeability of the distal intestine in Atlantic salmon. Seven experimental diets containing different levels of soya saponins were fed to seawater-adapted Atlantic salmon for 53 d. The diets included a fishmeal-based control diet, two fishmeal-based diets with different levels of added soya saponins, one diet containing 25% lupin kernel meal, two diets based on 25% lupin kernel meal with different levels of added soya saponins, and one diet containing 25% defatted soyabean meal. The effect on intestinal morphology, intestinal epithelial permeability and faecal DM content was examined. Fish fed 25% defatted soyabean meal displayed severe enteritis, whereas fish fed 25% lupin kernel meal had normal intestinal morphology. The combination of soya saponins and fishmeal did not induce morphological changes but fish fed soya saponins in combination with lupin kernel meal displayed significant enteritis. Increased epithelial permeability was observed in fish fed 25% defatted soyabean meal and in fish fed soya saponin concentrate independent of the protein source in the feed. The study demonstrates that soya saponins, in combination with one or several unidentified components present in legumes, induce an inflammatory reaction in the distal intestine of Atlantic salmon. Soya saponins increase the intestinal epithelial permeability but do not, per se, induce enteritis.     

Physiological effects of thermomechanical newsprint mill effluent on Atlantic salmon (Salmo salar L.)

Anadramous Atlantic salmon (Salmo salar L.) returning to Exploits River, Newfoundland, Canada, to spawn encounter low concentrations of thermomechanical pulp (TMP) effluent as they migrate upstream past an integrated newsprint mill. Various physiological responses of adult Atlantic salmon from the Exploits River were studied under laboratory conditions. The effects of a 6-h exposure to 0%, 6%, 12%, or 25% (v/v) TMP effluent or an increasing concentration gradient of effluent (0-25%) on cardiac output (Q ), critical swimming performance (U(crit)), hematocrit, and blood glucose, cortisol, lactate, and osmolality were examined. Relative to other treatment groups, Q during routine and low-level activity was 7-10% higher in fish exposed to at least 12% effluent. The 25% exposure group had a distinctly lower U(crit) and scope for increase in Q than the other treatment groups. These findings suggest that effluent exposure elevates physiological maintenance and repair costs, resulting in a reallocation of energetic resources.     

Interaction of soyasaponins with plant ingredients in diets for Atlantic salmon, Salmo salar L

The effects of combining soyasaponins with plant ingredients on intestinal function and fish health were investigated in an 80 d study with Atlantic salmon (270 g) distributed thirty each into twenty-four tanks with seawater. Soyasaponins were supplemented (2 g/kg) to diets with maize gluten (MG), pea protein concentrate (PPC) and sunflower (SFM), rapeseed (RSM) or horsebean meals. A diet with soyabean meal (SBM) and another with wheat gluten and soyasaponins served as reference diets. Marked soyasaponin effects were observed when combined with PPC. This combination induced inflammation in the distal intestine (DI) similar to SBM, reduced feed intake, apparent digestibility of lipid, most amino acids and ash, decreased bile salt levels in intestinal chyme and decreased leucine aminopeptidase (LAP) activity but increased trypsin activity in the DI. No enteritis was observed in other diet groups, but small consistent negative soyasaponin effects were seen on lipid and fatty acid digestibility, faecal DM and LAP activity of the DI. Soyasaponin combination with RSM reduced digestibility of all nutrients including minerals. The mineral effect was also seen for SFM, whereas with MG and SFM a positive soyasaponin effect on feed intake was observed. Caution should be exercised to avoid ingredient combinations giving high saponin levels, a condition that appears to be a key factor in diet-induced enteritis together with certain plant ingredients.     

Accumulation and elimination of dietary arsenobetaine in two species of fish, Atlantic salmon (Salmo salar L.) and Atlantic cod (Gadus morhua L.)

Despite the fact that marine fish contain relatively high concentrations of the naturally occurring arsenic compound arsenobetaine, little is known about the disposition of arsenobetaine in fish. We investigated the accumulation, distribution, and elimination of dietary arsenobetaine in Atlantic salmon (Salmo salar L.) and Atlantic cod (Gadus morhua L.), with the focus on muscle, liver, and kidney tissues. The fish were exposed to dietary arsenobetaine (24.7 +/- 0.6 microg As/g feed) for three months, followed by a three-month depuration period. The two species showed marked differences in the accumulation and elimination of arsenobetaine. Total arsenic concentrations in Atlantic salmon increased significantly in muscle, liver, and kidney, whereas in Atlantic cod, a significant increase in total arsenic concentration was observed only in muscle. Elimination kinetics in muscle were distinct between the two species, with elimination half-lives from muscle tissue estimated at approximately 77 d in Atlantic cod and 37 d in Atlantic salmon, resulting in an absorption efficiency approximately twofold higher in Atlantic cod (15 +/- 1%) compared to that in Atlantic salmon (8 +/- 1%). The differences in arsenobetaine disposition studied in Atlantic salmon and Atlantic cod contribute to explain the differences in arsenic levels observed among marine fish.     

The optimum dietary indispensable amino acid pattern for growing Atlantic salmon (Salmo salar L.) fry

To determine the optimum indispensable (I) amino acid (AA) balance in Atlantic salmon (Salmo salar L.) fry, a single protocol established for the pig was adapted. The balance was calculated from the reduction in N gain after replacing about 45% of a single IAA by a mixture of dispensable AA in isonitrogenous diets. We confirmed that the mixture of AA simulating the AA pattern of cod-meal protein and gelatine (46:3, w/w) was used with the same efficiency as cod-meal protein and gelatine. From the deletion experiment an optimum balance between the IAA was derived. Expressed relative to lysine = 100, the optimal balance was: arginine 76 (SE 0.2), histidine 28 (SE 2.2), methionine + cystine 64 (SE 1.7), phenylalanine + tyrosine 105 (SE 1.6), threonine 51 (SE 2.4), tryptophan 14 (SE 0.7), valine 59 (SE 1.7). No estimates were made for isoleucine and leucine. Expressed as g/16 g N, the optimal balance was: arginine 4.0 (SE 0.0), histidine 1.5 (SE 0.1), lysine 5.3 (SE 0.2), methionine + cystine 3.4 (SE 0.1), phenylaline + tyrosine 5.6 (SE 0.1), threonine 2.7 (SE 0.1), tryptophan 0.7 (SE 0.0), valine 3.1 (SE 0.1). This AA composition is close to that of the Atlantic salmon whole-body, but using it as an estimation of the IAA requirements may lead to an overestimation of the branched-chain AA requirements and an underestimation of aromatic and S-containing AA requirements. The results are discussed in accordance with the key assumptions associated with the model used (broken-line model, IAA efficiencies and maintenance requirements).     

Variation in the nutritional composition of farmed Atlantic salmon (Salmo salar L.) fillets with emphasis on EPA and DHA contents

The increase in the global popularity and production of farmed Atlantic salmon (Salmo salar L.) has led to compositional changes in their feeds that can potentially diminish their nutritive value. Thus, the aim of the study was to compare the lipid, protein, fatty acid (omega-3) and mineral contents of salmon fillet portions available in the UK and estimate their contribution towards consumer dietary intake levels. Twenty pre-packaged fresh salmon fillets, encompassing all ranges (value, standard, premium and organic) and farmed origins (Scotland and Norway) were purchased from 10 main UK-wide retailers and analysed for their nutritional compositions. Lipid contents were between 11.2–16.3 % wet weight (ww), except the Retailer 10 value product which was significantly lower due to a high proportion of tail pieces. No difference in protein contents (17.5–20.2 % ww) were observed between fillets. However, fatty acid profiles showed marked variations between samples with marker fatty acids 18:1n-9 (24.3–42.0 %), 18:2n-6 (8.3–15.1 %) and 18:3n-3 (2.6–8.1 %) reflecting the differing levels of vegetable oil inclusion and eicosapentaenoic and docosahexaenoic acids (EPA + DHA, 5.6–16.6 %) indicating the level of marine oils included within salmon feeds. Consequently, EPA + DHA contents varied from 0.88 to 2.36 g EPA + DHA.130 g⁻¹ flesh ww, equivalent to supplying 26–67 % of the recommended 3.5 g EPA + DHA weekly intake suggested for optimal cardiac health in adults. Similarly, selenium contents differed significantly between samples delivering between 13.9–55.5 % and 17.3–69.3 % of the 75 and 60 μg day⁻¹ UK intake for males and females, respectively. Additionally, EPA + DHA and selenium contents were both affected by farmed origin, reflecting differences in production strategies of the two salmon producing nations. Overall, the study highlights the contrasting nutritional profiles of farmed salmon fillets available to consumers based on retailer requirements (healthy versus sustainable product) and how this can affect the recommended dietary intakes from a human nutrition perspective.     

Farmed Atlantic salmon (Salmo salar L.) is a good source of long chain omega‐3 fatty acids

The limited amount of fish oils available has resulted in an increased use of vegetable oils in the feed for farmed fish, such as Atlantic salmon. The aim of this study was to evaluate how this has affected the fatty acid (FA) composition of farmed Atlantic salmon and to compare the FA composition of 20 farmed Atlantic salmon with that of 20 wild Atlantic salmon caught in the sea. Fillet fat of farmed salmon was about 12%, which is twice that found in wild salmon. The FA composition of the farmed salmon confirmed a substantial inclusion of vegetable oils in the feed, but the total amount of long chain n‐3 FA present in the fillets of farmed salmon was higher than in the wild fish. A standard portion of farmed Atlantic salmon fillet (200 g) provides approximately 2 g of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which is four times higher than the minimum daily intakes recommended by the UK's Scientific Advisory Committee on Nutrition and the World Health Organisation.     

Health risks associated with the migration of Atlantic salmon (Salmo salar L.): an epidemiological surveillance programme in Northern Spain

The authors present the results of an epidemiological surveillance programme involving Atlantic salmon (Salmo salar) populations in north-eastern Spain. The study investigates the risk factors critical in disease transmission, which include the presence of potentially pathogenic micro-organisms, the innate cell-mediated immune response of the salmon, and the potential interactions between the two. Also taken into account are the biological and migratory cycles of these salmon populations. The results showed that Aeromonas salmonicida and Saprolegnia parasitica were the micro-organisms that played the most important role in the transmission of the disease by virtue of their pathological characteristics, their occurrence in large numbers, and their capacity to transmit disease. With regard to the cell-mediated immune response, the authors observed a wide variability between individuals in some cell groups, as well as certain seasonal and age differences. These may be factors that predispose the animals to infection or to becoming asymptomatic carriers, with the attendant risk of transmitting infection in the course of migration. This could be especially relevant in the case of salmon infected with A. salmonicida and S. parasitica, since it has been observed that such salmon have a decreased number of leucocytes compared to non-infected salmon.     

Maintenance threonine requirement and efficiency of its use for accretion of whole-body threonine and protein in Atlantic salmon (Salmo salar L.) fry

Eighteen groups of seventy Atlantic salmon (Salmo salar L.) fry (initial mean body weight 0.8 (sd 0.01) g) were fed on semi-purified diets containing graded levels of l-threonine (Thr) in 15 litres aquaria at a temperature of 14.5+/-1 degrees C. Doses of Thr represented 1, 31, 41, 51, 62, 72, 83 and 93 % of its ideal level for optimum protein deposition. Indispensable amino acids other than Thr were included in the same proportion (on a g/16 g N basis) as in the Atlantic salmon fry whole-body carcass. Following 36 d of feeding and a 36 h fast, fry were killed for whole-body protein and amino acid analysis. Weight gain (r2 0.98), protein accretion (r2 0.97), and Thr accretion (r2 0.97) were linear (P<0.01) functions of Thr intake. Slope of the Thr accretion regression line showed that the efficiency of Thr utilisation above maintenance was 76 %. At zero Thr intake, fry lost 5.4 mg Thr/kg body weight0.75 per d. The Thr maintenance requirement was 7.2 mg/kg body weight0.75 per d and the Thr requirement for growth was 66 mg for 1 g protein deposition. Increasing doses of Thr resulted in increased (P<0.05) concentrations of histidine and lysine, and decreased concentrations of isoleucine in whole-body protein. The maintenance need for Thr represented 13.4 % of the total need for Thr. The data suggest that efficiency of Thr utilisation above maintenance is constant at all levels of Thr intake between 1 and 93 % of the level required for optimum protein deposition.     

Oral vaccination of Atlantic salmon (Salmo salar) against salmonid rickettsial septicaemia

Effective oral immunization systems may be very helpful to the salmon industry, particularly during the seawater growth stages in which vaccination through injection is not possible. During the seawater growing stage, fish become more susceptible to several types of disease, due to the natural decay of vaccine-induced immune responses. In this study, we demonstrate the immune response and efficacy of a new salmonid rickettsial septicaemia (SRS) oral vaccine, developed using MicroMatrix™ Technology. The vaccine, which is administered together with daily feed ration, induces a specific immune response at local and systemic levels. Anti-Piscirickettsia salmonis specific antibodies were detected as soon as 300 degree-days after vaccination. Furthermore, oral vaccination was able to protect fish against a lethal pathogen challenge when administered either as a primary vaccination or as a booster for an injected vaccine. Results show that oral vaccination is an efficacious treatment for the prevention of SRS outbreaks throughout the salmon culture period.     

Superior protection conferred by inactivated whole virus vaccine over subunit and DNA vaccines against salmonid alphavirus infection in Atlantic salmon (Salmo salar L.)

Salmonid alphavirus 3 (SAV-3) is an emerging pathogen in Norwegian salmon farming and causes severe annual losses. We studied the immunogenicity and protective ability of subunit and DNA vaccines based on E1 and E2 spike proteins of salmonid alphavirus subtype 3 (SAV-3), and compared these to an experimental inactivated, whole virus (IWV) vaccine in Atlantic salmon. The antigens were delivered as water-in-oil emulsions for the subunit and inactivated vaccines and non-formulated for the DNA vaccines. The IWV and the E2 subunit prime-boost groups had circulating neutralizing antibodies at challenge, correlating with high protection against lethal challenge and 3-log(10) reduction of virus titer in heart for the IWV group. Prime-boost with E1 subunit vaccine also conferred significant protection against mortality, but did not correlate with neutralizing antibody levels. Protection against pathology in internal organs was only seen for the IWV group. Prime-boost with E1 and E2 DNA vaccines showed marginal protection in terms of reduction of viral replication in target organs and protection against mortality was not different from controls. The IWV group showed significant upregulation of IFNγ and IL2 mRNA expression at 4 weeks post challenge possibly indicating that other mechanisms in addition to antibody responses play a role in mediating protection against infection. This is the first report comparing the immunogenicity and protection against mortality for IWV vaccines and spike protein subunit and DNA vaccines against salmonid alphavirus infection in Atlantic salmon. The IWV vaccine has superior immunogenicity over sub-unit and DNA vaccines.     

Differential effects of mercurial compounds on the electroolfactogram (EOG) of salmon (Salmo salar L.)

The effects on the salmon (Salmo salar L.) electroolfactogram (EOG) of the two mercurials, mercuric chloride (HgCl2) and methylmercuric chloride (CH3HgCl), were studied. The EOG responses were evoked by stimulating the olfactory epithelium with 340 microM L-alanine for 10 sec every second minute during a 1-hr period. Each EOG response consisted of an initial peak component followed by a sustained component with an amplitude about 40% below the peak value. Three experimental series, each comprising six fish, were carried out. In the first series, the rosette was irrigated solely with artificial "fresh water." In the two other series, a 5-min exposure to mercury (HgCl2 or CH3HgCl, at 10(-5) M) was included after 10 min and a 15-min exposure after 45 min. The mercuric ion (Hg2+) eliminated the peak response within 2 min and suppressed the sustained response to about 35%. During the subsequent irrigation with mercury-free fresh water, both EOG components regained about 50% of their initial amplitudes. In contrast, methylmercury induced a steady and parallel decline of both the peak and the sustained responses, which were not reversed by rinsing the epithelium with fresh water. The results of this study demonstrate the vulnerability of the olfactory receptor function in fish to mercury exposure. Also, they show the very different effects of inorganic and organic mercurials upon the EOG.     

PLGA/PLA micro- and nanoparticle formulations serve as antigen depots and induce elevated humoral responses after immunization of Atlantic salmon (Salmo salar L.)

Novel vaccine delivery systems are highly needed to improve the salmon aquaculture industry. Although particles of biocompatible polymers such as poly(lactic-co-glycolic) acid (PLGA) have long been considered promising candidates for delivery of immunogenic compounds, few studies have addressed their use as vaccine carriers in Atlantic salmon (Salmo salar L.). Investigating their ability to retain/depot antigen and induce time and dosage dependent adaptive humoral responses to immunization, we here present a basic study of the adjuvantic properties PLGA and PLA particles may have in salmon vaccines. A model antigen (human gamma globulin, HGG) was co-encapsulated with β-glucan in nanoparticles (<1000 nm) and microparticles (∼8 μm) of different chemical compositions. Atlantic salmon were immunized with (a) PLGA or PLA particle entrapped antigen (12 different treatment groups), (b) antigen and β-glucan in PBS, (c) an oil-based formulation or (d) nanoparticles (NPs) or microparticles (MPs) combined with the oil-adjuvanted formulation. ELISA analysis showed that NPs and MPs were capable of inducing elevated antibody responses at day 60 and 75 post immunization, but the antibody levels were reduced at day 90 and 120. In contrast, oil-based formulations, either alone or in combination with NPs or MPs resulted in strong antibody responses at all sampling time points. Comparable dosage dependent increase in antibody responses was observed when administering antigen with β-glucan either in PBS, entrapped in NPs or MPs, or in an oil-adjuvanted formulation. However, as the antigen doses were increased, MPs and the oil-based formulation gave the strongest responses. Antigen presence in the blood, organ package/injection site, kidney, carcass and the whole body was quantified by radiotracing of I¹²⁵-labelled HGG at day 7 and 36 post immunization. At both sampling time points, the highest radioactivity levels were measured from the whole-body and organ package/injection site in groups injected with MPs and oil-based formulations, indicating that these formulations resulted in superior antigen retention. Interestingly, NPs were found to accumulate in the kidney, a result that corroborated with in vitro uptake of NPs in a DC/Mφ-like cell line from Atlantic salmon.     

Replacement of fish oil with rapeseed oil in diets of Atlantic salmon (Salmo salar) affects tissue lipid compositions and hepatocyte fatty acid metabolism

Duplicate groups of Atlantic salmon post-smolts were fed five practical-type diets in which the added lipid was 100% fish oil [FO; 0% rapeseed oil (0% RO)], 90% FO + 10% RO (10% RO), 75% FO + 25% RO (25% RO), 50% FO + 50% RO (50% RO) or 100% RO, for a period of 17 wk. There were no effects of diet on growth rate or feed conversion nor were any histopathological lesions found in liver, heart, muscle or kidney. The greatest accumulation of muscle lipid was in fish fed 0% RO, which corresponded to significantly lower muscle protein in this group. The highest lipid levels in liver were found in fish fed 100% RO. Fatty acid compositions of muscle lipid correlated with RO inclusion in that the proportions of 18:1(n-9), 18:2(n-6) and 18:3(n-3) all increased with increasing dietary RO (r = 0.98-1.00, P < 0.013). The concentrations of eicosapentaenoic acid [20:5(n-3)] and docosahexaenoic acid [22:6(n-3)] in muscle lipid were significantly reduced (P < 0.05), along with total saturated fatty acids, with increasing dietary RO. Diet-induced changes in liver fatty acid compositions were broadly similar to those in muscle. Hepatic fatty acid desaturation and elongation activities, measured using [1-(14)C] 18:3(n-3), were increased with increasing dietary RO. Limited supplies of marine fish oils require that substitutes be found if growth in aquaculture is to be maintained such that fish health and product quality are not compromised. Thus, RO can be used successfully as a substitute for fish oil in the culture of Atlantic salmon in sea water although at levels of RO >50% of dietary lipid, substantial reductions occur in muscle 20:5(n-3), 22:6(n-3) and the (n-3)/(n-6) polyunsaturated fatty acid (PUFA) ratio, which will result in reduced availability of the (n-3) highly unsaturated fatty acids that are beneficial for human health.