Atlantic Salmon Post-smolts Research Articles

Nutritional evaluation of an aqueous-processed canola protein concentrate in diets of Atlantic Salmon Salmo salar at different life stages

A novel aqueous processing method was applied on canola defatted meal to produce a protein concentrate (>76 % crude protein, dry matter basis). This aqueous-processed canola protein concentrate (ACPC) was evaluated in three experiments with Atlantic salmon. The objectives were to (1) estimate the nutrient digestibility of ACPC in juvenile fish, (2) appraise its nutrient digestibility in post-smolt salmon, and (3) evaluate the long-term effect of ACPC on growth, survival, nutrient utilization, body composition, fillet pigmentation and gut histology of post-smolt salmon. In the first study, one Reference and one test diet, which was a blend of 70 % Reference and 30 % ACPC (70:30 ratio), were fed to juvenile salmon (57 ± 7 g) and feces were collected using settling columns. In the second study, post-smolt salmon (228 ± 4 g) were fed one Reference and three test diets, which consisted of three Reference to ACPC ratios: 90:10, 80:20 and 70:30. Feces were collected by manual stripping. The long-term study (168 days) was designed as a 2 × 4 full factorial analysis with two types of formula (Americas, with processed animal proteins (PAPs), and Europe, without PAPs) and four inclusion levels of ACPC (0, 10, 15, 20 %) fed to post-smolt Atlantic salmon (228 ± 5 g). Fish were bulk weighed every 28 or 56 days. Distal intestine and fillet were sampled (10 fish/tank) at the end for histological examination and color measurement, respectively. All diets (43–49 % crude protein, 18–25 % crude lipid depending on studies) were extruded and fed to apparent satiation. The apparent digestibility coefficients (ADCs) of ACPC protein in juvenile and post-smolt salmon were 89 % and ≥ 94 %, respectively. The essential amino acids ADCs in juvenile and post-smolt salmon were 83–99 % and 91–99 %, respectively. ACPC protein digestibility was not significantly affected by its inclusion level (P = 0.470). In the third experiment, growth (TGC: 0.143–0.160) and feed intake (668–780 g/fish) were not significantly affected by ACPC inclusion (P ≥ 0.10). Feed conversion ratios varied between 1.04 and 1.08. An unexpected occurrence of disease caused 1.6 % mortality over three weeks during the 24-week long-term study and survival was significantly higher in salmon fed ACPC-containing diets (P < 0.001). ACPC inclusion significantly increased fillet redness (P < 0.001) and increased villus length of distal intestine (P < 0.001). In conclusion, ACPC was a safe and nutritive protein alternative in salmon diets, regardless of inclusion level.

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.

Current vaccination strategy against Piscirickettsia salmonis in Chile based only on the EM-90 genogroup shows incomplete cross-protection for the LF-89 genogroup

Piscirickettsia salmonis, the primary bacterial disease in Chilean salmon farming, necessitates a constant refinement of control strategies. This study hypothesized that the current vaccination strategy for SRS control in the Chilean Atlantic salmon aquaculture industry, which has been in place since 2017 (ALPHA JECT® 5.1 plus LiVac®), solely relies on vaccines formulated with the EM-90 genogroup of P. salmonis (PS-EM-90), triggering a partial cross-immunity response in fish infected with the LF-89 genogroup (PS-LF-89). Relative Percent Survival (RPS) and cell-mediated immune (CMI) response were evaluated in Atlantic salmon post-smolts vaccinated with the standard vaccination strategy but challenged with both PS-EM-90 and PS-LF-89, in addition to other vaccination strategies considering primo vaccination and booster with other commercial vaccines and the possible enhancing effects of the combination with a natural immunomodulator (PAQ-Xtract®) administered orally. The intraperitoneal (I.P.) challenge was performed after 2395°-days (DD) after the start of the immunostimulant delivery, 1905 DD after the primo vaccination, and 1455 DD after the booster vaccination. Unvaccinated fish showed 73.6 and 41.7 % mortality when challenged with PS-EM-90 and PS-LF-89, respectively. Fish infected with PS-LF-89 died significantly faster (21 days post-infection, dpi) than fish challenged with PS-EM-90 (28 dpi) (p = 0.0043) and had a higher probability of death (0.4626) than fish challenged with PS-EM-90. RPS had a significant positive correlation with the PS-EM-90 load of the P. salmonis genogroup (r = 0.540, p < 0.01) but not with the PS-LF-89 load (r = 0.155, p > 0.05). This demonstrated that the immunization strategies were more effective in lowering PS-EM-90 loads, resulting in higher survival rates in fish challenged with PS-EM-90. The current industry vaccination strategy recorded a 100 % RPS when fish were challenged with PS-EM-90, but the RPS dropped significantly to 77 % when fish were challenged with PS-LF-89, meaning that the strategy did not show complete cross-protection. But after adding PAQ-Xtract®, the RPS improved from 77 % to 92 % in fish that were vaccinated with the standard method but then challenged with PS-LF-89. The most effective vaccination strategy was based on LiVac® as primo vaccination and ALPHA JECT® 5.1 plus LiVac® as booster vaccination, with or without PAQ-Xtract®, in both PS-EM-90 (100 %) and PS-LF-89 (96 %) challenged fish. The serum concentration of anti-P. salmonis IgM did not show a correlation with the protection of immunization strategies expressed in survival. Low serum IL-12 and high serum IFNγ concentrations showed a correlation with higher bacterial loads and lower survival. Aggregate analysis showed a significant correlation between higher numbers of CD8+ cells in the head-kidney, higher fish survival, and a lower bacterial load. The immunization strategies were safe for fish and induced only mild microscopic lesions in the gut. Taken together, our results help to better understand the biological interaction between P. salmonis and post-smolt vaccinated Atlantic salmon to deepen the knowledge on vaccine-induced protection, CMI immune response, and cross-immunity applied to improve the current immunization strategy for SRS control in the Chilean salmon industry.

Evaluating the suitability of nitrate-nitrogen levels for post-smolt Atlantic salmon Salmo salar production in RAS with assistance from heart rate bio-loggers

Previous onsite research determined that post-smolt Atlantic salmon Salmo salar growth, health, and welfare were unaffected by exposure to 99 mg/L nitrate-nitrogen (NO3-N) in freshwater recirculating aquaculture systems (RAS). A subsequent study was conducted to determine if higher concentrations negatively impact Atlantic salmon within a similar size range. Salmon production metrics were compared in triplicate RAS with NO3-N levels increasing from 100 to 250 mg/L (“high”) vs. 50–100 mg/L NO3-N (“low”). An average feed loading rate of 2.53 kg feed/m3 makeup water/day was maintained to facilitate natural NO3-N accumulation up to 75–100 mg/L, and sodium nitrate was continuously dosed to achieve higher concentrations. All-female diploid and all-female triploid Atlantic salmon were comingled in six replicated RAS, resulting in 227 fish/tank with an initial mean weight of 0.35 kg. Six diploid salmon with surgically implanted heart rate bio-loggers were included in each RAS. Continuously logged heart rates were matched with frequently measured NO3-N levels to determine the onset of a physiological response. At the end of the 6-month study, salmon exposed to the high and low NO3-N conditions weighed 1.84 ± 0.05 and 1.91 ± 0.02 kg, respectively (p > 0.05). Diploid and triploid salmon growth rates, feed conversion ratios, maturation prevalence, survival, fin condition, and cataract scores were unaffected (p > 0.05). However, salmon heart rates were generally higher in the high NO3-N treatment after concentrations exceeded 150 mg/L NO3-N. Higher plasma chloride, hematocrit, and hemoglobin levels (p < 0.05) consistent with an adaptive response to a stressor were also measured in salmon from the high NO3-N RAS during this period. These findings suggest that NO3-N concentrations < 150 mg/L do not affect post-smolt Atlantic salmon under similar operating conditions, including freshwater RAS without denitrification technologies and water hardness >300 mg/L as CaCO3. Additional research is required to determine if higher NO3-N concentrations are suitable for Atlantic salmon production over longer exposure periods and with naturally produced NO3-N levels imparted by reduced water exchange. Feed- and nitrogen-loading rates conducive to maintaining the tested NO3-N levels are reported.

Salmonid Rickettsial Septicemia (SRS) disease dynamics and Atlantic salmon immune response to Piscirickettsia salmonis LF-89 and EM-90 co-infection

In Chile, Piscirickettsia salmonis contains two genetically isolated genogroups, LF-89 and EM-90. However, the impact of a potential co-infection with these two variants on Salmonid Rickettsial Septicemia (SRS) in Atlantic salmon (Salmo salar) remains largely unexplored. In our study, we evaluated the effect of P. salmonis LF-89-like and EM-90-like co-infection on post-smolt Atlantic salmon after an intraperitoneal challenge to compare changes in disease dynamics and host immune response. Co-infected fish had a significantly lower survival rate (24.1%) at 21 days post-challenge (dpc), compared with EM-90-like single-infected fish (40.3%). In contrast, all the LF-89-like single-infected fish survived. In addition, co-infected fish presented a higher presence of clinical lesions than any of the single-infected fish. The gene expression of salmon immune-related biomarkers evaluated in the head kidney, spleen, and liver showed that the EM-90-like isolate and the co-infection induced the up-regulation of cytokines (e.g., il-1β, ifnγ, il8, il10), antimicrobial peptides (hepdicin) and pattern recognition receptors (PRRs), such as TLR5s. Furthermore, in serum samples from EM-90-like and co-infected fish, an increase in the total IgM level was observed. Interestingly, specific IgM against P. salmonis showed greater detection of EM-90-like antigens in LF-89-like infected fish serum (cross-reaction). These data provide evidence that P. salmonis LF-89-like and EM-90-like interactions can modulate SRS disease dynamics in Atlantic salmon, causing a synergistic effect that increases the severity of the disease and the mortality rate of the fish. Overall, this study contributes to achieving a better understanding of P. salmonis population dynamics.

Post-smolt Atlantic salmon can regulate buoyancy in submerged sea-cages by gulping air bubbles

Salmon lice (Lepeophtheirus salmonis) are the most severe health challenge facing Atlantic salmon (Salmo salar) aquaculture. Infestations threaten farmed and wild salmonids, drive up production costs, and limit industry growth. Submersible sea-cages can reduce infestation pressure by holding salmon below the surface layers where lice larvae are most abundant. However, submergence leads to loss of buoyancy over time, as salmonids must periodically refill their swim bladders by gulping air. Pockets of air held at depth within ‘air domes’ enable salmon to gulp air, but air domes are large and unwieldy structures. A more intuitive solution may be to deliver a stream of air bubbles at depth, as bubbling is commonly used in sea-cages to promote vertical mixing, and salmon interact with these bubbles. Here, we conducted a pilot study to test whether newly sea-transferred salmon would use bubbles for buoyancy regulation. We conducted 3 trials in sequence, each involving 15–19 days of continuous submergence, and recorded buoyancy-related behaviors in 1 cage with full surface access, 1 cage submerged without air, and 2 cages submerged with air bubbling. Bubbling was provided continuously in the first two trials, before being reduced to 90 min day−1 in the third trial. Salmon submerged with bubbling maintained their swim bladder volume and swimming speed at similar levels to the cage with full surface access throughout all trials, indicating that they successfully refilled using bubbles. The same parameters deteriorated in salmon in the cage submerged without air. Acclimation to bubbles took ∼3 days whether at the surface or after submergence, indicating that several days of bubbling before submergence may be beneficial. Some fish accumulated fluid in the swim bladder during submergence (more severe in fish submerged without bubbling). Together, the results suggest that bubbling can improve outcomes for salmon undergoing prolonged submergence. Moreover, bubbling offers an alternative to air domes that is less technically demanding, provides a greater spatial distribution of air, and supports welfare through behavioral enrichment.

Swimming at Increasing Speeds in Steady and Unsteady Flows of Atlantic Salmon Salmo salar: Oxygen Consumption, Locomotory Behaviour and Overall Dynamic Body Acceleration.

The swimming performance of cultured finfish species is typically studied under steady flow conditions. However, flow conditions are mostly unsteady, for instance, as experienced in sea pens in exposed sea areas. Using a Loligo swim tunnel, we investigated the effects of swimming in steady and unsteady flows at increasing swimming speeds on post-smolt Atlantic salmon. Oxygen consumption (MO2), locomotory behaviour, and overall dynamic body acceleration (ODBA), as determined with implanted acoustic sensor tags, were compared between both flow conditions. Results were obtained for mean swimming speeds of 0.2 to 0.8 m.s-1 under both flow conditions. Sensor tags that were implanted in the abdominal cavity had no significant effects on MO2 and locomotory parameters. The MO2 of fish swimming in unsteady flows was significantly higher (15-53%) than when swimming in steady flows (p < 0.05). Significant interaction effects of ODBA with flow conditions and swimming speed were found. ODBA was strongly and positively correlated with swimming speed and MO2 in unsteady flow (R2 = 0.94 and R2 = 0.93, respectively) and in steady flow (R2 = 0.91 and R2 = 0.82, respectively). ODBA predicts MO2 well over the investigated range of swimming speeds in both flow conditions. In an unsteady flow condition, ODBA increased twice as fast with MO2 compared with steady flow conditions (p < 0.05). From these results, we can conclude that (1) swimming in unsteady flow is energetically more costly for post-smolt Atlantic salmon than swimming in steady flow, as indicated by higher MO2, and (2) ODBA can be used to estimate the oxygen consumption of post-smolt Atlantic salmon in unsteady flow in swim tunnels.

Exposure to sublethal levels of H2S does not influence the short-term growth or feed utilization in post-smolt Atlantic salmon (Salmo salar) during restrictive feeding

Hydrogen sulfide (H2S) related mass mortality events in land-based recirculating aquaculture systems (RAS) are associated with the sudden release of substantial amounts of H2S, resulting in acutely toxic exposure. However, the production and release of H2S can also occur at slower constant rates, leading to continuous exposure to sublethal H2S concentrations, the effects of which are unknown. Here, we examined growth rates, feed conversion ratios, and apparent nutrient digestibility in post-smolt Atlantic salmon, following a 10-day exposure to one of two sublethal concentrations of H2S, compared against a control group. The H2S concentrations of the low exposure (LE) and moderate exposure (ME) groups throughout the trial were 1.8 ± 0.8 μg/L and 4.1 ± 1.9 μg/L, respectively. Neither exposure regime affected growth, feed utilization, or the apparent nutrient digestibility. The results suggest that Atlantic salmon tolerate exposure to sublethal H2S concentrations for 10 days, without a discernible effect on the production performance. For aquaculture systems, specifically RAS, this means that while higher concentrations of H2S should be strictly avoided, the presence of low H2S concentrations for a limited period might not be detrimental to fish welfare or production. The RAS environment is complex, and the potential combined effects of H2S and factors pertinent to RAS may influence fish performance differently in a production setting.

Inshore and offshore marine migration pathways of Atlantic salmon post-smolts from multiple rivers in Scotland, England, Northern Ireland, and Ireland.

The migratory behavior of Atlantic salmon (Salmo salar) post-smolts in coastal waters is poorly understood. In this collaborative study, 1914 smolts, from 25 rivers, in four countries were tagged with acoustic transmitters during a single seasonal migration. In total, 1105 post-smolts entered the marine study areas and 438 (39.6%) were detected on a network of 414 marine acoustic receivers and an autonomous underwater vehicle. Migration pathways (defined as the shortest distance between two detections) of up to 575 km and over 100 days at sea were described for all 25 populations. Post-smolts from different rivers, as well as individuals from the same river, used different pathways in coastal waters. Although difficult to generalize to all rivers, at least during the year of this study, no tagged post-smolts from rivers draining into the Irish Sea were detected entering the areas of sea between the Hebrides and mainland Scotland, which is associated with a high density of finfish aquaculture. An important outcome of this study is that a high proportion of post-smolts crossed through multiple legislative jurisdictions and boundaries during their migration. This study provides the basis for spatially explicit assessment of the impact risk of coastal pressures on salmon during their first migration to sea.

Exogenous growth hormone (GH) increases dimension and reduces strength of vertebrae in Atlantic salmon (Salmo salar)

Growth hormone (GH) administration increases bone strength in rats, mainly through alterations of the dimensional proportions of vertebrae. However, similar knowledge is lacking in other clades. In order to pursue this, Atlantic salmon were fed diets with high or low vitamin C and E and administered a slow-release implant of bovine GH.While neither the diets or the GH treatment affected growth, GH reduced condition factor and carcass-somatic index, and increased the hepatosomatic and visceral-somatic indexes. GH had an impact on the dimensional proportions of the vertebrae and increased the relative dorso-ventral and lateral vertebra diameters. Moreover, GH decreased the mechanical strength of the vertebra, both with (modulus of elasticity, failure point) and without (stiffness) normalizing the mechanical data for the dimensions of the vertebrae. Diet did not impact on the vertebra morphometry or mechanical strength. RNA-seq of vertebra centra show that several genes associated with bone growth and remodelling were affected by GH, while there was an interactive effect between diet vitamin level and GH on several genes associated with lipid metabolism.The data may suggest that exogenous GH affects the bone cellular mechanisms of Atlantic salmon postsmolts in such a way that their vertebrae increase in diameter but not length, not disturbing growth in body length. The GH and diet effects on lipid metabolism associated genes indicate a complex regulation of the lipids stored inside the vertebral column by endocrine and nutritional factors.

Effects of regional coordination of salmon louse control in reducing negative impacts of salmonid aquaculture on wild salmonids

Parasitic salmon lice (Lepeophtheirus salmonis) are a constraint to the sustainable growth of salmonids in open net pens, and this issue has caused production to level off in recent years in the most aquaculture-intensive areas of Norway. The maximum allowed biomass at a regional level is regulated by using the so-called “traffic light” system, where salmon louse-induced mortality of migrating wild salmon post-smolts is evaluated against set targets. As a case study, we have investigated how a specific aquaculture-intensive area can reduce its louse levels sufficiently to achieve a low impact on wild salmon. Analyses of the output from a virtual post-smolt model that uses data on the reported number of salmon lice in fish farms as key input data and estimates the salmon louse-induced mortality of wild out-migrating Atlantic salmon post-smolts, suggested that female louse abundance on the local farms must be halved in spring to reach the goal implied by the traffic light system. The outcome of a modelling scenario simulating a proposed new plan for coordinated production and fallowing proved beneficial, with an overall reduction in louse infestations and treatment efforts. The interannual variability in louse abundance in spring, however, increased for this scenario, implying unacceptably high louse abundance when many farms were in their second production year. We then combined the scenario with coordinated production with other louse control measures. Only measures that reduced the density of farmed salmonids in open cages in the study area resulted in reductions in salmon louse infestations to acceptable levels. This could be achieved either by stocking with larger fish to reduce exposure time or by reducing fish numbers, e.g. by producing in closed units.

Blue mussel (Mytilus edulis) silage, a possible low trophic marine protein source for Atlantic salmon (Salmo salar L.)

Blue mussel (Mytilus edulis) could be a promising marine protein source in fish feeds and is of great interest since it can be cultivated along the Norwegian coastline. However, the use of blue mussels in feeds is dependent on developing suitable preservation and processing methods to produce a feed grade raw material. The present studies were conducted to investigate whether blue mussel silage could be used in the feed for Atlantic salmon post-smolt. Two feeding experiments were conducted using the same reference diet with FM inclusion of 25%, giving a mix of ∼59–63% plant-based ingredients vs ∼34–36% marine ingredients to simulate a standard grower feed for salmon post-smolts in SW. In experiment 1, fish were fed diets containing three different inclusion levels of blue mussel silage (BMS 3, 7, and 11%), a diet containing blue mussel meal (BMM) (12%) as well as the reference feed. In this experiment, the fish that were fed a diet containing BMS had a decline in both weight gain and condition factor when compared to the fish given the reference and BMM. The daily feed intake was similar in all groups, but the feed conversion ratio (FCR) increased in the fish fed BMS. The inclusion of BMS and BMM did not affect the digestibility of nutrients, but reduced retention of whole-body lipid and protein retention was observed. Salmon given BMS in the diet also had lower iron (Fe) concentrations in liver and whole body, indicating lower Fe uptake, irrespective of inclusion level. These findings were followed up in a second feeding experiment aiming to investigate whether different processing methods of blue mussel silage could influence the bioavailability of iron, as well as feed utilization and growth. The reference feed was formulated similar to the feed in exp. 1. Additionally, fish were fed diets containing BMM (9%) and the same batch of BMS (9%) used in exp. 1 as well as two diets containing new productions of BMS (9%) using either a lower acid level or only formic acid at the same level. In experiment 2, no differences were seen in weight gain, feed intake, FCR, nutrient retention or body composition between fish given BMS and reference diet. The lower Fe status observed in experiment 1 was not seen in the second study. In both experiments, there were no differences in fish welfare indicators between the group of fish fed with BMS, BMM and the reference group.The present results show that blue mussel silage can be used in the diet for Atlantic salmon, however, the different processing and preservation methods to produce BMS influence the nutritional properties and consequently growth performance and feed utilization of Atlantic salmon post-smolts.

Northern shrimp (Pandalus borealis) protein hydrolyzate and oil promote flesh pigmentation and growth in post-smolt Atlantic salmon (Salmo salar)

ABSTRACT This study investigated the use of alternative ingredients obtained from Pandalus borealis by-products on flesh pigmentation and growth in post-smolt Salmo salar (initial weight of ~ 300 g). Four experimental diets were fed to fish in triplicate tanks for 16 weeks: a control diet and three other diets containing either synthetic astaxanthin, shrimp hydrolyzate, or shrimp oil. Fish that were fed the shrimp hydrolyzate diet exhibited higher red hue (based on L*a*b* data) when compared to fish fed the two other pigmented diets and had higher final weight. In addition, shrimp oil was found to be as efficient as artificial astaxanthin to increase the intensity of flesh pigmentation in Atlantic salmon. The shrimp hydrolyzate tested in our study provides an excellent source of astaxanthin and protein for Atlantic salmon. Likewise, shrimp oil could also constitute a natural source of astaxanthin and oil to this species.

Mucosal and systemic physiological changes underscore the welfare risks of environmental hydrogen sulphide in post-smolt Atlantic salmon (Salmo salar)

Atlantic salmon (Salmo salar) might encounter toxic hydrogen sulphide (H2S) gas during aquaculture production. Exposure to this gas can be acute or chronic, with heightened levels often linked to significant mortality rates. Despite its recognised toxicity, our understanding of the physiological implications of H2S on salmon remains limited. This report details the mucosal and systemic physiological consequences in post-smolt salmon reared in brackish water at 12 ppt after prolonged exposure to elevated H2S levels over 4 weeks. The fish were subjected to two concentrations of H2S: 1 µg/L (low group) and 5 µg/L (high group). An unexposed group at 0 µg/L served as the control. Both groups exposed to H2S exhibited incremental mortality, with cumulative mortality rates of 4.7 % and 16 % for the low and high groups, respectively. Production performance, including weight and condition factors, were reduced in the H2S-exposed groups, particularly in the high group. Mucosal response of the olfactory organ revealed higher tissue damage scores in the H2S-exposed groups, albeit only at week 4. The high group displayed pronounced features such as increased mucus cell density and oedema-like vacuoles. Transcriptome analysis of the olfactory organ unveiled that the effects of H2S were more prominent at week 4, with the high group experiencing a greater magnitude of change than the low group. Genes associated with the extracellular matrix were predominantly downregulated, while the upregulated genes primarily pertained to immune response. H2S-induced alterations in the metabolome were more substantial in plasma than skin mucus. Furthermore, the number of differentially affected circulating metabolites was higher in the low group compared to the high group. Five core pathways were significantly impacted by H2S regardless of concentration, including the phenylalanine, tyrosine, and tryptophan biosynthesis. The plasma levels of phenylalanine and tyrosine were reduced following exposure to H2S. While there was a discernible distinction in the skin mucus metabolomes among the three treatment groups, only one metabolite – 4-hydroxyproline – was significantly impacted by H2S. Furthermore, this metabolite was significantly reduced in the plasma and skin mucus of H2S-exposed fish. This study underscores that prolonged exposure to H2S, even at concentrations previously deemed sub-lethal, has discernible physiological implications that manifest across various organisational levels. Given these findings, prolonged exposure to H2S poses a welfare risk, and thus, its presence must be maintained at low levels (<1 µg/L) in salmon land-based rearing systems.

Determining early marine survival and predation by endothermic predators on acoustically tagged Atlantic salmon (Salmo salar) post-smolts

Many Atlantic salmon ( Salmo salar) populations have experienced significant declines for decades throughout North America and Europe. Mortality due to marine mammal predation during their early marine life could be an important factor contributing to these declines and limiting their population recoveries. However, quantifying predation events, and particularly the extent of marine mammal predation on Atlantic salmon, remains a challenge. In this study, we estimated the contribution of mesothermic and endothermic species predation to the mortality of Atlantic salmon post-smolts during their early marine life using acoustic telemetry. Predation events were inferred from changes in temperatures and depths experienced by acoustically tagged hatchery-reared Atlantic salmon smolts. No salmon were consumed by mesothermic predators, with most endothermic predation events being classified as marine mammals. Post-smolt mortality during the study period was low overall in both years (13.1%–16.7%), with endothermic predation accounting for 33.1%–42.9% of all marine mortality events (5.2%–5.6% mortality). Our results suggest that the current low return of adult Atlantic salmon observed in this area in recent years was not heavily influenced by endothermic predation on post-smolts in the first weeks at sea.

Isolation of intake mediated effects demonstrate that the phenomic benefits of dietary omega-3 are nominal to Atlantic salmon when reared in a challenging (hypoxic) environment

The present study was designed to examine whether challenging environmental conditions (such as hypoxia) influence a fish's response to diet by changing responses to nutrient utilisation or down-regulation of appetite. To examine this, post-smolt Atlantic salmon (138.8 ± 0.65 g/fish; mean ± SD) were fed sub- (2.6% TFA; Ln3) and optimal (6.0% TFA; Hn3) levels of long-chain polyunsaturated omega-3 fatty acids (n-3 LC-PUFA) when maintained under either normoxic (8.1 ± 0.6 mg/L) or hypoxic (6.8 ± 0.5 mg/L) environmental conditions. To control for the anticipated effects of hypoxia on appetite and food intake, two additional treatments included the two diets that were pair-fed to the same intake levels as in the hypoxic treatments. The reduction in dissolved oxygen was accompanied by an increase in dissolved CO2 and decline in seawater pH, with no change in total alkalinity. After 138 days of the study the fish increased their average weight to between 616 and 720 g/fish subject to treatment. Hypoxia had a significant negative impact on final weight and weight gain, and these were the result primarily of effects on feed intake. No effects of treatments on feed conversion were observed. Significant diet x oxygen interactions were observed on whole-body lipid levels, but no other proximate compositional parameters. Sub-clinical effects of hypoxia and ration on several plasma chemistry parameters were observed, but few responses were diet related. Hypoxia affected plasma total protein, cholesterol, and red blood cell counts. Diet digestibility was typically not affected by hypoxia or ration, though a range of diet related effects were evident. Nutrient retention data showed evidence of elongation and desaturation of dietary short-chain PUFA, with clear effects of diet affecting the deposition of both omega-6 and omega-3 fatty acids. This study builds on findings from earlier studies, but crucially now include controls for the feed intake effects to clarify the roles of hypoxia on the nutrient and intake mediated effects. As a result of these controls, it is now clearer that many of the phenomic responses to long-chain omega-3 by Atlantic salmon are nominal and are largely unaffected by challenging environmental conditions like hypoxia. The inclusion of a series of pair-fed treatments, matching the feed intake levels of corresponding hypoxic treatments, shows that the effects of n-3 LC-PUFA appear to be largely intake mediated effects for Atlantic salmon post-smolts.

Influence of Krill Meal on the Performance of Post-Smolt Atlantic Salmon That Are Fed Plant-Based and Animal-Based Fishmeal and Fish Oil-Free Diets

The purpose of this study was to determine the influence of krill meal (KM) inclusion at various levels (0%, 2.5%, 5%) in plant-based and animal-based feeds, that were fishmeal (FM) and fish oil (FO) free, on Atlantic salmon growth. A FM/FO feed containing 0% KM was the control. Using a 2 × 3 factorial approach, diets were randomly assigned to one of 28 0.5 m3 flow-through tanks (n = 4 tanks per diet) initially stocked with 60 fish (148.4 ± 12.9 g; 23.6 ± 0.8 cm; condition factor (K) = 1.16 ± 0.08) each. Salmon were fed for 90 days using automatic feeders ad libitum. On day 45, stocking densities were reduced to 45 fish per tank by the random removal of 15 individuals to remove any potential of density affecting growth through the trial end. Water temperature, oxygen saturation, pH, and salinity throughout the trial were 11.8 °C, 103.5%, 7.38, and 32.0 g L−1, respectively. Fish fed plant-based feed without KM were lighter (p &lt; 0.05) than all other groups at day 45 and 90, but those fed a plant-based feed with KM had comparable growth and feed intake compared to that of fish fed the control diet. Irrespective of the presence of KM, animal-based feeds achieved comparable weight growth (p &gt; 0.05) to the control and 5% KM plant-based groups, with KM increasing feed intake (p &lt; 0.05). Between day 45 and 90, feed conversion ratios increased in all groups except the control and 0% KM plant-based group, while specific growth rates (SGRs) decreased for all except the 0% KM plant-based diet. Between-group differences (p &lt; 0.05) were also noted for the thermal growth coefficient. No differences were recorded in visceral or intestinal weight, and whole-body lipid levels were identical, proportional for all groups. Although differences (p &lt; 0.05) were apparent in the concentrations of individual fillet fatty acids between groups, a 75 g serving size of any treatment would be sufficient to exceed daily intake recommendations for EPA + DHA. This trial determined that benefit, in terms of feed intake and growth performance, was gained when KM was added to plant-based feeds. However, no such advantage was observed when KM was used with animal-based feeds.

Growth Performance, Nutrient Digestibility, and Retention in Atlantic Salmon, Salmo salar L., Fed Diets with Fermented Sugar Kelp, Saccharina latissima

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.

The fate of intracoelomic acoustic transmitters in Atlantic salmon (Salmo salar) post-smolts and wider considerations for causal factors driving tag retention and mortality in fishes

Acoustic telemetry is a widely used method in assessing behavioural dynamics in fishes. Telemetry transmitters (tags) are often surgically implanted in the coelom of the animal with limited in situ testing and sometimes only assuming that they have minimal rates of post-release tag shedding and mortality. However, fish are capable of expelling tags and mortalities do occur following release, with the mechanism (s) underlying these effects not well-understood. The purpose of this research was to address causal factors underlying tag expulsion and tagging mortality in fishes. We conducted an empirical assessment of tag retention and post-surgical mortality rates in post-smolt Atlantic salmon (Salmo salar) fitted with dummy (non-transmitting) acoustic tags over a 92-day monitoring period. This was complimented with a meta-analysis of factors affecting tag retention and post-surgical mortality rates in the wider literature. Post-smolt salmon had high rates of tag expulsion (54.8%), impaired growth, and a foreign body response evident but exhibited low rates of mortality following tag implantation (≤ 5.1%). The meta-analysis showed that mortality was generally low across all studies (12.4%) and was largely unaffected by model cofactors. Tag retention rates were high among the studies investigated here (86.7%) and had a weak negative relationship with tag:body mass ratios. Our results suggest that while mortality is often low among tagging studies, including this one, caution must be exercised in assessing stationary tag location data as they may represent an expelled tag rather than a mortality event. Our results also indicate that tag dimensions are not nearly as important as the tag:body mass ratio.

Migration patterns and navigation cues of Atlantic salmon post-smolts migrating from 12 rivers through the coastal zones around the Irish Sea.

The freshwater phase of the first seaward migration of juvenile Atlantic salmon (Salmo salar) is relatively well understood when compared with our understanding of the marine phase of their migration. In 2021, 1008 wild and 60 ranched Atlantic salmon smolts were tagged with acoustic transmitters in 12 rivers in England, Scotland, Northern Ireland and Ireland. Large marine receiver arrays were deployed in the Irish Sea at two locations: at the transition of the Irish Sea into the North Atlantic between Ireland and Scotland, and between southern Scotland and Northern Ireland, to examine the early phase of the marine migration of Atlantic salmon smolts. After leaving their natal rivers' post-smolt migration through the Irish Sea was rapid with minimum speeds ranging from 14.03 to 38.56 km.day-1 for Atlantic salmon smolts that entered the Irish Sea directly from their natal river, to 9.69-39.94 km.day-1 for Atlantic salmon smolts that entered the Irish Sea directly from their natal estuary. Population minimum migration success through the study area was strongly correlated with the distance of travel, populations further away from the point of entry to the open North Atlantic exhibited lower migration success. Post-smolts from different populations experienced different water temperatures on entering the North Atlantic. This was largely driven by the timing of their migration and may have significant consequences for feeding and ultimately survivorship. The influence of water currents on post-smolt movement was investigated using data from previously constructed numerical hydrodynamic models. Modeled water current data in the northern Irish Sea showed that post-smolts had a strong preference for migrating when the current direction was at around 283° (west-north-west) but did not migrate when exposed to strong currents in other directions. This is the most favorable direction for onward passage from the Irish Sea to the continental shelf edge current, a known accumulation point for migrating post-smolts. These results strongly indicate that post-smolts migrating through the coastal marine environment are: (1) not simply migrating by current following (2) engage in active directional swimming (3) have an intrinsic sense of their migration direction and (4) can use cues other than water current direction to orientate during this part of their migration.