Atlantic Salmon Fry Research Articles

Flavobacterium facile sp. nov., isolated from water system of Atlantic salmon (Salmo salar) fry cultured in Chile.

Strain T-12T, an orange, Gram-stain-negative, non-motile, rod-shaped strain, was isolated in November 2013 from water samples collected from an Atlantic salmon (Salmo salar) fry culturing system at a fish farm in Chile. Phylogenetic analysis based on 16S rRNA sequences (1394 bp) revealed that strain T-12T belonged to the genus Flavobacterium, showing close relationships to Flavobacterium bernardetii F-372T (99.48 %) and Flavobacterium terrigena DS-20T (98.50 %). The genome size of strain T-12T was 3.28 Mb, with a G+C content of 31.1 mol%. Genome comparisons aligned strain T-12T with Flavobacterium bernardetii F-372T (GCA_011305415) and Flavobacterium terrigena DSM 17934T (GCA_900108955). The highest digital DNA-DNA hybridization (dDDH) values were 42.6 % with F. bernardetii F-372T (GCA_011305415) and 33.9 % with F. terrigena DSM 17934T (GCA_900108955). Pairwise average nucleotide identity (ANI) calculations were below the species cutoff, with the best results with F. bernardetii F-372T being: ANIb, 90.33 %; ANIm, 91.85 %; and TETRA, 0.997 %. These dDDH and ANI results confirm that strain T-12T represents a new species. The major fatty acids were iso-C15 : 0 and C15 : 1ω6с. Detected polar lipids included phospholipids (n=2), aminophospholipid (n=1), aminolipid (n=1) and unidentified lipids (n=2). The predominant respiratory quinone was menaquinone MK7 (80 %) followed by MK-6 (20 %). Phenotypic, chemotaxonomic, and genomic data support the classification of strain T-12T (=CECT 30410T=RGM 3222T) as representing a novel species of Flavobacterium, for which the name Flavobacterium facile sp. nov. is proposed.

Phage therapy minimally affects the water microbiota in an Atlantic salmon (Salmo salar) rearing system while still preventing infection

Excessive usage of antibiotics threatens the bacterial diversity in the microbiota of animals. An alternative to antibiotics that has been suggested to not disturb the microbiota is (bacterio)phage therapy. In this study, we challenged germ-free and microbially colonized yolk sac fry of Atlantic salmon with Flavobacterium columnare and observed that the mere presence of a microbiota protected the fish against lethal infection. We then investigated the effect of phage- or oxytetracycline treatment on fish survival and rearing water bacterial community characteristics using 16S rRNA gene amplicon sequencing. Phage treatment led to an increased survival of F. columnare-challenged fish and reduced the relative amounts of the pathogen in the water microbiota. In the absence of F. columnare, phage treatment did not affect the composition or the α-diversity of the rearing water microbiota. In the presence of the phage’s host, phage treatment induced minor changes to the bacterial community composition, without affecting the α-diversity. Surprisingly, oxytetracycline treatment had no observable effect on the water microbiota and did not reduce the relative abundance of F. columnare in the water. In conclusion, we showed that phage treatment prevents mortality while not negatively affecting the rearing water microbiota, thus suggesting that phage treatment may be a suitable alternative to antibiotics. We also demonstrated a protective effect of the microbiota in Atlantic salmon yolk sac fry.

Long-term persistence of piscine orthoreovirus-1 (PRV-1) infection during the pre-smolt stages of Atlantic salmon in freshwater

Piscine orthoreovirus (PRV) causes heart and skeletal muscle inflammation (HSMI) in farmed Atlantic salmon. During salmon production cycles, HSMI has predominantly been observed after seawater transfer. More recently, better surveillance and longitudinal studies have detected occurrences of PRV-1 in freshwater broodstock farms and hatcheries. However, very little is known about the viral kinetics of PRV-1 or disease development of HSMI during these pre-smolt stages. In this study, we conducted a long-term PRV-1 challenge experiment to examine the profile of viral load, infectiousness and/or clearance in Atlantic salmon during their development from fry to parr stage. Atlantic salmon fry (mean weight: 1.1 ± 0.19 g) were infected with PRV-1 (high virulent variant) via intraperitoneal (IP) injection. The viral load reached a peak at 2–4 weeks post-challenge (wpc) in heart and muscle tissues. The virus was detected at relatively high levels in whole blood, spleen, and head kidney tissues until 65 wpc. Heart and muscle lesions typical of HSMI were clearly observed at 6 and 8 wpc but then subsided afterwards resolving inflammation. Innate and adaptive immune responses were elicited during the early/acute phase but returned to basal levels during the persistent phase of infection. Despite achieving high viremia, PRV-1 infection failed to cause any mortality during the 65-week virus challenge period. Cohabitation of PRV-1 infected fish (10 and 31 wpc) with naïve Atlantic salmon fry resulted in very low or no infection. Moreover, repeated chasing stress exposures did not affect the viral load or shedding of PRV-1 at 26 and 44 wpc. The present findings provide knowledge about PRV-1 infection in juvenile salmon and highlight the importance of continued monitoring and management to prevent and mitigate the PRV-1 infection in freshwater facilities.

The stability and composition of the gut and skin microbiota of Atlantic salmon throughout the yolk sac stage.

The bacterial colonization of newly hatched fish is important for the larval development and health. Still, little is known about the ontogeny of the early microbiota of fish. Here, we conducted two independent experiments with yolk sac fry of Atlantic salmon that were (1) either reared conventionally, with the eggs as the only source for bacteria (egg-derived microbiota; EDM) or (2) hatched germ-free and re-colonized using lake water (lake-derived microbiota; LDM). First, we characterized the gut and skin microbiota at 6, 9, and 13 weeks post hatching based on extracted RNA. In the second experiment, we exposed fry to high doses of either a fish pathogen or a commensal bacterial isolate and sampled the microbiota based on extracted DNA. The fish microbiota differed strongly between EDM and LDM treatments. The phyla Proteobacteria, Bacteroidetes, and Actinobacteria dominated the fry microbiota, which was found temporarily dynamic. Interestingly, the microbiota of EDM fry was more stable, both between replicate rearing flasks, and over time. Although similar, the skin and gut microbiota started to differentiate during the yolk sac stage, several weeks before the yolk was consumed. Addition of high doses of bacterial isolates to fish flasks had only minor effects on the microbiota.

Experimental transmission of piscine orthoreovirus-1 (PRV-1) in different life stages of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta)

Piscine orthoreovirus -1 (PRV-1) causes the disease heart and skeletal muscle inflammation (HSMI) in farmed Atlantic salmon, and the virus has been detected in wild anadromous Atlantic salmon and brown trout. However, the infection prevalence, viral kinetics, and disease severity in different life stages of Atlantic salmon and brown trout are unknown. The current study aimed to evaluate and compare susceptibility to PRV-1 infection and development of HSMI in different life stages of anadromous Atlantic salmon (Salmo salar) and brown trout (Salmo trutta). We challenged Atlantic salmon and brown trout fry, parr, and post-smolts with PRV-1 by bath, cohabitation, or IP injection. The kinetics of viral infection and disease development were evaluated by RT-qPCR, in situ hybridization, and histology. Our results indicated that PRV-1 infection prevalence and viral kinetics depend on the developmental stage and challenge method in both Atlantic salmon and brown trout. All developmental stages of Atlantic salmon and brown trout can be infected with PRV-1. However, brown trout showed a lower infection prevalence, with positive cases exhibiting only mild infections without any pathological changes in the target organs, while all life stages of Atlantic salmon developed heart lesions characteristic of HSMI. These results strongly suggest that brown trout are less susceptible to PRV-1 infection than Atlantic salmon and further confirm the species-specific susceptibility and disease development for PRV-1 infection.

Black Soldier Fly Full-Fat Meal in Atlantic Salmon Nutrition – Part A: Effects on Growth Performance, Feed Utilization, Selected Nutriphysiological Traits and Production Sustainability in Fries

Abstract Black soldier fly larva meal (BSFM) application as an innovative feed material has been developed in aquaculture nutrition. However, recent scientific literature is still lacking in the case of insect meal effects on different stages of salmonid fish, and the vast majority of research is focused on farmed Atlantic salmon. Therefore, the holistic approach was undertaken in the cycle of two studies carried out: on fries and pre-smolts. This study aimed to evaluate the effect of fish meal (FM) replacement by full-fat BSFM in Atlantic salmon fry diets on the growth performance, feed utilization, crude protein and crude fat digestibility, somatic indices, histomorphology of intestines, and environmental sustainability. Additionally, pellet quality was assessed. Two experiments were performed on 360 Atlantic salmon fries – each of which lasted 60 days. For the first experiment, 240 individuals were distributed to four experimental groups: six replicates (tanks) per group and 10 fish per tank. For the second experiment, 120 individuals were distributed to four experimental groups: three replicates (tanks) per group and 10 fish per tank were used. The following groups were applied: CON – without the addition of full-fat BSFM and with 39% FM; BSFM5 – with 5% addition of full-fat BSFM and 36.1% FM; BSFM10 – with 10% full-fat BSFM and 33.3% FM; BSFM15 – with 15% addition of full-fat BSFM and 30.3% FM. The present study showed satisfactory results of BSFM inclusion at the levels of 5% and 10% as a replacement for fish meal in feeds for Atlantic salmon fries. No negative effects on growth performance, feed utilization, protein and fat digestibility, or intestinal structure were recorded. Simultaneously, insect meal inclusion in Atlantic salmon diets significantly improved the environmental sustainability of the feed. In the case of BSFM15, decreases in body weight gain, protein lipid efficiency ratio, and lipid efficiency ratio, as well as increases in feed conversion ratio, were observed. Therefore, the use of shares higher than 10% of BSFM in Atlantic salmon fries may not be recommended.

Long-term microbial community structures and dynamics in a commercial RAS during seven production batches of Atlantic salmon fry (Salmo salar)

The microbiota of recirculating aquaculture systems (RAS) is of major importance for optimal fish health. However, the microbial communities in commercial RAS are highly complex and more knowledge is needed to potentially control and maintain beneficial microbial communities for good fish production. In this study we monitored microbial communities in a commercial RAS producing Atlantic salmon fry (Salmo salar) during seven consecutive production batches. The water of rearing tanks and the water sump downstream of the biofilter/upstream of the UV, as well as biofilm of the wall of the rearing tanks and the fixed bed biofilter were analysed using 16S rRNA gene amplicon sequencing to elucidate the spatial-temporal microbial dynamics. The results showed that the microbiota composition of water and biofilm varied within and between the production batches, and that the fallowing periods had a substantial effect on the microbial communities. The correlation of the water and biofilm microbiota to fish presence in the system was confirmed by supervised machine learning. Shifts in the composition of the microbiota were identified in conjunction with variations in organic matter loading both during production and fallowing. In addition, variables like oxygen saturation, biomass, and feed type, showed good correlation with variations in the water microbiota composition. Although microbiota changed at fallowing, the microbiota returned to similar compositions during the production phases and was especially evident for the water microbiota. This indicates that the development of microbiota composition is strongly dictated by the similar selection pressure in the system. Nitrifying communities were dominated by Nitrospira, and the third most abundant Nitrospira OTUs were related to the comammox Nitrospira nitrificans. The microbial communities in the biofilter biofilm and water were significantly different but shared abundant taxa and followed the same temporal microbial dynamics and indicates an interaction between the biofilter biofilm and the suspended bacteria. CFU analysis showed that the fraction of rapid-growing bacteria was significantly higher in the rearing water than in the water sump upstream the UV disinfection, indicating that disinfection upstream the rearing tanks allowed for growth of opportunistic bacteria. A community with considerable potential for opportunistic regrowth can have consequences for the microbial water quality and the resistance against pathogen invasion The absence of an in-line disinfection step or placing the disinfection unit upstream the biofilter might provide better microbial water quality and a more resilient system against pathogen proliferation.

Growth and mortality of sympatric Atlantic salmon and brown trout fry in fluctuating and stable flows

AbstractSub‐daily fluctuations in streamflow may have adverse effects on the biota downstream of dams in hydropeaking‐regulated rivers. Although the stranding of salmonid fry is one documented effect of hydropeaking, little is known about the species‐specific behavioural and subsequent growth effects of sub‐daily flow fluctuations. We investigated the effects of sub‐daily flow fluctuation on growth, mortality and behaviour of sympatric Atlantic salmon (Salmo salar) and brown trout (S. trutta) fry (29–34 mm) in a laboratory experiment. The fluctuating flow treatment negatively affected growth and increased mortality for trout but not for salmon. The level of aggressive behaviour was similar in the fluctuating‐ and stable‐flow treatments. Within the fluctuating flow treatment, there was a trend that more fishes were visibly active above the substrate during low than high flow. These findings suggest that hydropeaking‐induced flow fluctuations may affect fry of different salmonid species in different ways and that brown trout fry may be more vulnerable to hydropeaking effects than Atlantic salmon fry. It can therefore be important to consider the possibility of divergent reactions by different fish species under hydropeaking situations and to incorporate species‐specific strategies to conserve culturally and economically relevant riverine fish species.

Immersion challenge model for Flavobacterium psychrophilum infection of Atlantic salmon (Salmo salar L.) fry.

Flavobacterium psychrophilum is the causative agent of bacterial cold-water disease (CWBD) and rainbow trout fry syndrome (RTFS), which affect salmonids. To better understand this pathogen and its interaction with the host during infection, including to support the development of resistant breeds and new vaccines and treatments, there is a pressing need for reliable and reproducible immersion challenge models that more closely mimic natural routes of infection. The aim of this present study was to evaluate a challenge model developed previously for rainbow trout for use in Atlantic salmon. First, preliminary challenges were conducted in Atlantic salmon (n=120) and rainbow trout (n=80) fry using two F.psychrophilum isolates collected from each fish species, respectively; fish had been pretreated with 200 mg/L hydrogen peroxide for 1h. Thereafter, the main challenge was performed for just one F.psychrophilum isolate for each species (at 2 × 107 CFU/mL) but using larger cohorts (Atlantic salmon: n=1187; rainbow trout: n=2701). Survival in the main challenge was 81.2% in Atlantic salmon (21 days post-challenge) and 45.3% in rainbow trout (31 days post-challenge). Mortalities progressed similarly during the preliminary and main challenges for both species, demonstrating the reproducibility of this model. This is the first immersion challenge model of F.psychrophilum to be developed successfully for Atlantic salmon.

In-water electric stunning as a humane commercial method for culling juvenile salmonids

Culling excess juvenile fish is a necessary and routine part of hatchery management. This is often achieved with an overdose of anaesthetic, which protects fish welfare at the time of killing. However, the use of anaesthetics means the resulting fish carcasses must be treated as controlled waste, which can be costly to dispose of. Electric stunning and killing of cull fish allows the carcasses to be sold as a protein source or disposed of as normal organic waste. For electric stunning and killing to protect welfare, unconsciousness should be achieved immediately and last until death. In this paper, we report on two trials investigating the efficacy of a commercial stunner designed for culling juvenile salmonids. To test whether unconsciousness was achieved immediately, Atlantic salmon smolts and fry in a small tank were exposed to an electric field similar to that inside the stunner for 1 s. Subsequent behavioural observation determined that they had been rendered unconscious. In the second experiment, Atlantic salmon parr and smolts were observed for up to 17 min after transiting the commercial stunner to ensure that unconsciousness persisted long enough for death to supervene. Of the 640 fish observed (both in air and water) none showed any signs of recovery. The commercial system and the scaled-down tank used electric fields of 3.1 V/cm and 2.36 V/cm respectively. We conclude that in-water electric stunning can deliver instant and lasting unconsciousness for salmonid juveniles and is well suited for culling operations.

Assessing the toxicity of peracetic acid to early Atlantic salmon Salmo salar life‐stages

Bacterial and fungal pathogens in recirculating aquaculture and egg incubation systems can cause elevated mortality and decreased production. Peracetic acid (PAA) is a relatively low-cost, safe, and effective disinfectant; however, its toxicity to early life stages of the Atlantic salmon (Salmo salar) has not been assessed. The 24-h LC50 value of PAA was determined for three early life stages of Atlantic salmon: eyed eggs, fry (~0.17 g), and fingerlings (~16.3 g). LC50 values were calculated using the Trimmed Spearman-Karber (TSK) method and Toxicity Relationship Analysis Program (TRAP). TRAP LC50 values for eyed eggs treated for 5 and 10 min were 781.5 and 485.0 mg/L PAA, respectively, while TSK LC50 values for eyed eggs treated for 5 and 10 min were 771.1 and 462.1 mg/L PAA, respectively. TRAP LC50 values for fry and fingerlings were 4.0 and 5.3 mg/L PAA, respectively, while TSK LC50 values for fry and fingerling were 4.1 and 5.3 mg/L PAA, respectively. The no observed effect concentration (NOEC) values for eyed eggs treated for 5 and 10 min were 500 and 300 mg/L PAA, respectively. These LC50 and NOEC values provide guidance for developing safe PAA treatment protocols Atlantic salmon eggs, fry, and/or fingerlings.

Elucidating bacterial adhesion to mucosal surface by an original AFM approach

BackgroundFish skin represents an ancient vertebrate mucosal surface, sharing characteristics with other mucosal surfaces including those of the intestine. The skin mucosa is continuously exposed to microbes in the surrounding water and is therefore important in the first line defense against environmental pathogens by preventing bacteria from accessing the underlying surfaces. Understanding the microbe-host interactions at the fish skin mucosa is highly relevant in order to understand and control infection, commensalism, colonization, persistence, infection, and disease. Here we investigate the interactions between the pathogenic bacteria Aeromonas salmonicida (A. salmonicida) and Yersinia ruckeri (Y. ruckeri), respectively, and the skin mucosal surface of Atlantic salmon fry using AFM force spectroscopy.ResultsThe results obtained revealed that when retracting probes functionalized with bacteria from surfaces coated with immobilized mucins, isolated from salmon mucosal surfaces, rupture events reflecting the disruption of adhesive interactions were observed, with rupture strengths centered around 200 pN. However, when retracting probes functionalized with bacteria from the intact mucosal surface of salmon fish fry no adhesive interactions could be detected. Furthermore, rheological measurements revealed a near fluid-like behavior for the fish fry skin mucus. Taken together, the experimental data indicate that the adhesion between the mucin molecules within the mucous layer may be significantly weaker than the interaction between the bacteria and the mucin molecules. The bacteria, immobilized on the AFM probe, do bind to individual mucins in the mucosal layer, but are released from the near fluid mucus with little resistance upon retraction of the AFM probe, to which they are immobilized.ConclusionThe data provided in the current paper reveal that A. salmonicida and Y. ruckeri do bind to the immobilized mucins. However, when retracting the bacteria from intact mucosal surfaces, no adhesive interactions are detected. These observations suggest a mechanism underlying the protective function of the mucosal surface based on the clearing of potential threats by adhering them to loosely attached mucus that is subsequently released from the fish skin.

Microbial community dynamics in a commercial RAS for production of Atlantic salmon fry (Salmo salar)

Recirculating aquaculture systems (RAS) harbour complex microbial communities which can have an impact on the growth and development of the reared fish. This study aimed to improve our understanding of microbial community dynamics in a RAS involving three production batches of Atlantic salmon fry and parr during a period of 20 months. Water for analysis of microbiota was sampled at different positions in the RAS, and we also examined the effect of UV treatment on the water microbiota. Microbial communities were characterized by 16S rDNA amplicon sequencing of water samples taken directly upstream and downstream of the UV treatment unit and from three of the rearing tanks. In total 6 sampling events were made during a 20-month period. The study showed that: 1) Two of the production batches had a highly similar water microbiota despite disinfection of the system between the batches and rearing fish of different stages. In contrast, the first production batch showed a different water microbiota with variable composition through the system and over time. A more immature biofilter in the first batch may explain these differences. 2) The full-flow UV treatment directly upstream the rearing tanks had no observable effect on the community composition of the water microbiota in the different sampling positions in the RAS. This was likely a consequence of the low hydraulic retention time (HRT) (23 min) in rearing tanks, low bacterial regrowth in the fish tanks and community changes throughout the RAS loop. 3) The disinfection effect on viable bacterial densities in the water directly downstream of the UV treatment was around 89%, when the water was clear. Regrowth of bacteria following disinfection was low compared to those reported for marine RAS with UV disinfection and long HRT in fish tanks. The study shows that UV disinfection can be used to efficiently reduce bacterial density without compromising the microbial water quality in the fish tanks in RAS with low HRT.

Early life stress causes persistent impacts on the microbiome of Atlantic salmon

Farmed fish are commonly exposed to stress in intensive aquaculture systems, often leading to immune impairment and increased susceptibility to disease. As microbial communities associated with the gut and skin are vital to host health and disease resilience, disruption of microbiome integrity could contribute to the adverse consequences of stress exposure. Little is known about how stress affects the fish microbiome, especially during sensitive early life stages when initial colonisation and proliferation of host-associated microbial communities take place. Therefore, we compared the effects of two aquaculture-relevant early-life stressors on the gut and skin microbiome of Atlantic salmon fry (four months post hatching) using 16S rRNA amplicon sequencing. Acute cold stress applied during late embryogenesis had a pronounced, lasting effect on the structure of the skin microbiome, as well as a less consistent effect on the gut microbiome. Follow-up targeted qPCR assays suggested that this is likely due to disruption of the egg shell microbial communities at the initial stages of microbiome colonisation, with persistent effects on community structure. In contrast, chronic post hatching stress altered the structure of the gut microbiome, but not that of the skin. Both types of stress promoted similar Gammaproteobacteria ASVs, particularly within the genera Acinetobacter and Aeromonas, which include several important opportunistic fish pathogens. Our results demonstrate the sensitivity of the salmon microbiome to environmental stressors during early life, with potential associated health impacts on the host. We also identified common signatures of stress in the salmon microbiome, which may represent useful microbial stress biomarkers.

Effects of dietary phospholipids on early stage Atlantic Salmon (Salmo salar) performance: A comparison among phospholipid sources

The present study aimed to confirm that additional phospholipids are needed in Atlantic Salmon (Salmo salar) fry diet for optimal performance. Furthermore, comparisons were made among soy lecithin and two marine-based phospholipids- krill oil and marine phospholipids extracted from fishmeal. First feeding salmon juveniles had significantly reduced mortality and increased growth when marine-based phospholipids (both krill and fish-sourced) were supplemented to high fishmeal diets (baseline control diet and a low phospholipid control diet). Fish fed diets with added soy lecithin had lower but not significantly different growth rates compared to fish fed diets supplemented with marine-based phospholipids, however, growth was also not significantly different than the baseline control. Thirty days after first feeding, differences in growth were no longer significant among supplemented diets and the control diet. However, both survival and growth for the low phospholipid baseline control remained poor, indicating a lowered but still present requirement for dietary phospholipids at this stage. Results show marine-based phospholipids, despite original source (fish versus krill), perform similarly, while soy lecithin has a tendency to perform slightly lower in the first-feeding juvenile stage. Marine-based ingredients are sustainable, but limited; therefore, it is important that many numerous, effective ingredient sources are available for the aquaculture industry.

Modulations of lipid metabolism and development of the Atlantic salmon (Salmo salar) fry in response to egg-to-fry rearing conditions.

In stocking program, the use of artificial incubation conditions in hatcheries from the fertilisation of eggs to the release of unfed fry could reduce their ability to adapt to the natural environment. This study evaluates the effects of three factors on the fitness and physiology of salmon fry at their emergence, the origin of water (river vs drilling), the type of support in the incubator (support matrix vs plastic sheets) and the type of incubators (Californian vs vertical trays), and compares them to a semi-natural incubation method in river. Key biological functions including nutritional and immune status were compared among experimental conditions using biometric parameters, lipid composition and gene expression analyses. Our findings demonstrated that fry incubated in vertical trays supplied with river water had no significant difference in growth and lipid composition compared to those in semi-natural incubators. Besides, fry incubated on a substrate matrix in Californian trays exhibited phenotypic characteristics closest to those incubated in river. This support matrix improved fish growth, lipid consumption and distribution compared to fry on plastic sheets. Moreover, the large amounts of several PUFAs in these fry could allow a better membrane fluidity ensuring a better adaptation to temperature variation under cold conditions. In addition, drilling water improved the survival rate compared to river water due to lower numbers of fine particles, known to be responsible for the clogging of eggs. To conclude, using a substrate combined with drilling water in artificial incubators could increase fry fitness and its adaption to wild life.

Morphopathology and gill recovery of Atlantic salmon during the parasitic detachment of Margaritifera margaritifera.

During the conservation aquaculture of the freshwater mussel Margaritifera margaritifera, fish health has become a concern due to the need of mussel larvae (glochidia) to parasitize the salmonid gills and metamorphose into juveniles. However, there is a lack of information about the impact on fish during the juvenile detachment and the subsequent gill healing. To evaluate the morphopathological changes and gill recovery after the parasitism of M. margaritifera, 51 Atlantic salmon fry (Salmo salar), infested with around 22 larvae/fish g, were necropsied during the synchronized detachment of the mussel juveniles, and gills were assessed by stereomicroscopy and by light and scanning electron microscopy. Salmon showed no clinical signs during the trial and gills recovered their normal morphology almost completely in a short time, suggesting a minimal impact on fish health after glochidiosis. In this sense, the non-erosive droplet detachment and the goblet cell hyperplasia favoured an effective gill remodelling mediated by apoptosis, polarization and cell shedding of the gill epithelia, providing insights to the defence, clearing and healing mechanisms of the gill. These morphopathological techniques could also be implemented to preserve fish welfare and to optimize the artificial breeding programmes of endangered freshwater mussels.

Impact of down-stream processing on functional properties of yeasts and the implications on gut health of Atlantic salmon (Salmo salar)

Yeasts are becoming popular as novel ingredients in fish feeds because of their potential to support better growth and concomitantly ensure good fish health. Here, three species of yeasts (Cyberlindnera jadinii, Blastobotrys adeninivorans and Wickerhamomyces anomalus), grown on wood sugars and hydrolysates of chicken were subjected to two down-stream processes, either direct heat-inactivation or autolysis, and the feed potential of the resulting yeast preparations was assessed through a feeding trial with Atlantic salmon fry. Histological examination of distal intestine based on widening of lamina propria, showed that autolyzed W. anomalus was effective in alleviating mild intestinal enteritis, while only limited effects were observed for other yeasts. Our results showed that the functionality of yeast in counteracting intestinal enteritis in Atlantic salmon was dependent on both the type of yeast and the down-stream processing method, and demonstrated that C. jadinii and W. anomalus have promising effects on gut health of Atlantic salmon.

Reducing mortality associated with opportunistic infections in Atlantic salmonSalmo salarfry using hydrogen peroxide and peracetic acid

Developing efficacious protocols for applying water disinfectants to reduce opportunistic pathogen-associated mortalities during the Atlantic salmon Salmo salar fry stage would be highly beneficial for producers. Atlantic salmon fry (0.47 g ± 0.02) were exposed to daily stressors over four weeks while providing daily 30-min bath treatments of 15 mg/L hydrogen peroxide (H2O2), 0.2 mg/L peracetic acid (PAA) or 0.5 mg/L PAA. Survival was tracked, and skin and gill samples were collected at 2 and 4 weeks for histopathology. Moribund fish were regularly assessed via wet-mount microscopy, with organisms resembling Saprolegnia spp. routinely observed on gills of affected fish. Tanks treated with H2O2 had significantly (p < 0.05) higher survival (83.7% ± 1.7) compared to controls (69.5% ± 5.2) while no significant differences were observed between either PAA treatments (76.6% ± 0.6 and 77.4% ± 3.0 survival in the 0.2 mg/L and 0.5 mg/L PAA groups, respectively) and controls. Interestingly, no significant differences were noted among treatments for waterborne Saprolegnia spp. concentrations through qPCR quantification. Lower total suspended solids (TSS) were observed in both PAA treatment groups; no other water quality differences were noted. No treatment impacts were observed through histopathology at either sampling point. These results suggest that, at the dosage and treatment regime tested, H2O2 can be a safe and efficacious water treatment for reducing Atlantic salmon fry opportunistic infection-associated mortality during periods of physical and environmental stress. Assessments of alternative PAA treatment regimens should also be considered in future research aimed at reducing early life-stage mortality in Atlantic salmon.

Common-garden comparison of relative survival and fitness-related traits of wild, farm, and hybrid Atlantic salmon Salmo salar parr in nature

When escapee farmed Atlantic salmon Salmo salar interbreed with wild fish, the introgression of maladaptive genes can lower wild population productivity and alter key life history traits. To date, only a few European studies have compared wild, farm, and hybrid salmon under common conditions in the wild, isolating the influence of genetics on survival and fitness-related traits. Here, we examined the performance of experimentally derived Atlantic salmon fry from 4 cross types (wild, farm, and reciprocal F1 hybrids) during the first summer of growth at 3 locations in southern Newfoundland. Overall survival was high, with the cross type rank order consistent across sites (mean percent recaptured: wild-mother hybrids 26.2% ≈ wild 26.0% &gt; farm 19.2% &gt; farm-mother hybrids 12.8%). Wild fish were smaller than wild-mother hybrids and farm fish, though differed less in size from farm-mother hybrids. At 2 out of 3 sites, wild-mother hybrids were larger than wild and farm-mother hybrid fish but had only a small size advantage over farm fish. Shape differences were small and mainly related to body depth, with the largest differences between wild and farm fish. Wild-mother hybrids had fewer parr marks than other cross types at a single site, and though differences in the size of marks were minimal, farm fish tended to have the narrowest marks. Overall, these results show that genetic differences exist for fitness-related traits among wild, farm, and hybrid juveniles, even over short temporal scales and under favourable environmental conditions, and may contribute to patterns of reduced farm-mother hybrid and feral farm survival in the wild.