The Impact of Exposure Dosage and Host Genetics on the Shedding Kinetics of Flavobacterium psychrophilum in Rainbow Trout.

Flavobacterium psychrophilum, the causative agent of bacterial cold water disease (BCWD), is one of the leading pathogens in rainbow trout (Oncorhynchus mykiss) aquaculture. To date, there is little knowledge of the transmission kinetics of F. psychrophilum over the course of infection. In particular, how transmission is affected by host genotype and pathogen exposure dosage are not well studied. In order to fill in these knowledge gaps, we exposed two divergently selected lines of rainbow trout (ARS-Fp-R and ARS-Fp-S) to a range of dosages of F. psychrophilum (strain CSF117-10). We then measured mortality and bacterial shedding to estimate transmission risk at multiple time points since initial infection. As dosage increased, the number of fish shedding and the amount of bacteria shed increased ranging from 0% to 100% and 103 to 108 cells fish-1 h-1, respectively. In addition, we found that disease resistance (survival) was not correlated with transmission risk blocking, in that 67% of fish which shed bacteria experienced no clinical disease. In general, fish mortality began on Day 3, peaked between Days 5-7 and was higher in the ARS-Fp-R line. Results from this study could be used to develop epidemiological models and improve disease management, particularly in the context of aquaculture and selective breeding.

Passage efficiency through fishways of species of the family Cyprinidae and their management implications for fragmented rivers.

The contemporary management of fragmented river systems is in a trade-off between the societal benefits of instream barriers (e.g. hydropower, flood risk management) and the ecological harms of their adverse impacts on fish populations. The consequent fragmentation can be mitigated through fishway construction, with mitigation performance measured using species-specific passage rates and efficiencies. There is, however, a bias in passage efficiency studies towards diadromous fishes and, although fish of the Cyprinidae family play a significant role in the fish assemblages of rivers worldwide, their passage efficiencies are poorly understood. Here, systematic review and meta-analyses assessed the passage efficiencies of cyprinid fishes through fishways that have been measured using telemetry methods. Passive integrated transponder (PIT) telemetry was the most common evaluation method of passage efficiency due to their high read rates and relatively low costs versus alternative telemetry methods. These methods revealed cyprinid passage efficiencies were highest through vertical slot fishways and lowest through nature-like constructions, with overall passage rates comparing favourably to anadromous salmonid fishes. Fish were most active during spring and summer, with passage and associated movements often related to spawning. Passage rates of non-native fishes were also higher than for native fishes. Despite the growing acknowledgment of how fishways influence potamodromous fish dispersal and distribution in rivers, passage data remain scarce, preventing managers and policy-makers from making informed decisions on optimal passage solutions for multiple fish species in highly fragmented rivers.

Distinct food-web transfers of 137Cs to fish in river and lake ecosystems: A case study focusing on masu salmon in the Fukushima evacuation zone.

This study was conducted to elucidate the spatial and size variations, and food-web transfer of 137Cs in freshwater fish in the upper reaches of the Ukedo River system, a highly contaminated river system flowing through the Fukushima evacuation zone. Fish collection and environmental surveys were conducted in the summer of 2020 at five forest rivers and at the Ogaki Dam reservoir (an artificial lake) with different air dose rates (mean 0.20-3.32 µSv/h). From the river sites, two salmonid species (masu salmon and white-spotted charr) were sampled, with masu salmon generally exhibiting higher 137Cs concentrations, ranging widely (10.6 Bq/kg-wet to 13.0 kBq/kg-wet) depending on the fish size (size effect) and site. The 137Cs concentrations in masu salmon were explained by the air dose rates, 137Cs concentrations in water, sediments (excluding the lake site), and primary producers, with site-specific variations. In the rivers, masu salmon (fluvial type with parr marks) mainly fed on terrestrial insects with higher 137Cs concentrations compared with those of aquatic insects, indicating that 137Cs was transferred mainly to fish through the allochthonous forest food-web during summer. In the lake, masu salmon (lake-run type with larger size and silvery body coloration) mainly preyed on smaller fish with lower 137Cs concentrations, demonstrating that 137Cs is transferred to fish through the autochthonous lake food-web with biomagnification. Differences in 137Cs concentrations among masu salmon (mean 441 Bq/kg-wet) and other fish species (mean 74.8 Bq/kg-wet to 2.35 kBq/kg-wet) were also found in the lake. The distinct 137Cs transfers to river and lake fish were supported by stable isotope analysis: δ15N and δ13C values enriched stepwisely through the food-webs were, respectively, higher and lower in the lake. Our results obtained using multiple approaches clearly revealed the distinct food-web transfer of 137Cs in river and lake ecosystems. These findings can contribute to prediction of radioactive contamination in freshwater fish in the Fukushima evacuation zone.

Gone with the flow: Whitefish egg drift in relation to substrate coarseness under a range of flow velocities.

Most major rivers in Europe have been dammed for hydropower or other purposes. Such river alterations have decimated natural reproduction of many migratory fish species, including that of the anadromous European whitefish, Coregonus lavaretus, which is now maintained by extensive stocking programmes. In addition to stocking, limited natural reproduction may occur downstream of dams, where peak flow bouts from the dams threaten to flush the eggs into unsuitable habitats. Here, we assessed the effects of water flow velocity and substrate coarseness on downstream drift of whitefish eggs under laboratory conditions. The experiment's two different gravel substrates retained eggs better than cobble or sand substrates; the water velocity needed for notable egg drift was higher for the gravel substrates. The results indicate that egg drift is one of the factors that should be considered when evaluating the effects of hydropower plant operations. Moreover, measures mitigating the effects of the artificial flow regimes should incorporate the type and coarseness of the riverbed's substrate.

Growth of Oncorhynchus mykiss (Rainbow Trout) through a Recirculation System in the Foothills of the Extreme North of Chile.

Given the overexploitation of fisheries and the growing consumption of sea products, aquaculture is emerging as an alternative to meet the demand for protein at regional, national, and global levels. In northern Chile, the foothills of the Andes offer an opportunity for sustainable economic diversification. In this study, results of a rainbow trout (Oncorhynchus mykiss) culture in a recirculation system are presented, analyzing its growth and performance under altitude conditions. The research was carried out in Copaquilla, a small area in the foothills of northern Chile, 3000 m above sea level. Five thousand 15 g juvenile trout were acquired and transported by land from the Rio Blanco fish farm, successfully traveling more than 2100 km. During the 20-month-long culture, several growth parameters were evaluated, including specific growth rate, percentage of weight growth, feed conversion factor, survival, and Fulton's condition factor. All these parameters were within the normal growth range for trout, evidencing good-quality growth of the organisms. The results offer evidence that rainbow trout farming at 3000 m above sea level is viable and presents a viable performance, opening new opportunities for aquaculture in northern Chile.

Nanopore sequencing provides snapshots of the genetic variation within salmonid alphavirus-3 (SAV3) during an ongoing infection in Atlantic salmon (Salmo salar) and brown trout (Salmo trutta).

Frequent RNA virus mutations raise concerns about evolving virulent variants. The purpose of this study was to investigate genetic variation in salmonid alphavirus-3 (SAV3) over the course of an experimental infection in Atlantic salmon and brown trout. Atlantic salmon and brown trout parr were infected using a cohabitation challenge, and heart samples were collected for analysis of the SAV3 genome at 2-, 4- and 8-weeks post-challenge. PCR was used to amplify eight overlapping amplicons covering 98.8% of the SAV3 genome. The amplicons were subsequently sequenced using the Nanopore platform. Nanopore sequencing identified a multitude of single nucleotide variants (SNVs) and deletions. The variation was widespread across the SAV3 genome in samples from both species. Mostly, specific SNVs were observed in single fish at some sampling time points, but two relatively frequent (i.e., major) SNVs were observed in two out of four fish within the same experimental group. Two other, less frequent (i.e., minor) SNVs only showed an increase in frequency in brown trout. Nanopore reads were de novo clustered using a 99% sequence identity threshold. For each amplicon, a number of variant clusters were observed that were defined by relatively large deletions. Nonmetric multidimensional scaling analysis integrating the cluster data for eight amplicons indicated that late in infection, SAV3 genomes isolated from brown trout had greater variation than those from Atlantic salmon. The sequencing methods and bioinformatics pipeline presented in this study provide an approach to investigate the composition of genetic diversity during viral infections.

Comparing feces collection methods for evaluating the apparent digestibility coefficient of brewers' spent yeast in juvenile Atlantic salmon (Salmo salar).

Brewer's spent yeast (BSY), derived from Saccharomyces cerevisiae used in beer production, is a valuable protein source for aquafeeds. Estimations of apparent digestibility coefficients (ADC) for nutrients in BSY are crucial for its inclusion in aquafeeds. ADC estimations for Saccharomyces cerevisiae protein in rainbow are hardly comparable from a methodological point of view, whereas the ADC estimations for BSY protein in Atlantic salmon are only based on stripped feces, which are known to produce underestimations. Therefore, new determinations of ADC of BSY nutrients are necessary for the inclusion of this ingredient in practical diets for salmonids. This study is focused on determining unbiased ADC values for protein and energy from BSY in juvenile Salmo salar. To reduce systematic biases, fecal samples were collected using stripping and decantation methods, which are known to produce under-and overestimations, respectively. 780 fish (25.16 ± 4.88 g) were stocked in six tanks. A reference diet (50% protein, 20% lipid, 1% Cr2O3) was provided to three tanks, and a test diet (70,30 reference diet to BSY) to the other three. ADC for BSY protein was 84.70 ± 1.04% (decantation) and 70.50 ± 4.03% (stripping). For gross energy, stripped feces yielded an ADC of 52.04 ± 5.30%, while decantation resulted in 63.80 ± 1.17%. Thus, ADC estimates were taken as the average of the stripping-value and the decantation-value, resulting in 77.6% for BSY crude protein, which is appreciably higher than previously measured values in S. salar fed undisrupted S. cerevisiae, and in 57.9% for gross energy.

Assessing Movements between Freshwater and Saltwater by Brown Trout (Salmo trutta L.) Based on Otolith Microchemistry.

By analyzing otolith microchemistry, we examined the use of freshwater and marine environments by brown trout Salmo trutta L. that spawn in the Swedish River Emån and migrate to the Baltic Sea. We estimated the time juveniles spent in freshwater and the number of times the fish returned to freshwater, presumably to spawn. Twenty-six percent of the fish migrated to sea by 1 year of age. However, 13% spent less than one year in the river. Most brown trout (48%) migrated to the sea between 1 and 2 years of age. On average, brown trout, which averaged 4.4 years in age (range 3-6 years), returned to freshwater 2.3 times, and there was an inverse relationship between time spent in freshwater after hatching and the number of visits to freshwater. Our results do not support the classical life history pattern, where brown trout spend one or more years in freshwater before migrating to the sea. Here, we found evidence that part of the population leaves freshwater during their first year. While the cause for precocial migration in the River Emån is not known, our results from this permanently flowing river do not support the idea proposed for other Baltic Sea populations, where the risk of drought has been suggested to be the cause.

Introgression affects Salmo trutta juvenile life-history traits generations after stocking with non-native strains.

Introgression of non-native conspecifics changes the genetic composition of wild populations, potentially leading to loss of local adaptations and fitness declines. However, long-term data from wild populations are still relatively few. Here, we studied the effects of introgression in a Danish brown trout (Salmo trutta, L.) population, subjected to intensive stocking with domesticated hatchery fish of non-native origin. We used wild-caught genetically wild and admixed trout as well as fish from the partly domesticated hatchery strain used for stocking the river up until ~15 years prior to this study, to produce 22 families varying in hatchery/wild admixture. Following a replicated common-garden experiment conducted in fish tanks from first feeding through 23 weeks at 7, 12, and 16°C, we observed a significant positive relationship between family admixture and fish size upon termination, an effect observed through all levels of admixture. Furthermore, the admixture effect was most distinct at the higher rearing temperatures. Although the hatchery strain used for stocking had been in culture for ~7 generations, it had not been deliberately selected for increased growth. These data thus demonstrate: (i) that growth had increased in the hatchery strain even in the absence of deliberate directional selection for this trait, (ii) that the increasing effect of admixture by temperature could represent inadvertent selection for performance in the hatchery strain at higher temperatures, and most significantly, (iii) that despite undergoing up to five generations of natural selection in the admixed wild population, the genetically increased growth potential was still detectable and thus persistent. Our findings suggest that altered growth patterns and potentially their cascading effects are of importance to the severity of hatchery/wild introgression, especially under changing-climate scenarios and are of general significance to conservation practitioners seeking to evaluate long-term effects of intra-specific hybridization including under recovery.

Local variation in stress response of juvenile anadromous brown trout, Salmo trutta.

Habitat fragmentation may cut off anadromous salmonids from parts of their potential native habitat and separate previously connected populations. Understanding the consequences of this is vital for fish management and prioritization of restoration activities. Here, we show that there is a significant difference in the body morphology, physiological stress response, and aspects contributing to aerobic capacity between juvenile anadromous brown trout, Salmo trutta, collected at a downstream site and an upstream site, separated by 2 km and several challenging stream sections, in a small unfragmented stream system in western Sweden. Following a standardized stress test, there were significant differences between fish from the upstream and downstream sites (plasma cortisol concentration, plasma osmolality, hematocrit, hemoglobin concentration, and mean corpuscular hemoglobin concentration). Plasma glucose concentration did not significantly differ between fish from the two sites. Fish from the upstream site had larger spleen mass, although there was no evidence of differences in ventricle mass or proportion of compact ventricular myocardium. These physiological differences indicate local variation in stress response and highlight the importance of considering local trait variation in river management. If a section of the river becomes fragmented or degraded, and there are differences in the juveniles in different parts of the river, the consequence for the population might be larger than the proportional loss of habitat.

MAVS disruption impairs downstream signaling and results in higher virus replication levels of salmonid alphavirus subtype 3 but not infectious pancreatic necrosis virus in vitro.

The mitochondrial anti-viral signaling (MAVS) protein is an intermediary adaptor protein of retinoic acid-inducible gene-1 (RIG-I) like receptor (RLR) signaling, which activates the transcription factor interferon (IFN) regulatory factor 3 (IRF3) and NF-kB to produce type I IFNs. MAVS expression has been reported in different fish species, but few studies have shown its functional role in anti-viral responses to fish viruses. In this study, we used the transcription activator-like effector nuclease (TALEN) as a gene editing tool to disrupt the function of MAVS in Chinook salmon (Oncorhynchus tshawytscha) embryonic cells (CHSE) to understand its role in induction of interferon I responses to infections with the (+) RNA virus salmonid alphavirus subtype 3 (SAV-3), and the dsRNA virus infectious pancreatic necrosis virus (IPNV) infection. A MAVS-disrupted CHSE clone with a 7-aa polypeptide (GVFVSRV) deletion mutation at the N-terminal of the CARD domain infected with SAV-3 resulted in significantly lower expression of IRF3, IFNa, and ISGs and increased viral titer (1.5 log10) compared to wild-type. In contrast, the IPNV titer in MAVS-disrupted cells was not different from the wild-type. Furthermore, overexpression of salmon MAVS in MAVS-disrupted CHSE cells rescued the impaired type I IFN-mediated anti-viral effect against SAV-3.

Thermal stressors during embryo incubation have limited ontogenic carryover effects in brook trout.

Winter climate is changing rapidly in northern latitudes, and these temperature events have effects on salmonid thermal biology. Stressors during winter egg incubation could reduce hatching success and physiological performance of fall-spawning fishes. Here we quantified the potential for ontogenic carryover effects from embryonic thermal stress in multiple wild and hatchery-origin populations of brook trout (Salvelinus fontinalis), a temperate ectotherm native to northeastern North America. Fertilized eggs from four populations were incubated over the winter in the laboratory in four differing thermal regimes: ambient stream-fed water, chronic warming (+2 °C), ambient with a mid-winter cold-shock, and short-term warming late during embryogenesis (to stimulate an early spring). We examined body size and upper thermal tolerance at the embryonic, fry (10 weeks post-hatch and 27-30 weeks post-hatch) and gravid adult (age 2+) life stages (overall N = 1482). In a separate experiment, we exposed developing embryos to acute seven-day heat stress events immediately following fertilization and at the eyed-egg stage, and then assessed upper thermal tolerance (CTmax) 37 weeks post-hatch. In all cases, fish were raised in common garden conditions after hatch (i.e., same temperatures). Our thermal treatments during incubation had effects that varied by life stage, with incubation temperature and life stage both affecting body size and thermal tolerance. Embryos incubated in warmer treatment groups had higher thermal tolerance; there was no effect of the mid-winter melt event on embryo CTmax. Ten weeks after hatch, fry from the ambient and cold-shock treatment groups had higher and less variable thermal tolerance than did the warmer treatment groups. At 27-30 post-hatch and beyond, differences in thermal tolerance among treatment groups were negligible. Collectively, our study suggests that brook trout only exhibit short-term carryover effects from thermal stressors during embryo incubation, with no lasting effects on phenotype beyond the first few months after hatch.

Red and melanized focal changes in the white skeletal muscle of farmed rainbow trout Oncorhynchus mykiss.

Fillet discoloration by red and melanized focal changes (RFCs and MFCs) is common in farmed Atlantic salmon Salmo salar. In farmed rainbow trout Oncorhynchus mykiss, similar changes have been noted, but their prevalence and histological characteristics have not been investigated. Thus, we conducted a study encompassing 1293 rainbow trout from 3 different farm sites in Norway, all examined at the time of slaughter. Both macroscopic and histological assessments of the changes were performed. Reverse transcription (RT)-qPCR analyses and in situ hybridization (ISH) were used to detect the presence and location, respectively, of potential viruses. Only 1 RFC was detected in a single fillet, while the prevalence of MFCs ranged from 1.46 to 6.47% between populations. The changes were predominantly localized in the cranioventral region of the fillet. Histological examinations unveiled necrotic myocytes, fibrosis, and regeneration of myocytes. Melano-macrophages were found in the affected areas and in myoseptal adipose tissue. Organized granulomas were observed in only 1 fish. Notably, the presence of inflammatory cells, including melano-macrophages, appeared lower compared to what has been previously documented in Atlantic salmon MFCs. Instead, fibrosis and regeneration dominated. RT-qPCR and ISH revealed the presence of piscine orthoreovirus 1 (PRV-1) and salmonid alphavirus (SAV) in skeletal muscle. However, these viruses were not consistently associated with lesioned areas, contrasting previous findings in Atlantic salmon. In conclusion, rainbow trout develop MFCs of a different character than farmed Atlantic salmon, and we speculate whether the observed pathological differences are contributing to their reduced occurrence in farmed rainbow trout.

Virus-specific antibody secreting cells reside in the peritoneal cavity and systemic immune sites of Atlantic salmon (Salmo salar) challenged intraperitoneally with salmonid alphavirus.

The development and persistence of antibody secreting cells (ASC) after antigenic challenge remain inadequately understood in teleosts. In this study, intraperitoneal (ip) injection of Atlantic salmon (Salmo salar) with salmonid alphavirus (WtSAV3) increased the total ASC response, peaking 3-6 weeks post injection (wpi) locally in the peritoneal cavity (PerC) and in systemic lymphoid tissues, while at 13 wpi the response was only elevated in PerC. At the same time point a specific ASC response was induced by WtSAV3 in PerC and systemic tissues, with the highest frequency in PerC, suggesting a local role. Inactivated SAV (InSAV1) induced comparatively lower ASC responses in all sites, and specific serum antibodies were only induced by WtSAV3 and not by InSAV1. An InSAV1 boost did not increase these responses. Expression of immune marker genes implies a role for PerC adipose tissue in the PerC immune response. Overall, the study suggests the Atlantic salmon PerC as a secondary immune site and an ASC survival niche.

Inland freshwater aquaculture in a warming world.

Climate change potentially threatens the sustainable production of highly valued cold-water fish species in flow-through systems, such as salmonids. By analysing the relationship of water temperature to hydrological characteristics, air temperature, solar exposure, and precipitation, this study predicted temperature dynamics of five temperate cold-water aquaculture facilities under four projected climate change scenarios. Air temperature was found to be directly associated with facility site water temperature, and based on rational assumptions, two of the five facilities were predicted to face critical warming by mid-century. Extreme precipitation events induced acute short-term increases in water temperature of up to 5 °C. Significantly lower warming, roughly equal to the projected climate change-induced increase, was seen with artificial shading lowering temperature by 1 °C. Complementary niche modelling revealed that 37-77 % of current cold-water facilities will likely incur suboptimal climate conditions by the end of the century. Shading of raceways, more efficient water use, and disease management are proposed as key actions to preserve cold-water aquaculture.

Gene Function is a Driver of Activin Signaling Pathway Evolution Following Whole-Genome Duplication in Rainbow Trout (Oncorhynchus mykiss).

The genomes of plant and animal species are influenced by ancestral whole-genome duplication (WGD) events, which have profound impacts on the regulation and function of gene networks. To gain insight into the consequences of WGD events, we characterized the sequence conservation and expression patterns of ohnologs in the highly duplicated activin receptor signaling pathway in rainbow trout (RBT). The RBT activin receptor signaling pathway is defined by tissue-specific expression of inhibitors and ligands and broad expression of receptors and Co-Smad signaling molecules. Signaling pathway ligands exhibited shared expression, while inhibitors and Smad signaling molecules primarily express a single dominant ohnolog. Our findings suggest that gene function influences ohnolog evolution following duplication of the activin signaling pathway in RBT.

Comparing the migration behavior and survival of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) smolts.

Many organisms rely on migrations between habitats to maximize lifetime fitness, but these migrations can be risky due to a suite of factors. In anadromous salmonids, the smolt migration from fresh water to sea is a critical life stage, during which smolts can experience high mortality from multiple sources. This study investigated the migratory behavior and survival of Atlantic salmon (Salmo salar) and anadromous brown trout (Salmo trutta) smolts during their seaward migration using acoustic telemetry between March and May 2021. Due to the extinction of wild salmon in the River Gudenaa after the construction of the Tange hydropower plant, this study used hatchery-reared salmon originating from a nearby Danish river. A total of 75 hatchery-reared salmon smolts, 75 hatchery-reared trout smolts, and 75 wild trout smolts were tagged with acoustic transmitters and released into River Gudenaa, Denmark. The downstream movements of tagged fish were monitored using acoustic receivers deployed in the river and fjord. Hatchery-reared trout initiated migration first, followed by hatchery-reared salmon, with wild trout being the last to migrate. There was no difference in riverine progression rates among the three smolt groups, but noticeable differences emerged once in the fjord: trout (wild and hatchery) slowed down, whereas hatchery-reared salmon maintained their speed. Riverine migration was predominantly nocturnal for all smolts; however, daytime migration increased at the fjord arrays. Day-of-year significantly influenced diurnal patterns in the river and fjord, where daytime migration increased later in the year. Hatchery-reared salmon and wild trout had reasonably good overall survival from river to sea entry (≥66%), whereas hatchery-reared trout had poor survival (c.26%). The fjord was the major bottleneck for survival of hatchery-reared trout. We found no strong evidence for differences in progression rate or diurnal patterns between wild and hatchery-reared trout to explain the lower survival. This study demonstrates that salmon and trout differ in their life-history strategy already in the post-smolt phase, and that stocking is a sub-optimal strategy to aid wild populations.

The Effect of an Attenuated Live Vaccine against Salmonid Rickettsial Septicemia in Atlantic Salmon (Salmo salar) Is Highly Dependent on Water Temperature during Immunization.

Salmonid Rickettsial Septicemia (SRS), caused by the bacterium Piscirickettsia salmonis, is the main reason for antibiotic usage in the Chilean aquaculture industry. In 2016, a live attenuated vaccine (ALPHA JECT LiVac® SRS, PHARMAQ AS) was licensed in Chile and has been widely used in farmed salmonids since then. In experimental injection and cohabitation laboratory challenge models, we found that the vaccine is effective in protecting Atlantic salmon (Salmo salar) for at least 15 months against P. salmonis-induced mortality. However, the protection offered by the vaccine is sensitive to temperature during immunization. Fish vaccinated and immunized at 10 °C and above were well protected, but those immunized at 7 °C and 8 °C (the lower end of the temperature range commonly found in Chile) experienced a significant loss of protection. This temperature-dependent loss of effect correlated with the amount of vaccine-strain RNA detected in the liver the first week after vaccination and with in vitro growth curves, which failed to detect any growth at 8 °C. We found that good vaccine efficacy can be restored by exposing fish to 15 °C for the first five days after vaccination before lowering the temperature to 7 °C for the remaining immunization period. This suggests that maintaining the correct temperature during the first few days after vaccination is crucial for achieving a protective immune response with ALPHA JECT LiVac® SRS. Our results emphasize the importance of temperature control when vaccinating poikilothermic animals with live vaccines.

Consistent Negative Correlations between Parasite Infection and Host Body Condition Across Seasons Suggest Potential Harmful Impacts of Salmincola markewitschi on Wild White-Spotted Charr, Salvelinus leucomaenis.

Assessing the impacts of parasites on wild fish populations is a fundamental and challenging aspect of the study of host-parasite relationships. Salmincola, a genus of ectoparasitic copepods, mainly infects salmonid species. This genus, which is notorious in aquaculture, damages host fishes, but its impacts under natural conditions remain largely unknown or are often considered negligible. In this study, we investigated the potential impacts of mouth-attaching Salmincola markewitschi on white-spotted charr (Salvelinus leucomaenis) through intensive field surveys across four seasons using host body condition as an indicator of harmful effects. The prevalence and parasite abundance were highest in winter and gradually decreased in summer and autumn, which might be due to host breeding and/or wintering aggregations that help parasite transmissions. Despite seasonal differences in prevalence and parasite abundance, consistent negative correlations between parasite abundance and host body condition were observed across all seasons, indicating that the mouth-attaching copepods could reduce the body condition of the host fish. This provides field evidence suggesting that S. markewitschi has a potential negative impact on wild white-spotted charr.

Effect of pancreas disease vaccines on infection levels and virus transmission in Atlantic salmon (Salmo salar) challenged with salmonid alphavirus, genotype 2.

Salmonid alphavirus (SAV) causes pancreas disease (PD), which negatively impacts farmed Atlantic salmon. In this study, fish were vaccinated with a DNA-PD vaccine (DNA-PD) and an oil-adjuvanted, inactivated whole virus PD vaccine (Oil-PD). Controls were two non-PD vaccinated groups. Fish were kept in one tank and challenged by cohabitation with SAV genotype 2 in seawater. Protection against infection and mortality was assessed for 84 days (Efficacy study). Nineteen days post challenge (dpc), subgroups of fish from all treatment groups were transferred to separate tanks and cohabited with naïve fish (Transmission study 1) or fish vaccinated with a homologous vaccine (Transmission study 2), to evaluate virus transmission for 26 days (47 dpc). Viremia, heart RT-qPCR and histopathological scoring of key organs affected by PD were used to measure infection levels. RT-droplet digital PCR quantified shedding of SAV into water for transmission studies. The Efficacy study showed that PD associated growth-loss was significantly lower and clearance of SAV2 RNA significantly higher in the PD-DNA group compared to the other groups. The PD-DNA group had milder lesions in the heart and muscle. Cumulative mortality post challenge was low and not different between groups, but the DNA-PD group had delayed time-to-death. In Transmission study 1, the lowest water levels of SAV RNA were measured in the tanks containing the DNA-PD group at 21 and 34 dpc. Despite this, and irrespective of the treatment group, SAV2 was effectively transmitted to the naïve fish during 26-day cohabitation. At 47 dpc, the SAV RNA concentrations in the water were lower in all tanks compared to 34 dpc. In Transmission study 2, none of the DNA-PD immunized cohabitants residing with DNA-PD-vaccinated, pre-challenged fish got infected. In contrast, Oil-PD immunized cohabitants residing with Oil-PD-vaccinated, pre-challenged fish, showed infection levels similar to the naïve cohabitants in Transmission study 1. The results demonstrate that the DNA-PD vaccine may curb the spread of SAV infection as the DNA-PD vaccinated, SAV2 exposed fish, did not spread the infection to cohabiting DNA-PD vaccinated fish. This signifies that herd immunity may be achieved by the DNA-PD vaccine, a valuable tool to control the PD epizootic in farmed Atlantic salmon.