Farmed Rainbow Trout Oncorhynchus Mykiss Research Articles

First case of ichthyophonosis in farmed rainbow trout Oncorhynchus mykiss in Serbia

First case of ichthyophonosis in farmed rainbow trout Oncorhynchus mykiss in Serbia

In Vivo Experiments Provide Evidence That Flavobacterium psychrophilum Strains Belonging to Multilocus Sequence Typing Clonal Complex ST191 Are Virulent to Rainbow Trout.

Flavobacterium psychrophilum, the causative agent of bacterial coldwater disease (BCWD), causes significant economic losses worldwide, particularly in farmed Rainbow Trout Oncorhynchus mykiss. Over the last decade, multilocus sequence typing has revealed >30 clonal complexes (CCs) globally, comprised of >320 F. psychrophilum sequence types (STs). Despite the large number of CCs worldwide, CC-ST10, which is currently the largest CC affecting Rainbow Trout, has been the primary focus of F. psychrophilum virulence studies, leaving the role of other CCs as primary causes of BCWD epizootics unclear. To this end, fingerling Rainbow Trout were experimentally challenged with F. psychrophilum strains belonging to the CC now recognized as the second largest in the world (CC-ST191) alongside CC-ST10 strains. Cumulative percent mortality was 100% in 7-month-old Rainbow Trout and between 27.8% and 61.1% in 8-month-old Rainbow Trout. All examined F. psychrophilum STs were virulent to Rainbow Trout, and no significant differences in virulence between CC-ST10 and CC-ST191 were detected. Due to their wide distribution and high pathogenic potential, both CC-ST191 and CC-ST10 F. psychrophilum strains are excellent candidates for further research aimed at preventing and controlling BCWD.

Flavobacterium bernardetii sp. nov., a possible emerging pathogen of farmed rainbow trout (Oncorhynchus mykiss) in cold water

Identification of novel Flavobacterium species from fish samples has growing importance due to their notorious effects on freshwater fish species. During the course of studying farmed rainbow trout (Oncorhynchus mykiss, Walbaum, 1792) samples exhibiting clinical signs in Rize, Turkey, two isolates were recovered, and their taxonomic positions were revealed based on a genome-based polyphasic taxonomy approach. Pairwise sequence analyses of the 16S rRNA genes confirmed that strains F-372T and F-408 belonged to the genus Flavobacterium. Strain F-372T exhibited the highest identity level of 98.37% with Flavobacterium terrigena DSM 17934T (=KCTC 12761T) while the 16S rRNA gene sequence identity between strain F-372T and F-408 was 99.8% A polyphasic taxonomy approach including phenotypic characterization as well as genomic and chemotaxonomic analyses was employed to confirm the novelty of the strains in the genus Flavobacterium. The major fatty acids were isoC15:0 and C15:1 ω6c for both strains while their predominant menaquinone was MK-6. Both strains have genome size of 3.3 Mb. The genomic DNA G + C content of the strains was 30.89 mol%. In addition, strain F-372T was found to carry 68 high confidence (HC) functional genes encoding AMR and virulence factors implying a possible correlation between those factors and mortality in the rainbow trout. Phylogenomic analysis and whole genome nucleotide sequences confirmed that the isolates represent a new species for which the name Flavobacterium bernardetii sp. nov. is proposed, with F-372T as the type strain (=JCM 34204T = KCTC 82264 T).

Vannella mustalahtiana sp. nov. (Amoebozoa, Vannellida) and rainbow trout nodular gill disease (NGD) in Russia.

An outbreak of nodular gill disease (NGD) in farmed rainbow trout Oncorhynchus mykiss (Walbaum, 1792) was recorded in Ladoga Lake (Karelia, north-western Russia) in Spring 2020. The disease was characterised by typical clinical signs including asphyxiation, distended opercula, loss of activity and swimming upside down under the water surface. Maximum monthly mortality was 15.2%. The histological examination of the gills showed deformation and clubbing of lamellae, epithelial hypertrophy and hyperplasia, lamellar fusion and fusion of filaments. Granulomas were located within the epithelial layer and/or rose above its surface. Light microscopic in vivo observations of the mucus smears from the affected gills revealed numerous amoeboid protists demonstrating a flattened body when adhering to the substratum, and blunt, radiating pseudopodia when afloat. Based on these morphological characters, these amoebae could be assigned to the Discosea (Amoebozoa), and analyses of their small subunit rRNA gene sequences showed that they belonged to the genus Vannella Bovee, 1965. The results reported herein support the designation of a new species, V. mustalahtiana sp. nov. Despite having been isolated from the gills of a freshwater fish, the species belongs to a clade of Vannella comprising mostly species isolated from marine and brackish water habitats. These findings may be essential for the aetiology and treatment of the disease.

Swim bladder mycosis in farmed rainbow trout Oncorhynchus mykiss caused by Phoma herbarum and experimental verification of pathogenicity.

In this study, spontaneous swim bladder mycosis was documented in a farmed fingerling rainbow trout from a raceway culture system. At necropsy, the gross lesions included a thickened swim bladder wall, and the posterior portion of the swim bladder was enlarged due to massive hyperplasia of muscle. A microscopic wet mount examination of the swim bladder contents revealed abundant septate hyphae, and histopathological examination showed periodic acid-Schiff-positive mycelia in the lumen and wall of the swim bladder. Histopathological examination of the thickened posterior swim bladder revealed muscle hyperplasia with expansion by inflammatory cells. The causative agent was identified as Phoma herbarum through morphological analysis and DNA sequencing. The disease was reproduced in rainbow trout fingerlings using intraperitoneal injection of a spore suspension. Necropsy in dead and moribund fish revealed extensive congestion and haemorrhages in the serosa of visceral organs and in liver and abdominal serosanguinous fluid. Histopathological examination showed severe hepatic congestion, sinusoidal dilatation, Kupffer cell reactivity, leukostasis and degenerative changes. Fungi were disseminated to the liver, pyloric caeca, kidney, spleen and heart. Although infections caused by Phoma spp. have been repeatedly reported in fish, species identification has been hampered by extensive taxonomic changes. The results of this study confirmed the pathogenicity of P. herbarum in salmonids by using a reliably identified strain during experimental fish infection and provides new knowledge regarding the course of infection.

Activity of metabolic enzymes in farmed rainbow trout Oncorhynchus mykiss Walb. Affected by bacterial septicemia: the effect of feed additives

The effect of feed additive including antioxidant dihydroquercetin and polysaccharide arabinogalactan on the activity of metabolic enzymes in muscles and liver of artificially grown rainbow trout Oncorhynchus mykiss Walb., affected by bacterial infection,was investigated. The results of the study indicated an increase in the resistance of trout to the action of bacterial infection with the enrichment of the diet with the studied bioactive components, including, apparently, through the activation of metabolic pathways of synthesis of energy and reducing equivalents.

Activity of Metabolic Enzymes in Farmed Rainbow Trout Oncorhynchus mykiss Walb. Affected by Bacterial Septicemia: The Effect of Food Additives.

The effect of food additive including antioxidant dihydroquercetin and polysaccharide arabinogalactan on the activity of metabolic enzymes in muscles and liver of artificially grown rainbow trout Oncorhynchus mykiss affected by bacterial infection was investigated. The results of the study indicated an increase in the resistance of trout to the action of bacterial infection with the enrichment of the diet with the studied bioactive components, apparently mediated, among other factors, by the activation of metabolic pathways of synthesis of energy and reducing equivalents.

Genomic Diversity and Evolution of the Fish Pathogen Flavobacterium psychrophilum.

Flavobacterium psychrophilum, the etiological agent of rainbow trout fry syndrome and bacterial cold-water disease in salmonid fish, is currently one of the main bacterial pathogens hampering the productivity of salmonid farming worldwide. In this study, the genomic diversity of the F. psychrophilum species is analyzed using a set of 41 genomes, including 30 newly sequenced isolates. These were selected on the basis of available MLST data with the two-fold objective of maximizing the coverage of the species diversity and of allowing a focus on the main clonal complex (CC-ST10) infecting farmed rainbow trout (Oncorhynchus mykiss) worldwide. The results reveal a bacterial species harboring a limited genomic diversity both in terms of nucleotide diversity, with ~0.3% nucleotide divergence inside CDSs in pairwise genome comparisons, and in terms of gene repertoire, with the core genome accounting for ~80% of the genes in each genome. The pan-genome seems nevertheless “open” according to the scaling exponent of a power-law fitted on the rate of new gene discovery when genomes are added one-by-one. Recombination is a key component of the evolutionary process of the species as seen in the high level of apparent homoplasy in the core genome. Using a Hidden Markov Model to delineate recombination tracts in pairs of closely related genomes, the average recombination tract length was estimated to ~4.0 Kbp and the typical ratio of the contributions of recombination and mutations to nucleotide-level differentiation (r/m) was estimated to ~13. Within CC-ST10, evolutionary distances computed on non-recombined regions and comparisons between 22 isolates sampled up to 27 years apart suggest a most recent common ancestor in the second half of the nineteenth century in North America with subsequent diversification and transmission of this clonal complex coinciding with the worldwide expansion of rainbow trout farming. With the goal to promote the development of tools for the genetic manipulation of F. psychrophilum, a particular attention was also paid to plasmids. Their extraction and sequencing to completion revealed plasmid diversity that remained hidden to classical plasmid profiling due to size similarities.

Morphological and free amino acid profile variability, as a tool for stock identification among farmed rainbow troutOncorhynchus mykissof different origin

As a tool for stock identification the body morphology and the free amino acid (FAA) profile variability were determined among farmed rainbow trout Oncorhynchus mykiss (Walbaum 1792) from different geographical areas throughout Greece, using geometric morphometrics and Reversed Phase-High-Performance Liquid Chromatography. Significant intraspecific body shape variation was detected among the fish stocks of different origin. Canonical Variate Analysis distinguished populations into two groups being consistent to a great extent with a previous population genetics study. General linear models did not support an association of haplotypes and diet with shape. However, shape was significantly correlated with origin, water temperature, geological-chemical-climatic zones, histidine and arginine. Therefore, morphological variation was primarily driven by environmentally induced differences among these zones being the result of phenotypic plasticity. Environmental cues and rearing conditions played the pivotal role in comparison to broodstock origin. Specimens presented different FAA profiles depending on their geographical origin. Principal Component Analysis showed a division between eastern and western Greece which might be owed to the existence of climatic fluctuations affecting the abiotic factors. This combined approach offers an important tool for stock identification thus assisting environmental risk assessments in evaluation of potential ecological effects and fostering research excellence in sustainable aquaculture management.

Microarray-predicted marker genes and molecular pathways indicating crowding stress in rainbow trout (Oncorhynchus mykiss)

Stocking density is considered as a key factor determining the productivity of fish aquaculture systems. The transcriptomic response to crowding stress is, however, still poorly investigated. We aimed to identify potential biomarker genes via microarray analyses to gain insight into molecular pathways modulated by density-induced stress in farmed rainbow trout Oncorhynchus mykiss. Transcriptome profiling in the liver, kidney, and gills was complemented by analysis of classical plasma parameters and behavioural observations. Individuals of two trout strains were exposed for nine days to definite stocking densities, ~10kg/m3 (low density); ~30kg/m3 (moderate); ~60kg/m3 (elevated); and ~120kg/m3 (high). Only the PYGMA gene, encoding the key enzyme of glycogenolysis, was upregulated in gills and kidney, while about 90 genes were differentially regulated in the liver of trout exposed to high density compared with the moderate-density group. Pathway enrichment analyses predicted the upregulation of HIF1A signalling in liver, contributing to glucose homeostasis during stress conditions. This is consistent with the increased plasma glucose levels detected in crowding-stressed trout. The mTOR and PI3K/AKT signalling pathways were impaired in liver at higher stocking densities indicating stress at the cellular level. Contrary to our expectations, protein ubiquitination and cholesterol biosynthesis pathways were found to be underexpressed. Cholesterol is a precursor of the stress hormone cortisol, whose plasma concentrations were indeed within the normal physiological range in all treatment groups. In addition, we did not observe any aggressive interactions, which are known to be triggered through cortisol release. Taken together, we identified a relatively modest number of rainbow trout-specific crowding markers suggesting that farmed rainbow trout have gained enhanced stress-tolerant phenotypes through decades of selective breeding. Statement of relevanceThe comprehensive recording of animal-based biomarkers is becoming increasingly important for the further development of the aquaculture industry with importance for fish well-being and consumer opinion. Previous studies on crowding stress in rainbow trout have used single or multi-gene approaches to assess the relevance of putative biomarkers, but transcriptomic approaches to define crowding stress-induced pathways in salmonids are overdue. The present study compares the transcriptomic profiles of rainbow trout exposed to different stocking densities and correlates these data sets with classical plasma parameters and behavioural observations.

Using physiological and zootechnical profiles to evaluate welfare in farmed rainbow trout Oncorhynchus mykiss (Walbaum) under stressful conditions

The welfare of farmed animals, and more recently of farmed fish, is a growing concern in current society. Changes in living conditions represent one of the most stressful situations in fish production. It is crucial to develop tools to evaluate welfare in farmed fish throughout the various situations observed during the production cycle. The present work aims to select relevant parameters in order to evaluate the welfare of trout, by describing the physiological and zootechnical profiles of rainbow trout Oncorhynchus mykiss (Walbaum) populations during adaptation to a new environment. Ten days after transferring seven groups of farmed trout to a laboratory setting, a principal component analysis was performed on five parameters: plasma cortisol level (PCL), hepatosomatic index (HI), growth (G), food intake (FI) and food conversion ratio (FCR). PCL and FCR were positively correlated and were both negatively correlated with FI and G. FI and G were positively correlated. HI was not correlated with any other parameter. PCL was considered as being at basal levels. Descriptive statistics and the principal component analysis suggest measuring PCL, G, FI and FCR to be a relevant strategy for evaluating the welfare of trout when adapting to a new environment. The study demonstrates the relevance of this approach in evaluating specific profiles related to the welfare of farmed animals.

Susceptibility of Koi and Yellow Perch to infectious hematopoietic necrosis virus by experimental exposure.

Infectious hematopoietic necrosis virus (IHNV) is a novirhabdoviral pathogen that originated in western North America among anadromous Pacific salmonids. Severe disease epidemics in the late 1970s resulting from IHNV's invasion into farmed Rainbow Trout Oncorhynchus mykiss in North America, Asia, and Europe emphasized IHNV's ability to adapt to new hosts under varying rearing conditions. Yellow Perch Perca flavescens and Koi Carp Cyprinus carpio (hereafter, "Koi") are aquaculture-reared fish that are highly valued in sport fisheries and the ornamental fish trade, respectively, but it is unknown whether these fish species are vulnerable to IHNV infection. In this study, we exposed Yellow Perch, Koi, and steelhead (anadromous Rainbow Trout) to IHNV by intraperitoneal injection (10(6) PFU/fish) and by immersion (5.7×10(5) PFU/mL) for 7 h, and monitored fish for 28 d. The extended immersion exposure and high virus concentrations used in the challenges were to determine if the tested fish had any level of susceptibility. After experimental exposure, Yellow Perch and Koi experienced low mortality (<6%) compared with steelhead (>35%). Virus was found in dead fish of all species tested and in surviving Yellow Perch by plaque assay and quantitative reverse transcription polymerase chain reaction (qPCR), with a higher prevalence in Yellow Perch than Koi. Infectious virus was also detected in Yellow Perch out to 5 d after bath challenge. These findings indicate that Yellow Perch and Koi are highly resistant to IHNV disease under the conditions tested, but Yellow Perch are susceptible to infection and may serve as possible virus carriers.

Acute dermatitis in farmed trout: an emerging disease

A new skin condition, known as puffy skin disease (PSD), emerged in farmed rainbow trout Oncorhynchus mykiss (Walbaum) in 2002. The number of new cases increased considerably from 2006. Clinical signs include white or grey skin patches, which become raised and red with excessive mucous production and scale loss. Fish are inappetant and lose condition. Histologically, the key feature is epithelial hyperplasia. We undertook a questionnaire study of trout farmers in England and Wales to investigate prevalence and risk factors. PSD was reported on 37% (n = 49) of rainbow trout sites, located in 28 river catchments. The increase in cases from 2006 onwards was mirrored by the increase in red mark syndrome (RMS). Prevalence and severity of PSD were highest in the summer months. The presence of PSD was associated with RMS (OR = 9.7, P < 0.001). Sites receiving live rainbow trout in the previous 12 months were considerably more likely to have PSD (OR = 5.3. P < 0.01), which suggests an infectious aetiology. The size of affected fish and prevalence varied between farms, indicating that farm-level factors are important. Future research should further investigate the aetiology of PSD and practices to manage the disease.

High oxygen consumption rates and scale loss indicate elevated aggressive behaviour at low rearing density, while elevated brain serotonergic activity suggests chronic stress at high rearing densities in farmed rainbow trout

The effect of stocking density on indicators of welfare has been investigated by several studies on farmed rainbow trout Oncorhynchus mykiss. However, the densities at which welfare are compromised remain ambiguous. Here three different stocking density treatments were selected based on the results of a previous study, where levels of crowding where determined using the spatial distribution of fish in two-tank systems. An un-crowded low density of 25kgm−3, the highest density accepted by the fish without showing indications of crowding stress of 80kgm−3 as the intermediate density, and the highest density accepted by the fish showing indications of crowding stress of 140kgm−3 as the high density were investigated. The aim of the present study was to examine the effect of being held at these densities on indicators of welfare. This was achieved through oxygen consumption measurements using automated respirometry, recording fin erosion, determining scale loss and analysing plasma cortisol and brain serotonergic activity levels. The results obtained in the present study indicated that at the lowest density the fish had the space and opportunity to display their natural aggressive behaviour and that the fish held at the highest density were exposed to a situation of confinement.

Utilising spatial distribution in two-tank systems to investigate the level of aversiveness to crowding in farmed rainbow trout Oncorhynchus mykiss

In aquaculture, fish are exposed to a range of unfavourable environmental conditions. Amongst these, stocking density has attracted considerable attention as inappropriate densities may compromise welfare and negatively impact production. However, the recommendations for stocking remain elusive. The aim of the present study was to apply a novel method to investigate a level of crowding that indicated aversiveness in rainbow trout (Oncorhynchus mykiss). In a two-tank system, where two identical tanks were connected via a doorway, it was observed that social behaviour controlled the distribution of the fish between the tanks. Fish were stocked at equal quantities in each tank of the system. The doorway was opened and the fish moved between the two tanks. Typically, this resulted in one tank being occupied by a few highly aggressive dominant individuals (“dominant” tank) and the majority of the fish occupying the second tank (“crowded” tank). Here, the potential of this unequal spatial distribution for quantifying aversion to crowding was explored. Fish were stocked in three two-tank systems at a total density of 20, 40 and 80kgm−3 respectively. The number of fish in each tank was determined every three days throughout the duration of the experiment and the percentage of fish in the “crowded” tank was used as an indicator of the distribution pattern in the two-tank systems. The results indicated a negative relationship between the total density stocked (20, 40 and 80kgm−3) and the percentage of fish in the “crowded” tank. A subsample of individuals was sacrificed for blood and brain samples every three days from the “crowded” tank, prior to the fish count. The neuroendocrine indicators of stress, elevated serotonergic activity levels which were not associated with high plasma levels of cortisol, suggested chronic stress in the fish at the highest total density stocked (80kgm−3). Taken together, these results indicated that a level of aversiveness to crowding had been reached at the highest density stocked, where the mean absolute density, irrespective of time of day, observed in the “crowded” tank was 126.5±3.7kgm−3.

Experimental Infection of Australian Freshwater Fish with Epizootic Haematopoietic Necrosis Virus (EHNV)

The ranavirus, epizootic hematopoietic necrosis virus (EHNV), is endemic to southern Australia with natural outbreaks resulting in mass mortality events in wild Redfin Perch Perca fluviatilis (also known as Eurasian Perch) and less severe disease in farmed Rainbow Trout Oncorhynchus mykiss. To further investigate the host range for EHNV, 12 ecologically or economically important freshwater fish species from southeastern Australia were exposed experimentally to the virus. A bath-challenge model at 18 ± 3°C was employed with limited use of intraperitoneal inoculation to determine if a species was likely to be susceptible to EHNV. Of the species tested, Murray-Darling Rainbowfish Melanotaenia fluviatilis and Dewfish Tandanus tandanus (also known as Freshwater Catfish) were considered to be potentially susceptible species. EHNV was isolated from approximately 7% of surviving Eastern Mosquitofish Gambusia holbrooki, indicating this widespread alien fish species is a potential carrier. The infection of Silver Perch Bidyanus bidyanus and Macquarie Perch Macquaria australasica and the lack of infection in Murray Cod Maccullochella peelii peelii and Golden Perch Macquaria ambigua ambigua after exposure to EHNV via water confirmed earlier data from Langdon (1989). Five other species of native fish were potentially not susceptible to the virus or the fish were able to recover during the standard 35-d postchallenge observation period. Overall, it appeared that EHNV was less virulent in the present experimental model than in previous studies, but the reasons for this were not identified. Received May 21, 2012; accepted November 1, 2012.

Isolation and Characterization of Flavobacterium johnsoniae from Farmed Rainbow Trout Oncorhynchus mykiss

Flavobacterium johnsoniae was isolated from farmed rainbow trout Oncorhynchus mykiss in Korea, and its biochemical and molecular characterization was determined. Yellow-pigmented bacterial colonies were isolated from 18 of 64 fish samples (28.1%) on trypticase soy agar plates, and their biochemical profiles were characterized by API 20E and API 20NE test kits. F. johnsoniae was identified by biochemical phenotyping of factors including rapid gliding motility, Gram-negative condition, oxidase- and catalase-positive status, Congo red absorption, nitrate reduction, <TEX>${\beta}$</TEX>-galactosidase production, acid production from glucose, and gelatin and casein hydrolysis. PCR and subsequent sequencing of 16S rRNA confirmed that the yellow-pigmented colonies were most similar to F. johnsoniae. The alignment analysis of 16S rRNA sequences also showed that all 18 rainbow trout isolates had highly similar homologies (97-99% identity). One isolate was selected and named FjRt09. This isolate showed 98% homology with previously reported F. johnsoniae isolates, and in phylogenetic analysis was more closely grouped with F. johnsoniae than with F. psychrophilum, F. columnare, or F. branchiophilum. This is the first report on the occurrence and biochemical characterization of F. johnsoniae isolated from rainbow trout in Korea.

Phenotypical and genetic characterization of Yersinia ruckeri strains isolated from recent outbreaks in farmed rainbow trout Oncorhynchus mykiss (Walbaum) in Peru

A total of 30 strains of Yersinia ruckeri causing recent outbreaks in Peruvian trout culture systems, were studied by means of biochemical characteristics, serology, lipopolysaccharide (LPS) and outer membrane protein (OMP) analysis, and ERIC and REP PCR fingerprinting. All the Peruvian isolates were found to be fermentative, oxidase negative and positive for decarboxylation of lysine and ornithine and utilization of glucose and mannitol, allowing their presumptive identification as Y. ruckeri. Sequencing of the 16S rRNA gene confirmed that isolates were indeed Y. ruckeri (> 99.98% identity). Although most of the strains studied were motile and lipase positive corresponding to the biotype 1 of Y. ruckeri, 5 of these strains were negative from both tests, being identified as biotype 2. In addition, drug susceptibility tests determined high sensitivity to sulfamethoxazole/trimethoprim, oxytetracycline, ampicillin and enrofloxacin in all the isolates. Serologically, all the Peruvian strains studied were identified as belonging to the serotype O1 subgroup a. Analysis of the lipopolysaccharide (LPS) as well as total and outer membrane proteins (OMPs) profiles and the correspondent inmunoblotting, supported these results. Genotyping performed by means of ERIC- and REP-PCR determined major correlation of the Peruvian isolates with the type strain NCIMB 2194 T regardless of the biotype.

Isolation and Distribution of Bacterial Flora in Farmed Rainbow Trout from Mexico

Trout farming is a growing aquaculture industry in Mexico, with stock mainly supplied by the importation of eyed eggs. The aim of the present study was to determine the frequency of bacterial isolations in farmed rainbow trout Oncorhynchus mykiss from Mexico. Sixty-five farms distributed among seven states of Mexico were included in the study. Individual samples from gills, liver, spleen, intestine, and kidney were obtained from 563 apparently healthy fish. In total, 371 bacterial isolates were recovered from sampled fish; isolates of the genera Aeromonas, Edwardsiella, Enterobacter, Escherichia, Klebsiella, Plesiomonas, Pseudomonas, and Yersinia were identified. Aeromonads were the most frequently isolated bacteria. Renibacterium salmoninarum was not isolated from any of the sampled fish. Our results showed the presence of bacteria that are potential pathogens of both rainbow trout and humans.

Recruitment and effects of Discocotyle sagittata (Monogenea) infection on farmed trout

Farmed rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta were monitored over 3 years for infection with the blood-feeding gill fluke Discocotyle sagittata. Parasite transmission is seasonal: new infections take place during summer/autumn, and transmission is generally negligible during winter/spring. There are 2 sources of infection for naïve fish-of-the-year: limited invasion when fish are in the raceways by riverborne larvae originating external to the farm; and internally, within the farm, when 0+ fish are transferred to ponds previously occupied by older cohorts of infected fish. Thereafter, infection levels continue to increase in rainbow trout primarily through transmission within the farm. Prevalence rose to 100% in 1+ fish by the end of their second summer. In O. mykiss, mean abundance reached 194 worms/host for 1+ fish (up to 489 worms/host) and 160 worms/host for 2+ fish. By contrast, in S. trutta, parasite prevalence never exceeded 85% and, after the first year's invasions, infection levels decreased over time: in 1+ and 2+ brown trout, parasite mean abundance was <4 (maximum 15) worms/host. We present evidence of the detrimental effects of D. sagittata on the host: high burdens are associated with pale gills, decreased body condition and host mortality. Parasite burdens become overdispersed during the warmer part of the year, as prevalence and mean abundance increase. However, the degree of parasite overdispersion decreases over winter; we cannot distinguish whether decreased aggregation is due to parasite losses from infected fish (including immune-mediated parasite mortality) or parasite-induced host mortality.