Trout Fry Research Articles

Identification of a novel type II-C Cas9 from the fish pathogen Flavobacterium psychrophilum.

Flavobacterium psychrophilum is the causative agent of rainbow trout fry syndrome and bacterial cold-water disease in salmonid fish worldwide. As an important fish pathogen, F. psychrophilum is frequently exposed to multiple invading genetic elements in natural environments. Endonuclease Cas9 provides bacteria with adaptive interference against invading genetic elements. Previous studies revealed that several F. psychrophilum strains harbored a type II-C Cas9 called Fp1Cas9, but little is known about the potential role of this endonuclease against invading genetic elements. In this work, we identified a gene encoding a novel type II-C Cas9 called Fp2Cas9 from F. psychrophilum strain CN46. Through bacterial RNA sequencing, we demonstrated active transcription of both Fp2Cas9 and pre-crRNAs in strain CN46. Bioinformatics analysis further revealed that the transcription of Fp2Cas9 and pre-crRNAs was driven by a newly integrated promoter sequence and a promoter element embedded within each CRISPR repeat, respectively. To formally demonstrate that Fp2Cas9 and associated crRNAs yielded functional interference in strain CN46, a plasmid interference assay was performed, resulting in adaptive immunity to target DNA sequences in Flavobacterium bacteriophages. Phylogenetic analysis demonstrated that Fp2Cas9 was present only in several F. psychrophilum isolates. Phylogenetic analysis revealed that this novel endonuclease was probably acquired through horizontal gene transfer from the CRISPR-Cas9 system in an unidentified Flavobacterium species. Comparative genomics analysis further showed that the Fp2Cas9 was integrated into the type II-C CRISPR-Cas locus in strain CN38 instead of the original Fp1Cas9. Taken together, our results shed light on the origin and evolution of Fp2Cas9 gene and demonstrated that this novel endonuclease provided adaptive interference against bacteriophage infections.

Pathogenicity of two lineages of infectious hematopoietic necrosis virus (IHNV) to farmed rainbow trout (Oncorhynchus mykiss) in South Korea

In May 2015, a high mortality event in farmed rainbow trout occurred in Jeollabuk-do province in Korea. Histopathological analysis revealed necrosis in the kidney, liver, branchial arch, and gills of moribund fish, and infectious hematopoietic necrosis virus (IHNV) was detected in the lesions by immunohistochemistry. Cytopathic effects were observed in EPC, FHM, and RTG-2 cell lines after inoculation with kidney and spleen tissues and IHNV was detected by reverse transcription polymerase chain reaction (PCR). The amplified PCR product was sequenced, and phylogenetic analysis placed IHNV in the JRt Nagano group. Both in vivo and in vitro trials were performed to compare the virulence properties between RtWanju15 isolate, which causes 100% mortality in imported fry, and a previous isolate RtWanju09 of the JRt Shizuoka group isolated from eggs of healthy broodfish. In vivo challenge with high dose on specific pathogen free (SPF) rainbow trout fry performed in Denmark with isolates RtWanju09, RtWanju15 and DF04/99 isolates showed a survival rates of 60%, 37.5% and 52.5% (average), respectively without statistical difference. The replication efficiency of the two isolates in the in vitro challenge was similar.

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.

Validation of a QTL for Flavobacterium psychrophilum resistance in rainbow trout Oncorhynchus mykiss

Flavobacterium psychrophilum is the causative agent of Bacterial Cold Water Disease (BCWD)/ Rainbow Trout Fry Syndrome (RTFS) in rainbow trout Oncorhynchus mykiss. The disease is associated with significant mortality in trout farms and thereby responsible for severe economical losses in this part of the aquaculture industry. Vaccination of the very young life cycle stages is not successful due to the immature development of the adaptive immunity in yolksac larvae and early fry. This explains the extensive usage of antibiotics for control of BCWD/RTFS. Selective breeding of RTFS resistant fish may be a solution, and identification of markers associated with natural susceptibility/resistance to the disease may elevate breeding efforts towards more robust strains. We suggest a QTL (SNP Affx-88941461) for partial disease resistance on chromosome 25 (Omy25) based on our experimental F. psychrophilum challenges of outbred fish and subsequent GWAS analyses. The favourable SNP allele was designated Q, whereas q indicated the unfavourable allele. We validated this QTL in two subsequent challenge experiments by F. psychrophilum exposure of QQ, Qq and qq trout. In the first trial we produced trout carrying at least one favourable allele (QQ and Qq) associated with partial resistance by fertilizing trout eggs from non-selected (outbred) females with sperm from homozygous QQ males. In the second trial we also produced homozygous offspring by fertilizing trout eggs from QQ females with sperm from QQ males. The resistance profiles of these groups were then determined in the laboratory by triplicate or duplicate challenge experiments (common garden water bath exposure to F. psychrophilum), which showed a significantly higher survival in trout carrying the favourable allele. Field observations supplemented the laboratory studies. Under farm conditions QQ and Qq trout showed a higher survival rate compared to qq trout. In the present study chromosome 25 (Omy25) QTL was found associated with a partial resistance to F. psychrophilum. However, we recommend to include additional SNPs in future breeding studies because several QTL are associated with resistance.

Aerial Application of Organic Pellets Eliminates Lake Trout Recruitment from a Primary Spawning Reef in Yellowstone Lake

AbstractInvasive Lake Trout Salvelinus namaycush in the Yellowstone Lake ecosystem have been gillnetted since 1995 to suppress the population and allow for recovery of native Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri. Although gillnetting is effective (Lake Trout population growth rate λ ≤ 0.6 during 2012–2022), the effort only targets free‐swimming, age‐2 and older Lake Trout. We developed a complementary suppression method using organic (soy and wheat) pellets to cause Lake Trout embryo mortality and reduce recruitment from spawning areas. The entire Carrington Island spawning reef (0.5 ha) was aerially treated with 3.56 and 3.00 kg/m2 of pellets in 2019 and 2020, respectively. Pellet decomposition caused dissolved oxygen concentrations to decline to lethal levels at 20 cm depth in the substrate, and pellets mostly dissipated from the reef within 12 d. Lake Trout fry trap CPUE was reduced to zero after ice‐off each spring after the treatments. Prior to the treatments, 71 fry were captured during 58 trap‐nights of effort in 2017–2019. After the treatments, no fry were captured during 273 trap‐nights in 2020 and 2021. Lake Trout CPUE in large‐mesh gill nets set near Carrington Island in September did not decline during 2017–2021 and fry were again trapped on the reef in spring 2022, suggesting that adults were not deterred from spawning there in the years after the pellet treatments. Complementary methods that increase mortality of prerecruits may allow for a reduction in gill‐netting effort and the long‐term costs of maintaining Lake Trout population suppression in Yellowstone Lake. Treatment of spawning areas may improve suppression efficiency for Lake Trout and invasive fish populations elsewhere because entire cohorts are targeted while immobile and temporarily concentrated in relatively small areas.

Secreted peptidases contribute to virulence of fish pathogen Flavobacterium columnare.

Flavobacterium columnare causes columnaris disease in freshwater fish in both natural and aquaculture settings. This disease is often lethal, especially when fish population density is high, and control options such as vaccines are limited. The type IX secretion system (T9SS) is required for F. columnare virulence, but secreted virulence factors have not been fully identified. Many T9SS-secreted proteins are predicted peptidases, and peptidases are common virulence factors of other pathogens. T9SS-deficient mutants, such as ΔgldN and ΔporV, exhibit strong defects in secreted proteolytic activity. The F. columnare genome has many peptidase-encoding genes that may be involved in nutrient acquisition and/or virulence. Mutants lacking individual peptidase-encoding genes, or lacking up to ten peptidase-encoding genes, were constructed and examined for extracellular proteolytic activity, for growth defects, and for virulence in zebrafish and rainbow trout. Most of the mutants retained virulence, but a mutant lacking 10 peptidases, and a mutant lacking the single peptidase TspA exhibited decreased virulence in rainbow trout fry, suggesting that peptidases contribute to F. columnare virulence.

The efficacy of new oral vaccine feeds against Salmonid novirhabdovirus in rainbow trout

Salmonid novirhabdovirus (IHNV) causes infectious haematopoietic necrosis (IHN) in salmonid species. Despite an injectable plasmid-based DNA vaccine of the glycoprotein (G) gene is effective, there are no oral vaccines for mass vaccination of rainbow trout (Oncorhynchus mykiss) fry. Recombinant baculoviruses were generated, used in cabbage looper (Trichoplusia ni) insect larvae to produce IHNV G and IHNV G-C5a proteins. Western blotting and chemiluminescence assays confirmed the expression of recombinant proteins, which were added to the fish feeding and top-coated with unflavored gelatin binder. Commercial rainbow trout were fed with experimental diets containing either IHNV G or IHNV G-C5a proteins for 2 weeks, and boosted 4 weeks after. Four weeks post-booster, fish were challenged with IHNV by immersion. Survival upon the infection challenge was evaluated. Spleen were sampled at 7 and 14 days post infection (dpi). Non-vaccinated and IHNV G fed trout reached a mortality of 91.7 and 97.6%, and 70.9 and 88.4%, respectively at 8 and 15 dpi. The IHNV G-C5a fed group exhibited a reduced mortality of 51.2% at 8 dpi, reaching 81.7% at 15 dpi, suggesting some level of antiviral protection. The individual viral load was measured by RT-qPCR detection of IHNV N gene, showing no significant difference across experimental groups. The transcription modulation of selected immune response markers was evaluated across experimental groups, including Type I IFN-a, Mx-1, CD4, and IgM. Further study is needed to assess how new oral vaccines may become effective to mitigate IHNV pathogenesis in juvenile trout by modulating the host immune response to protect towards IHNV exposure.

Effects of Flavobacterium psychrophilum extracellular products and lipopolysaccharide antigens on the antioxidant enzyme system activity of rainbow trout (Oncorhynchus mykiss) fry

The aim of this study was to investigate the response of different doses of lipopolysaccharide (LPS) and extracellular product (ECP) antigens (25, 50, 100, 250, and 500 µg) obtained from Flavobacterium psychrophilum to changes in oxidative stress parameters (catalase [CAT], superoxide dismutase [SOD], glutathione peroxidase [GPx], glucose 6-phosphate dehydrogenase [G6PD], glutathione reductase [GR], and malondialdehyde [MDA]) in juvenile rainbow trout. For this purpose, 6 different doses of F. psychrophilum (102–106) were applied to the fish, and the lethal dose 50 % (LD50) value was determined as 1.33·104. Changes in MDA levels and antioxidant enzymes were determined by applying 25 % of the LD50 value to fish for 14 days. Accordingly, while F. psychrophilum increased GR, G6PD, GPx, and SOD activity, it decreased MDA levels and CAT activity. Initially, 25, 50, 100, 250, and 500 µg doses of ECP and LPS were applied to the fish; after 21 days, a 25 % dose of LD50 was applied to these fish. Fourteen days later, the fish were taken, and the changes in MDA levels and antioxidant enzymes were examined. As a result, it was determined that while ECP and LPS application decreased GR, G6PD, GPx, and SOD activity, there was an increase in MDA levels and CAT activity.

Flavobacterium psychrophilum: Response of Vaccinated Large Rainbow Trout to Different Strains.

Although Flavobacterium psychrophilum is recognized as the causative pathogen of rainbow trout fry syndrome (RTFS), often resulting in high fry mortality, it is also responsible for bacterial cold water disease (BCWD) in large and older rainbow trout (Oncorhynchus mykiss). These older fish do not experience high mortality, but sustain, through the shedding of bacteria, a constant infection pressure at farm level, which exposes fry to an unnecessary infection risk. We have produced and assessed the immunogenicity of an experimental injection BCWD vaccine, which may be used to decrease the shedding of bacteria from older fish. A total of 800 fish were i.p.-injected: 200 fish received the bacterin with adjuvant, 200 fish received the bacterin alone, 200 fish received adjuvant alone and 200 fish were injected with physiological saline. Blood samples were taken at day 0 and at three different time points (4, 8 and 14 weeks) post-vaccination. Plasma antibody levels were measured by ELISA for reactivity against both the homologous F. psychrophilum vaccine strain (serotype Fd) and heterologous strains (serotype Th). Significantly elevated antibody titers were found against all serotypes in vaccinated fish. Welfare parameters associated with the vaccination process were evaluated by analyzing trout plasma samples for six different biochemical parameters, but no adverse effects associated with injection were indicated. The study suggests that an injection vaccine containing formalin-inactivated whole cells of F. psychrophilum (serotype Fd), adjuvanted with FIA, may also induce protection against heterologous strains. We advocate for, as the next step, the performance of field trials evaluating if the vaccination of older rainbow trout will (1) reduce the infection pressure in farms, (2) elevate the general health level in all groups and (3) minimize F. psychrophilum infection in fry at farm level. This may reduce the need for the administration of antibiotics in all age classes.

The cohabitation of Chinook salmon (Oncorhynchus tshawytscha) with trout populations in two important recreational fishing rivers in southern Chile

A study was carried out on two important recreational fishing rivers in southern Chile, the Petrohué and the Puelo, to assess the population structure of the salmonid species inhabiting both rivers. Five river sectors were surveyed on four dates between April 2016 and February 2017. In the Petrohué River, 2400 fish were sampled (42.9% rainbow trout Oncorhynchus mykiss; 23.8% brown trout Salmo trutta, and 33.3% Chinook salmon Oncorhynchus tshawytscha), while in the Puelo River, 1972 fish were examined (51.6% rainbow trout; 30.4% brown trout and 18% Chinook salmon). Fry and fingerling stages accounted for the highest proportion of fish collected by electrofishing. In the Petrohué and Puelo rivers, rainbow trout fry and fingerlings represented 96.3 and 99.2% of the fish sampled, respectively; 96.8 and 97.1% in the case of brown trout, while for Chinook salmon, parr and pre-smolts represented 97% of the fish examined in the Petrohué River and 98.3% in the Puelo River. Rainbow and brown trout shared the same habitats and food items in both rivers, and the two oldest trout recorded 5+ and 6+years. Parr and pre-smolt Chinook salmon were present mainly in the winter and spring seasons and were aged 1+ and 2+, respectively. There were no differences in the food items recorded from the juvenile fish sampled in both rivers. The mature, returning adult Chinook salmon aged between 1.5+ and 1.6+ years were devoid of food and were recorded over the autumn season with the highest abundance in the Petrohué River. The study did not identify any adverse competition or negative interactions among the three main salmonid species sharing the same habitat.

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.

Thermal tolerance in cutthroat trout of the southern Rocky Mountains

With temperatures expected to rise across the southern Rocky Mountains, the ability of native fishes to tolerate stream warming has become a critical concern for those tasked with preserving coldwater species. We used common garden experiments to evaluate the thermal tolerance of cutthroat trout ( Oncorhynchus clarkii) fry from five populations important to managers representing three sub-species. Critical thermal maxima (CTMs) were evaluated through traditional exposure trials, while optimal growth and ultimate upper incipient lethal temperatures (UUILTs) were examined over the course of 21-day trials at six static temperature treatments. Whereas CTMs differed among populations (mean = 27.91 °C, SD = 0.35 °C), UUILTs did not (mean = 24.40 °C, SD = 0.04 °C). Comparison of cubic temperature-growth functions to the traditional quadratic functions showed that adding a third-order term for temperature can improve model fit, and revealed substantial differences in optimal growth temperatures (15.4–18.3 °C). Knowledge of these thermal tolerance thresholds will help to predict the consequences of a warming climate, identify suitable habitats for repatriation, and inform water quality temperature standards established to protect these fish into the future.

Lowered nutritional quality of prey decrease the growth and biomolecule content of rainbow trout fry.

Diet quality is crucial for the development of offspring. Here, we examined how the nutritional quality of prey affects somatic growth and the lipid, carbohydrate, protein, amino acid, and polyunsaturated fatty acid content of rainbow trout (Oncorhynchus mykiss) fry using a three-trophic-level experimental setup. Diets differed especially in their content of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are physiologically essential polyunsaturated fatty acids for a fish fry. Trout were fed with an artificial diet (fish feed, DHA-rich), marine zooplankton diet (krill/Mysis, DHA-rich), or freshwater zooplankton diet (Daphnia, Cladocera, DHA-deficient). The Daphnia were grown either on a poor, intermediate, or high-quality algal/microbial diet simulating potential changes in the nutritional prey quality (EPA-content). Trout fed with the fish feed or marine zooplankton entirely replaced their muscle tissue composition with compounds of dietary origin. In contrast, fish tissue renewal was only partial in fish fed any Daphnia diet. Furthermore, fish grew five times faster on marine zooplankton than on any of the Daphnia diets. This was mainly explained by the higher dietary contents of arachidonic acid (ARA), EPA, and DHA, but also by the higher content of some amino acids in the marine zooplankton than in the Daphnia diets. Moreover, fatty acid-specific carbon isotopes revealed that trout fry could not biosynthesize ARA, EPA, or DHA efficiently from their precursors. Our results suggest that changes in the zooplankton and macroinvertebrate communities' structure in freshwater habitats from DHA-rich to DHA-poor species may reduce the somatic growth of fish fry.

Assessment of the impact of density on the growth of fry (Oncorhynchus mikiss) at the salmon farming station in the Middle Atlas Morocco

The objective of this study is to determine the impact of the load of the nursery ponds (density) on the growth of the rainbow trout fry (Oncorhynchus mikiss), this study was conducted at the fish farming station of Ras Al Ma Ifrane, Morocco. The results obtained for the densities D1 (7.48 Kg / m3), D2 (9.31 Kg / m3) and D3 (11.21 Kg / m3) during this nursery phase which lasted 30 days, the results indicate that the zootechnical parameters evaluated vary according to the density of the fry in different nursery ponds. In average, the final weight of the fry for D1 is 12.73g, for D2, the final average weight is 11.39g against only 9.56g for ponds with density D3. As such, the growth in size of the fry evolves in the same way as the growth in weight.

Effects of Potassium Diformate (KDF) on Growth Performance and Some Physiological Parameters of Cultured Rainbow Trout (Oncorhynchus mykiss) in Commercial–Scaled Net Cages

This study was conducted to evaluate the feed utilization and protein effect ratios of combined and single diet administration of potassium di formate (KDF) in juvenile rainbow trout (Oncorhynchus mykiss). 600.000 trout fry were stocked as 50.000 in each cage and were fed 3 meals a day in the range of 25-80 grams and two meals a day up to 80-250 grams (until harvest time) and fed with commercial feeds until satiation. 3 kg/ton (T1), 5kg/ton (T2) and 7 kg/ton (T3) KDF were added to the diets and given to fish for 11 months. In the study, when the specific growth rates (SGR) of fish were compared, the results obtained in the T1, T2 and T3 groups were generally higher than in the control group. While no significant difference was found between T1 and T2 groups (p>0.05), there is a significant difference between T3, the other experimental groups and the control (p<0.05). In the study, a significant difference was found between the feed conversion rate (FCR of all groups, (p<0.05)). When the protein efficiency ratio (PER) was compared to the control group and the other groups, a significant difference was detected, while the same situation was found among the KDF supplemented groups (p<0.05). While the survival rate (SR) was significantly different between the control and experimental groups (p<0.05), there was no statistical difference between the experimental groups (p>0.05). The results of the study showed that the application of 7 kg/ton KDF can be considered a useful feed additive and a growth promoter for rainbow trout juveniles.

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.

Egg Hatchability and Egg Thiamine Concentrations in Lake Trout from the Northern Refuge of Lake Michigan

AbstractThe Northern Refuge of Lake Michigan was established in 1985 under the auspices of the Great Lakes Fishery Commission as part of an intensely managed effort to restore a self‐sustaining Lake Trout Salvelinus namaycush population to the lake. Despite the stocking of millions of Lake Trout into the refuge and survival of these stocked fish to adulthood, signs of natural recruitment by Lake Trout in the refuge have been very sparse. To ascertain whether low egg viability or poor fry survival could be limiting natural recruitment by Lake Trout in the refuge, hatching rates of eggs taken from Lake Trout caught in the Northern Refuge during October 2019 were determined in the laboratory. Moreover, survival rates of the hatched‐out fry were determined along with the thiamine concentrations in the fry. In addition, thiamine concentrations in eggs of ovulating Lake Trout caught in the Northern Refuge during 2019–2021 were determined. Egg hatching rate in the laboratory averaged 93%. Survival rate of Lake Trout fry up to 5 weeks after hatching averaged 97%. Mean (SE) thiamine concentrations in eggs of Lake Trout caught in 2019, 2020, and 2021 were 19.1 (3.6), 5.9 (1.3), and 6.2 (0.4) nmol/g, respectively; these levels are well above the threshold concentration of 2.3 nmol/g, above which the likelihood of experiencing mortality due to thiamine deficiency complex (TDC) is low. Thiamine concentrations in the Lake Trout fry at the end of the laboratory experiment averaged 11.2 nmol/g, and abnormal behavior in the fry was not observed. We conclude that TDC does not appear to represent a major impediment to Lake Trout rehabilitation in the Northern Refuge at present, although TDC may have contributed to the lack of detectable natural recruitment in the past.

Time-course transcriptome analyses of spleen in rainbow trout (Oncorhynchus mykiss) post-Flavobacterium psychrophilum infection.

Flavobacterium psychrophilum, the etiological agent of bacterial coldwater disease and rainbow trout fry syndrome, causes considerable losses in salmonid aquaculture globally. Systemic F. psychrophilum infections in rainbow trout (Oncorhynchus mykiss) lead to a range of clinical signs, including ulcerative lesions in the skin and muscle and splenitis. Previous studies offered an integrative analysis of the skeletal muscle response to F. psychrophilum infection in rainbow trout. However, little is known about the molecular mechanism of immune response in the spleen, which is an important immune organ of rainbow trout. Here, we investigated the time-course splenic transcriptome profiles in uninfected rainbow trout (CK) and F. psychrophilum–infected rainbow trout at day 3 and day 7 (D3, D7) by RNA-seq analyses. Among the 7,170 differentially expressed genes (DEGs) in the three comparisons (D3 vs. CK, D7 vs. CK, D3 vs. D7), 1,286 DEGs showed consistent upregulation or downregulation at D3 and D7 and were associated with pattern recognition, acute-phase response, complement cascade, chemokine and cytokine signaling, and apoptosis. The Real time quantitative PCR (RT-qPCR) analysis of eight DEGs confirmed the accuracy of the RNA-Sequencing (RNA-seq) data. Our results reflected a general process from pathogen recognition to inflammatory cytokine generation and delineated a putative Toll-like receptor signaling pathway in rainbow trout spleen, following F. psychrophilum infection. Taken together, these results provide new insights into the molecular mechanism of the immune response to F. psychrophilum infection and are a valuable resource for future research on the prevention and control of bacterial coldwater disease during salmon culture.

Reversible mutations in gliding motility and virulence genes: A flexible and efficient phage defence mechanism in Flavobacterium psychrophilum.

Flavobacteria are among the most important pathogens in freshwater salmonid aquaculture worldwide. Due to concerns regarding development of antibiotic resistance, phage therapy has been proposed as a solution to decrease pathogen load. However, application of phages is challenged by the development of phage resistance, and knowledge of the mechanisms and implications of phage resistance is therefore required. To study this, 27 phage-resistant isolates of F. psychrophilum were genome sequenced and characterized to identify genetic modifications and evaluate changes in phenotypic traits, including virulence against rainbow trout. Phage-resistant isolates showed reduction or loss of gliding motility, proteolytic activity, and adhesion to surfaces, and most isolates were completely non-virulent against rainbow trout fry. Genomic analysis revealed that most phage-resistant isolates had mutations in genes associated with gliding motility and virulence. Reversal of these mutations in a sub-set of isolates led to regained motility, proteolytic activity, virulence and phage susceptibility. Although costly, the fast generation of phage resistance driven by single, reversible mutations likely represents a flexible and efficient phage defence mechanism in F. psychrophilum. The results further suggest that phage administration in aquaculture systems to prevent F. psychrophilum outbreaks selects for non-virulent phage-resistant phenotypes.

Stress response of rainbow trout fry after exposure to an elevated concentration of carbon dioxide based on transcriptome analysis

Stress response of rainbow trout fry after exposure to an elevated concentration of carbon dioxide based on transcriptome analysis