Strains Of Rainbow Trout Research Articles

Ancestry and genetic structure of resident and anadromous rainbow trout (Oncorhynchus mykiss) in Argentina.

Since the first introduction from North America more than a century ago, rainbow trout (Oncorhynchus mykiss) have rapidly established self-sustaining populations in major river basins of Patagonia. Many generations later, only the freshwater resident life history is expressed in the Chubut and Negro rivers of northern Argentinian Patagonia, whereas both the resident and anadromous life histories are found in the Santa Cruz River of southern Argentina. Despite previous studies that have tried to identify the sources of these introduced populations, uncertainty still exists. Here we combined data from many single-nucleotide polymorphisms and microsatellite loci in O. mykiss populations from Argentina and North America to evaluate putative source populations, gene flow between Argentinian river basins, and genetic diversity differences between Argentinian and North American populations. We found that populations from northern and southern Patagonia are highly differentiated and have limited gene flow between them. Phylogeographic analysis also confirmed that they have separate origins, with the northern populations most closely related to the domesticated rainbow trout strains that are raised worldwide and the Santa Cruz River populations most closely related to North American populations from California and Oregon that have an anadromous component. In addition, fish with different life histories in the Santa Cruz River were found to constitute a single interbreeding population. No evidence was found of reduced genetic variation in introduced rainbow trout, suggesting multiple contributing sources. In spite of these advances in understanding, significant questions remain regarding the origins and evolution of the introduced O. mykiss in Patagonia.

Generation and genetic analysis of a rainbow trout (Oncorhynchus mykiss) clonal line produced by gynogenesis

The efficiency of heat-shock treatments for producing gynogenetic double haploids was evaluated using a rainbow trout (Oncorhynchus mykiss) strain locally adapted in Brazil (2002 broodstock). Eleven and 15 females produced by conventional reproduction (FC) or meiotic gynogenesis (FG) were used as egg donors, respectively. Oocytes from each female were randomly divided into five uneven lots which were submitted to different treatments: (C2n) Normal diploid control, (H) Haploid control, (G-Me) Meiotic gynogenetic, (G-Mi) Mitotic gynogenetic, and (4 n) Control of suppression of the 1st cell cleavage. Homozygous yellow-colored (Dominant albino - DA) males were used as semen donors. Gynogenesis was induced by insemination of eggs with DA semen genetically inactivated by ultraviolet irradiation. Diploidization of gynogenic individuals was induced by an early heat shock to block 2nd polar body extrusion (G-Me), or a late heat shock to suppress first mitosis (G-Mi). Overall mean survival rates to eyed-egg and first feeding stages of treated groups, relative to respective C2n control group were: 19.21 and 14.90%, 13.02 and 8.09%, and 29.94 and 16.48%, for treatments G-Me, G-Mi and 4 n, respectively. When compared with C2n, all treatments showed lower survival rates at both stages. Survival of G-Me and G-Mi progeny from FG females were significantly higher when compared to progeny from FC females for both stages. Progenies from two different females observed to be 100% homozygous in analyzed informative microsatellite markers were used in subsequent cycles of gynogenic reproduction. One line originally derived from a single gynogenic female was reproduced for nine successive generations of meiotic gynogenetic reproduction. Forty animals from the ninth successive generation of gynogenetic reproduction were analyzed with a low-density SNP (Single Nucleotide Polymorphism) panel. All individuals were observed to be homozygous for identical alleles at all SNPs, therefore indicating that the established mitotic gynogenetic line is composed of 100% clonal individuals.

Comparative Growth Performance of Genetically Improved, Chinese, and Local Strains of Rainbow Trout (Oncorhynchus mykiss) in Mid-hill Nepal

Aims: This study aimed to evaluate the growth performance of rainbow trout fry (Oncorhynchus mykiss) derived from the best performing families in a previous experiment.
 Study Design: Experimental (Original Research Article).
 Place and Duration of Study: Fisheries Research Station (FRS), Trishuli Nepal, between April 2021 and June 2021.
 Methodology: A total of 2700 fry with similar initial weights were randomly assigned to three groups based on their origin: Farmers Trout (T1:1.39±0.16g), Chinese Trout (T2: 1.40±0.06g), and Genetically Improved Trout (T3: 1.19±0.1g), each group replicated thrice with 300 fish per tank. The parameters assessed were weight gain, feed conversion ratio, and survival rate. All fish were fed farm-made feed for 90 days with fortnightly sampling.
 Results: The Genetically Improved trout group exhibited the highest weight gain (14.22±0.51g), followed by the Chinese trout (13.08±0.24g), and Farmers trout (10.77±0.82g) groups (P=.01). The specific growth rate followed a similar trend, with the Genetic group showing the highest value of 4.28±0.08% and the Farmers group showing the lowest value of 3.60±0.07% (P=.01). However, there was no significant difference in the feed conversion ratio among the groups, which ranged from 0.77 to 1.07. Conversely, the Chinese group's fry demonstrated a significantly higher survival rate (77.94±3.63%) compared to the Genetic (72.33±1.09%) and Farmers (59.28±4.6%) groups (P=.02).
 Conclusion: The weight gain and growth rate of genetically enhanced rainbow trout is considerably higher and may enhance the production and profitability of the rainbow culture in Nepal. However, the performance of genetically improved trout requires more validation in farmer’s raceways.

Effectiveness of Mysis diluviana as a Substitute for Fishmeal in Formulated Aquafeed for Fingerling Rainbow Trout

The present study explored the potential of wild Mysis diluviana, a widespread freshwater crustacean that is also invasive in many lakes, to serve as a substitute for fishmeal in the formulation of salmonid aquafeed. M. diluviana, harvested from Okanagan Lake in British Columbia, Canada, had a high protein content, with crude protein making up 53% of its dry body weight; lipids formed 32% of dry M. diluviana body weight. Fingerlings from two distinct strains of rainbow trout were fed for 60 days with one of three diet treatments: experimental feeds in which 25% or 50% of the fishmeal ingredient was replaced with M. diluviana and a feed containing no M. diluviana. Feeds containing M. diluviana supported the same survivorship and similar or better fingerling growth as the fishmeal‐only feed, including fingerlings of a trout strain that are highly selective feeders. The amino acid and fatty acid composition of fingerling tissues at the end of the experiment was mostly indistinguishable among the three feed treatments. In addition, the tissue composition of fingerlings, such as the EAA : NEAA ratio and the amounts of SFA, MUFA, and PUFA, did not fully match the biochemical composition of their feeds, likely due in part to the capacity of freshwater salmonids for bioconversion of some amino acids into other less available amino acids. M. diluviana was therefore found to be an effective source of protein and lipid for fingerling rainbow trout and an effective substitute for marine fishmeal in trout aquafeed.

Trout Production in District Ganderbal – An Analysis of Economic Challenges

The present study carried during the year 2022-23, worked on the impact of the introduction of genetically improved strains of rainbow trout on the economy of the fish farmers, in comparison to the normal strains. The economy of 40 farmers in groups of 20 Normal stock (NS) and Genetically Improved Stock (GIS) each was conducted and interviewed using a pre-tested questionnaire. The investigation showed that fixed capital investment on sample farm was Rs.1.25 lakh per raceway in both the set ups. The major share of investment was for the construction of raceway, which accounted for 68.53% of total investment on sample farms. The share of total variable cost was 58.37% in NS and 48.50 in GIS per cent and fixed cost was 41.62 in NS and 51.49 in GIS group. Feed was the major cost component accounting for about 46.43 per cent of the total cost in NS, while as it was reduced to 37.15% in GIS, thereby encouraging as a key factor in deciding profitability of trout farming. Resource use efficiency showed that feed was overutilized while labor was under-utilized in NS, while as it was optimally utilized in GIS group. The returns to scale in trout production of 1.25 indicates increasing returns to scale in trout production. High price of feed and seed are found to be the major constraints in trout culture. The B-C ratio worked out to be 1.37 in NS group and 1.87 in GIS group, indicating the economic feasibility of trout farming in the Kashmir valley, and better economic profile of farmers farming GIS strain.

Standard metabolic rate differs between rainbow trout (Oncorhynchus mykiss) growth forms

In variable environments, repeatable phenotypic differences between individuals provide the variation required for natural selection. The pace-of-life syndrome (POLS) provides a conceptual framework linking individual physiology and life histories to behaviour, where rapidly growing individuals demonstrate higher rates of resting or “standard” metabolic rate (SMR). If differences in SMR are consistent between fast- and slow-growing individuals, these differences may be important to capture in bioenergetic relationships used to describe their growth, energy acquisition, and allocation. We compared growth rates and SMR between a domesticated and wild strain of rainbow trout ( Oncorhynchus mykiss (Walbaum, 1792)) using intermittent flow respirometry. Though mass-scaling exponents were similar between strains, mass-scaling coefficients of SMR for fast-growing rainbow trout were 1.25 times higher than those for slower growing fish. These observed differences in mass-scaling coefficients between fast- and slow-growing rainbow trout were consistent with data extracted from several other studies. Bioenergetic estimates of consumption for domestic strain fish increased as the difference in SMR and wild strain fish increased, and increased as activity level increased. Our results indicate patterns of SMR consistent with POLS, and suggest that strain-specific SMR equations may be important for applications to active populations (i.e., field observations).

Concentration, Life Stage, Feeding, Density, Flow, and Strain Effects on Formalin Sensitivity in Rainbow Trout (Oncorhynchus mykiss).

Formalin is one of the most widely used and effective chemotherapeutic compounds for treatment of fungal infections and external parasites of fish eggs and fish. However, exposure to formalin can cause mortality in eggs and fingerlings, dependent upon the concentration used and the rearing conditions in which fish are treated. Additionally, strains within a species can exhibit differential susceptibility to formalin. Four experiments were conducted to evaluate the differential sensitivity to formalin of four rainbow trout (Oncorhynchus mykiss) strains in both the egg and fingerling life stages. Eggs were exposed to concentrations of 1667, 2000, and 5000 ppm formalin, and sensitivity differed among the strains when formalin concentration exceeded 2000 ppm. Exposure to higher formalin concentrations (i.e., 5000 ppm) as eggs did not increase mortality when fish were re-exposed to concentrations of 0, 167, 250, or 500 ppm formalin at 77 mm total length (TL). Fish size affected formalin sensitivity, with larger fish (128 mm TL) exhibiting higher rates of mortality than fish ≤ 77 mm TL when exposed to 250 ppm formalin. The effects of crowding, feeding, flow, and density on the formalin sensitivity of 77 mm TL fish were also investigated. Mortality increased in fish crowded away from the inflow to prevent contact with formalin as it entered the tank, potentially the result of an increase in density index within the crowded tanks. Feeding fish on the day they were treated caused mortality to increase by 5.4 to 8.8% in fish exposed to 167 and 250 ppm formalin, respectively, and mortality differed by strain. Reducing flows by half resulted in doubled to quadrupled mortality, and increased densities resulted in increased mortality in some strains but not others. Hatchery managers should consider what effect rearing conditions and formalin concentrations might have on the strain of fish being treated, prior to large-scale treatment.

Influence of lactic-acid bacteria feed supplementation on free amino acid levels in serum and feces of rainbow trout (Oncorhynchus mykiss, Walbaum 1792)

IntroductionThe influence of intestinal microbiota, particularly lactic-acid bacteria, on digestion and nutrient metabolic activities has been extensively studied. In this research, we investigated the effects of supplementing the feed of rainbow trout (O. mykiss) with P. acidilactici or L. lactis probiotics. A Control group without probiotic supplementation was also included. The study aimed to evaluate growth, feed conversion indices, free amino acid levels in serum and feces, and the relative gene expression of amino acid solute carrier transporters (SLC).MethodsRainbow trout weighing 73.9±10 g were fed with a basal commercial diet supplemented with mono-strains of P. acidilactici or L. lactis at a dosage of 5 x 108 CFU per gram of feed for 60 days. The trout were reared in 100-liter tanks with independent closed recirculation systems, with a water replacement rate of 80% of the total volume daily and a controlled temperature of 15 ± 0.3 °C. The proximate composition of the basal diet consisted of crude protein (49.8%), fat (32.1%), fiber (1.31%), ash (13%), and moisture (8.3%) on a dry matter basis. The diet was provided at a rate of 2% of bodyweight per day.ResultsNo significant differences (p > 0.05) were observed in growth and feed conversion indices between the experimental groups. However, significant differences in free amino acid levels in feces and serum samples (p ≤ 0.05) were observed. The P. acidilactici group exhibited significantly higher levels of glutamate, lysine, proline, and tyrosine in feces samples, and higher levels of arginine, iso/leucine, phenylalanine, proline, serine, threonine, and valine in serum samples compared to the Control group. Additionally, the supplemented probiotic groups showed significant downregulation (p ≤ 0.05) of theslc6a19 and slc7a9 genes, which encode transporters for neutral and cationic amino acids.DiscussionThe autochthonous rainbow trout strain, P. acidilactici, demonstrated more pronounced effects on amino acid levels in feces and serum compared to the non-autochthonous L. lactis strain. However, the increased amino acid levels, particularly in the serum, resulting from P. acidilactici supplementation, did not lead to improved trout growth or feed conversion. Nonetheless, lactic-acid bacteria supplementation induced measurable metabolic effects, as evidenced by elevated levels of specific amino acids in the serum and feces.Future research should focus on evaluating these probiotic effects using specialized diets and considering the observed metabolic effects in this study.

The adaptation of rainbow trout to warmer water: Oxidative damage in the germinal line

Contemporary evolution was observed in a feral rainbow trout (Oncorhynchus mykiss) population of a thermal stream (Valcheta) in Northern Patagonia, in terms of juvenile thermal tolerance and preferred temperature. Other authors showed that high-temperature treatment applied to male rainbow trout juveniles increased the thermal tolerance in the next generation. This implies a high mutation rate and/or a modified epigenetic inheritance. Comparisons were made among a) a rainbow trout strain adapted in terms of upper thermal tolerance and higher preferred temperature (Valcheta stream), b) a wild temperate stream population (Guillelmo stream), and c) two temperate farmed strains. We examined: Oxidative damage (lipid peroxidation) and activities of antioxidant enzymes; Catalase (CAT), Glutathione S-Transferases (GST), and Superoxide Dismutase (SOD), in liver, testicle, and spermatozoa. Semen fatty acid composition, sperm morphology, sperm motility, and fertilization performance in samples before and after the application of cryopreservation procedures were also evaluated. The observed responses, mainly related to the sperm membrane, reinforces the idea that ROS can affect the germinal line of male rainbow trout juveniles subjected to high water temperature. Our results suggest that the acquired thermal tolerance traits may be part of a wide spectrum of novel characteristics produced as a consequence of an enhanced mutation rate and/or a different DNA methylation pattern, induced by higher temperatures and mediated by ROS.

Biotic and Abiotic Factors Affecting Short‐Term Survival of Two Age‐0 Rainbow Trout Strains in Colorado Streams

AbstractBoth biotic and abiotic factors can influence the survival and growth of age‐0 salmonids. Diseases, such as whirling disease, can also affect salmonid demographics and population dynamics. Here, we conducted a supplementary analysis and evaluated specific stream characteristics that may have been responsible for the differences in growth and survival of two whirling disease resistant Rainbow Trout Oncorhynchus mykiss strains observed by Avila et al. (2018). We used regression modeling to analyze the influence of the biotic and abiotic characteristics of nine streams on the short‐term apparent survival and growth of two Rainbow Trout strains, 5,000 German Rainbow Trout and 5,000 German Rainbow Trout × Colorado River Rainbow Trout in each stream. Akaike's information criterion (AICc) model selection was used to identify the factors that most affected short‐term survival and growth. Average stream temperature had the largest (positive) effect, βtemp = 0.060, on short‐term survival. Rainbow Trout strain, average stream temperature (βtemp = 1.55), competitor biomass (βcompetitor biomass = −0.002), and predator number (βpredator number = 0.01) additively affected short‐term growth. Our results indicate that both biotic and abiotic factors are important short‐term determinants of Rainbow Trout poststocking performance and may account for the differences in survival and growth that we observed among stocking locations.

Does Genetic Background of Rainbow Trout Impact Growth and Feed Utilisation following Fishmeal Substitution by Partly Defatted Insect Meal (Hermetia illucens) or Microalgae Powder (Arthrospira platensis)?

A growth trial was carried out to evaluate differences in feed utilisation and growth performance in response to substitution of fishmeal (FM) by either partly defatted insect meal or microalgae powder in different strains of rainbow trout. Three iso-nitrogenous and iso-energetic diets were fed to 495 juvenile rainbow trout with initial weight of 150 ± 37 g. Whereas, the control diet included 20% FM, the two experimental diets were free of FM. Instead of that, an equal amount of spray-dried Spirulina powder (SP) or partly defatted Hermetia meal (HM) was applied as alternative protein source. Feed utilisation and growth response were investigated in a commercial breed (TL) and three local strains (HK3, HK7, and HK8) over a period of 56 days using three replicates per diet. Diets were fed relative to fish body weight throughout at a constant rate of 1.0%. Although growth was comparable among diets, growth and feed utilisation differed between breeds. The strain TL tended to express the highest specific growth rate, however, associated with lower feed conversion. Protein utilisation was superior in local strains irrespective of diet, most pronounced between strains HK7 and TL. Due to large differences in initial body weights, compensatory growth might have affected the study’s outcome. Both alternative protein sources showed to be adequate to fully replace FM in diets for rainbow trout. Improved adaption to diets including high levels of HM or SP might be achieved by selective breeding.

Poststocking Survival and Myxospore Evaluation of Whirling Disease‐Resistant Rainbow Trout Strains

AbstractThe ensuing spread of whirling disease following its initial introduction in the United States in the late 1950s caused drastic declines in populations of Rainbow Trout Oncorhynchus mykiss. The German Rainbow (GR) strain has strong resistance to Myxobolus cerebralis, the parasite causing whirling disease, but also has an extensive history as a domesticated food‐production fish, resulting in traits that are presumed to be maladaptive for survival in the wild. The wild Harrison Lake Rainbow (HAR) strain exhibits reduced infection severity compared with other wild Rainbow Trout strains, but resistance is lower than the GR strain and was considered insufficient to meet management goals. Prior research suggests that crosses of GR with susceptible strains create offspring with Myxobolus cerebralis resistance and survival rates intermediate to the parental strains in lotic environments, but analysis on lentic environments with larger predators has been limited. We evaluated survival rates and infection severities of fingerling Rainbow Trout stocked into a lentic environment from four different strains with theoretically varying resistances to Myxobolus cerebralis: GR, HAR, and an F1 [GR × HAR(50:50)] and F2 [GR × HAR(75:25)] crossed strain between the GR and HAR strains. Using Seber dead‐recovery models, we found that both the HAR and GR × HAR(50:50) strains had the highest survival rate, followed sequentially by the GR × HAR(75:25) and GR. Whirling disease resistance, as estimated from infection probability and severity, increased with increasing GR strain genetic background. To reduce spore loads and increase resistance in managed Rainbow Trout populations, we suggest stocking strains like the GR × HAR(50:50) that have respectable whirling disease resistance and high survival rates, enabling wild recruitment with increased cost‐efficiency of stocking.

Stress response of juvenile rainbow trout reared at three densities

High rearing densities (kg of fish per rearing unit volume) can impact fish health and growth. This study evaluated the stress levels (as indicated by blood glucose) and growth of juvenile Shasta strain rainbow trout (Oncorhynchus mykiss) reared at three densities in 1.8-m diameter circular tanks for 111 days. Initial and final rearing densities for each treatment were: low (4.48 and 47.56 kg/m3), medium (5.96 and 60.60 kg/m3), and high (7.49 and 72.42 kg/m3). Throughout the experiment, blood glucose levels from individual fish were not significantly different among the three density treatments. At the end of the experiment, total tank weights and gain were significantly greater in the high-density treatment than the medium-density treatment, which in turn was significantly greater than the low-density treatment. However, percent gain was significantly greater in the low-density treatment. Feed conversion ratio was significantly lower in the high-density treatment compared to the low-density treatment, with the ratio in the medium-density treatment similar to the other two treatments. The results of this study indicate that higher rearing densities do not negatively impact juvenile Shasta strain rainbow trout stress or hatchery rearing performance.

Factors Affecting the Binding of Diltiazem to Rainbow Trout Plasma: Implications for the Risk Assessment of Pharmaceuticals in Aquatic Systems.

The accumulation of organic toxicants in fish plasma, and how they partition between the bound and unbound fraction once absorbed, are important metrics in models that seek to predict the risk of such contaminants in aquatic settings. Rapid equilibrium dialysis of diltiazem, an ionizable weak base and important human pharmaceutical contaminant of freshwaters, was conducted with rainbow trout (Oncorhynchus mykiss) plasma. The effect of fed state, fish sex, fish strain/size, and dialysis buffer pH on the binding of radiolabeled diltiazem (9 ng ml-1 ) was assessed. In fed fish, 24.6%-29.5% of diltiazem was free, unbound to plasma proteins. Although starvation of fish resulted in a decrease in plasma protein, the bound fraction of diltiazem remained relatively constant. Consequently, the protein-bound concentration of diltiazem increased with length of starvation. In general, rainbow trout strain was a significant factor affecting plasma binding, although the two strains tested also differed markedly in size. Dialysis buffer pH significantly influenced plasma binding, with a higher unbound diltiazem fraction at pH 6.8 than pH 8.0. These data indicate that empirical measures of plasma binding in fish are important for accurate risk assessment and that the physiological status of a fish is likely to impact its sensitivity to toxicants such as diltiazem. Environ Toxicol Chem 2022;41:3125-3133. © 2022 SETAC.

Differences in energy utilisation between a lean and fat strain of rainbow trout (Oncorhynchus mykiss)

A good understanding of the utilisation of energy in fish diets is important for accurate feed formulation in aquaculture. One of the primary reasons for differences in the utilisation of digestible energy between fish species are differences that exist in the composition of growth (fat versus protein gain). However, it has also been observed that composition of growth can differ between genetic strains within a single fish species. The main focus of the current experiment was to investigate whether the genetic background of different strains of rainbow trout affects the relationship between digestible energy intake and retained energy. To test this, two different commercial trout strains were selected based on their differences in body fat content (a Lean-Strain and Fat-Strain) and therefore expected differences in composition of growth. Furthermore, this research investigated whether such a potential strain difference in the relationship between digestible energy and retained energy was dependent on the type of non-protein energy in the diet (a Carb-Diet versus a Fat-Diet). Three feeding levels were used in order to estimate the utilisation efficiency of digestible energy for retained energy, leading to a 2 by 2 by 3 factorial design. The results of this study showed that the relationship between digestible energy and retained energy was affected by both strain and diet, but not by an interaction effect between these two factors. Firstly, it was observed that the utilisation efficiency of digestible energy for growth (kgDE) was higher in the Fat-Strain (72% in the Lean-Strain versus 87% in the Fat-Strain) which may be related to a higher potential for fat deposition. This higher kgDE in the Fat-Strain was however balanced by a higher maintenance requirement (47 kJ/kg0.8 per day versus 28 kJ/kg0.8 per day) leading to a similar retained energy between strains in the current trial. Secondly, it was shown that the exchange of dietary carbohydrates for dietary fat on an isoenergetic basis also increased kgDE (74% for the Carb-Diet versus 85% for the Fat-Diet). The lack of an interaction effect between strain and diet showed that kgDE in both strains was affected by the exchange of carbohydrates for fat on an isoenergetic basis in a similar way. The results of the current trial demonstrated that both dietary macronutrient composition and the composition of growth of specific trout strains should be accounted for in calculating the true net available energy for fish in feed formulation.

Efficacy of Utilization of All-Plant-Based and Commercial Low-Fishmeal Feeds in Two Divergently Selected Strains of Rainbow Trout (Oncorhynchus mykiss): Focus on Growth Performance, Whole-Body Proximate Composition, and Intestinal Microbiome.

The present study aimed to investigate the growth performance, whole-body proximate composition, and intestinal microbiome of rainbow trout strains when selected and non-selected for weight gain on all-plant protein diets. A 2x2 factorial design was applied, where a selected (United States) and a non-selected (ITA) rainbow trout strain were fed using either an all-plant protein (PP) or a commercial low-FM diet (C). Diets were fed to five replicates of 20 (PP) or 25 (C) fish for 105 days. At the end of the trial, growth parameters were assessed, and whole fish (15 pools of three fish/diet) and gut samples (six fish/diet) were collected for whole-body proximate composition and gut microbiome analyses, respectively. Independent of the administered diet, the United States strain showed higher survival, final body weight, weight gain, and specific growth rate when compared to the ITA fish (p < 0.001). Furthermore, decreased whole-body ether extract content was identified in the PP-fed United States rainbow trout when compared to the ITA strain fed the same diet (p < 0.001). Gut microbiome analysis revealed the Cetobacterium probiotic-like genus as clearly associated with the United States rainbow trout, along with the up-regulation of the pathway involved in starch and sucrose metabolism. In summary, the overall improvement in growth performance and, to a lesser extent, whole-body proximate composition observed in the selected rainbow trout strain was accompanied by specific, positive modulation of the intestinal microbiome.

Differences in composition and fatty acid contents of different rainbow trout (Oncorhynchus mykiss) strains in similar and contrasting rearing conditions

Fatty acid (FA) composition and contents of fillets of seven strains of rainbow trout Oncorhynchus mykiss, reared in the same farm and sampled at the same time were studied. Three strains were compared with those from another farm as well as with outbreed and wild fish of the same fish species. In general, FA composition of farmed fish reflected their diet composition. Nevertheless, a genetics factor (belonging to a certain strain), also was found to contribute to fish FA composition. Rainbow trout of strain Steelhead appeared to have comparatively higher ability to regulate their FA composition, in particular, to retain high contents of physiologically important eicosapentaenoic (20:5n-3, EPA) and docosahexaenoic (22:6n-3, DHA) acids in biomass despite variations of their diet contents. Differences in contents of the EPA + DHA of different strains, found in this study, likely provided evidence for a potential of selective breeding of rainbow trout strains with a high EPA and DHA, which are of key importance for humans' diet. Besides, FA markers, which allow differentiating wild and farmed rainbow trout, were revealed.

Multi-tissue RNAseq reveals genetic and temporal differences in acute response to viral (IHNV) infection among three selected lines of rainbow trout with varying resistance

Utilizing RNA-seq, this study compared the transcriptomic responses of three improved strains (VSel, PSel, and CSel) of rainbow trout fry during acute stages of challenge with infectious hematopoietic necrosis virus (IHNV). The VSel strain has been selected for resistance against the specific strain of IHNV used in our challenge, PSel has undergone selection for utilization of plant-protein based feeds and previously has shown elevated non-specific disease resistance despite no disease related selection pressures, and the final strain, CSel, is a commercial strain that has been domesticated for several years but has not been selected for specific viral disease resistance. Following a 21-day IHNV challenge, Kaplan-Meier survival estimator curves and cumulative percent mortality (CPM) showed significant differences in IHNV resistance across strains: VSel - 19.3 ± 5.0%, PSel - 67. ± 3.03%, CSel - 94.6 ± 4.1% CPM. To evaluate acute responses to IHNV infection, whole blood, as well as samples from the kidney, liver, and intestine, were collected at 0, 4, 12, 24, and 48 h post infection (hpi). Serum lysozyme activity, a marker of non-specific innate immunity, showed strain and temporal effects during the acute infection phase with PSel showing the highest activity at 0 and 48 hpi. Differential gene expression responses were detected, with varying degrees, in all tissues, both between strains, as well as across acute timepoints within strains. The VSel strain showed upregulation for a particular subset of viral recognition genes during early infection timepoints and rather limited upregulation of immune genes later, while maintaining and reactivating metabolic pathways. The CSel strain showed a downregulation of metabolic related genes and a limited upregulation of immune genes, while the PSel strain showed similar downregulation of metabolic genes during acute infection, yet when compared to the CSel strain, showed a more robust innate immune response. Evaluation of upregulated immune response genes, as well as interferon-related genes showed the PSel strain to have the greatest number of uniquely upregulated immune genes in both the kidney and intestine, with CSel and PSel showing a similar number of such genes upregulated in liver. A moderate number of immune response genes were shared between PSel and CSel in all tissues, though both PSel and VSel showed a high number of uniquely overexpressed immune response genes in the kidney, and PSel showed the highest number of uniquely upregulated interferon related genes in the intestine. Overall, the VSel response was unique from the CSel with very little overlap in activated immune responses. Findings from this study highlight the disparity in IHNV resistance among genetic strains of rainbow trout, while identifying molecular mechanisms underlying differences in disease phenotypes. Furthermore, our results on trout strains with distinct selection backgrounds yields comparative insights into the adaptive gains brought about by selection programs for pathogen-specific disease resistance, as well as the non-specific immune enhancement associated with selection for utilization of plant-based diets.

Dual resistance to Flavobacterium psychrophilum and Myxobolus cerebralis in rainbow trout (Oncorhynchus mykiss, Walbaum).

Aquatic pathogens are a major concern for fish hatchery production, fisheries management, and conservation, and disease control needs to be addressed. Two important salmonid pathogens are Myxobolus cerebralis and Flavobacterium psychrophilum that cause whirling disease and bacterial coldwater disease (BCWD), respectively. Innate disease resistance is a potential option for reducing disease‐related mortality in hatchery‐reared rainbow trout (Oncorhynchus mykiss, Walbaum). Two experiments were conducted to assess pathogen resistance of first‐generation (F1) rainbow trout created by crossing M. cerebralis‐ and F. psychrophilum‐resistant strains. In the first experiment, we exposed two rainbow trout strains and one F1 cross to six treatments: control (no exposure), mock injection, F. psychrophilum only, M. cerebralis only, F. psychrophilum then M. cerebralis, and M. cerebralis then F. psychrophilum. Results indicated that the F1 cross was not resistant to either pathogen. In the second experiment, we exposed five rainbow trout strains and four rainbow trout crosses to F. psychrophilum. The second experiment indicated that at least one rainbow trout cross was F. psychrophilum‐resistant. Achieving dual resistance may be possible using selective breeding but only some multigenerational strains are suitable candidates for further evaluation.

Sudden Changes in Water Hardness Do Not Impact Short-Term Rainbow Trout Survival

Fish transferred from hard water to soft water, as can occur during the stocking of hatchery-reared fish into natural environments, experience many physiological stressors that can cause mortality. This study consisted of four trials examining the effects of direct transfer from hard (345 mg/L CaCO3) to soft water on mortality and glucose stress response of two different sizes and strains of rainbow trout Oncorhynchus mykiss. The first trial subjected 90 g Shasta strain rainbow trout to a direct transfer to soft water (70 or 160 mg/L as CaCO3). The second trial used the same strain and size trout, with transfer to lower hardness values of either 0 or 35 mg/L. The third and fourth trials used 3 to 5 g Arlee strain rainbow trout. The third trial transferred the fish from 345 mg/L hardness water to water at a hardness of either 0 or 35 mg/L. The fourth trial added a secondary temperature stressor of 20 °C (versus the experimental standard of 11 °C) in combination with a hardness level of 0 mg/L. In every trial, survival was not significantly different among all treatments. Sudden and extreme changes in water hardness, even in combination with increased water temperature, did not cause trout mortality. There was also no significant difference in blood glucose over time among any of the treatments in the first three trials. In the fourth trial, glucose values did significantly vary among the treatments at 48 and 168 h after the start of the trial. However, all glucose values were relatively close to the basal level. These results indicate that changes in water hardness likely do not impact the survival of rainbow trout, negating the need for tempering or acclimation.