Trout Salmo Research Articles

Great Cormorants Phalacrocorax carbo in the Subalpine Soča River System, Slovenia: The Possible Effect of Avian Predators in a Sensitive Biogeographic Region for Fish

The foraging of Great Cormorants Phalacrocorax carbo in Alpine rivers is a relatively recent phenomenon. Wintering populations in Slovenia increased in the 1980s. In the 1990s, the increase affected the upper sections of rivers where the habitats of Grayling Thymallus thymallus and trout Salmo and Oncorhynchus spp. are located. The upper section of the Soča River system is known as one of the biogeographic hotspots for (sub)alpine rivers, where several endemic or rare fish species are found. The aim of this study is to evaluate the diet of the Cormorants in this area and to compare it to some other threats to the fish fauna of the river. A total of 75 Cormorant stomachs were examined. Of these, 68 (90.7%) contained fish or recognisable fish remains. The birds' diet consisted of eight species of fish: Marble Trout, Rainbow Trout, Common Nase, Vairone, Common Chub, Italian Chub, Italian Barbel and Bullhead. The increase in numbers and the diet of Cormorants in the river basin of Soča is discussed in the context of the increase of other threats to this ecosystem, such as fish stocking, gravel extraction and tourism, especially angling and water sports. A more comprehensive environmental assessment of all these factors is needed, as their increase runs parallel with the arrival of the Cormorants in the upper section of the river. There is therefore reason to believe that Cormorants are only a small part of a larger problem in the conservation of the river ecosystem.

Genome-wide alternative splicing profile in the posterior kidney of brown trout (Salmo trutta) during proliferative kidney disease

BackgroundThe cnidarian myxozoan parasite Tetracapsuloides bryosalmonae causes chronic proliferative kidney disease (PKD) in salmonids. This parasite is a serious threat to wild and cultured salmonids. T. bryosalmonae undergoes intra-luminal sporogonic development in the kidney of brown trout (Salmo trutta) and the viable spores are released via urine. We investigated the alternative splicing pattern in the posterior kidney of brown trout during PKD.ResultsRNA-seq data were generated from the posterior kidney of brown trout collected at 12 weeks post-exposure to T. bryosalmonae. Subsequently, this data was mapped to the brown trout genome. About 153 significant differently expressed alternatively spliced (DEAS) genes, (delta PSI = 5%, FDR P-value < 0.05) were identified from 19,722 alternatively spliced events. Among the DEAS genes, the least and most abundant alternative splicing types were alternative 5′ splice site (5.23%) and exon skipping (70.59%), respectively. The DEAS genes were significantly enriched for sodium-potassium transporter activity and ion homeostasis (ahcyl1, atp1a3a, atp1a1a.1, and atp1a1a.5). The protein-protein interaction network analysis enriched two local network clusters namely cation transporting ATPase C-terminus and Sodium/potassium ATPase beta chain cluster, and mixed inclusion of Ion homeostasis and EF-hand domain cluster. Furthermore, the human disease-related salmonella infection pathway was significantly enriched in the protein-protein interaction network.ConclusionThis study provides the first baseline information about alternative splicing in brown trout during PKD. The generated data lay a foundation for further functional molecular studies in PKD - brown trout infection model. The information generated from the present study can help to develop therapeutic strategies for PKD in the future.

Sex-Specific Life History Affected by Stocking in Juvenile Brown Trout

Salmonids are a socioeconomically and ecologically important group of fish that are often managed by stocking. Little is known about potential sex-specific effects of stocking, but recent studies found that the sexes differ in their stress tolerances already at late embryonic stage, i.e., before hatchery-born larvae are released into the wild and long before morphological gonad formation. It has also been speculated that sex-specific life histories can affect juvenile growth and mortality, and that a resulting sex-biassed demography can reduce population growth. Here we test whether juvenile brown trout (Salmo trutta) show sex-specific life histories and whether such sex effects differ in hatchery- and wild-born fish. We modified a genetic sexing protocol to reduce false assignment rates and used it to study the timing of sex differentiation in a laboratory setting, and in a large-scale field experiment to study growth and mortality of hatchery- and wild-born fish in different environments. We found no sex-specific mortality in any of the environments we studied. However, females started sex differentiation earlier than males, and while growth rates were similar in the laboratory, they differed significantly in the field depending on location and origin of fish. Overall, hatchery-born males grew larger than hatchery-born females while wild-born fish showed the reverse pattern. Whether males or females grew larger was location-specific. We conclude that juvenile brown trout show sex-specific growth that is affected by stocking and by other environmental factors that remain to be identified.

Density-dependence and environmental variability have stage-specific influences on European grayling growth

Fish somatic growth is indeterminate and can be influenced by a range of abiotic and biotic variables. With climate change forecast to increase the frequency of warming and unusual discharge events, it is thus important to understand how these variables currently influence somatic growth and how that might differ for specific age-classes and/ or life stages. Here, we used a 17-year dataset from a chalk stream in southern England to identify the abiotic and biotic influences on the growth of juvenile, sub-adult and adult life stages of European grayling (Thymallus thymallus), a cold-water riverine salmonid. The results revealed that interannual variations in grayling growth were well described by annual- and site-specific abiotic and biotic explanatory variables. We found divergent responses between life stages to increased temperature and unusual discharge during the main growth period with, for example, elevated temperatures related to increased juvenile growth but reduced sub-adult growth, and high discharge events related to increased sub-adult growth yet reduced juvenile growth. Conversely, stage-specific grayling abundance negatively influenced growth at each life stage, though only juvenile growth was impacted by the abundance of a competitor species, brown trout (Salmo trutta). These results emphasise the merits of testing a wide range of environmental and biological explanatory variables on fish growth, and across life stages. They also reveal the importance of maintaining high habitat heterogeneity in rivers to ensure all life stages can reduce their competitive interactions and have access to adequate flow and thermal refugia during periods of elevated environmental stress.

Isolation of Chromatophores from Brown Trout (Salmo trutta) Skin

As new techniques for single-cell analysis are evolving, there is a growing need for streamlined and flexible protocols for single-cell suspension preparation and single-cell isolation. Based on previous research on the model species zebrafish (Danio rerio), a novel protocol for the isolation of chromatophores from brown trout (Salmo trutta) skin was developed and specific gene expression in chromatophore types evaluated with qPCR. This is the first report on a protocol for the isolation of chromatophores from the skin of salmonids. The method is simple and fast (taking about 90 min), requires no special equipment and is in line with the high ethical standards required in animal research, as no animals need to be sacrificed. In addition, skin biopsies can be taken from the same fish at multiple time points, allowing monitoring of chromatophore differentiation and their involvement in the formation of pigmentation patterns. The protocol should be effective for the vast majority of salmonid species due to similarities in skin morphology.

Predatory ability and abundance forecast the ecological impacts of two aquatic invasive species

Characterising interspecific interaction strengths, combined with population abundances of prey and their novel predators, is critical to develop predictive invasion ecology. This is especially true of aquatic invasive species, which can pose a significant threat to the structure and stability of the ecosystems to which they are introduced. Here, we investigated consumer-resource dynamics of two globally-established aquatic invasive species, European green crab (Carcinus maenas) and brown trout (Salmo trutta). We explored the mediating effect of prey density on predatory impact in these invaders relative to functionally analogous native rock crab (Cancer irroratus) and Atlantic salmon (Salmo salar), respectively, feeding on shared prey (Mytilus sp. and Tenebrio molitor, respectively). We subsequently combined feeding rates with each predator’s regional abundance to forecast relative ecological impacts. All predators demonstrated potentially destabilising Type II functional responses towards prey, with native rock crab and invasive brown trout exhibiting greater per capita impacts relative to their trophic analogues. Functional Response Ratios (attack rates divided by handling times) were higher for both invasive species, reflecting greater overall per capita effects compared to natives. Impact projections that incorporated predator abundances with per capita effects predicted severe impacts by European green crabs. However, brown trout, despite possessing higher per capita effects than Atlantic salmon, are projected to have low impact owing to currently low abundances in the sampled watershed. Should brown trout density increase sixfold, we predict it would exert higher impact than Atlantic salmon. Such impact-forecasting metrics and methods are thus vital tools to assist in the determination of current and future adverse impacts associated with aquatic invasive species.

Genetic and correlative light and electron microscopy evidence for the unique differentiation pathway of erythrophores in brown trout skin

Based on their cell ultrastructure, two types of erythrophores in the spotted skin regions of brown trout (Salmo trutta) were previously described. To test the hypothesis regarding the origin of a new cell type following genome duplication, we analysed the gene and paralogue gene expression patterns of erythrophores in brown trout skin. In addition, the ultrastructure of both erythrophore types was precisely examined using transmission electron microscopy (TEM) and correlative light microscopy and electron microscopy (CLEM). Ultrastructural differences between the sizes of erythrophore inclusions were confirmed; however, the overlapping inclusion sizes blur the distinction between erythrophore types, which we have instead defined as cell subtypes. Nevertheless, the red spots of brown trout skin with subtype 2 erythrophores, exhibited unique gene expression patterns. Many of the upregulated genes are involved in melanogenesis or xanthophore differentiation. In addition, sox10, related to progenitor cells, was also upregulated in the red spots. The expressions of paralogues derived from two genome duplication events were also analysed. Multiple paralogues were overexpressed in the red spots compared with other skin regions, suggesting that the duplicated gene copies adopted new functions and contributed to the origin of a new cell subtype that is characteristic for red spot. Possible mechanisms regarding erythrophore origin are proposed and discussed. To the best of our knowledge, this is the first study to evaluate pigment cell types in the black and red spots of brown trout skin using the advanced CLEM approach together with gene expression profiling.

Interacting Effects of Graphene Oxide Derivatives and Trace Metals on Freshwater Brown Trout (Salmo Trutta L.) Behaviour at Different Stages of Development

Interacting Effects of Graphene Oxide Derivatives and Trace Metals on Freshwater Brown Trout (Salmo Trutta L.) Behaviour at Different Stages of Development

Evidence of unidirectional gene flow in a fragmented population of Salmo trutta L.

Selection, genetic drift, and gene flow affect genetic variation within populations and genetic differences among populations. Both drift and selection tend to decrease variation within populations and increase differences among populations, whereas gene flow increases variation within populations but leads to populations being related. In brown trout (Salmo trutta L.), the most important factor in population fragmentation is disrupted river-segment connectivity. The main goal of the study was to use genetic analysis to estimate the level of gene flow among resident and migratory brown trout in potential hybridization areas located downstream of impassable barriers in one river basin in the southern Baltic Sea region. First, spawning redds were counted in the upper river basin downstream of impassable barriers. Next, samples were collected from juveniles in spawning areas located downstream of barriers and from adults downstream and upstream of barriers. Subsequently, genetic analysis was performed using a panel of 13 microsatellite loci and the Salmo trutta 5 K SNP microarray. The genetic differentiation estimated between the resident form sampled upstream of the barriers and the anadromous specimens downstream of the barriers was high and significant. Analysis revealed that gene flow occurred between the two forms in the hybridization zone investigated and that isolated resident specimens shared spawning grounds with sea trout downstream of the barriers. The brown trout population from the river system investigated was slightly, internally diversified in the area accessible to migration. Simultaneously, the isolated part of the population was very different from that in the rest of the basin. The spawning areas of the anadromous form located downstream of the barriers were in a hybridization zone and gene flow was confirmed to be unidirectional. Although they constituted a small percentage, the genotypes typical upstream of the barriers were admixed downstream of them. The lack of genotypes noted upstream of the barriers among adult anadromous individuals might indicate that migrants of upstream origin and hybrids preferred residency.

A comparison of trends in population size and life history features of Atlantic salmon (Salmo salar) and anadromous and non-anadromous Brown trout (Salmo trutta) in a single catchment over 116\xa0years

We use a long time series of catch abundance from a recreational fishery over 116 years to look for population trends in Atlantic salmon, and anadromous (sea trout) and non-anadromous (brown) trout for a single catchment, Loch Lomond, west central Scotland. Year strongly predicted variation in catches but catch effort did not meaningfully increase explained variation. Salmon showed periods of increasing and decreasing trends, for sea trout and brown trout there was an overall declining trend. Since 1952, Lomond salmon population trends differed from both wider Scotland and southern Europe, indicating that the Lomond population is partially buffered from drivers of change in salmon populations more widely. In contrast Lomond sea trout showed a similar declining trend to that of populations from the wider west of Scotland over this period. The Lomond populations showed some evidence of shorter-term cycling patterns; the drivers for which are unknown. Body size in salmon and sea trout declined but increased in brown trout; salmon returned to freshwater later, and the relative proportion of all caught trout that were anadromous increased across the time series. This study shows a long and protracted period of fundamental change to populations of these two species over 116 years.

Interspecific competition reduces the performance of Atlantic salmon (Salmo salar): implications for restoration programs

Competition from non-native salmonids is potentially impairing efforts to restore Atlantic salmon (Salmo salar) to parts of their historical range. In three separate meta-analyses, we collected 104 effect sizes from 25 published papers to quantify the effect of both native and non-native salmonids on the performance (i.e., behaviour, habitat use, growth and survival) of Atlantic salmon. The presence of other species had negative effects on the performance of Atlantic salmon; in particular, non-native species and brown trout (Salmo trutta), whether native or non-native, had the most negative effects. Contrary to our predictions, the negative effects of other species were not exacerbated in laboratory compared to field studies and did not increase with total salmonid abundance or the relative body size of the competitors. However, most studies in our analyses were conducted under laboratory conditions and at densities much higher than found in nature. Thus, a realistic assessment of the potential success of restoration programs when interspecific competitors are present should include more studies conducted under natural conditions.

Assessment of Stocking Activities on the Native Brown Trout Populations from Nestos River (Southern Balkans) Inferred by mtDNA RFLP and Sequencing Analyses

Alien fish introductions, conducted towards the ichthyofauna enhancement in local drainages, have been occasionally proved harmful for the indigenous freshwater fish populations. The present study was designed to assess the impact of stocking activities, carried out in the past decades with fingerlings originating from Acheloos river hatcheries, on the native trout (Salmo sp.) populations of Nestos River, Greece. Trout specimens collected from several tributaries of Nestos River and were analyzed by means of PCR-RFLP and sequencing targeting the mitochondrial ND5-ND6 genes and the entire control region, respectively. It should be mentioned that trouts from Acheloos mainly belong to the marmoratus mitochondrial DNA (mtDNA) lineage, while the autochthonous trouts from Nestos belong to the Adriatic lineage. Both methodologies demonstrated that most samples from the three tributaries located at the lower part of Nestos constitute offspring of the fingerlings transferred from Acheloos hatcheries. Therefore, these tributaries have been strongly affected by stocking activities with a potential complete loss of their autochthonous trout. On the other hand, it seems that trout populations from higher altitude tributaries have not been affected by stockings. Hence, efforts should be undertaken in order to prevent the prevalence of the non-indigenous translocated Salmo in higher altitude tributaries, in conjunction with a management plan designed for the total trout populations from the area, speaking of which it has been recently included to the National Park of Rodopi Mountains.

Temporal analysis shows relaxed genetic erosion following improved stocking practices in a subarctic transnational brown trout population

Maintaining standing genetic variation is a challenge in human-dominated landscapes. We used genetic (i.e., 16 short tandem repeats) and morphological (i.e., length and weight) measurements of 593 contemporary and historical brown trout (Salmo trutta) samples to study fine-scale and short-term impacts of different management practices. These had changed from traditional breeding practices, using the same broodstock for several years, to modern breeding practices, including annual broodstock replacement, in the transnational subarctic Pasvik River. Using population genetic structure analyses (i.e., Bayesian assignment tests, DAPCs, and PCAs), four historical genetic clusters (E2001A-D), likely representing family lineages resulting from different crosses, were found in zone E. These groups were characterized by consistently lower genetic diversity, higher within-group relatedness, lower effective population size, and significantly smaller body size than contemporary stocked (E2001E) and wild fish (E2001F). However, even current breeding practices are insufficient to prevent genetic diversity loss and morphological changes as demonstrated by on average smaller body sizes and recent genetic bottleneck signatures in the modern breeding stock compared to wild fish. Conservation management must evaluate breeding protocols for stocking programs and assess if these can preserve remaining natural genetic diversity and morphology in brown trout for long-term preservation of freshwater fauna.

Preliminary observations of Dades trout (Salmo multipunctatus) spawning in a High Atlas mountain spring

Preliminary observations of Dades trout (Salmo multipunctatus) spawning in a High Atlas mountain spring

Population consequences of climate change through effects on functional traits of lentic brown trout in the sub-Arctic

Climate-induced plasticity in functional traits has received recent attention due to the immense importance phenotypic variation plays in population level responses. Here, we explore the effect of different climate-change scenarios on lentic populations of a freshwater ectotherm, the brown trout (Salmo trutta L.), through climate effects on functional traits. We first parameterize models of climate variables on growth, spawning probability and fecundity. The models are utilized to inform a dynamic age-structured projection matrix, enabling long-term population viability projections under climate and population density variation. Ambient temperature and winter conditions had a substantial effect on population growth rate. In general, warmer summer temperatures resulted in faster growth rates for young fish but ended in smaller size at age as fish got older. Increasing summer temperatures also induced maturation at younger age and smaller size. In addition, we found effects of first-year growth on later growth trajectories for a fish, indicating that environmental conditions experienced the first year will also influence size at age later in life. At the population level, increasing temperatures average (up to 4 °C increase in areas with mean summer temperature at approximately 12 °C) resulted in a positive effect on population growth rate (i.e. smaller but more fish) during climate simulations including increasing and more variable temperatures.

Morphological and molecular characterization of a novel Myxobolus species from the gastrointestinal tract of brown trout (Salmo trutta) in Spain.

The genus Myxobolus Bütschli, 1882 is the largest group within the class Myxosporea and includes 905 nominal species, 18 of which have been found to infect fish belonging to the family Salmonidae. In the present study, microscopic analysis enabled detection of myxospores in 43 of 613 (7.0%) gastrointestinal tracts from brown trout (Salmo trutta) captured in several rivers in the northwest of the Iberian Peninsula. Measurement of the whole myxospores, polar capsules and other morphological characteristics, together with identification of the site of infection, has led us to propose a novel salmonid-myxobolid species, Myxobolus compostellanus n. sp. Molecular analysis of the small subunit ribosomal RNA (SSU-rRNA) gene yielded the same consensus sequence of 2039bp in 14 fish specimens. A BLAST search indicated 97.6% similarity to Myxobolus neurobius. Phylogenetic analysis revealed that M. compostellanus n. sp. is clustered with other salmonid-infecting myxobolids. The present findings contribute to the existing knowledge about the genus Myxobolus, providing both morphological and molecular data on a novel species of Myxobolus found to infect the gastrointestinal tract of salmonids, M. compostellanus n. sp. in the brown trout (S. trutta).

Detection of fish pathogen Saprolegnia parasitica in environmental DNA samples by droplet digital PCR

Oomycetes are fungal-like microorganisms parasitic towards a large number of plant and animal species. Genera from order Saprolegniales, such as Saprolegnia and Aphanomyces, cause devastating infections of freshwater animals. Saprolegnia parasitica is a widely distributed oomycete pathogen that causes saprolegniosis, a disease responsible for significant economic losses in aquaculture, as well as declines of natural populations of fish and other freshwater organisms. Despite its negative impact, no monitoring protocol for S. parasitica has been established to date. Thus, we aimed to develop a droplet digital PCR (ddPCR) assay for the detection and quantification of S. parasitica in environmental DNA samples. Saprolegnia parasitica-specific primers were designed to target internal transcribed spacer region 2 (ITS 2), based on the alignment of ITS sequences of S. parasitica and a range of Saprolegnia spp., as well as other oomycetes. The specificity of primers was tested using genomic DNA of S. parasitica (as positive control) and DNA of non–S. parasitica oomycete isolates, as well as trout/crayfish DNA (as negative control). The primers specifically amplified a segment of the ITS region of oomycete pathogen S. parasitica, while no amplification (i.e. no positive droplets) was obtained for closely related Saprolegnia spp. (e.g. Saprolegnia sp. 1 and S. ferax) and other more distantly related oomycetes. Next, the limit of detection (LOD) of the assay was established by using serial dilutions of the S. parasitica genomic DNA. The determined sensitivity of the assay was high: LOD was 15 fg of pathogen’s genomic DNA per µL of the reaction mixture. Assay performance was further assessed with environmental DNA samples isolated from water from the trout farms and natural environments, as well as (ii) biofilm from the host surface (swab samples). Water samples were collected from 21 different locations in Croatia, while swab samples were collected from S. parasitica host/carrier species: (i) skin and eggs of the rainbow trout (Oncorhynchus mykiss Walbaum, 1792) and brown trout (Salmo trutta Linnaeus, 1758), and (ii) cuticle of signal crayfish (Pacifastacus leniusculus Dana, 1852) and narrow clawed crayfish (Pontastacus leptodactylus Eschscholtz, 1823). Samples were classified into agent levels A0 to A6, depending of the number of S. parasitica ITS copies per ng of total DNA. Saprolegnia parasitica was detected in 76 % of water samples (16/21) and the range of pathogen’s ITS copies in positive samples was between 0.02 and 14 copies/ng of total DNA (agent levels A1 to A3). Regarding the swab samples, S. parasitica load was significantly higher in diseased trout than in those with healthy appearance: 9375 vs 3.28 S. parasitica copies/ng of total swab DNA (average agent level A6 vs. A2, respectively). Despite the fact that none of the sampled crayfish had signs of infection, the pathogen was detected in all tested cuticle swabs. Swabs of P. leniusculus, a known S. parasitica host, had significantly higher S. parasitica load than swabs of P. leptodactylus, S. parasitica carrier: 390 vs 83 S. parasitica copies/ng (agent level A5 vs. A4, respectively). In conclusion, our results demonstrate the applicability of the newly developed ddPCR assay in monitoring and early detection of S. parasitica in aquaculture facilities and natural freshwater environments. This would help in a better understanding of S. parasitica ecology and its effects on the host populations.

An eDNA-based method for monitoring a salmonid infectious disease: Development and application

In the current context of global change, freshwater species are increasingly exposed to emerging infectious diseases (Okamura and Feist 2011). As an example, the Proliferative Kidney Disease (PKD) has emerged in salmonid fish during the last two decades, both in Europe and North America, causing important losses in aquaculture and worrying declines of several wild salmonid populations (Sudhagar et al. 2019). It is caused by Tetracapsuloides bryosalmonae, a myxozoan parasite with a complex life cycle involving two hosts: salmonids (intermediate host) and bryozoans (primary host). As PKD development strongly depends upon water temperature and quality, it is expected that global change could lead to more outbreaks (Okamura et al. 2011). Current monitoring of fish parasite load and infection status relies on histological observation or T. bryosalmonae DNA amplification out of kidney samples, involving fish euthanasia, and thus relatively small sample sizes when inferring infection prevalence. As large-scale screening of this parasite infections are required to better understand PKD dynamics, we have developed a non-lethal method for T. bryosalmonae detection in fish host based on the biological fact that T. bryosalmonae spores can be excreted from infected fish into the water through urine (Hedrick et al. 2004). This novel approach based on the detection of T. bryosalmonae DNA in fish urine was developed on wild brown trout (Salmo trutta), a species known to be an intermediate host of T. bryosalmonae and for releasing infective spores (only towards bryozoan host) through urine (Okamura et al. 2011). Applying this method, we have been able to map wild brown trout infection prevalence across 50 sites at the foothill of French Pyrenees and to identify the main environmental drivers of this disease.

Seasonal fluctuations of the serum basal biochemical profile in spawning and postspawning West Balkan trout (Salmo farioides, Salmonidae): Implications for fish biology and aquaculture

ABSTRACT Endangerment of fish habitats worldwide is a global problem. Breeding fish in restocking hatcheries is important for bioconservation of many fishes. Analysis of biochemical and hematological parameters provides important information on spawning performance and fish health. Variations of different serum biochemical constituents and increase in body length and mass of West Balkan trout were analyzed during spawning and postspawning. High body weight deviations of males were found in both periods. The body length and weight of males is higher compared to females. After spawning, higher growth was observed in males. Biochemical values are higher in females in relation to males. Low values of glucose, chloride, and potassium are present in the spawning phase. Decreases in protein, triglycerides, cholesterol, AST, sodium, and calcium levels were found in postspawning. Serum biochemical constituents of females vary significantly compared to males. Serum markers indicate a strong association with metabolic processes, which allows for better nutritional control and management of environmental factors, especially the presence of organic particles in the broodstock.

Competition among juvenile brown trout, grayling, and landlocked Atlantic salmon in flumes — predicting effects of interspecific interactions on salmon reintroduction success

Worldwide declines in salmonid populations have generated major interest in conservation and restoration of wild populations and riverine habitats. Species reintroductions to previous habitats raise questions about their potential impact on these systems. In River Klarälven, landlocked Atlantic salmon (Salmo salar) have been extinct from upper reaches for over 50 years due to hydropower dams. Here we study competitive interactions among juvenile salmon, grayling (Thymallus thymallus), and brown trout (Salmo trutta) that occur in the upper reaches of the river. We examine foraging rates, aggression, and activity of juvenile fish in allopatry at three different densities and in sympatry with one or both potential competitors in laboratory flumes. Salmon captured prey less frequently in the presence of brown trout and grayling, whereas grayling and brown trout were unaffected by salmon, but affected each other. Grayling was the most aggressive and active species, whereas salmon the least. Consequently, reintroduction of salmon probably will have little impact on grayling and brown trout, whereas grayling and brown trout could affect the success of reintroducing salmon.