Brown Trout Research Articles

Parasitic mussels induce upstream movement in their fish Original Articlehosts: early evidence of extended phenotype

Abstract Parasites often have a large impact on their hosts and can alter host phenotype to increase their own fitness, a phenomenon known as extended phenotype. Studies demonstrating extended phenotype for non-trophically transmitted parasites are scarce. Unionid mussels have a parasitic life stage adapted to parasitize fish which can affect host behaviour, habitat use and growth rates, raising the question if parasitic freshwater mussels can also manipulate their host fish to compensate for downstream dispersal and to reach habitats favourable for newly excysted juvenile mussels. Wild-caught, parasite-naïve juvenile brown trout (Salmo trutta) were PIT-tagged, and half of the individuals were infested with parasitic larvae from the freshwater pearl mussel (Margaritifera margaritifera), all individuals were then returned to their home stream. During the following year, trout were tracked to investigate movement and habitat use, and also periodically recaptured to measure growth and body condition factor. The infested trout showed significantly higher upstream movement than non-infested trout and were more often recaptured in stream sections with slow-moving shallow water, particularly during the parasite excystment period (270 days post infestation). These data suggest that the juvenile mussels were successfully transported an average of 170 m upstream from the host trout release points to stream sections favourable for adult mussels. Infested trout survived as well as the non-infested, but had a significantly lower specific growth rate than non-infested trout. These results indicate a first example of extended phenotype in unionid mussels and highlight the importance of understanding glochidia-induced changes to host fish behavioural ecology.

Anthropogenic sources and transfer of trace metals from the environment to brown trout (Salmo trutta) in mountainous rivers.

Anthropogenic sources and transfer of trace metals from the environment to brown trout (Salmo trutta) in mountainous rivers.

Subtle ecological effects of the invasive signal crayfish (Pacifastacus leniusculus) on Iberian fish communities

The introduction of non-native species can disrupt the structure of communities and affect the functioning of freshwater ecosystems, disturbing various taxonomic groups, including fish. This study aimed to assess the ecological effects of the recent introduction of the signal crayfish Pacifastacus leniusculus in the fish communities in the Rabaçal and Tuela River basins (Montesinho Natural Park and adjacent areas; NE of Portugal), an area with very low human disturbance. A total of 34 sites (18 invaded and 16 non-invaded) were sampled in the summer of 2022 and comparisons were made between invaded and non-invaded sites concerning abundance, biomass, richness and diversity of fish communities. In addition, we compared the physiological condition of the sampled species. A total of 2307 fishes belonging to six different species were collected and results indicated almost no negative (except diversity) effects at the community level. However, species-specific analysis revealed a decrease in the abundance of the Iberian chub Squalius carolitertii in invaded sites. We also observed a lower physiological condition of the brown trout Salmo trutta. Although the ecological effects appear to be subtle, ongoing monitoring of signal crayfish populations and their effects on fish communities is essential, using this study as a baseline for future comparisons. Additionally, urgent management measures—such as controlling and containing signal crayfish populations and raising local awareness—should be considered, given the conservation significance of the study area and the potential negative ecological effects of this non-native species.

Combined Effects of Toxic Metals and Oxidative Stress on the Development and Health of Sea Trout (Salmo Trutta L.).

Contaminants in the environment pose a considerable threat to biodiversity, ecological balance, and the health of both wildlife and humans, particularly through the transfer of these harmful substances via fish in the food chain. This study focused on the developmental stages of sea trout (Salmo trutta L.) in both riverine and Baltic Sea environments, with the aim of exploring how chemical element accumulation influences oxidative stress biomarkers in these species. The findings revealed notable age- and tissue-specific patterns in the accumulation of chemical elements in sea trout. Specifically, higher levels of lead (Pb), arsenic (As), mercury (Hg), and tin (Sn) were detected in the muscle tissues of adult trout, while cadmium (Cd) primarily accumulated in the gills, particularly in smolts. These results underscore the influence of both age and tissue type on the bioaccumulation of contaminants in the trout, highlighting how the accumulation of toxic elements contributes to increased oxidative stress in the fish. This oxidative stress, reflected by increased lipid peroxidation (TBARS) and carbonyl derivatives of oxidatively modified proteins, was closely related to the presence of contaminants such as Cd, Pb, As, Hg, and Sn. Gills, which are directly exposed to waterborne pollutants, exhibited significantly higher levels of oxidative damage compared to muscle tissue, consistent with the greater accumulation of metals in this organ. Despite higher total antioxidant status (TAS) in muscle tissue, both muscle and gill tissues of adult trout showed signs of considerable oxidative stress, indicating the cumulative effects of prolonged exposure to these contaminants. The study highlights the detrimental consequences of chemical element contamination on the health of trout, with a particular emphasis on oxidative damage, and calls for effective environmental management to protect aquatic species from the long-term effects of exposure to contaminants. Furthermore, the correlation and regression analysis conducted revealed significant patterns, demonstrating positive correlations between the accumulation of Cd, Pb, and As in the gills of adult trout, and between Pb and oxidative stress markers in smolts. Additionally, the analysis indicated that mercury contributes significantly to oxidative damage.

Does wetland restoration create an ecological trap for migrating Brown trout smolts?

Does wetland restoration create an ecological trap for migrating Brown trout smolts?

Native Yellowstone cutthroat trout Oncorhynchus virginalis bouvieri Bendire 1882 growth and survival in a headwater stream primarily driven by warming stream temperatures, with non-native brown trout Salmo trutta Linnaeus 1758, posing an additional threat to survival

Warming rivers and interactions with non-native species impact salmonid species globally. Understanding how hydroclimatic conditions synergistically and independently interact with non-native species is critical for effectively managing salmonids into the future. We used a 10-year mark-recapture dataset to assess how native Yellowstone cutthroat trout Oncorhynchus virginalis bouvieri Bendire 1882 (hereafter YCT) and non-native brown trout Salmo trutta Linnaeus 1758 growth rates and apparent survival were affected by hydroclimatic conditions and (for YCT) the presence of brown trout in a tributary. Growth (YCT) and survival (both species across size classes) were negatively related to warming stream temperatures. Brown trout growth was positively related to increasing daily streamflow variability (a proxy for streamflow), but this variable was not included in the top YCT growth model. Density dependent effects appeared to be nonexistent (growth) or weakly positive (survival). When sympatric with brown trout, YCT displayed worse survival than allopatric YCT across environmental conditions. Broadly, we found native and non-native trout respond to different hydroclimatic conditions that shift with changing climatic conditions; and brown trout represent an additional threat to YCT survival.

Early life development and sex determination of brown trout affected by treated wastewater discharge.

Early life development and sex determination of brown trout affected by treated wastewater discharge.

Responses of fish to nationwide improvements in the water quality of a densely populated and heavily modified country over four decades.

Globally, fish have been severely affected by the widespread, chronic degradation of fresh waters, with a substantial proportion of species declining in abundance or range in recent decades. This has especially been the case in densely populated countries with an industrial heritage and intensive agriculture, where the majority of river catchments have been affected by deteriorations in water quality and changes in land use. This study used a spatially and temporally extensive dataset, encompassing 16,124 surveys at 1180 sites representing a wide range of river typologies and pressures, to examine changes in the fish populations of England's rivers over four decades (1980s-2010s). The analyses revealed gradual, nationwide increases in mean fish species richness and diversity across the range of pressure gradients. In the majority of cases, increases were most pronounced in the 1980s, since when any further changes have been comparatively minor, but there were no declining trends across the full time series. There were also temporal, nationwide changes in fish assemblage structure, driven largely by variations in the densities of brown trout Salmo trutta or roach Rutilus rutilus, but no consistent increases in the abundance of sensitive, pollution-intolerant species in response to improvements in wastewater treatment and, consequently, water quality. Although the increases in fish species richness and diversity over the last four decades are encouraging, subtle and contrasting changes in the abundance of a range of species require further investigation, and causal relationships between fish assemblage structure and putative drivers should be modelled at a national scale. This study is the first to examine long-term, nationwide trends in the freshwater fish populations of England, and significantly advances our understanding of the ecological health of rivers in densely populated and heavily modified countries.

Environmental DNA surveys can determine in-stream dominance of non-native brook charr Salvelinus fontinalis over native brown trout Salmo trutta

Environmental DNA surveys can determine in-stream dominance of non-native brook charr Salvelinus fontinalis over native brown trout Salmo trutta

Behavioural plasticity in circadian foraging patterns increases resistance of brown trout populations to environmental change

Stream-dwelling salmonids in the low-latitude and -altitude margins of their range are particularly threatened by climate change. However, they possess a variety of evolutionary, plastic, and behavioural mechanisms that provide resistance against rapid changes in their environment. Behavioural plasticity can be important under rapid environmental change because it is relatively fast and flexible. In particular, salmonids can exhibit flexible diel activity patterns in response to new environmental conditions, but the consequences of this capability for long-term population persistence in the face of climate change remain unclear. We used an individual-based model to simulate the trajectory of a brown trout population at the warmest edge of its range under three environmental-change scenarios of increasing warming and streamflow reduction. We assessed (1) how simulated trout responded behaviourally to climate change by modifying their circadian foraging patterns, and (2) how much this behavioural plasticity buffered the population-level consequences of environmental change. Our simulations showed that under current conditions trout of different age classes segregated foraging both temporally and spatially. The most consistent response to environmental change was more diurnal feeding in all age classes and under all scenarios, with the strength of this response increasing with the severity of change. In addition, total daily foraging activity increased in all age classes. A second experiment indicated that virtual populations of individuals capable of flexible circadian feeding were more resistant to environmental change than populations restricted to fixed feeding patterns. Thus, our computational experiment suggests that the ability of fish to adaptively select when as well as where to feed, well-documented at the individual level in the empirical literature, could potentially buffer the demographic impacts of long-term environmental change.

The History and Distribution of Brown Trout, Salmo trutta Linnaeus, 1758, in Nova Scotia, New Brunswick, and Prince Edward Island

The History and Distribution of Brown Trout, Salmo trutta Linnaeus, 1758, in Nova Scotia, New Brunswick, and Prince Edward Island

Age matters: exploring differential effects of antimicrobial treatment on gut microbiota of adult and juvenile brown trout (Salmo trutta)

BackgroundAntibiotics and antiparasitics are essential tools in controlling infectious disease outbreaks in commercial aquaculture. While the negative effects of antimicrobials on the gut microbiome of various farmed fish species are well documented, the influence of underlying host factors, such as age, on microbiome responses remains poorly understood. This is especially evident for peracetic acid, whose impact on the gut microbiome has not yet been studied. Understanding how microbiome dynamics vary by host age is critical to improving antibiotic stewardship in aquaculture. In this study, juvenile and sexually mature brown trout (Salmo trutta) were used as a model to investigate the age-dependent effects of florfenicol and peracetic acid on the gut microbiome using a 16S rRNA metabarcoding approach.ResultsFish age significantly shaped taxonomic composition and microbial co-occurrence network structure of the gut microbiome, regardless of treatment. Juvenile trout exhibited greater microbiome volatility and a stronger response to both florfenicol and peracetic acid compared to adult fish, with disruptions persisting up to 11 days post-treatment. Temporal dynamics were also evident, with microbial shifts characterized by a decline in beneficial commensals like Cetobacterium and Lactococcus. Although overall abundance recovered by 18 days post-treatment, network positions of key microbial community members remained altered, particularly in juvenile fish. Opportunistic pathogens, including Aeromonas and Streptococcus, were enriched and assumed more central roles within the microbial networks in treated fish.ConclusionThe initial composition of the gut microbiome in brown trout is strongly influenced by fish age, which in turn affects the microbiome’s response to antibiotic disruption. Juveniles displayed higher susceptibility to microbiome perturbation, and although recovery was observed at the community level, network properties remained altered. This study also provides the first evidence that external peracetic acid application can disrupt gut microbial communities. Since compositional shifts are often linked to functional alterations, even short-term disruptions may have important consequences for host health in developing fish. These findings emphasize the importance of considering gut microbial community structure in relation to fish age in aquaculture management practices.

Effects of Brook Trout Invasion on Behavioral and Dietary Shifts in Brown Trout.

Behavioral variation within a population is generally maintained by frequency dependent selection, allowing various personalities to coexist. Bolder individuals usually engage in more risky behaviors that can gain fitness benefits such as growth under certain conditions. Therefore, it has been suggested that there should be a link between personality and dietary niches, but the results so far are inconsistent. In addition, the equilibrium of the distribution of behavioral traits and the trophic niche of native populations may shift following the introduction of an invasive species. Here, using the invasive brook trout (Salvelinus fontinalis) and native brown trout (Salmo trutta) as model species in two different natural streams in Sweden, we aimed to test whether (1) the trophic niche of native brown trout living in allopatry and brown trout living in sympatry with brook trout differ and (2) bolder brown trout individuals utilize a different foraging niche. Our results suggest that there is a dietary niche convergence between brown trout and brook trout, which likely is a result of brook trout invasion, but that the trophic niche of native brown trout varies across streams, possibly due to differing invasion impacts (varying ratio of brook trout to brown trout). We also found a strong positive correlation between trophic position and personality of brown trout irrespective of the presence of brook trout.

Viral Pathogens in Free‐Living Salmonids: Aquaculture and Ecosystem Implications

ABSTRACTSalmonids play vital ecological and economic roles and have been introduced to many regions worldwide. When not held captive in farms, they can be classified as free‐living salmonids, a group that includes native, feral, and naturalized populations, as well as fish that have escaped from aquaculture facilities. Compared to farmed salmonids, knowledge of viral infections affecting free‐living populations remains limited, creating a significant gap in understanding the ecological impacts of interactions between aquaculture and natural ecosystems. To address this knowledge gap, we reviewed global reports and summarized the information on viruses infecting free‐living salmonids. Most viral detections reported in the reviewed studies were found in native wild salmonids, with escaped salmon ranking second in detection frequency. Atlantic salmon (Salmo salar), sockeye salmon (Oncorhynchus nerka), and brown trout (Salmo trutta) were the most commonly reported host species. The most frequently reported viruses included infectious hematopoietic necrosis virus (IHNV), piscine orthoreovirus (PRV), infectious pancreatic necrosis virus (IPNV), and infectious salmon anemia virus (ISAV). Evidence from some studies suggests a bidirectional transmission of viruses between farmed and free‐living salmonids, highlighting the complex interplay between these groups. Beyond their potential role as reservoirs for aquaculture pathogens, free‐living salmonids may also be negatively impacted by viruses from farmed fish, contributing to the decline of native populations. By enhancing our understanding of the prevalence and impacts of viral infections in free‐living salmonids, we can inform future research and management strategies to protect the health of both farmed and wild fish populations.

Acute cardiorespiratory effects of 6PPD-quinone on juvenile rainbow trout (Oncorhynchus mykiss) and arctic char (Salvelinus alpinus).

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-quinone) is an environmental transformation product of the widely used rubber tire antioxidant, 6PPD. Found in stormwater runoff, 6PPD-quinone has been reported to cause acute lethality at ≤1 μg/L in salmonids like coho salmon, rainbow trout, and brook trout. Conversely, other species such as Arctic char and brown trout are insensitive, even when exposed to significantly greater concentrations (3.8-50 μg/L). Sensitive species exhibit symptoms such as gasping, spiraling, increased ventilation, and loss of equilibrium, suggesting a possible impact on cardiorespiratory physiology. This study investigated sublethal 6PPD-quinone toxicities, focusing on cardiovascular and metabolic effects in two salmonids of varying sensitivity: a sensitive species, rainbow trout (Oncorhynchus mykiss) and a tolerant species, Arctic char (Salvelinus alpinus). Fish were exposed to measured concentrations of 0.59 or 7.15 μg/L 6PPD-quinone, respectively, in respirometry chambers for 48 h to assess temporal changes in resting oxygen consumption compared to unexposed controls. Following exposure, cardiac ultrasound and electrocardiography characterized cardiac function in vivo, while blood gas analysis examined blood composition changes. In both species, changes in resting oxygen consumption were observed. In rainbow trout only, a decrease in end systolic volume and an increase in passive ventricular filling, cardiac output, and PR interval length were observed, indicating cardiac stimulation. Cardiorespiratory symptoms observed following rainbow trout exposure might partly be driven by a significant increase in methemoglobin, resulting in an impaired ability to oxygenate tissues. This study is the first to examine the effects of 6PPD-quinone exposure on the cardiorespiratory system of salmonid fishes and provides information invaluable to a better understanding of the mechanism of 6PPD-quinone toxicity.

Streamlining boldness measurement in fish: A practical approach to field studies.

Streamlining boldness measurement in fish: A practical approach to field studies.

Metabarcoding Versus Species‐Specific Primers to Estimate Salmo trutta Biomass and Density in Mountain Streams

ABSTRACTEnvironmental DNA (eDNA) is a cost‐efficient, noninvasive method to monitor fish populations, but the quantitative aspect of this technique (e.g., estimating biomass or densities) remains underexplored. Few studies have established relationships between fish DNA concentration and biomass/density. Here, we investigate the relationship between eDNA concentration (copies per liter) and trout biomass and densities estimated by electrofishing in mountain streams of Picos de Europa National Park (Spain). We assessed eDNA effectiveness in inferring biomass/density using 18S rRNA (18S) and cytochrome c oxidase I (COI) metabarcoding, and quantitative PCR with a COI‐specific Salmo trutta primer, each performed with different datasets from the same sampling points. Salmonidae eDNA concentration positively correlates with trout biomass and density. Both 18S and specific‐COI markers showed a significant increase in DNA concentration as trout biomass and density rose in electrofishing surveys. However, general COI did not exhibit significant trout DNA concentration and biomass/density relationships, despite providing greater taxonomic resolution at the species level. Further analysis exploring eDNA concentration and biomass/densities across different trout size classes (fry, juvenile, and adult) revealed that juvenile trout biomass contributed the most to the observed eDNA concentration–biomass/density relationship. Our results suggest that DNA concentration estimated from metabarcoding, when using an appropriately selected primer, can reliably indicate trout biomass and density in these mountain streams where trout is the dominant species. Although quantitative PCR showed similar trends, it had lower explanatory power. This study highlights the importance of integrating a quantitative framework in metabarcoding for ecological monitoring and biodiversity assessments. Factors such as amplicon length, genetic region, marker specificity, or fish size class can influence the relationship between sequencing reads and electrofishing data. This methodology could aid the conservation and management of fish populations and other communities, though further research is needed to extend these results and assess eDNA detection reliability.

Genetic Monitoring of Brown Trout Released Into a Novel Environment: Establishment and Genetic Impact on Natural Populations.

Translocations are carried out either unintentionally or intentionally for conservation or management reasons. In both cases, translocated populations may genetically impact natural populations via introgression. Understanding how genetic background may affect an establishment in a novel environment and the potential risks for native populations is important for biodiversity conservation. Here, using a panel of 96 SNPs, we monitor the establishment of two genetically and ecologically distinct brown trout populations released into a mountain lake system in central Sweden where trout did not occur prior to the release. The release was carried out in 1979, and we monitor the establishment over the first three decades (5-6 generations) in seven lakes downstream of the release site. We find that extensive hybridization has occurred, and genes from both populations exist in all lakes examined. Genes from the population that was nonmigratory in its native environment have remained to a higher degree in the area close to the release site, while genes from the population that was more migratory in its native habitat have spread further downstream. All established populations exhibit higher levels of genetic diversity than the released populations. Natural, stream-resident brown trout populations occur ~15 km downstream of the release site and below a waterfall that acts as an upstream migration barrier. Released fish have spread genes to these populations but with low introgression rates of 3%-8%. Recently adopted indicators for monitoring genetic diversity were partly able to detect this introgression, emphasizing the usefulness of genetic indicators in management. The SNP panel used in this study provides a similar picture as previously used allozymes, showing that older marker systems with fewer loci may still be useful for describing the population structure.

Differences in trophic niches and life-history traits between brook trout and brown trout in alpine lake food webs

Abstract Trophic ecology of lacustrine salmonid populations is well-documented in North America and northern Europe, highlighting their role in aquatic ecosystems. However, there is a notable gap in comparative studies on the trophic niches and life-history traits of salmonids in Iberian alpine lakes, which are increasingly impacted by introduced fishes. We compared the trophic ecology and life-history traits of brook trout (Salvelinus fontinalis) and brown trout (Salmo trutta) in two alpine lakes of central Spain, providing insights into their ecological roles and potential responses to environmental pressures. Compared to brown trout, brook trout displayed a broader dietary niche (based on isotopes and stomach contents), higher reliance on terrestrial food resources, higher δ13C values and higher trophic position. Thus, the higher trophic position of top predator salmonids suggests a slightly longer food chain in the lake with brook trout. Brook trout showed a higher trophic plasticity indicated by an ontogenetic dietary shift from zooplanktivory to more generalist foraging on terrestrial prey. Brook trout displayed life-history traits such as early maturation, which likely facilitate successful establishment in alpine lakes. Our study suggests that the trophic plasticity and adaptive life-history traits likely support the successful establishment of non-native salmonids in alpine lake food webs.

The Molecular Monitoring of an Invasive Freshwater Fish, Brown Trout (Salmo trutta), Using Real-Time PCR Assay and Environmental Water Samples.

Salmo trutta, commonly known as brown trout, is an invasive species that has established itself in various regions, including South Korea, where it poses ecological risks to native freshwater fish populations. To enable natural habitat restoration, S. trutta needs to be monitored, but traditional monitoring techniques are associated with several limitations. Therefore, in this study, we aimed to apply a sensitive and specific real-time PCR (qPCR) assay using a set of primers and a hydrolysis probe specific to the mitochondrial cytochrome b gene of S. trutta. Environmental DNA (eDNA) was extracted from river-water samples collected downstream of the Soyang Reservoir and around the Uiam Reservoir between January and March 2023. The qPCR assays successfully detected S. trutta eDNA in 11 of the 24 samples, with high concentrations found upstream and downstream of the Soyang River. Our results demonstrate the effectiveness of qPCR assay for the S. trutta detection in aquatic environments and highlight its potential for monitoring the spread of this species, especially in areas that are difficult to survey using traditional methods. This molecular approach offers a more efficient tool for S. trutta population management, mitigating its impact on native biodiversity.