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Background: The salmonid pathogen Flavobacterium psychrophilum poses a significant economic threat to global aquaculture, yet our understanding of its genetic and phenotypic diversity remains incomplete across much of its geographic range. In this study, we characterise the genetic and phenotypic diversity of 70 isolates collected from rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta m. fario) from fish farms in the Czech Republic between 2012 and 2019 to compare their genomic content with all draft or complete genomes present in the NCBI database (n = 187). Results: The Czech isolates underwent comprehensive evaluation, including multiplex PCR-based serotyping, genetic analysis, antimicrobial resistance testing, and assessment of selected virulence factors. Multiplex PCR serotyping revealed 43 isolates as Type 1, 23 as Type 2, with sporadic cases of Types 3 and 4. Multi-locus sequence typing unveiled 12 sequence types (ST), including seven newly described ones. Notably, 24 isolates were identified as ST329, a novel sequence type, while 22 were classified as the globally-distributed ST2. Phylogenetic analysis demonstrated clonal distribution of ST329 in the Czech Republic, with these isolates lacking a phage sequence in their genomes. Antimicrobial susceptibility testing revealed a high proportion of isolates classified as non-wild type with reduced susceptibility to oxolinic acid, oxytetracycline, flumequine, and enrofloxacin, while most isolates were classified as wild type for florfenicol, sulfamethoxazole-trimethoprim, and erythromycin. However, 31 isolates classified as wild type for florfenicol exhibited minimum inhibitory concentrations at the susceptibility breakpoint. Conclusion: The prevalence of the Czech F. psychrophilum serotypes has evolved over time, likely influenced by the introduction of new isolates through international trade. Thus, it is crucial to monitor F. psychrophilum clones within and across countries using advanced methods such as MLST, serotyping, and genome sequencing. Given the open nature of the pan-genome, further sequencing of strains promises exciting discoveries in F. psychrophilum genomics. [ABSTRACT FROM AUTHOR]

Controlled sediment flushing operations (CSFOs) allow to recover reservoirs storage loss while rebalancing the sediment flux interrupted by dams but, at the same time, may cause unacceptable ecological impact. In this study, we investigated the responses of the food web of an upland stream to a CSFO, focusing on the effects of fine sediment deposition detected in three different mesohabitats, i.e., a pool, a riffle, and a step-pool. The field campaign lasted two years and included repeated measurements of fine sediment deposits, and sampling of periphyton, benthic macroinvertebrates and fishes. A moderate and patchy deposition occurred due to the CSFO with short and medium-term ecological impact on the lower trophic levels of the food web, which may affect the whole ecosystem functioning. The monitoring of all available mesohabitats in the investigated stream allowed to detect variations in the ecological response to CSFO, providing a more adequate assessment of the impact. As expected, sedimentation was larger in the pool but, in contrast to our hypotheses, the impact was lower and the recovery was longer for the benthic organisms inhabiting the riffle. In the case of fishes, no lethal impact of both brown trout and bullhead was recorded in the short term but the occurrence of longer lasting effects could not be excluded. To date, this is one of the few studies dealing with a detailed integrative assessment of the downstream impact of sediment management from reservoir on both abiotic and biotic components of stream ecosystem. [ABSTRACT FROM AUTHOR]

Cold‐water lakes situated in high latitudes and altitudes have pivotal socio‐ecological importance both globally and locally. However, they are increasingly threatened by multiple anthropogenic stressors, such as climate change, hydropower and invasive species. The development of efficient management strategies is therefore urgently needed and requires a comprehensive understanding of the factors influencing the biodiversity and ecological processes of these ecosystems. We provide a holistic knowledge base for informed future research and management by addressing the interplay between local and global environmental drivers of food webs in Arctic charr (Salvelinus alpinus, Salmonidae) and brown trout (Salmo trutta, Salmonidae) dominated cold‐water lakes in Fennoscandia. The trophic niche and population dynamics of these generalist top consumers provide extensive insights into the effects of natural and anthropogenic drivers on food webs in intensively studied Fennoscandian cold‐water lakes, covering marked biogeographical gradients in abiotic and biotic conditions. Drawing on a synthesis of existing literature, our focus is on three pivotal drivers: (1) lake location and connectivity, (2) lake area and morphometry and (3) fish community composition. These drivers significantly influence the complexity and the origin and flow of energy in lake food webs, and ultimately the size structure of the charr and trout populations. Furthermore, we highlight ongoing environmental changes in Fennoscandian cold‐water lakes caused by hydropower and invasive species. Finally, we identify crucial knowledge gaps and propose management actions for improving the future state of Fennoscandian cold‐water lake ecosystems and their charr and trout populations. [ABSTRACT FROM AUTHOR]

Traditional methods for collecting robust samples of recreational effort and catch data from offsite mail or telephone surveys are becoming increasingly difficult to obtain. Smartphone applications (apps) that allow recreational fishers to report their effort and catch are a potentially viable data collection tool, but may be biased. We evaluated demographics, effort, and catch of freshwater anglers in south‐west Western Australia by comparing independent data from concurrent phone‐recall and app‐diary surveys for a 12‐month period in 2017–2018. Survey modes differed, with higher proportions of app respondents angling in freshwater and being avid anglers. For both survey modes, unweighted mean effort (days fished) was higher in dams than rivers, but was higher from the phone‐recall survey for dams and rivers. The unweighted distribution of kept and released catches (number of fish per angler) of rainbow trout (Oncorhynchus mykiss) and redfin (Perca fluviatilis) were similar between survey modes, with higher retention of redfin and higher release of rainbow trout. Kept and released rates of freshwater cobbler (Tandanus bostocki) and brown trout (Salmo trutta) were less common in both survey modes. Lower response rates and higher survey participation by avid anglers (≥20 days per year) from the app‐diary survey may bias survey data. Such bias needs to be addressed when using digital data collection methods, along with improving recruitment and retention of app participants. [ABSTRACT FROM AUTHOR]

Since the last century, the aquatic ecosystems of Tierra del Fuego have been impacted by the introduction of exotic species, among which the North American beaver Castor canadensis and salmonids stand out for their dispersion and abundance. This study is part of the project Strengthening Governance for the Protection of Biodiversity Through the Formulation and Implementation of the National Strategy on Invasive Alien Species, for the eradication of C. canadensis from the Isla Grande de Tierra del Fuego (IGTdF). This work aimed to evaluate the effect of beaver plantation removal on different aspects of the ichthyofauna of the IGTdF. Ichthyological sampling was carried out before and after the beaver ponds in six sites spread across the island. Three species of salmonids (Salmo trutta, Oncorhynchus mykiss and Salvelinus fontinalis) were captured, and an average increase of 43% in their abundance, changes in their diet, and differences in their energy density (higher or lower depending on the species) was observed after the removal of the beaver ponds. No native fish species were recorded before or after the removal of the beaver pond. The results suggest that the beaver ponds impact positively salmonid populations. [ABSTRACT FROM AUTHOR]

Temperature is a key driver in determining species distribution and abundance across climatic and geographical gradients. This occurs through direct physiological effects of temperature on performance, but may also result from temperature effects on species interactions.In the current study we experimentally test for temperature‐dependent competition between juvenile anadromous Atlantic salmon and brown trout and evaluate the role of temperature in shaping the relative abundance of these species across 88 Norwegian rivers.When the two species were reared in allopatry their somatic growth rates were similar across a range of temperatures (4–16°C).When reared in sympatry, the growth of salmon was greatly reduced at the coldest temperature relative to when reared in allopatry, whereas this was not the case for trout. The effect of interspecific competition on growth was more similar for the two species at warmer temperatures. Thus, interspecific competition effects were strongly asymmetric only at the lowest temperature, with trout outcompeting salmon.The results from the experiment were reflected in qualitative patterns of relative abundance of salmon and trout in natural populations, which shifted from being trout dominated in rivers with low summer temperatures to being salmon dominated in warmer rivers.These results provide an empirical example of how temperature effects on the relative abundance of species can only be understood in light of information about temperature‐dependent species interactions. [ABSTRACT FROM AUTHOR]

Endangered trout populations can be monitored with a variety of methods, the selection of which should consider social constraints and environmental variables known to affect method effectiveness. Here, we confront the effectiveness of four monitoring methods (removal with electrofishing, ELE; underwater camera survey, UCS; streamside visual survey, SVS; visual surveys with angling, VSA) to estimate the relative abundance of three populations of the endangered Mediterranean brown trout. The trout counts obtained via different methods were well correlated (r = 0.65–0.72), providing a coherent description of the relative pool abundance across the methods. However, the methods were differently affected by environmental variables, depending on the age classes of trout. Specifically, the adult and subadult counts provided by ELE and VSA were negatively and positively affected by the maximum pool depth, respectively; adult and subadult counts of VSA and the SVS were positively affected by pool area; the juvenile counts provided by the UCS were positively affected by pool shade and negatively affected by water turbidity; juvenile counts provided by VSA were positively affected by shade. Variables such as pool depth, area, shading, water turbidity and proportion of age classes can be hardly controlled in monitoring programs, and their bias could be modelled. Different sampling methods provided similar information about relative abundance and appeared equivalent. While ELE could be selected to collect samples and biometric data, monitoring relative abundance with the UCS, VSA, and SVS appears more suited and can also involve citizen scientists. [ABSTRACT FROM AUTHOR]

Human‐mediated species introductions are contributing to the biotic homogenization of global flora and fauna. Despite extensive research, we lack simple methods of predicting how and where an introduced species will spread and establish, particularly in species with complex life histories in aquatic ecosystems. We predict that spread can be modeled simply using the characteristics of the invading population, specifically species growth rate and dispersal capacity. In addition, we predict that the establishment of introduced species should be explained by the characteristics of the receiving ecosystem. Using the brown trout (Salmo trutta) invasion on the Island of Newfoundland as a case study, we fit and test a reaction–diffusion model with brown trout population data collected from the literature. Next, we use statistical models to assess the influence of a suite of abiotic (conductivity, pH, turbidity, calcium), biotic (Atlantic salmon occurrence), and landscape (watershed relief, watershed area, distance to original introduction) variables on brown trout establishment (i.e., presence–absence) patterns. We find that observed coastal spread in Newfoundland is slow (~4.4 km/year), and that it lies on the lower end of the range of predictions made by the reaction–diffusion model parameterized based on the estimates of growth rate and movement from the literature (predicted spread range 1.4 to 92 km/year). Also, we did not find evidence for a relationship between abiotic or biotic variables and brown trout establishment. However, we did observe that landscape variables of the distance to trout introduction point and estuary area may explain establishment patterns along the south coast of Newfoundland. Our results suggest the importance of using population‐specific parameterization and the need to integrate regional landscape factors that are generally applicable across biological invasions (e.g., distance to introduction), and those that are more specific to the ecology of the invader (e.g., estuary area). Our study contextualizes the mechanisms that contribute to a slow invasion by an aquatic species with a complex life history and reveals that future studies need to integrate a variety of methods to elucidate the processes governing invasions. [ABSTRACT FROM AUTHOR]

As a predatory fish that migrates between freshwater and marine environments, the sea trout (Salmo trutta) is important in linking these systems. This study investigated movement patterns of sea trout in a coastal fjord at the Swedish Skagerrak region from August 2018 to January 2019, using acoustic telemetry, while assessing these against environmental variables across different spatial and temporal scales. Six acoustic receivers were deployed in the fjord and a river, which flows into the upper reaches of the fjord, with the aim of detecting 20 sea trout that had been tagged with acoustic transmitters. Sea surface temperature and winds (east–west) affected movement patterns of the sea trout the most, while changes in atmospheric pressure were also important, but to a lesser extent. Sea surface temperature and atmospheric pressure both had a positive relationship with the number of detections, whereas stronger winds from the east (i.e. from land) resulted in more detections than stronger winds from the west (i.e. from the ocean). In addition, interesting diel (day–night) movement patterns were observed for some fish. A significant positive correlation was also discovered between the weight of the fish and the number of detections. This study offers insight in movements of sea trout that use coastal habitats and how environmental conditions can affect movement patterns in a fjord system. To further our understanding of sea trout movement patterns and connectivity, tracking from river, through fjord, out to sea and at a longer time scale with more variation in fish size would be valuable to understand more about the complex movement dynamics of this important species. [ABSTRACT FROM AUTHOR]

This investigation compared the spatial ecology and population dynamics of brown trout Salmo trutta L. between reservoirs with (impact; Langsett Reservoir) and without (control; Grimwith Reservoir) barriers to fish movements into headwater tributaries, and the effectiveness of a fish pass intended to remediate connectivity. Passive integrated transponder (PIT) telemetry revealed that fish that emigrated from Langsett and Grimwith tributaries were 1–3 and 0–2 years old, respectively, and predominantly did so in spring and autumn‐early winter in both systems. Weirs at Langsett Reservoir appeared to thwart emigration rate (26%) relative to Grimwith Reservoir (85%). Acoustic telemetry (two‐dimensional positions) in the impacted reservoir revealed that the largest home range was in October–December (95% monthly activity space S.D. up to 26.9 ± 6.69 ha in November), activity was influenced by both month and time of day, and fish occupied shallow water depths (relative to reservoir depth), especially at night. Brown trout tagged in Grimwith and Langsett Reservoirs (42.9% and 64.1%, respectively) and fish tagged in the tributaries that emigrated (37.2% and 27.7%, respectively) were detected immigrating into tributaries throughout the year. At both reservoirs, peak immigration for ≥3‐year‐old trout occurred primarily in autumn‐early winter. Overall passage efficiency went from 3% prior to remediation to 14% after and there was no significant increase in fish densities following the construction of the fish pass. Fish were attracted towards and entered the fish pass under a wide range of river levels, but only succeeded in passing upstream during low levels, which are uncommon druing the main migration period. Overall, this investigation significantly furthers our understanding of brown trout spatial ecology and population dynamics in reservoirs and headwater tributaries.   [ABSTRACT FROM AUTHOR]

Current procedures to establish vertebral column regionalization (e.g., histology) in fish are time consuming and difficult to apply. The aim of this study was to develop a more rapid and accurate radiology‐based method for Atlantic salmon (Salmo salar). A detailed analysis of 90 animals (4 kg) led to the establishment of region‐specific radiographic hallmarks. To elucidate its transferability to other salmonid species, radiography was carried out in brown trout (Salmo trutta), Arctic char (Salvelinus alpinus), rainbow trout (Oncorhynchus mykiss), pink salmon (Oncorhynchus gorbuscha), and Chinook salmon (Oncorhynchus tshawytscha). This method was also evaluated for whole ungutted fish. The vertebral column of Atlantic salmon can be subdivided into five regions (R1–R5) based on anatomy: postcranial (R1, V1, and V2), abdominal (R2, V3–V26), transitional (R3, V27–V36), caudal (R4, V37–V53), and ural (R5, V54–V59). The following specific radiographic hallmarks allow the identification of regions: (i) lack of ribs in R1, (ii) modified parapophysis of the first vertebra of R3, (iii) prominent hemal spine of the first vertebra of R4, and (iv) the separated hemal spine of the most cranial pre‐ural vertebra of R5. These hallmarks were all transferable to the other salmonid species assessed. The results include a further description of various region‐specific characteristics in Atlantic salmon. The method was found applicable for sedated/whole ungutted fish, verifying it as quick and easy compared to other regionalization methods. The regions defined by radiology in this study agree with the vertebral column regions recently defined for Chinook salmon (O. tshawytscha). Thus, and considering the results of this study on various salmonid species, the currently developed regionalization protocol can be generally used for salmonids. [ABSTRACT FROM AUTHOR]

In Japan, stocked chum salmon (Oncorhynchus keta) fry may have become the perfect prey for non-native brown trout (Salmo trutta), which are popular targets of anglers. If this is the case, fry stocking which is intended to boost commercial fishing may be helping to sustain the populations of an invasive predator. We used dietary and biochemical analyses to examine whether brown trout quickly restore their nutritional status following wintertime declines by preying upon chum salmon fry that are stocked in spring. We targeted six rivers in Hokkaido, Japan, three with fry stocking and three without. Changes in brown trout condition factor, triglyceride contents in muscle and serum, serum insulin-like growth factor-1 (IGF-1; an indicator of short-term growth), and docosahexaenoic acid (DHA; an essential fatty acid abundant in fish) content in muscle were examined between before stocking and during the stocking period in the six rivers. Dietary analysis showed that brown trout preyed on fry during the stocking period in all stocked rivers. Their nutritional status tended to be higher during the stocking period than before stocking in stocked rivers, but not in unstocked rivers. These results suggest that the massive stocking of chum salmon fry provides brown trout with the perfect prey to quickly restore their nutritional status and fuel increased growth; this may therefore be a controversial issue among stakeholders. [ABSTRACT FROM AUTHOR]

Simple Summary: Otoliths, which are calcium carbonate structures found in the inner ears of fish, can be used to describe the use of freshwater and saltwater habitats during the lifespan of fish. This is performed by looking at the ratio of strontium to calcium along the radius of an otolith, where a high ratio indicates the use of saltwater and a low ratio indicates the use of freshwater. We studied brown trout (Salmo trutta L.) from the Swedish River Emån. We found that most brown trout, as expected, spent the first 1–2 years of their lives in freshwater before migrating to the Baltic Sea. We also found that 13% spent considerably less than one year in the river, which was not expected based on classical life history patterns for brown trout. In addition, we found that brown trout, which ranged from 3 to 6 years of age, returned to freshwater 2.3 times, and the amount of time spent in freshwater after hatching was negatively related to the number of visits to freshwater. Previous studies have explained that precocial migration occurs when there is a risk of drought. This explanation cannot apply to the permanently flowing River Emån. By analyzing otolith microchemistry, we examined the use of freshwater and marine environments by brown trout Salmo trutta L. that spawn in the Swedish River Emån and migrate to the Baltic Sea. We estimated the time juveniles spent in freshwater and the number of times the fish returned to freshwater, presumably to spawn. Twenty-six percent of the fish migrated to sea by 1 year of age. However, 13% spent less than one year in the river. Most brown trout (48%) migrated to the sea between 1 and 2 years of age. On average, brown trout, which averaged 4.4 years in age (range 3–6 years), returned to freshwater 2.3 times, and there was an inverse relationship between time spent in freshwater after hatching and the number of visits to freshwater. Our results do not support the classical life history pattern, where brown trout spend one or more years in freshwater before migrating to the sea. Here, we found evidence that part of the population leaves freshwater during their first year. While the cause for precocial migration in the River Emån is not known, our results from this permanently flowing river do not support the idea proposed for other Baltic Sea populations, where the risk of drought has been suggested to be the cause. [ABSTRACT FROM AUTHOR]

When investigating potential causes of apparent declines in European brown trout angling catch rates over recent decades, fisheries management procedures are rarely considered. Using a 20‐year dataset of brown trout stocking and angling catch rates, we found a significant decline in brown trout angling catch rates in streams of East Bohemia, Czechia, were well predicted by a reduction in the number of brown trout stocked at catchable size (i.e. age‐3+). Increased stocking of smaller sized brown trout, and recent sporadic efforts to stock large brown trout, had no effect on catch rates. Our findings suggest that brown trout catch rates were related to ‘put‐and‐take’ stocking, in which non‐native rainbow trout gradually replaced brown trout in both stocking and catches. Our study stresses the importance of considering historical fisheries management, a sometimes‐neglected factor, when studying angling catches of economically important species. [ABSTRACT FROM AUTHOR]

Stocking can affect population density but may influence fish growth responses to changes in environmental conditions. In a multi‐year field experiment, we investigated the interactive effects of density and temperature on the growth of young‐of‐the‐year anadromous brown trout in streams stocked with wild‐origin hatchery‐reared brown trout. Fish origin was the predominant factor influencing growth rate, thereby highlighting the importance of discriminating between wild and hatchery‐reared individuals. Growth was positively related to temperature (degree‐days) and negatively related to density, with the growth response to temperature modulated by density. Temperature had a more pronounced effect on juvenile growth at low density than at high density. The reduced growth response to temperature caused by density could reduce population resilience and should be considered in management and conservation strategies. [ABSTRACT FROM AUTHOR]

Fish involved in telemetry studies are usually handled, anaesthetized, and subjected to internal tag implantation, all of which have the potential to disrupt the fish's physiology, migratory patterns, food‐seeking behavior, growth, and survival. As fish tagging is a widespread standard method in aquaculture, it is crucial to gain a better understanding of the short‐ and long‐term effects on various aspects of fish welfare. The experimental outcomes of each phase of the surgical process and how it affects the fish's ability to recover during the post‐tagging period, particularly in small salmonids, are poorly understood. Thus, it is essential to measure the extent of these adverse effects to precisely extrapolate differences in surgical intervention for tagged fish compared to their unmanipulated counterparts. In our study, we explored the post‐tagging impacts on wild‐captured brown trout (Salmo trutta) (1+) juveniles. We examined the time effect on fish recovery after each phase of the surgical procedure, which included anaesthetized fish, followed by surgery with/without internal tagging, within the first 2 and 25 h post‐surgery. Common blood parameters (glucose levels and haematocrit) and endpoints related to swimming behavior, including fish movement, direction, and social distancing aspects, were studied. The findings of the study indicated no significant changes in glucose and haematocrit levels over the observational period. Fish subjected to anesthesia and tagging procedures exhibited a significant decrease in swimming activity, recovering to baseline levels 1 h after anesthesia and 24 h after surgical (tagging) procedures in recirculating aquaculture systems. Tagged fish showed less social proximity than non‐tagged fish in the first 2 h post‐surgery. A significant effect size was observed between nonsurgical (anesthesia) and surgical (non‐tagged/tagged) groups, highlighting a more pronounced impact associated with surgical tag interventions. Our data indicate that the magnitude of behavioral response was significantly influenced by tag weight (~1.4% body/tag weight ratio in water) among manipulated fish groups. Consequently, our study reveals that wild‐captured S. trutta juveniles, tagged with commercially available low‐weight acoustic tags (V6, 69 kHz), experienced negative effects on swimming performance. In surgical studies, acknowledging potential influences is crucial for accurately inferring fish physiological and behavioral status. Emphasizing fish recovery potential in both short‐ and long‐term periods is essential for quantifying tagging effects. Future research should prioritize exploring alternative tagging technologies and refining methodologies, with a particular focus on assessing telemetry's impact on socioeconomically relevant small salmonids. [ABSTRACT FROM AUTHOR]

Populations of anadromous brown trout, also known as sea trout, have suffered recent marked declines in abundance due to multiple factors, including climate change and human activities. While much is known about their freshwater phase, less is known about the species' marine feeding migrations. This situation is hindering the effective management and conservation of anadromous trout in the marine environment. Using a panel of 95 single nucleotide polymorphism markers we developed a genetic baseline, which demonstrated strong regional structuring of genetic diversity in trout populations around the English Channel and adjacent waters. Extensive baseline testing showed this structuring allowed high‐confidence assignment of known‐origin individuals to region of origin. This study presents new data on the movements of anadromous trout in the English Channel and southern North Sea. Assignment of anadromous trout sampled from 12 marine and estuarine localities highlighted contrasting results for these areas. The majority of these fisheries are composed predominately of stocks local to the sampling location. However, there were multiple cases of long‐distance movements of anadromous trout, with several individuals originating from rivers in northeast England being caught in the English Channel and southern North Sea, in some cases more than 1000 km from their natal region. These results have implications for the management of sea trout in inshore waters around the English Channel and southern North Sea. [ABSTRACT FROM AUTHOR]

Introgression of non‐native conspecifics changes the genetic composition of wild populations, potentially leading to loss of local adaptations and fitness declines. However, long‐term data from wild populations are still relatively few. Here, we studied the effects of introgression in a Danish brown trout (Salmo trutta, L.) population, subjected to intensive stocking with domesticated hatchery fish of non‐native origin. We used wild‐caught genetically wild and admixed trout as well as fish from the partly domesticated hatchery strain used for stocking the river up until ~15 years prior to this study, to produce 22 families varying in hatchery/wild admixture. Following a replicated common‐garden experiment conducted in fish tanks from first feeding through 23 weeks at 7, 12, and 16°C, we observed a significant positive relationship between family admixture and fish size upon termination, an effect observed through all levels of admixture. Furthermore, the admixture effect was most distinct at the higher rearing temperatures. Although the hatchery strain used for stocking had been in culture for ~7 generations, it had not been deliberately selected for increased growth. These data thus demonstrate: (i) that growth had increased in the hatchery strain even in the absence of deliberate directional selection for this trait, (ii) that the increasing effect of admixture by temperature could represent inadvertent selection for performance in the hatchery strain at higher temperatures, and most significantly, (iii) that despite undergoing up to five generations of natural selection in the admixed wild population, the genetically increased growth potential was still detectable and thus persistent. Our findings suggest that altered growth patterns and potentially their cascading effects are of importance to the severity of hatchery/wild introgression, especially under changing‐climate scenarios and are of general significance to conservation practitioners seeking to evaluate long‐term effects of intra‐specific hybridization including under recovery. [ABSTRACT FROM AUTHOR]

The European Commission requested scientific and technical assistance in the preparation of a EU‐wide baseline survey of antimicrobial resistance (AMR) in bacteria from aquaculture animals. It is recommended that the survey would aim at estimating the occurrence of AMR in Aeromonas spp. isolated from Atlantic Salmon (Salmo salar), European seabass (Dicentrarchus labrax) and trout (Salmo trutta, Salvelinus fontinalis, Oncorhynchus mykiss) intended to consumption, at harvesting (at farm/slaughter), at the EU level and in addition, at estimating the occurrence and diversity of AMR of Escherichia coli, Enterococcus faecium, Enterococcus faecalis, Vibrio parahaemolyticus and Vibrio alginolyticus in blue mussel (Mytilus edulis) and Mediterranean mussel (Mytilus galloprovincialis) from production areas and at dispatch centres at the EU level. These technical specifications define the target populations, the sample size for the survey, sample collection requirements, the analytical methods (for isolation, identification, phenotypic susceptibility testing and further genotypic analysis of some of the bacteria targeted) and the data reporting requirements. The data to be reported by the EU Member States to support this baseline survey are presented in three data models. The results of the survey should be reported using the EFSA reporting system. This publication is linked to the following EFSA Journal article: http://onlinelibrary.wiley.com/doi/10.2903/j.efsa.2024.8896/full [ABSTRACT FROM AUTHOR]

Dams fragment streams, alter hydrology and habitat, and facilitate establishment of nonnative species worldwide to the detriment of native biota. Understanding and mitigating these effects to conserve and restore stream fish assemblages has relied on short‐ and long‐term datasets to assess acute and chronic change through time, craft management strategies, and measure remediation success. We used sampling records collected over a 29‐year period (1993–2021) to examine likely causes of fish assemblage change in the Cache la Poudre River, Colorado, USA. Numerous low‐head dams have reduced connectivity and altered flow, temperature, and habitat in the transition zone, a reach that historically supported rare and sensitive taxa valuable to regional biodiversity. We found diversity, distribution, and abundance of native species declined since the early 1990s, with formerly rare taxa extirpated and some common species becoming rare. Native taxa remained numerically dominant in warmer downstream reaches most affected by streamflow diversion but were incrementally reduced in richness and abundance upstream of low‐head dams without fishways. Concurrently, nonnative Brown Trout Salmo trutta increased in distribution and abundance, dominating upstream reaches that receive cooler and more stable flows, and expanding into downstream reaches where they were formerly absent, with likely negative consequences for native fishes. In the absence of mitigation, these collective effects, plus recent wildfire disturbance and future water development, will continue to degrade stream fish assemblages in our study area, and worldwide, where resource managers face the often‐competing interests of conserving native species, providing recreational fisheries, and meeting increasing water demands. [ABSTRACT FROM AUTHOR]

Aim: The British Isles have been worked for millennia to extract metal ores to feed industrial development, leaving a legacy of mine water pollution that continues to impact freshwater communities in many regions. Brown trout (Salmo trutta L.) have long been observed to persist in these metal‐impacted systems as apex predators, with previous studies showing a small number of impacted populations to be highly genetically divergent. We sought to understand the scale of genetic diversity across regions and the repeatability of genetic divergence in trout populations affected by metal pollution. Location: We examined four mine water‐impacted regions across the British Isles: west Wales, northeast England, southwest England and southeast Ireland. Methods: We employed a panel of 95 SNP loci to screen 1236 individuals from 71 sites representing paired metal‐impacted and clean sites from across the four regions. From these, we obtained diversity statistics, assessed genetic structuring of populations and modelled historical demographic scenarios to understand which factors most credibly explain genetic variation in divergent populations. Results: We evidenced hierarchical population structure in the regions studied, in line with expectations from phylogeographic history. However, in a hierarchical analysis of genetic structuring the first level of differentiation was driven by the divergence of the metal‐impacted trout of Cornwall in southwest England. Within regions we observed reduced genetic diversity and repeated patterns of local genetic sub‐structuring between paired samples from metal‐impacted and relatively clean sites. Demographic history analyses suggested the timing of these splits to be relatively recent and to be associated with periods of peak mining activity. Main conclusions: Our findings demonstrate distinct patterns of genetic isolation and reduced diversity arising from legacy pollution in freshwater ecosystems, with impacts being most apparent where both chemical pollution and physical barriers are present. Management should focus on the amelioration of mine water wash‐out and the removal of barriers to fish movement to safeguard genetic diversity in impacted populations. [ABSTRACT FROM AUTHOR]

Probiotics play an important role in animal production, providing health benefits to the host by improving intestinal microbial balance. In this study, we added three different probiotics, Saccharomyces cerevisiae (SC), Bacillus licheniformis (BL), and lactic acid bacteria (LAB), and compared them with the control group (CON), to investigate the effects of probiotic supplementation on growth performance, gut microbiology, and gut flora of S. trutta. Our results showed that feeding probiotics improved the survival, growth, development, and fattening of S. trutta. Additionally, probiotic treatment causes changes in the gut probiotic community, and the gut flora microorganisms that cause significant changes vary among the probiotic treatments. However, in all three groups, the abundance of Pseudomonas, Acinetobacter, and Rhizophagus bacterial genera was similar to that in the top three comparative controls. Furthermore, differences in the composition of intestinal microbiota among feed types were directly associated with significant changes in the metabolomic landscape, including lipids and lipid-like molecules, organic acids and derivatives, and organoheterocyclic compounds. The probiotic treatment altered the gut microbiome, gut metabolome, and growth performance of S. trutta. Using a multi-omics approach, we discovered that the addition of probiotics altered the composition of gut microbiota, potentially leading to modifications in gut function and host phenotype. Overall, our results highlight the importance of probiotics as a key factor in animal health and productivity, enabling us to better evaluate the functional potential of probiotics. [ABSTRACT FROM AUTHOR]

Atlantic salmon (Salmo salar), brown trout (Salmo trutta), and steelhead trout (Oncorhynchus mykiss) kelts have important conservation value for population resilience. Nonetheless, relative to other salmonid life-stages, knowledge on their behavior and survival is poor. This is especially true for kelt downstream migration in rivers fragmented by hydropower plants (HPP), even though the physiological and survival consequences are severe when functional connectivity is not provided. Here, the existing information about kelt downstream migration past HPP was summarized, beginning with an overview of iteroparous salmonid lifecycles. Then, the importance of kelts for conservation and the threats they encounter while migrating was discussed. Finally, the current corpus of peer-reviewed literature and reports focused on kelt migration in these systems was presented. Kelt studies have focused on five main themes: (1) postspawning survival, (2) migration delays before dam passage, (3) passage selection and guidance efficiency, (4) passage mortality, and (5) the swimming behaviors of kelts during migration. Overall, there was a paucity of information about kelts and several information gaps and needs for future research. This work should inform scientists and river managers on the conservation of salmonids in regulated rivers, including the development of passage solutions for safe downstream migration. [ABSTRACT FROM AUTHOR]

Freshwater habitats and species have been lost because of river constructions, particularly regarding hydropower plants. Such losses may be mitigated, for example, by providing compensatory habitats. We present a case which had the objective to promote reproduction of a key species. Imatra City Brook in Finland was constructed in 2014 at a hydropower dam as a new reproduction area for brown trout (Salmo trutta L.). In the planning phase, the objective was to maximize suitable habitat types and areas with limited discharge. Monitoring shows promising densities of brown trout juveniles within only three years, inspiring hope for high smolt production in the main river. The increase in total macroinvertebrate abundances and taxa numbers in the second year of monitoring showed that species richness was comparable to that in natural rivers, and the brook was in a good ecological state. This case confirms the expected high potential of spawning and rearing channels to improve migratory fish stocks and compensate for lost habitats in constructed rivers. [ABSTRACT FROM AUTHOR]

Abstract The thermotactic response of brown trout (Salmo trutta) was examined with the goal to investigate potential effects of the emerging temperature-dependent fatal trout disease PKD (proliferative kidney disease). First the differences in cold-water preferences of two forms of brown trout, lacustrine (migratory) and riverine, were determined. Second, it was studied whether this preference was changed in fish infected with PKD. The experiment involved a one-week habituation period at 14 °C in a two-chamber runway followed by a week of 3 °C temperature difference between the two runways. The fish could freely move between lanes via an opening at the end where food was provided. The temperature manipulation was repeated twice, and there were 3 trials per experimental group. All fish developed a clear spatial preference in the test. Lacustrine trout demonstrated a preference for warmer water, while riverine trout preferred cooler water. This may increase the risk to PKD in the lacustrine form. Most strikingly, riverine trout experimentally exposed to Tetracapsuloides bryosalmonae, the parasite that causes PKD, demonstrated stronger cold-seeking behaviour than control fish. Cold seeking behaviour suggests the occurrence of a disease-induced behavioural chill response, which may play an important role in disease recovery. This demonstrates the significance of protecting river connectivity and cold-water sanctuaries as management strategies for preserving salmonid populations in a warming climate.

Brown trout is a partially migratory salmonid that makes use of diverse habitats to maximise growth and fitness. One of the most substantial threats to brown trout is infection with pathogens from open net‐pen fish farming, which creates hotspots for pathogen reproduction and transmission. Western Norway is a global hotspot for both fish farming and wild salmonids, which generates conflicts due to the impacts of the farms on the behaviour, survival, and fitness of salmonids that overlap with farming activities. In this study, we tagged adult brown trout (>35 cm) at two spatiotemporal intervals that corresponded to two different life history stages: springtime in the river when trout were completing overwintering and summer in the fjord when trout were in their marine feeding phase. The tagging revealed three different behaviours, fish that remained in freshwater, fish that migrated between freshwater and the fjord, and fish that remained in the estuary. Although some trout moved >100 km to the outer fjord areas, most trout remained relatively close to the river. Depth sensor transmitters in a subset of trout also revealed that the trout remained in the upper water column. Most of the horizontal and vertical movements therefore resulted in spatial overlap with fish farming for the migratory trout, but not for resident trout that remained in the estuary or in freshwater. Findings reveal the challenges of managing a fish with such behavioural plasticity but the urgency of recognising how important inner fjord habitats are for migratory brown trout. [ABSTRACT FROM AUTHOR]

Habitat fragmentation may cut off anadromous salmonids from parts of their potential native habitat and separate previously connected populations. Understanding the consequences of this is vital for fish management and prioritization of restoration activities. Here, we show that there is a significant difference in the body morphology, physiological stress response, and aspects contributing to aerobic capacity between juvenile anadromous brown trout, Salmo trutta, collected at a downstream site and an upstream site, separated by 2 km and several challenging stream sections, in a small unfragmented stream system in western Sweden. Following a standardized stress test, there were significant differences between fish from the upstream and downstream sites (plasma cortisol concentration, plasma osmolality, hematocrit, hemoglobin concentration, and mean corpuscular hemoglobin concentration). Plasma glucose concentration did not significantly differ between fish from the two sites. Fish from the upstream site had larger spleen mass, although there was no evidence of differences in ventricle mass or proportion of compact ventricular myocardium. These physiological differences indicate local variation in stress response and highlight the importance of considering local trait variation in river management. If a section of the river becomes fragmented or degraded, and there are differences in the juveniles in different parts of the river, the consequence for the population might be larger than the proportional loss of habitat. [ABSTRACT FROM AUTHOR]

Increasing rates of hybridization and introgression in managed populations of freshwater fish are a major threat to the long-term viability of native species. The conservation challenge begins with identifying native gene pools. For brown trout (Salmo trutta Linnaeus, 1758) in the Upper Danube drainage, this task is complicated by the presence of both naturally and anthropogenically induced admixture of highly divergent lineages (Atlantic and Danubian). Herein, a ddRADseq protocol was used to type 377 individuals from 24 populations in the Upper Danube in Austria and Germany, and from reference populations from adjacent drainages and commercial hatcheries. High genetic differentiation at small geographic scales was found among pure Danubian-lineage populations, especially in the Kalkalpen National Park (Austria). In the Upper Danube drainage of Germany, as well as in the Rhine and Elbe drainages, brown trout populations were predominantly of Atlantic-lineage origin – as were those of all commercial hatcheries. Most populations, however, showed various degrees of admixture between Danubian and Atlantic lineages, hypothesized to be the result of both natural and anthropogenic processes. We highlight the conservation value of pure Danubian-lineage populations, and the challenges promoting conservation of naturally admixed populations, while discouraging continued stocking and admixture via management activities. [ABSTRACT FROM AUTHOR]

The study investigated if gonad maturation in triploid brown trout, Salmo trutta, was entirely suppressed or only delayed, and if triploids could interbreed with diploid counterparts. Ten percent of the total number of 3‐year‐old triploid S. trutta, 15% of 4‐year‐old fish, and 17% of 5‐year‐old fish produced semen. Three and 4 years old triploid fish did not produce eggs, but 15% of the 5‐year‐old fish did so. The quantity and sperm motility of triploid semen did not differ from diploids, but the sperm concentration was significantly lower. When diploid eggs were fertilized with triploid semen (3n × 2n crosses), the percentage of eyed stage embryos, of hatched larvae, and of normal‐shaped larvae did not differ from the diploid controls. Circa 90% of 3n × 2n crosses had a ploidy level of 2.4n. In the remaining percentage of 3n × 2n crosses, the ploidy level was ≥2n and <2.4n. In sperm competition experiments where diploid eggs were fertilized with a mixture of diploid and triploid semen, 52% of the originating larvae had a ploidy level of 2n, 43% of 2.4n, and 5% of the fish were not exactly classified. From the start of feeding to an age of 248 days, the mortality rate of 3n × 2n interploid crosses and of 2n × 2n controls was similar. The growth of interploid crosses was significantly higher than that of controls. In triploid mature females, the egg mass per kilogram of body weight was significantly lower than in diploids. The mass of the non‐hardened eggs and the percentile weight increase during hardening did not differ from diploid eggs. When triploid eggs were fertilized with diploid semen (2n × 3n crosses), the development rate to normal hatched larvae was less than 10%. All originating larvae had a ploidy level of 3n. From the start of feeding to an age of 248 days, 2n × 3n crosses had a higher mortality rate (15%) than diploid controls (<5%). Growth of this type of interploid crosses was reduced in comparison to controls. Therefore, triploids introduced into natural waters for recreational fisheries or escaping from farms may interbreed with diploid counterparts. This not only alters the genotypes of local populations but also changes the ploidy levels. [ABSTRACT FROM AUTHOR]

Graphene-based nanomaterials are exceptionally attractive for a wide range of applications, raising the likelihood of the release of graphene-containing nanoparticles into aquatic environments. The growing use of these carbon nanomaterials in different industries highlights the crucial need to investigate their environmental impact and evaluate potential risks to living organisms. The current investigation evaluated the nanotoxicity of graphene (nanoflakes) and graphene oxide (GO) nanoparticles on the cardiorespiratory responses (heart rate, gill ventilation frequency), as well as the swimming and nesting behavioral parameters of early stage larvae and juvenile salmonids. Both short-term (96 h) and long-term (23 days) exposure experiments were conducted using two common species: brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss). The findings demonstrated notable alterations in fish nesting behavior, swimming performance, and cardiorespiratory functions, indicating the potential toxicity of nanoparticles. This impact was observed at both physiological and whole-organismal levels in salmonids at early stages. Future investigations should explore different types of nanocarbons and their potential enduring effects on fish population structure, considering not only individual survival but also broader aspects of development, including feeding, reproductive, and other social dynamics. [ABSTRACT FROM AUTHOR]

Huchen or Danube salmon Hucho hucho (Linnaeus 1758) is one of the largest salmonid species in the world and the largest species native to the Danube basin in Europe. In Serbia, this species inhabits the Drina river system, as well as the upper reaches of the River Ibar. It has already been introduced into the rivers Moravica and Đetinja, which are part of the Zapadna Morava catchment area. Most recently, huchen was stocked from its native habitat into the rivers Jerma and Nišava in the Južna Morava river system, and into the River Mlava which flows into the Danube. Huchen is listed as an endangered (EN) species on the IUCN Red List of Threatened Species and is protected in Serbia, and a Conservation Action Plan has already been adopted. In this report, we present the first record of this species in the River Grza, which is a part of the Velika Morava river system, outside its native range. The ichthyofauna of this recipient river consists of brown trout Salmo trutta L. and Eurasian minnow Phoxinus phoxinus L. In a small river of a fragile ecosystem such as this, the introduction of huchen as an apex predator can seriously harm the species of the native fish community, which warns of the illegal stocking measures currently in process. [ABSTRACT FROM AUTHOR]

In recent decades, rapid technological advancements have led to a growing influence of anthropogenic electromagnetic fields on aquatic ecosystems. These fields predominantly stem from energy infrastructure facilities such as wind farms, high-voltage lines, underwater transmission cables, and power plants. As a result, there has been increasing interest in understanding the effects of heightened electromagnetic fields on aquatic ecosystems and their inhabitants, including developing fertilized eggs. This study focused on examining the effects of electromagnetic fields generated by high-voltage lines and power transformers, which are integral to transmission nodes and power plants, on the directional axis of symmetry in sea trout Salmo trutta (Linnaeus, 1758) embryos and their survival rates. The research was carried out at "PGE Dolna Odra in Nowy Czarnów", one of Poland's largest power plants (53°12′34′′ N, 014°28′16′′ E). Two experimental sites were selected, with electromagnetic field values ranging from 1.36 to 5.28 μT at the first site and 8.97 to 14.18 μT at the second site. A control sample was also established in an area unaffected by anthropogenic electromagnetic fields. The study findings revealed that electromagnetic fields originating from energy infrastructure facilities significantly affect the spatial orientation of sea trout embryos. The embryos' response was influenced to be influenced by both the strength and direction of the electromagnetic fields. At the first test site, lower field values caused deviations from the natural alignment axis in the geomagnetic field (north-south), resulting in a shift towards the primary field source. In contrast, the second test site, with higher electromagnetic field values, showed no significant spatial orientation in embryo alignment. Additionally, the study highlighted that higher electromagnetic field values accelerated the rate of embryogenesis but also led to an increased mortality rate among sea trout embryos. [ABSTRACT FROM AUTHOR]

Laboratory studies on embryos of salmonids, such as the brown trout (Salmo trutta), have been extensively used to study environmental stress and how responses vary within and between natural populations. These studies are based on the implicit assumption that early life‐history traits are relevant for stress tolerance in the wild. Here we test this assumption by combining two data sets from studies on the same 60 families. These families had been experimentally produced from wild breeders to determine, in separate samples, (1) stress tolerances of singly kept embryos in the laboratory and (2) growth of juveniles during 6 months in the wild. We found that growth in the wild was well predicted by the larval size of their full sibs in the laboratory, especially if these siblings had been experimentally exposed to a pathogen. Exposure to the pathogen had not caused elevated mortality among the embryos but induced early hatching. The strength of this stress‐induced change of life history was a significant predictor of juvenile growth in the wild: the stronger the response in the laboratory, the slower the growth in the wild. We conclude that embryo performance in controlled environments can be a useful predictor of juvenile performance in the wild. [ABSTRACT FROM AUTHOR]

Rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta fario) are popular salmonid species that are reared for sport and recreational activities worldwide. In India, they were introduced and successfully established in the late 19th and early 20th centuries by the European settlers. However, until now, no studies have analysed the genetic integrity of wild trout populations in India. Therefore, this study aimed to analyse the genetic integrity of the wild rainbow trout populations from south India, one wild rainbow trout population from north India, and one wild brown trout population from north India. Genetic diversity studies revealed low genetic diversity in all the population with genetic bottlenecks in two trout populations from south India and disruption of alleles in the populations from north India. The results showed that the south Indian trout populations are in a comparatively poor condition than the north Indian trout populations, and stocking efforts have recently been carried out to enhance the genetic diversity of south Indian trout populations. [ABSTRACT FROM AUTHOR]

Objective: Populations of Brook Trout Salvelinus fontinalis have declined across their native range in North America due to a combination of habitat loss, exploitation, and introductions of nonnative salmonids. Brown Trout Salmo trutta have been widely introduced into streams across Brook Trout's native range and likely displace Brook Trout from suitable habitat. We evaluated the population response of Brook Trout to Brown Trout removal in Maple Dale Creek, a Wisconsin Driftless Area stream, relative to a nearby control stream with similar habitat and sympatric Brook Trout and Brown Trout. Variation in mean July stream temperature (11.6–16.4°C) among survey sites also allowed us to examine whether summer stream temperature was related to the population response of Brook Trout to Brown Trout removal. Methods: Between 2019 and 2023, we completed 33.7 km of single‐pass electrofishing during 56 site visits and removed 20,495 Brown Trout from 7.1 km of Maple Dale Creek upstream of an existing fish passage barrier. Concurrently, annual salmonid population estimates were completed on Maple Dale Creek and a nearby reference stream. Result: By 2023, biomass and density of Brown Trout age 1 and older (age 1+) had decreased to 1% or less of 2019 levels. Concurrently, age‐1+ Brook Trout biomass increased by a factor of 5.5 and density by a factor of 9.7. In contrast, age‐1+ Brook Trout density and biomass in our control stream remained low and relatively unchanged. Brook Trout population response to Brown Trout removal was positive across all four sites but exhibited a positive correlation to mean July stream temperature. Conclusion: Our results add to the evidence that Brown Trout displace Brook Trout populations from stream habitat, highlight the utility of electrofishing removal of Brown Trout in restoring Brook Trout populations where downstream fish passage barriers are present, and suggest that Brown Trout suppression of Brook Trout is positively related to summer stream temperature. Impact statementBrook Trout have declined across their native range, in part due to displacement by nonnative salmonids. We used a before–after–control–impact study design to evaluate the population response of Brook Trout to nonnative Brown Trout removal in a Wisconsin Driftless Area stream. Our results support previous findings that Brown Trout suppress Brook Trout populations in streams, highlight the utility of electrofishing in removing Brown Trout when a downstream fish passage barrier is present, and suggest a positive association between summer stream temperature and degree of Brook Trout population suppression by Brown Trout. [ABSTRACT FROM AUTHOR]

This study investigates the effect of large woody debris (LWD) on the abundance of juvenile Atlantic salmon (Salmo salar, L.) and anadromous brown trout (Salmo trutta, L.) in semi‐alluvial side channels of the river Aurlandselva (Norway) using point electrofishing and microhabitat mapping. Not the presence of LWD, but stream bed shelter availability and the distance to spawning grounds affected the fish abundance (fish/point), independent of other habitat components. LWD showed only an effect on fish abundance when in interaction with other habitat components. This discrepancy can be explained by the availability of cavities in the shelter‐rich coarse substrate which provide sufficient cover and territory for juvenile fish at the given carrying capacity of river Aurlandselva. Whilst LWD may be most effective to provide shelter in lowland streams (bed slope <0.005), maintaining or restoring shelter‐rich coarse substrates should be considered a key priority in steeper salmonid rivers and associated semi‐fluvial streams. [ABSTRACT FROM AUTHOR]

The gut microbiota plays a pivotal role in upholding intestinal health, fostering intestinal development, fortifying organisms against pathogen intrusion, regulating nutrient absorption, and managing the body's lipid metabolism. However, the influence of different cultivation modes on the growth indices and intestinal microbes of Salmo trutta fario remains underexplored. In this study, we employed high-throughput sequencing and bioinformatics techniques to scrutinize the intestinal microbiota in three farming modes: traditional pond aquaculture (TPA), recirculating aquaculture (RA), and flow-through aquaculture (FTA). We aimed to assess the impact of different farming methods on the water environment and Salmo trutta fario's growth performance. Our findings revealed that the final weight and weight gain rate in the FTA model surpassed those in the other two. Substantial disparities were observed in the composition, relative abundance, and diversity of Salmo trutta fario gut microbiota under different aquaculture modes. Notably, the dominant genera of Salmo trutta fario gut microbiota varied across farming modes: for instance, in the FTA model, the most prevalent genera were SC-I-84 (7.34%), Subgroup_6 (9.93%), and UTCFX1 (6.71%), while, under RA farming, they were Bacteroidetes_vadinHA17 (10.61%), MBNT15 (7.09%), and Anaeromyxoactor (6.62%). In the TPA model, dominant genera in the gut microbiota included Anaeromyxobacter (8.72%), Bacteroidetes_vadinHA17 (8.30%), and Geobacter (12.54%). From a comparative standpoint, the genus-level composition of the gut microbiota in the RA and TPA models exhibited relative similarity. The gut microbiota in the FTA model showcased the most intricate functional diversity, while TPA farming displayed a more intricate interaction pattern with the gut microbiota. Transparency, pH, dissolved oxygen, conductivity, total dissolved solids, and temperature emerged as pivotal factors influencing Salmo trutta fario gut microbiota under diverse farming conditions. These research findings offer valuable scientific insights for fostering healthy aquaculture practices and disease prevention and control measures for Salmo trutta fario, holding substantial significance for the sustainable development of the cold-water fish industry in the Qinghai–Tibet Plateau. [ABSTRACT FROM AUTHOR]

Our understanding of the environmental behavior, bioaccumulation and concentrations of chlorinated paraffins (CPs) and Dechloranes (Dec) in the Arctic environment is still limited, particularly in freshwater ecosystems. In this descriptive study, short chain (SCCPs) and medium chain (MCCPs) CPs, Dechlorane Plus (DP) and analogues, and polychlorinated biphenyls (PCBs) were measured in sediments, benthic organisms, three-spined stickleback (Gasterosteus aculeatus), Arctic char (Salvelinus alpinus) and brown trout (Salmo trutta) in two Sub-Arctic lakes in Northern Norway. Takvannet (TA) is a remote lake, with no known local sources for organic contaminants, while Storvannet (ST) is situated in a populated area. SCCPs and MCCPs were detected in all sediment samples from ST with concentration of 42.26--115.29 ng/g dw and 66.18--136.69 ng/g dw for SCCPs and MCCPs, respectively. Only SCCPs were detected in TA sediments (0.4--5.28 ng/g dw). In biota samples, sticklebacks and benthic organisms showed the highest concentrations of CPs, while concentrations were low or below detection limits in both char and trout. The congener group patterns observed in both lakes showed SCCP profiles dominated by higher chlorinated congener groups while the MCCPs showed consistency in their profiles, with C14 being the most prevalent carbon chain length. Anti- and syn-DP isomers were detected in all sediment, benthic and stickleback samples with higher concentrations in ST than in TA. However, they were only present in a few char and trout samples from ST. Dec 601 and 604 were below detection limits in all samples in both lakes. Dec 603 was detected only in ST sediments, sticklebacks and 2 trout samples, while Dec 602 was the only DP analogue found in all samples from both lakes. While there were clear differences in sediment concentrations of DP and Dec 602 between ST and TA, differences between lakes decreased with increasing δ15N. This pattern was similar to the PCB behavior, suggesting the lake characteristics in ST are playing an important role in the lack of biomagnification of pollutants in this lake. Our results suggest that ST receives pollutants from local sources in addition to atmospheric transport. [ABSTRACT FROM AUTHOR]

Changes in the number, structure, and function of mitochondria during the early life stages of animals can play an important role for an organism's metabolic rate, growth, and health. Previous studies have shown that juvenile brown trout (Salmo trutta) subjected to elevated temperatures during the embryonic stage respond phenotypically with a reduced metabolic rate. The aim of this study was to explore if embryonic temperature affects the mitochondria content of young brown trout and as such explains the previously found differences in metabolic rates. Here, we optimize a quantitative PCR (qPCR) method for the mitochondria cytochrome c oxidase subunit I gene, and then use the method as a proxy for mitochondrial DNA content. We hypothesize that young trout subjected to elevated temperatures during the embryonic stage respond phenotypically with a reduced mitochondrial DNA content. To test this hypothesis, we subjected brown trout to either control ambient (4.4 ± 1.5 °C) or elevated temperatures (7.1 ± 0.6 °C) during embryogenesis. Subsequently, we extracted DNA from liver and white muscle tissue of juvenile brown trout from the two different incubation temperature treatments and successively optimized qPCR for mitochondrial DNA. We found that the amount of mitochondria DNA in liver tissue was 18 times higher than in white muscle tissue, but there was no significant difference in mitochondria content in liver or muscle tissue between brown trout exposed to elevated and ambient control temperatures during embryogenesis. We conclude that reduced metabolic rate is not likely associated with mitochondria DNA content. We also suggest that qPCR is a simple and cost-effective method to quantify mitochondria DNA in frozen and partly degraded tissue from different treatment groups and a useful proxy for identification of differences in mitochondria number. [ABSTRACT FROM AUTHOR]

Fragmentation of watercourses poses a significant threat to biodiversity, particularly for migratory fish species. Mitigation measures such as fishways, have been increasingly implemented to restore river connectivity and support fish migration. The effects of such restoration efforts are typically tested using telemetry and fisheries methods, which do not fully capture the broader population movements that may have important consequences for population viability. We performed a before‐and‐after control‐impact (BACI) study using genetic tools (SNPs) to investigate the effect of a newly implemented fishway, aiming to enhance upstream spawning migration of brown trout (Salmo trutta Linnaeus) in a reservoir with two headwater tributaries fragmented by man‐made weirs. Another reservoir with two barrier‐free tributaries was also analysed as a control. Our results showed that the isolated brown trout population was spawning in the reservoir before the installation of the fishway, and we found genetic structuring and differentiation between fragmented headwater tributaries before the fishway construction, but not in the control reservoir. Unexpectedly, after the fishway construction we observed signals consistent with increased genetic differentiation between populations of newly recruited juvenile fish in the reservoir tributary and fish in the reservoir. We propose this was caused by newly enabled philopatric behaviour of brown trout to their natal spawning tributary. In contrast, we did not find any genetic changes in the tributary without a fishway or in the barrier‐free reservoir system. Given the scarcity of similar studies, we advocate for an increased use of genetic analyses in BACI studies to monitor and evaluate the effect of efforts to restore habitat connectivity and inform future management strategies. [ABSTRACT FROM AUTHOR]

Salmonids are well known for their natal homing behaviour, meaning they return to breed in the same area where they originated. However, not all individuals return to their natal breeding grounds—a behavioural trait known as straying. The prevalence of straying is difficult to explore and therefore quantitative estimates for straying are seldom reported. In this study, otolith microchemistry and genetics were combined to investigate patterns of straying over ecological and evolutionary time, respectively, between neighbouring rivers flowing into Mariager fjord, Denmark. Otolith microchemistry was used to determine the river of origin for sea trout (Salmo trutta) upon their return to freshwater and 288 SNP markers were used to determine genetic structure among the rivers in the fjord. In this system, where the distance between rivers is short, otolith microchemistry achieved 80% accuracy in assigning juvenile brown trout to their natal river, thus allowing us to determine that approximately 43% of the adult sea trout had returned to non‐natal rivers to spawn, with a similar proportion of strayers and natal homers in all of the rivers. Genetic analysis further supported that there was substantial gene flow among individuals originating from different rivers, indicating that sea trout in Mariager fjord make up one population. The findings obtained from otolith microchemistry and genetics complement each other and provide further evidence that sea trout in this system migrate to non‐natal rivers and spawn there, which consequently affects the genetic structure of the population. [ABSTRACT FROM AUTHOR]

Effective management of migrating fish stocks requires an understanding of population structure and each stock's contribution to the fishery. Water bodies spanning across national borders pose a particular challenge. We investigated the genetic structure of 13 adfluvial brown trout populations from natal rivers and conducted a genetic mixed‐stock analysis of brown trout from the cross‐border Lake Pyaozero Basin in eastern Fennoscandia, using 16 microsatellite loci. Our analysis revealed a clear genetic structure of three distinct geographic groups, consisting of river‐specific populations. The Western and Southern groups, which included 11 rivers, contributed over 90% to the mixed stock in the lacustrine rearing area. This mixed stock of several river‐specific subpopulations renders poorly managed fishing in the area a significant threat to the entire native adfluvial brown trout population complex. [ABSTRACT FROM AUTHOR]

When benthic fish engage in predation, they often swim near the riverbank or close to hard rocks, where they are subjected to the combined effects of side and benthic walls. This study focuses on the brown trout and employs a three-dimensional numerical model to simulate the process of brown trout accelerating from a stationary state to a cruising state under the influence of wall effects. A self-developed subroutine algorithm is applied to solve the various hydrodynamic parameters of brown trout swimming. By varying the distance between the fish's center of gravity and the wall, this study explores the self-propelled performance and efficiency of fish swimming affected by the sidewall as well as the combined influence of the side and benthic walls. This study also reveals the hydrodynamic mechanism of wall effects that impact the performance of fish swimming in the body/caudal fin (BCF) mode. The results demonstrate that when the distance is less than 0.5 times the body length of the fish, swimming near the sidewall can enhance speed and thrust, but the swimming efficiency will reduce. Closer proximity to the benthic wall leads to increased power consumption and decreased efficiency, which is disadvantageous for fish swimming. The findings of this study reveal the unstable wall effects experienced by fish and offer insights for designing biomimetic underwater vehicles that leverage wall effects as well as for creating habitats that support fish swimming in the BCF mode. [ABSTRACT FROM AUTHOR]

Winter is an energetically challenging period for many animals in temperate regions because of the relatively harsh environmental conditions and reduction in food availability during this season. Moreover, stressors experienced by individuals in the fall can affect their subsequent foraging strategy and energy stores after exposure has ended, referred to as carryover effects. We used exogenous cortisol manipulation of wild juvenile brown trout (Salmo trutta) in the fall to simulate a physiological stress response and then investigated short‐term (2 weeks) and long‐term (4 months) effects on condition metrics (hepatosomatic index and water muscle content), diet (stomach contents and stable isotopes), and morphology during growth in freshwater. We revealed some short‐term impacts, likely due to handling stress, and long‐term (seasonal) changes in diet, likely reflecting prey availability. Unfortunately, we had very few recaptures of cortisol‐treated fish at long‐term sampling, limiting detailed analysis about cortisol effects at that time point. Nonetheless, the fish that were sampled showed elevated stable isotopes, suggestive of a cortisol effect long after exposure. This is one of few studies to investigate whether cortisol influences foraging and morphology during juvenile growth, thus extending the knowledge of proximate mechanisms influencing ecologically‐relevant phenotypes. Research highlights: Exogenous cortisol treatment did not affect condition, diet, or morphology of juvenile brown trout after 2 weeks or 4 months. However, few recaptures of cortisol treated fish at 4 months limit our conclusions but suggest nutritional effects. [ABSTRACT FROM AUTHOR]