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The 10th International Charr Symposium was held in Nikko, Japan, bringing together researchers from 11 countries to discuss the ecological and evolutionary studies of charr species, such as Arctic charr and lake charr. The symposium covered various topics, including migration patterns, feeding habits, and habitat differences among charr species. Researchers emphasized the importance of conservation and management of charr species worldwide, especially in the face of anthropogenic stressors like climate change and development. The symposium aimed to foster collaboration and knowledge-sharing among scientists to deepen our understanding of charr biology and contribute to their conservation. [Extracted from the article]

Underwater cameras were used to record and describe, for the first time, the spawning behaviour of the lake charr (Salvelinus umbla) in an Alpine stream (Oberer Lunzer Seebach, Austria). The results were then compared with other Salvelinus species reported in the literature. Unlike other Salvelinus species, lake charr females do not perform the typical undulating behaviour intended to ventilate and distribute eggs over gravel crevices after the spawning act. This behavioural distinction reveals a different evolutionary history and supports the lake charr being conferred a species status. The Salvelinus species' unique mode of spawning is discussed within an evolutionary framework. [ABSTRACT FROM AUTHOR]

In sympatry, juvenile brown trout (Salmo trutta) usually occupy the shallow and most productive areas (littoral zone) of lakes, while juveniles of Arctic charr (Salvelinus alpinus) are found in deeper, less productive parts. In contrast, when Arctic charr juveniles occur in allopatry, they often occupy shallow littoral areas as well. Habitat segregation has traditionally been interpreted as a trade-off between predation risk and energy gain, while the segregation of these two species has been explained by brown trout being more aggressive and competitively superior to Arctic charr. We hypothesize, however, that the marked habitat segregation between the two species may also be due to differences in predator avoidance. Accordingly, we conducted several laboratory tests, using Arctic charr and brown trout as potential predators. Live fish of the same species were offered as prey, either as small charr only, small trout only, or both small charr and small trout together. Artificial shelters were then introduced to examine the avoidance ability of prey fish against predatory fish. Our results showed that under these circumstances, access to shelters strongly decreased mortality in juvenile brown trout, but had no effect on juvenile Arctic charr mortality. Thus, the habitat segregation shown by sympatric juvenile Arctic charr and brown trout in lakes may be a consequence of interspecific differences in predator avoidance. [ABSTRACT FROM AUTHOR]

Migrations between spawning, growth and refuge habitats are critical for many fish species. Partially migratory populations of brook charr Salvelinus fontinalis (known as brook trout in the United States), in which a portion of the population migrates, were once widespread in the Great Lakes region of North America, but are now scarce and a rehabilitation priority. Great Lakes fishery managers lack a simple, non-lethal means to determine whether a large brook charr caught in a Great Lakes accessible stream reach represents a stream-resident fish or migrant that previously spent time foraging and growing in Great Lakes waters. We explored a relatively inexpensive and non-lethal stable isotope approach for documenting lake to stream movements of brook charr, using fin clips from brook charr captured in streams in fall prior to spawning. Using fin tissue from juvenile and adult coho salmon captured in Michigan tributaries to Lake Superior, we confirmed distinct stable isotope signatures indicative of prior stream and Lake Superior foraging. We identified brook charr from tributaries accessible to Lake Superior whose stable isotope signatures were similar to those of lake-captured brook charr, lake charr and splake (a brook charr–lake charr hybrid) and distinct from stable isotope signatures of brook charr in streams inaccessible to Lake Superior fishes, suggestive of an adfluvial migratory pattern. Brook charr from two streams had elevated δ15N values, indicative of previous residency in a downstream lake receiving water from a sewage treatment facility. Our findings demonstrate the utility of stable isotope analysis as a non-lethal technique for confirming adfluvial movements of brook charr from Lake Superior to streams. [ABSTRACT FROM AUTHOR]

The genetic identity and conservation status of lake ecotype of brown trout (Salmo trutta) populations in Slovenia's Alpine lakes Bled and Bohinj have been subjected to changes due to historical stocking practices. We aimed to evaluate the genetic differentiation and origins of these populations to advise for correct conservation strategies. Using microsatellite and mitochondrial DNA analysis on 365 trout specimens, we explored the genetic diversity and structure of these populations, comparing them with potential originators. Results revealed distinct genetic profiles between Bled and Bohinj lake trout, with Bohinj lake trout closely aligned with the native Danubian lineage and Bled lake trout showing a hybrid composition, significantly influenced by hatchery-introduced Atlantic lineage strains. The genetic distinction between these populations underscores the complexity of their conservation, showcasing the impact of human-mediated introductions on natural populations. The urgent need for conservation measures to preserve the Bohinj population's native genetic pool is highlighted, underlining the must to prevent the transfer of individuals from Lake Bled or elsewhere and to reconsider stocking practises in order to prevent further genetic dilution of these iconic European lake dwellers. [ABSTRACT FROM AUTHOR]

A recent spontaneous return of a native brown trout population in Lake Laka and its tributaries followed the gradual recovery of the Bohemian Forest (Czechia) from atmospheric acidification. The lake had hosted a vibrant brown trout population in the first half of the 20th century, which then became extinct around the 1960s due to strong atmospheric acidification, peaking in the 1980s. However, since the 1990s, a gradual reversibility in acidification has occurred. This positive trend was delayed after tree dieback in the lake catchment in the 2000s. The following detailed monitoring showed recurrent critical periods of low pH and ANC, and elevated concentrations of ionic Al. This was especially in spring, lasting until the late 2010s. Electrofishing from 2020 to 2022 indicated suitable conditions for permanent fish populations. In comparison to previous electrofishing (2005–2010), brown trout colonised Lake Laka and formed a stable population in the lake outflow. Downstream of the lake, the fish abundance remained similar to those some ten years earlier. In contrast to the brown trout, the bullhead was only found in the stream below the lake, as it was unable to migrate to the upper areas due to natural barriers. The recovery of the brown trout population in Lake Laka after the acidification phase is an example of the successful restoration of a disturbed aquatic ecosystem under pristine conditions. [ABSTRACT FROM AUTHOR]

Recent adaptive radiations provide experimental opportunities to parse the relationship between genomic variation and the origins of distinct phenotypes. Sympatric radiations of the charr complex (genus Salvelinus) present a trove for phylogenetic analyses as charrs have repeatedly diversified into multiple morphs with distinct feeding specializations. However, charr species flocks normally comprise only two to three lineages. Dolly Varden charr inhabiting Lake Kronotskoe represent the most extensive radiation described for the genus, containing at least seven lineages, each with defining morphological and ecological traits. Here, we perform the first genome-wide analysis of this species flock to parse the foundations of adaptive change. Our data support distinct, reproductively isolated lineages within the clade. We find that changes in genes associated with thyroid signaling and craniofacial development provided a foundational shift in evolution to the lake. The thyroid axis is further implicated in subsequent lineage partitioning events. These results delineate a genetic scenario for the diversification of specialized lineages and highlight a common axis of change biasing the generation of specific forms during adaptive radiation. [ABSTRACT FROM AUTHOR]

Identifying and characterizing spawning locations are paramount for the protection of critical fish habitats but can be challenging, particularly in remote locations. Using the underexplored oviduct‐tagging technique, we aimed to identify the timing and location of spawning for wild Arctic char (Salvelinus alpinus) and lake trout (Salvelinus namaycush) in two high‐Arctic lakes in Nunavut. Specifically, Innovasea V7 acoustic telemetry transmitters were inserted into the oviducts of 13 Arctic char and 4 lake trout, and the timing and location of tag expulsion were determined using a fine‐scale positioning system. Twenty Arctic char and 20 lake trout were also tagged with abdominal V16 transmitters, and 10 of them were paired with the oviduct tags, to further study the behavior of individual fish during the spawning season. Oviduct tags from four Arctic char and one lake trout could be used to assess the timing and location of spawning. Spawning anadromous Arctic char drastically reduced their activity and remained proximate to their presumed spawning location immediately before and for months after spawning. In contrast, a non‐anadromous (i.e., freshwater resident) Arctic char and a lake trout showed little to no reduction in activity around presumed spawning events. Because of the highlighted sedentary behavior of inferred spawning anadromous Arctic char implanted with both abdominal and oviduct tags, we could also infer potential spawning based on the behavior of individuals equipped only with abdominal tags. Spawning areas identified via telemetry also aligned well with Inuit knowledge of those lakes. This is the first field study to use acoustic oviduct and abdominal tags coupled with a fine‐scale positioning system. Despite a limited success rate of ejection, the study reveals the strong potential of the method to study spawning habitat and timing, particularly in remote areas. [ABSTRACT FROM AUTHOR]

Objective: Understanding recruitment dynamics is necessary to predict population‐level responses to exploitation, management actions, or anthropogenic influences. Fecundity is commonly used as a metric of recruitment dynamics and can guide successful management of fisheries. However, an individual female's fecundity is not constant over time; females resorb ovarian follicles to regulate fecundity as they approach spawning. This suggests that sampling for fecundity too early may produce inaccurate estimates of relative fecundity. In Yellowstone National Park, suppression of invasive Lake Trout Salvelinus namaycush reduced the abundance of mature fish by 92% between 2012 and 2022. The continued efficacy of this suppression effort requires accurate assessments of reproductive potential of the population that remains. Methods: We sought to determine whether the timing of ovarian follicle collection affected estimates of mean relative fecundity. We collected ovarian follicles from female Lake Trout, which are autumn spawners, between mid‐August and early October in 2021 and 2022. The number of ovarian follicles per sample was counted to obtain estimates of relative fecundity for each female. Result: We observed a 13% decline in estimated mean relative fecundity between individuals that were sampled before mid‐September and those that were sampled after mid‐September. Conclusion: Our data support strategic timing of fecundity sampling to best capture the true reproductive capability of a population, which is a key metric used in models that guide adaptive management of fishes. Impact statementFemale Lake Trout undergo fecundity reduction through follicular atresia, reducing the number of ovarian follicles that may be spawned, which suggests that fecundity estimates based on fish that are sampled too early in the reproductive cycle will be inaccurate. [ABSTRACT FROM AUTHOR]

The southeastern edge of the Lake Trout (Salvelinus namaycush) range extends to Nova Scotia (NS), Canada, including Sherbrooke Lake, Lunenburg County. These Lake Trout may be of native origin; however, the putatively native population was historically supplemented by stocking from nonindigenous strains. To determine genetic origin of this population, 25 Lake Trout from Sherbrooke Lake were used to compare genetic likeness to populations that had been stocked into the lake. Lake Trout of Sherbrooke Lake were determined to be highly genetically distinct from source populations in Ontario, Manitoba, New York, and Vermont, suggesting that the extant population is of native origin. To identify suitable habitat of Lake Trout in Sherbrooke Lake, acoustic telemetry was used to track the depth and temperature residency of 11 Lake Trout in tandem with measuring lake bathymetric and water column temperature and oxygen. During spring and summer, 2022, Lake Trout occupied a hypolimnetic residency, ranging from 11 to 22 m of depth and 8.5 to 10.0 °C. Suitable conditions for Lake Trout were available in the south basin. The main basin, containing ~ 80% of the lake's volume, was unstratified and unsuitable for Lake Trout. The effects of Hurricane Fiona caused the hypolimnion in the south basin to shrink via the thermal mixing of the epilimnion, forcing adult Lake Trout to a depth range of 16–18 m for ~ 1 month. During this time, suitable Lake Trout habitat comprised only ~ 0.5% of the total volume of Sherbrooke Lake. [ABSTRACT FROM AUTHOR]

ABSTRACT Adaptive radiation is considered to be a major source of biodiversity in charrs of the genus Salvelinus. These fishes demonstrate numerous examples of diversification in postglacial lakes and rivers resulting in the emergence of different ecomorphs, including piscivorous. Usually, the number of fish feeders does not exceed two, suggesting the existence of ecological and/or internal factors that prevent further diversification. Under these circumstances, rare instances of increases in the number of piscivorous morphs are of particular interest. Here, we present a comprehensive study of the piscivorous charr assemblage in Lake Kronotskoe, residence of the most diverse species flock of Salvelinus fishes. To unravel the diversity and ecological relationships of piscivorous charrs, we analysed their morphology and microsatellite DNA polymorphism, distribution within the lake, trophic preferences and parasite load, as well as trace element accumulation, stable isotope and free fatty acid ratios. We revealed that, apart from a few strictly benthivorous morphs, the lake is inhabited by four closely related but morphologically and genetically distinct morphs that exhibit a range of piscivorous specialisation: from opportunistic secondary predation to ultra‐specialised primary piscivory. They effectively exploit the spatiotemporal structure of the ecosystem and experience diversifying selection due to niche discordance. Each piscivorous morph develops specific phenotypic, ecological and life history traits that allow it to terminate weakly overlapping branches of the trophic chain and share the energy flows of the lake ecosystem. Our data suggest that Lake Kronotskoe facilitates high level of ecological opportunity that favours niche expansion and allows the piscivorous lineage to simultaneously realise the archetypical modes of ecological diversification available to fishes in cold‐water freshwaters. [ABSTRACT FROM AUTHOR]

Acoustic telemetry is a common tool used in fisheries management to estimate fish space use (i.e., occupancy) from a local habitat scale to entire systems. Numerous analytical models have been developed to estimate different aspects of fish movement from telemetry datasets, yet evaluations of model performance and comparisons among models are limited. Here, we develop a framework to evaluate model estimates of regional occupancy in large and fragmented systems using an acoustic receiver array in Lake Champlain. We simulated the tracks of 100 acoustically tagged fish using a random walk function and created detection events based on receiver positions and distance-based detection probability. Regional occupancy for the simulated data was estimated by six movement models that ranged in analytical complexity, and results were compared to the true distributions for each simulated track to evaluate model error. The six movement models included: (1) a basic residency index using detections alone; (2) a residency index using last-observation-carried-forward; (3) a centers of activity model; (4) linear and non-linear interpolations (i.e., least-cost paths); and (5 and 6) two dynamic Brownian bridge movement models generated using separate packages in R. We developed a model selection process to compare model performance and select the optimal analysis based on simulation error. This process showed significant differences in model performance among the six movement models based on model error. Overall, the model generating least-cost paths using linear and non-linear interpolations consistently provided the most accurate regional occupancy estimates. Based on these simulation results, we applied this model to a case study that evaluated patterns in the regional distribution of stocked lake trout (Salvelinus namaycush) in Lake Champlain, which demonstrated distinct regional occupancy of two stocked lake trout groups. These results demonstrate potential for large variability in interpretation of acoustic telemetry data for describing regional fish distribution dependent on the analytical method used. [ABSTRACT FROM AUTHOR]

Bioenergetics models are powerful tools used to address a range of questions in fish biology. However, these models are rarely informed by free‐swimming activity data, introducing error. To quantify the costs of activity in free‐swimming fish, calibrations produced from standardized laboratory trials can be applied to estimate energy expenditure from sensor data for specific tags and species. Using swim tunnel respirometry, we calibrated acceleration sensor‐equipped transmitting tags to estimate the aerobic metabolic rates (ṀO2) of lake trout (Salvelinus namaycush) at three environmentally relevant temperatures. Aerobic and swim performance were also assessed. Like other calibrations, we found strong relationships between ṀO2 and acceleration or swimming speed, and jackknife validations and data simulations suggest that our models accurately predict metabolic costs of activity in adult lake trout (~5% algebraic error and ~20% absolute error). Aerobic and swim performance metrics were similar to those reported in other studies, but their critical swimming speed was lower than expected. Additionally, lake trout exhibited a wide aerobic scope, suggesting that the avoidance of waters ≥15°C may be related to selection for optimal growing temperatures. The ability to quantify the free‐swimming energetic costs of activity will advance our understanding of lake trout ecology and may yield improvements to bioenergetics model. [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]

Understanding how invasive species affect the stability and function of ecosystems is critical for conservation. Here, we quantified the effect of an actively suppressed invasive species on the Yellowstone Lake ecosystem using a food web energetics approach. We compared energy flux, functional state, and stability of four food web states: a pre‐invasion network and three post‐invasion networks undergoing active invasive species suppression, namely, initial invasion, expansion, and decline. Invasion caused ≥ 25% change (±) in energy flux for most consumers, and total flux increased twofold post‐invasion. Flux to the species of conservation concern, Yellowstone cutthroat trout (Oncorhynchus virginalis bouvieri), was 2.8 times less post‐invasion versus pre‐invasion, whereas invasive lake trout (Salvelinus namaycush) flux was up to 17.3 times higher compared to the initial invasion network. The dominant functional state and food web stability did not change post‐invasion, likely due to introduction of a generalist predator and the stabilizing effect of suppression. Lake trout invasion in Yellowstone Lake caused large changes to energy flux, shifting dominant fluxes away from the species of conservation concern, despite not changing functional state or stability. We demonstrate that changes in energy flux may signal invasions in ecosystems, but functional state or stability may not necessarily reflect the magnitude of invasion influences. For invaded fish communities, a better understanding of how the invasive species control the food web beyond just the direct influence on prey can be achieved by investigating energy flux, functional state, and food web stability. Furthermore, evaluating the effect of suppression beyond the invasive species can demonstrate the far‐reaching value of suppression management actions for conservation. [ABSTRACT FROM AUTHOR]

Abstract Acoustic telemetry is a common tool used in fisheries management to estimate fish space use (i.e., occupancy) from a local habitat scale to entire systems. Numerous analytical models have been developed to estimate different aspects of fish movement from telemetry datasets, yet evaluations of model performance and comparisons among models are limited. Here, we develop a framework to evaluate model estimates of regional occupancy in large and fragmented systems using an acoustic receiver array in Lake Champlain. We simulated the tracks of 100 acoustically tagged fish using a random walk function and created detection events based on receiver positions and distance-based detection probability. Regional occupancy for the simulated data was estimated by six movement models that ranged in analytical complexity, and results were compared to the true distributions for each simulated track to evaluate model error. The six movement models included: (1) a basic residency index using detections alone; (2) a residency index using last-observation-carried-forward; (3) a centers of activity model; (4) linear and non-linear interpolations (i.e., least-cost paths); and (5 and 6) two dynamic Brownian bridge movement models generated using separate packages in R. We developed a model selection process to compare model performance and select the optimal analysis based on simulation error. This process showed significant differences in model performance among the six movement models based on model error. Overall, the model generating least-cost paths using linear and non-linear interpolations consistently provided the most accurate regional occupancy estimates. Based on these simulation results, we applied this model to a case study that evaluated patterns in the regional distribution of stocked lake trout (Salvelinus namaycush) in Lake Champlain, which demonstrated distinct regional occupancy of two stocked lake trout groups. These results demonstrate potential for large variability in interpretation of acoustic telemetry data for describing regional fish distribution dependent on the analytical method used.

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.

Increased knowledge of heritable traits in Atlantic Salmon (Salmo salar) is important to overcome bottlenecks in salmonid aquaculture. Atlantic salmonid populations, both landlocked and anadromous, represent an interesting model to gain insight into anadromy related traits, most notably, the probability to smoltify. While a previous study has identified several genomic regions diverging between anadromous and landlocked populations across the species range, the present study explores these data further with the aim to uncover if some of these genomic regions are linked to beneficial genetic traits associated with smoltification. In this study 17 of these loci were monitored in 669 anadromous salmon originating from 36 full-sibling families that had been reared under common garden conditions. The Smolt Index was calculated, using multiple visual markers, and provided a means of assessing smoltification stage. One SNP, located in Ssa04, showed a significant association with probability to smoltify, where individuals homozygous for the landlocked variant (LL) displayed a decrease in probability of smoltifying after one winter when compared with the homozygous for the anadromous variant (AA). This effect was independent of individual fish size. A separate common garden study comprising 200 individuals from either anadromous or landlocked strains showed that expression levels of ncor1, a thyroid mediator hormone located on the same chromosomal region (Ssa04), were significantly reduced in landlocked individuals post smoltification but remained constant in their anadromous counterparts. This study therefore suggests that while size is still the most important trigger for the induction of smoltification, there may also be an additional genetic component or trigger that has been 'lost' during the years deprived of SW transfer. In conclusion, the LL genotype identified here could potentially be used by the industry to delay smoltification and may also represent one of the first clues to the genetic regulation of smoltification in Atlantic salmon. [ABSTRACT FROM AUTHOR]

Coldwater‐adapted freshwater fishes, especially their populations along warm‐range margins, are most vulnerable to the climate oscillations associated with global warming. Stocking is a major strategy for avoiding the extinction of these species. However, while stocking can reverse the decline of isolated populations, it may also result in a loss of genetic diversity in the native local population due to the introgressive replacement of hatchery genes. To plan an adequate strategy for conserving locally adapted populations, the genetic impacts of stocking on native lineages should be evaluated from small river branches to wide‐ranging drainage areas. We investigated the population genetic structure of white‐spotted charr (Salvelinus leucomaenis) within its southern range (Lake Biwa basin, Japan). By applying genome‐wide SNP analysis to the population's genetic structure, we assessed the extent of genetic introgression resulting from stocking. White‐spotted charr in the Lake Biwa watershed constitutes a distinctive genetic group, within which apparent genetic differentiation was observed. The hatchery‐reared fish line commonly used for supplementation stocking in the catchment was discernable from the native population, enabling us to analyze genetic introgression across the entire drainage area. Admixed individuals resulting from hatchery introgression were observed in most of the stocked sites that showed relatively high heterozygosity and nucleotide diversity. However, their genetic differentiation was much lower than that of native populations. The supplementation history as well as the road availability contributed substantially to the introgression of hatchery genes. Populations with the native genetic structure remained in the upstream regions of the tested rivers. However, their heterozygosity and nucleotide diversity were low when compared with that of the populations with hatchery supplementation. Our results shed light on the genetic impacts of stocking on isolated native populations and suggest that conventional supplementation methods cannot preserve a unique biodiversity in the distribution margin. [ABSTRACT FROM AUTHOR]

Objective: Coldwater releases from reservoirs support economically valuable sport fisheries for salmonids in the southern United States. The tailwater of Lewis Smith Dam in northwestern Alabama supports the only coldwater fishery in the state, with monthly stockings of about 1500 catchable‐size (200–406 mm total length) Rainbow Trout Oncorhynchus mykiss. Historically, the return to angler creel has been low in this fishery, and few large Rainbow Trout are caught. Numerous potential predators of stocked trout exist in this tailwater, including Gulf‐strain Striped Bass Morone saxatilis, which are stocked annually in a downstream reservoir. Thus, the objective of this study was to quantify the amount of predation, which occurs on Rainbow Trout in the Smith Lake tailwater. Methods: Potential predators of Rainbow Trout were collected six times over a 3‐week period after stocking in March, May, June, and October 2018. Result: Rainbow Trout were found in the stomachs of Striped Bass, hybrid striped bass (White Bass Morone chrysops × Striped Bass), Largemouth Bass Micropterus nigricans, and Chain Pickerel Esox niger. Only Striped Bass exhibited substantial predation on stocked Rainbow Trout, with 75% of these fish having at least one trout in their stomach. Of all identifiable prey items, Rainbow Trout composed 82% of the Striped Bass diet by number across all four sample months and constituted more than 65% of the Striped Bass diet by number in each sample month. Bioenergetics models predicted that a population of 500 Striped Bass living continuously in the tailwater from March through October could consume all Rainbow Trout stocked during each month. If Striped Bass vacated the area or switched from trout to other diet items for one‐third of each month, they could still consume almost 60% of stocked trout. Conclusion: Results of this study demonstrate that Striped Bass are significant predators of Rainbow Trout in the Smith Lake tailwater and are likely one of the main factors mediating Rainbow Trout survival and persistence in this fishery. Impact statementLow return rates to anglers of stocked Rainbow Trout in an Alabama tailwater fishery are likely due to high rates of predation by native Gulf‐strain Striped Bass. This research demonstrates that Striped Bass in the tailwater could consume 60% or more of the stocked trout during each month. [ABSTRACT FROM AUTHOR]

Objective: This purpose of this paper is to describe the characteristics of recovered Lake Trout Salvelinus namaycush populations in Lake Superior by describing its population dynamics, ecology, and recent research and management activities since 1993, when Lake Trout were declared rehabilitated. Methods: Data from commercial fisheries, recreational fisheries, agency stocking reports, and natural resource agency fishery‐independent survey data along with published research findings on lean Lake Trout in Lake Superior between 1993 and 2022 were synthesized and reported. Result: Currently, Lake Trout populations are self‐sustaining and lightly exploited with only a few areas with elevated total mortality rates. The total annual mortality has been far below the target maximum range of 42–45%. Furthermore, stocking of hatchery Lake Trout is no longer necessary. Conclusion: We have learned from research and management experience that the regulatory role of Lake Trout in the Great Lakes is critical to proper ecosystem function. Thus, continued commitment from natural resource agencies to cooperate and implement effective management actions is required to preserve the accomplishments of lakewide recovery of Lake Trout populations. Impact statementThe Lake Trout is the keystone predator in deepwater areas of the Great Lakes, and its successful recovery in Lake Superior has restored ecosystem stability and valued fisheries. This paper synthesizes information that describes the status of a highly managed species after restoration objectives have been achieved and maintained. [ABSTRACT FROM AUTHOR]

Cisco (Coregonus artedi) are a widespread, cold‐water zooplanktivore native to North America. Although Cisco are generally referred to as an “obligate zooplanktivore,” there is some evidence that the species exhibits considerable variability in trophic niche. Here, we assessed how Cisco body size relates to trophic position, that is, trophic ontogeny. We analysed 13C and 15N isotopes from Cisco ranging from 127 to 271 mm in body length (n = 66) from Trout Lake, Vilas County, Wisconsin, USA. 15N isotopes showed smaller Cisco had a trophic position of ~3, which steadily increased to ~3.5 for larger Cisco. Further, 13C isotope signatures showed Cisco transitioned to be more pelagically reliant (lower 13C signatures). Using gillnet catch data, we found that larger Cisco were using deeper habitats than smaller Cisco. Our results support that Cisco have significant variability in trophic niche even though they are traditionally thought of as an obligate planktivore. Overall, we emphasize that researchers should be cautious when generalizing Cisco trophic function, particularly when considering the broader food web. [ABSTRACT FROM AUTHOR]

The in vitro biotransformation of phenol at 11 °C was studied using pre-spawn adult rainbow (Oncorhynchus mykiss) (RBT), brook (Salvelinus fontinalis) (BKT), and lake trout (Salvelinus namaycush) (LKT) hepatic microsomal preparations. The incubations were optimized for time, cofactor concentration, pH, and microsomal protein concentration. Formation of Phase I ring-hydroxylation and Phase II glucuronidation metabolites was quantified using HPLC with dual-channel electrochemical and UV detection. The biotransformation of phenol over a range of substrate concentrations (1 to 180 mM) was quantified, and the Michaelis–Menten kinetics constants, Km and Vmax, for the formation of hydroquinone (HQ), catechol (CAT), and phenylglucuronide (PG) were calculated. Species differences were noted in the Km values for Phase I enzyme production of HQ and CAT, with the following rank order of apparent enzyme affinity for substrate: RBT > BKT = LKT. However, no apparent differences in the Km for Phase II metabolism of phenol to PG were detected. Conversely, while there were no apparent differences in Vmax between species for HQ or CAT formation, the apparent maximum capacity for PG formation was significantly less in LKT than that observed for RBT and BKT. These experiments provide a means to quantify metabolic activation and deactivation of xenobiotics in fish, to compare activation and deactivation reactions across species, and to act as a guide for future predictions of new chemical biotransformation pathways and rates in fish. These experiments provided the necessary rate and capacity (Km and Vmax) inputs that are required to parameterize a fish physiologically based toxicokinetic (PB-TK) model for a reactive chemical that is readily biotransformed, such as phenol. In the future, an extensive database of these rate and capacity parameters on important fish species for selected chemical structures will be needed to allow the effective use of predictive models for reactive, biotransformation chemicals in aquatic toxicology and environmental risk assessment. [ABSTRACT FROM AUTHOR]

Hybridization with introduced or invasive species is a major threat and driver of population declines in native salmonids. The rainbow trout (Oncorhynchus mykiss) is a major introduced and invasive species, often establishing entrenched naturalized populations. Here, we characterize population genetic variation of naturalized rainbow trout in a subset of Lake Tahoe tributaries to assess potential homing of rainbow trout to specific streams for spawning, thereby informing native Lahontan cutthroat trout (Oncorhynchus clarkii henshawi) reintroduction. Diploid, reproductively viable rainbow trout of commonly raised hatchery strains were stocked into Lake Tahoe from the late 1800s until the early 2000s by California and Nevada fish and wildlife agencies. Since 2007, however, triploid individuals comprise the bulk of rainbow trout planted. Despite dispersal from stocking locations across regions, our analyses revealed variation in population differentiation among tributaries, with individuals from spatially proximate streams clustering across multiple population genetic analyses. Although subtle, we detected evidence for genetic differentiation among tributaries from the southern, western, and northern regions, including surprising structure involving a single tributary. These results illustrate the extent of differentiation within and among streams and could inform possibilities for and implications of rainbow trout removal and Lahontan cutthroat trout reintroduction into its historical habitat. [ABSTRACT FROM AUTHOR]

To fight climate change, it is crucial to have a precise knowledge of greenhouse gas (GHG) concentrations in the atmosphere and to monitor sources and sinks of GHGs. On global scales, satellites are an appropriate monitoring tool. For the validation of the satellite measurements and to tie them to the World Meteorological Organization (WMO) trace gas scale, ground-based Fourier transform infrared (FTIR) networks are used, which provide reference data. To ensure the highest-quality validation data, the network must be scaled to the WMO trace gas scale and have a very small site-to-site bias. Currently, the Total Carbon Column Observing Network (TCCON) is the de facto standard FTIR network for providing reference data. Ensuring a small site-to-site bias is a major challenge for the TCCON. In this work, we describe the development and application of a new method to evaluate the site-to-site bias by using a remotely controlled portable FTIR spectrometer as a travel standard (TS) for evaluating the consistency of columnar GHG measurements performed at different TCCON stations, and we describe campaign results for the TCCON sites in Tsukuba (Japan), East Trout Lake (Canada) and Wollongong (Australia). The TS is based on a characterized portable EM27/SUN FTIR spectrometer equipped with an accurate pressure sensor which is operated in an automated enclosure. The EM27/SUN is the standard instrument of the Collaborative Carbon Column Observing Network (COCCON). The COCCON is designed such that all spectrometers are referenced to a common reference unit located in Karlsruhe, Germany. To evaluate the long-term stability of the TS instrument, it is placed side-by-side with the TCCON instrument in Karlsruhe (KA) and the COCCON reference unit (the EM27/SUN spectrometer SN37, which is operated permanently next to the TCCON-KA site) between deployments to collect comparing measurements. At each of the visited TCCON sites, the TCCON spectrometers collected low-resolution (LR) (0.5 cm-1) and high-resolution (HR) (0.02 cm-1) measurements in an alternating manner. Based on the TS as a portable standard, the measurements are compared to the Karlsruhe site as a common reference. For Tsukuba and Wollongong, the agreement with the reference in Karlsruhe found for XCO2 is on the 0.1 % level for both the LR and HR measurements. For XCH4 , the agreement is at the 0.2 % level, with the low-resolution measurements showing a low bias at both sites and for both gases. For XCO , the deviations are up to 7 %. The reason for this is likely to be a known issue with the CO a priori profiles used by the TCCON over source regions. In East Trout Lake (ETL), the TCCON spectrometer broke down while the TS was en route to the station. Hence, no side-by-side comparison was possible there. An important auxiliary value for FTIR retrievals is the surface pressure. Using the pressure sensor in the TS, the surface pressure measurements at each site are also compared. The surface pressure analysis reveals excellent agreement (0.027, 0.135 and 0.094 hPa) for the Tsukuba, ETL and Wollongong sites. [ABSTRACT FROM AUTHOR]

In many rivers, downstream‐migrating salmonid smolts must pass multiple dams often with high losses as a result. Fish experience mortality both in dam and reservoir passage, and spilling water might allow fish to avoid turbine passage and hence increase migration survival. In River Klarälven, Sweden landlocked Atlantic salmon smolts migrate along a 180 km long reach passing eight dams. A previous telemetry study estimated an accumulated migration success of 16% under conditions with no or very little spill. Here we repeat this study, under a planned spill regime at a subset of hydropower dams. Overall passage success through the eight dams was 32%, which is greater than the 16% reported from the same river section in a year without spill. Most of this increase, however, was attributable to the situation at one dam, where spill constituted a large proportion of total discharge. In addition, we found that loss rates km−1 were similar over dammed reaches and the lentic habitats, but greater than in the free‐flowing reference reaches. Results for migration speed paralleled this result with the highest speeds observed in the free‐flowing reaches. [ABSTRACT FROM AUTHOR]

Non-native species have invaded most ecosystems and methods are needed to manage them, especially in locations with sensitive species where they cannot be easily extirpated. Gillnetting for invasive lake trout [Salvelinus namaycush (Walbaum, 1792)] in Yellowstone Lake, Yellowstone National Park, USA began in 1995 and their carcasses are deposited into deep areas. This suppression method was recently supplemented by adding carcasses to shallow (< 20 m) spawning sites during the autumn spawning period to decrease dissolved oxygen through decomposition, suffocating lake trout embryos. We measured ammonium concentrations (shallow and deep sites), algal biomass, and ammonium uptake by phytoplankton and periphyton (shallow sites only) to investigate the degree to which carcasses caused bottom-up effects. Ammonium concentrations increased in autumn and were higher at deep sites than shallow sites. Algal biomass and ammonium uptake did not increase after adding carcasses, suggesting minimal effects. Periphyton biomass was 9 times higher than phytoplankton, but phytoplankton demanded 4.5 times more ammonium. Returning lake trout carcasses to deep areas of the lake may cause a second algal bloom. Assessing how novel management techniques alter the environment helps managers develop the most successful mitigation strategies that are effective without causing adverse effects to other portions of the ecosystem. [ABSTRACT FROM AUTHOR]

We applied an ensemble of climate warming models to an iconic protected landscape (Algonquin Park, Ontario) and the seasonal temperature profile model for lakes to assess changes in brook trout (Salvelinus fontinalis) thermal habitat volume (THV) among lakes of different sizes in 30-year periods under two climate warming scenarios (RCP 4.5 and 8.5). Bayesian beta regression models show that lake size (surface area) and morphometry (dynamic lake ratio) are important factors in THV loss. THV loss increases as a function of the dynamic lake ratio (transition from bowl-shaped to dish-shaped lakes). The magnitude of this effect depends on the lake size category and the RCP scenario. Small (<100 ha) and medium (100–500 ha) dish-shaped lakes are projected to have greater THV loss in 2071–2100 (60%–100% of brook trout THV under RCP 8.5; 40%–70% under RCP 4.5) than large lakes (>500 ha) of similar shape. Climate warming projections for the balance of this century, regardless of the RCP category, will result in the loss of brook trout THV in lakes that range widely in size and morphometry. [ABSTRACT FROM AUTHOR]

The native Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri; YCT) of Yellowstone Lake, Yellowstone National Park, Wyoming, USA, comprise a metapopulation that inhabits an ecologically simple and uniquely pristine environment. A recent study Bayesian fit an age‐structured Leslie model that had a local climate index (total annual mean‐daily air temperatures >0°C at the lake's north shore) as its only driver (regulating age‐0 YCT survival) to data for a key YCT spawning stock (median fish age, 5 years) from 1977 to 1992, before predation from an illegally introduced and growing lake trout (Salvelinus namaycush) population (1994 discovery) measurably affected YCT population dynamics. Results showed the new climate regime that began in the late 1970s had an overarching effect and largely explained declining spawning runs, although a growing predation effect became evident near lake trout discovery and quickly became the major YCT population driver. In the present study, that model was likewise fitted to estimated abundances of age‐5 YCT (~75% mature) that had been annually (also 1977–1992) gill‐netted from Yellowstone Lake and thus characterized the metapopulation. Results showed age‐0 YCT survival declined when a climate index of 1440 was reached and exceeded. Metapopulation recovery to historically high levels requires that the climate index decline to and remain near or below 1440 and—as shown by another recent study—that the lake trout population be reduced to its mid‐1990s levels. The most recent time of evident YCT metapopulation persistence at historically high levels and broadest spatial extent occurred under those simultaneous conditions. [ABSTRACT FROM AUTHOR]

Endangered wild fish populations are commonly supported by hatchery propagation. However, hatchery‐reared fish experience very different selective pressures compared to their wild counterparts, potentially causing genotype‐by‐environment interactions (G × E) in essential fitness traits. We experimentally studied early selection in a critically endangered landlocked Atlantic salmon population, first from fertilization to the swim‐up stage in a common hatchery setting, and thereafter until the age of 5 months in two contrasting rearing environments. Swim‐up progeny were moved either to standard indoor hatchery tanks involving conventional husbandry or to seminatural outdoor channels providing only natural food. After the first summer, sampled survivors were assigned to their families by genotyping. Early survival until the swim‐up stage was mostly determined by maternal effects, but also involved significant variation due to sires and full‐sib families (potential genetic effects). High on‐growing survival in hatchery tanks (88.7%) maintained a more even distribution among families (relative share 1.5%–4.2%) than the seminatural environment (0.0%–5.4%). This heterogeneity was mostly maternal, whereas no independent paternal effect occurred. Heritability estimates were high for body size traits in both environments (0.62–0.69). Genetic correlations between the environments were significantly positive for body size traits (0.67–0.69), and high body condition in hatchery was also genetically linked to rapid growth in the seminatural environment (0.54). Additive and phenotypic growth variation increased in the seminatural environment, but scaling effects probably played a less significant role for G × E, compared to re‐ranking of genotypes. Our results suggest that not only maternal effects, but also genetic effects, direct selection according to the environmental conditions experienced. Consistently high genetic variation in growth implies that, despite its low overall genetic diversity and long history in captive rearing (>50 years), this landlocked Atlantic salmon population still possesses adaptive potential for response to change from hatchery rearing back to more natural conditions. [ABSTRACT FROM AUTHOR]

Bioenergetics models for fishes are useful for understanding ecological processes (e.g., survivorship, growth, and reproduction) and can also inform fisheries management. Yet, current bioenergetics models are unable to ascertain direct energetic costs associated with standard and active metabolism for wild, free-swimming fishes. The use of telemetry with accelerometer sensors, calibrated in the laboratory using swim tunnel respirometers, have made it possible to estimate field metabolic activity in wild fish. Our objectives were to determine seasonal thermal habitat use and habitat-dependent metabolic costs associated with standard, active, and maximum metabolism in a ~1400 ha multibasin lake in Québec, Canada. We implanted 47 wild, free-swimming Lake Trout (Salvelinus namaycush) with either acoustic transmitters equipped with temperature and depth sensors or an acceleration sensor. Three sets of water temperature loggers (one set for each basin) were deployed at 2, 4, 6, 10, and 18 or 20 m to measure seasonally available thermal habitat. Thermal profiles of lake water temperature varied among basins with the thermocline being ~5 m in the smallest basin (north basin) and ~7.5 m in the largest basin (east basin). Thermal habitat used by Lake Trout varied seasonally, coupled with seasonal and basin differences in standard and maximum metabolism. Daily active metabolism loosely followed seasonal changes in thermal habitat use but was largely unaffected by differences in thermal habitat use among capture basins. The theoretical scope-for-activity followed seasonal trends and was estimated to range between 47% and 74% of theoretical aerobic scope. Our observations suggest that available thermal habitats influence Lake Trout thermal habitat use, and thus metabolic costs associated with swimming. These changes in thermal habitats could have metabolic consequences for individuals in a population, resulting in altered fitness metrics (i.e., survival, growth, and/or reproduction). Reductions in the volume and availability of optimal thermal habitats for Lake Trout are likely to occur under climate change scenarios. Our study indicates that the species can adjust metabolic costs throughout the year even when thermal habitats appear to be limiting, which may inform future evidence-based management decisions. [ABSTRACT FROM AUTHOR]

Recent studies have linked ecosystem warming to decreased mean body size in aquatic ectotherms but have often relied on experimental approaches. This study explores how climate change modifies adult body sizes in natural communities for five culturally, commercially, and recreationally important freshwater fish species that vary in thermal preferences (lake trout (Salvelinus namaycush), lake whitefish (Coregonus clupeaformis), walleye (Sander vitreus), yellow perch (Perca flavescens), and smallmouth bass (Micropterus dolomieu)). Using data for >170 000 sampling records from >600 lakes spanning the entire province of Ontario, Canada, we employ a spatial analysis technique based on Integrated Nested Laplace Approximation to detect body size changes due to warming lake conditions. With warming conditions, cold-adapted species (lake whitefish) decreased in adult body size, while cool- and warm-adapted species (walleye and smallmouth bass) increased in adult body size. Our study, like others recently conducted in marine and freshwater systems, demonstrates that the responses of fish body sizes to warming are not a unidirectional shift and may be linked to the habitat and thermal preferences of individual fish species. [ABSTRACT FROM AUTHOR]

Thiamin is an essential water‐soluble B vitamin known for its wide range of metabolic functions and antioxidant properties. Over the past decades, reproductive failures induced by thiamin deficiency have been observed in several salmonid species worldwide, but it is unclear why this micronutrient deficiency arises. Few studies have compared thiamin concentrations in systems of salmonid populations with or without documented thiamin deficiency. Moreover, it is not well known whether and how thiamin concentration changes during the marine feeding phase and the spawning migration. Therefore, samples of Atlantic salmon (Salmo salar) were collected when actively feeding in the open Baltic Sea, after the sea migration to natal rivers, after river migration, and during the spawning period. To compare populations of Baltic salmon with systems without documented thiamin deficiency, a population of landlocked salmon located in Lake Vänern (Sweden) was sampled as well as salmon from Norwegian rivers draining into the North Atlantic Ocean. Results showed the highest mean thiamin concentrations in Lake Vänern salmon, followed by North Atlantic, and the lowest in Baltic populations. Therefore, salmon in the Baltic Sea seem to be consistently more constrained by thiamin than those in other systems. Condition factor and body length had little to no effect on thiamin concentrations in all systems, suggesting that there is no relation between the body condition of salmon and thiamin deficiency. In our large spatiotemporal comparison of salmon populations, thiamin concentrations declined toward spawning in all studied systems, suggesting that the reduction in thiamin concentration arises as a natural consequence of starvation rather than to be related to thiamin deficiency in the system. These results suggest that factors affecting accumulation during the marine feeding phase are key for understanding the thiamin deficiency in salmonids. [ABSTRACT FROM AUTHOR]

Bioenergetics models estimate ectotherm growth, production, and prey consumption – all key for effective ecosystem management during changing global temperatures. Based on species‐specific allometric and thermodynamic relationships, these models typically use the species' lab‐derived optimum temperatures (physiological optimum) as opposed to empirical field data (realized thermal niche) that reflect actual thermal experience. Yet, dynamic behavioral thermoregulation mediated by biotic and abiotic interactions may provide substantial divergence between physiological optimum and realized thermal niche temperatures to significantly bias model outcomes. Here, using the Wisconsin bioenergetics model and in‐situ year‐round temperature data, we tested the two approaches and compared the maximum attainable lifetime weight and lifetime prey consumption estimates for two salmonid species with differing life histories. We demonstrate that using the realized thermal niche is the better approach because it eliminates significant biases in estimates produced by the physiological optimum. Specifically, using the physiological optimum, slower‐growing Salvelinus namaycush maximum attainable lifetime weight was underestimated, and consumption overestimated, while fast‐growing Oncorhynchus tshawytscha maximum attainable weight was overestimated. While the physiological optimum approach is useful for theoretical studies, our results demonstrate the critical importance that models used by management utilize up‐to‐date system‐ and species‐specific field data representing actual in‐situ behaviors (i.e., realized thermal niche). [ABSTRACT FROM AUTHOR]

Offshore reefs in the Laurentian Great Lakes provide critical spawning habitats for native lake trout. Though many historic spawning reefs have been documented, thorough evaluation of the habitat they contain is rare. After decades of stocking, natural recruitment of lake trout has increased dramatically in the Illinois waters of Lake Michigan since 2010, reinforcing the need for information about the viability and distribution of spawning habitats.Using side‐scan sonar and underwater video, this study sought to map bottom substrates at Lake Bluff Reef and Gumby's Reef, offshore of the northern Illinois coast of Lake Michigan, to describe and quantify the availability of substrates suitable for lake trout spawning. Substrates were manually delineated in ArcGIS using side‐scan sonar imagery mosaics. Underwater video was used to verify substrate classifications through direct observation.Suitable spawning substrate consisting of fractured bedrock debris and piled boulder/cobble was observed at both reefs. Underwater video revealed the presence of lake trout at both reefs in the weeks leading up to the spawning season. The largest congregation of fish was observed in the vicinity of the shallowest points, steep slopes, and the largest patches of spawning substrate.The distribution, amount, and condition of spawning substrate available at each reef was in line with others studied in the region and their likely importance for lake trout spawning was reinforced by observed aggregations of mature fish.This study has expanded the inventory of well‐documented offshore reefs in the Illinois waters of Lake Michigan, enhancing appreciation of what is coming to be understood as the Illinois Reef Complex. Knowledge of the distribution and status of habitats critical to the life history of native fishes in the Great Lakes are necessary for managers to properly assess, manage, and conserve lake trout and other fish populations. [ABSTRACT FROM AUTHOR]

To fight climate change it is crucial to have a precise knowledge of Greenhouse Gas (GHG) concentrations in the atmosphere and to monitor sources and sinks of GHGs. On global scales, satellites are an appropriate monitoring tool. For the validation of the satellite measurements, and to tie them to the World Meteorological Organization (WMO) trace gas scale, ground based Fourier Transform Infrared (FTIR) networks are used, which provide reference data. To ensure the highest quality validation data, the network must be scaled to the WMO trace gas scale and have a very small site-to-site bias. Currently, the Total Carbon Column Observing Network (TCCON) is the de-facto standard FTIR network for providing reference data. To ensure a small site-to-site bias is a major challenge for the TCCON. In this work we describe the development and application of a new method to evaluate the site-to-site bias by using a remotely controlled portable FTIR spectrometer as a Travel Standard (TS) for evaluating the consistency of columnar GHG measurements performed at different TCCON stations and we describe campaign results for the TCCON sites in Tsukuba (Japan), East Trout Lake (Canada) and Wollongong (Australia). The TS is based on a characterized portable EM27/SUN FTIR spectrometer equipped with an accurate pressure sensor which is operated in an automated enclosure. The EM27/SUN is the standard instrument of the Collaborative Carbon Column Observing Network (COCCON). The COCCON is designed such that all spectrometers are referenced to a common reference unit located in Karlsruhe, Germany. To evaluate the long-term stability of the TS instrument, it is placed side-by-side with the TCCON instrument in Karlsruhe and the COCCON reference unit (the EM27/SUN spectrometer SN37, which is operated permanently next to the TCCON-KA site) between deployments to collect comparing measurements. At each of the visited TCCON sites, the TCCON spectrometers collected low-resolution (LR) (0.5 cm-1) and high-resolution (HR) (0.02 cm-1) measurements in an alternating manner. In East Trout Lake (ETL), the TCCON spectrometer broke down while the TS was en route to the station. Hence, no side-by-side comparison was possible there. For Tsukuba and Wollongong the agreement found for XCO2 is on the 0.1 % level. For XCH4 the agreement is at the 0.2 % level, with the low-resolution measurements showing a low bias at both sites and for both gases. For XCO the deviations are up to 7 %. The reason for this is likely to be an known issue with the CO a priori profiles used by TCCON over source regions. The pressure analysis reveals excellent agreement (0.027 hPa, 0.135 hPa, and 0.094 hPa) for the Tsukuba, ETL, and Wollongong sites. [ABSTRACT FROM AUTHOR]

Understanding and modeling water quality in a lake/reservoir is important to the effective management of aquatic ecosystems. The advantages and disadvantages of different water quality models make it challenging to choose the most suitable model; however, direct comparison of 1-D and 3-D models for lake water quality modeling can reveal their relative performance and enable modelers and lake managers to make informed decisions. In this study, we compared the 1-D model MINLAKE and the 3-D model EFDC+ for water temperature, ice cover, and dissolved oxygen (DO) simulation in three Minnesota lakes (50-m Carlos Lake, 23.5-m Trout Lake, and 5.6-m Pearl Lake). EFDC+ performed well for water temperature and DO simulation in the open water seasons with an average root mean square error (RMSE) of 1.32 °C and 1.48 mg/L, respectively. After analyzing the ice thickness with relevant data, it was found that EFDC+ calculates a shorter ice cover period and smaller ice thickness. EFDC+ does not consider snowfall for ice thickness simulation. The results also revealed that EFDC+ considers spatial variance and allows the user to select inflow/outflow locations precisely. This is important for large lakes with complex bathymetry or lakes having multiple inlets and outlets. MINLAKE is computationally less intensive than EFDC+, allowing rapid simulation of water quality parameters over many years under a variety of climate scenarios. [ABSTRACT FROM AUTHOR]

Yellowstone cutthroat trout inhabiting Yellowstone Lake have declined substantially over the past 25 years as a result of predation by invasive lake trout, the presence of whirling disease, and periods of persistent drought. We used otolith microchemistry to assess whether cutthroat trout recruitment patterns have changed in response to these environmental stressors. Though water chemistry variation among the 22 sampled spawning tributaries was low, we identified 9 distinct spawning stream clusters. Random forest models were developed for assessing relative recruitment, yielding a high classification accuracy of 84.4% for known-origin cutthroat trout fry otoliths and 79.0% for simulated otolith signatures based on water chemistry. Proportion of recruitment varied significantly between pre- and post-stressor samples (X2 = 15.40, P = 0.03). The majority of pre- (0.84) and post-stressor (0.77) recruitment occurred in the same three stream clusters, but there was a notable decrease in recruitment in streams with high whirling disease prevalence, and a notable increase from the tributary cluster without whirling disease and with low lake trout predation risk. Conservation efforts should be focused on protecting important spawning tributaries and improving cutthroat trout recruitment in spawning streams that in the past likely contributed much greater numbers of fish. [ABSTRACT FROM AUTHOR]

Assisted gene flow is increasingly used to combat severe population declines. However, the associated risks, such as outbreeding depression, are often insufficiently assessed. Here, we studied the impact of assisted gene flow on the fitness of a highly endangered landlocked salmon population (Salmo salar m. sebago) from the lake Saimaa complex (Finland), using an anadromous Atlantic salmon population (S. salar) as donor. We released individuals of both parental populations and their hybrids into seminatural streams, monitoring their survival under predation risk from Northern pike (Esox lucius) and their growth on a natural diet. Before release, we exposed half of the salmon to the parasite Diplostomum pseudospathaceum to study whether assisted gene flow affects infection susceptibility, which could indirectly shape predation susceptibility and growth. The parental populations differed in both studied traits and the hybrids showed intermediate values. Relative to the target landlocked salmon population, hybrids experienced 21–26% lower survival, but 1.4–2.2% higher growth. They also carried 0.6–2.8 more parasites than the landlocked salmon, contributing to survival differences. These findings indicate that assisted gene flow can induce both negative and positive fitness effects. We propose that the overall net effects of this conservation tool need to be carefully evaluated before its implementation. [ABSTRACT FROM AUTHOR]

Objective: Cormack–Joly–Seber (CJS) mark–recapture analysis was used to estimate the effect of travel distance and fuel costs on angling party persistence (i.e., survival = probability of remaining in the Lake Trout Salvelinus namaycush fishery from year‐to‐year) and visitation (i.e., detection = probability of detection at the access point) over a 14‐year period (2006–2019) for the Lake Trout recreational fishery in Lake Opeongo, Ontario, Canada. Methods: Boat identification numbers, positioned near the bow and present by law, were used as "tags" in the CJS analysis. Complete trip creel interviews at the sole access point provided information on trip characteristics. Result: The long‐term mark–recapture analysis of individual angling parties showed travel distance had a significant negative effect on party persistence among years and visitation within years. The distance effect was strongest for visitation, with visitation having a more negative relationship with distance than persistence. Persistence in the Lake Trout fishery was insensitive to fuel costs while the fuel cost covariate had a small though significant and negative effect on visitation. Time‐varying CJS models were ranked low with little model weight indicating the distance effect estimated for Lake Opeongo was a general phenomenon and not year dependent. Conclusion: Cormack–Joly–Seber mark–recapture analysis cleanly separated effects of travel distance and fuel cost between visitation versus persistence for Lake Trout angling parties. Prior to this analysis, travel costs were typically associated with visiting a recreational fishery, while here we have shown that travel costs can be associated with persisting as anglers in a fishery independent of visitation patterns. Long‐term monitoring of angling parties provides unique insights into angling patterns that can aid in travel cost estimation. Impact statementWe followed individual Lake Trout angling parties for 14 years at a single access point fishery. Our findings showed greater travel costs primarily resulted in reduced visitation within years, with a smaller negative effect on persistence as anglers among years. Visitation to a fishery and persistence as anglers in a fishery are different features of travel costs. [ABSTRACT FROM AUTHOR]

Extinctions occur naturally in all environments, but rates have accelerated rapidly during the Anthropocene, especially in fresh water. Despite supporting many fish species of conservation importance, there has never been a formal assessment of their extinction risks in Britain, which has impeded their inclusion in relevant legislation and policy. This study therefore used the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species™ Categories and Criteria to conduct the first systematic assessment of the extinction risks and threats facing the native freshwater and diadromous fishes of Britain. In addition, national assessments were produced for England, Scotland and Wales, reflecting the level at which environmental policy decisions are taken in Britain.Seven species were categorized as being threatened with extinction at the regional level, with European eel Anguilla anguilla and allis shad Alosa alosa classified as Critically Endangered, Atlantic salmon Salmo salar, vendace Coregonus albula and European whitefish Coregonus lavaretus classified as Endangered, and Arctic charr Salvelinus alpinus and twaite shad Alosa fallax classified as Vulnerable. In addition, burbot Lota lota was classified as Regionally Extinct, ferox trout Salmo ferox was categorized as Data Deficient, and 25 species were categorized as Least Concern. European sturgeon Acipenser sturio and houting Coregonus oxyrinchus, although probably native, qualified as only vagrants in fresh water, so were categorized as Not Applicable.The assessments provide objective baselines against which future changes can be determined, and a key evidence base to support policy and management decisions for the conservation of freshwater and diadromous fish species and their habitats in Britain. It is recommended that the assessments are repeated every 10 years, which would enable changes in conservation status, the effectiveness of policies and where targeted interventions may be required to be examined using the Red List Index. [ABSTRACT FROM AUTHOR]

The endangered endemic species Sevan trout (Salmo ischchan Kessler, 1877) is under the threat of extinction and its survival is dependent on restocking by smolts. Thus, there is an urgent need to find an effective solution for restocking wild populations. After developing a theoretical methodology to reveal the most suitable parts of spawning rivers for natural reproduction, we consider the incubation of eggs in the wild as an appropriate method for the validation of the results of the theoretical assessment. Thus, we estimate the eggs' survival differences in the wild and microcosm conditions which found significant differences in the sac-fry emergence rate in the zones assessed as "partly suitable" and "unsuitable". Also, the results show that the emergence of eggs in the wild was less than in the microcosm conditions and the success rate of emergence of eyed eggs in the wild was higher than that of the green eggs. The results of one-way ANOVA, correlation and general linear mixed model analyses have in general proven the theoretical assessment but raise some questions on the completeness of using a set of abiotic criteria as the range of success during different experiments varies significantly. [ABSTRACT FROM AUTHOR]

Salvelinus namaycush (Lake Trout) occur in the Canadian Maritime Provinces of New Brunswick and Nova Scotia, and are absent in Prince Edward Island. Lake Trout are assumed to be native in parts of New Brunswick, but their ancestry in Nova Scotia is unknown. Starting in 1886, stocking was used to spread Lake Trout across lakes in the Maritime Provinces to bolster existing populations and to create new fishing opportunities. The success of most stocking efforts was not well documented, and it is presumed that many of these efforts failed. After the early stocking from which some populations persist, little effort has been made to understand the distribution, stock structure, or ancestry of Lake Trout in this region. This review is a synthesis of Lake Trout distribution in the Canadian Maritime Provinces, including a comprehensive account of stocking history, proposed native population occurrence, and identification of populations with variable ancestry. At present, Lake Trout occur in 20 lakes in New Brunswick (19 confirmed and 1 potential), and 4 lakes in Nova Scotia (2 confirmed and 2 potential). Native Lake Trout may persist in 12 of 15 New Brunswick lakes and 3 of 9 Nova Scotia lakes that historically contained ancestral Lake Trout populations. Of these lakes with historical populations that persist to this day, 10 in New Brunswick and 1 in Nova Scotia were supplemented with introduced stock; therefore, only 2 lakes in New Brunswick and 2 in Nova Scotia are likely to still contain purely native Lake Trout. By improving the understanding of population structure and origins, this synthesis provides information that is critical for the effective management and conservation of native Lake Trout in the eastern Maritime region. [ABSTRACT FROM AUTHOR]

Many Norwegian rivers and lakes are regulated for hydropower, which affects freshwater ecosystems and anadromous fish species, such as sea trout (Salmo trutta). Lakes are an important feature of many anadromous river systems. However, there is limited knowledge on the importance of lakes as habitat for sea trout and how hydropower affects the behaviour of sea trout in lakes. To investigate this, we conducted an acoustic telemetry study. A total of 31 adult sea trout (532 ± 93 mm total length) were captured by angling in river Aurlandselva, Norway, and tagged between July 20 and August 12, 2021. The tags were instrumented with accelerometer, temperature, and depth sensors, which provided information on the sea trout's presence and behaviour in lake Vassbygdevatnet. Our results indicate that there was a large prevalence of sea trout in the lake during the spawning migration, and that the sea trout were less active in the lake compared to the riverine habitats. An increase in activity of sea trout in the lake during autumn might indicate that sea trout spawn in the lake. However, the discharge from the high-head storage plant into the lake did not affect the depth use or activity of sea trout in the lake. Furthermore, the large prevalence of spawners in the lake during autumn will likely cause an underestimation of the size of the sea trout population in rivers with lakes during annual stock assessment. In conclusion, our results could not find evidence of a large impact of the discharge on the behaviour of sea trout in the lake. [ABSTRACT FROM AUTHOR]

Identifying potential links between food web structure and animal body condition is fundamental for predicting the long‐term persistence of populations under rapidly changing environments. Northern lakes, and the cold‐water adapted species that inhabit them, are particularly vulnerable to a warming climate.We explored relationships among body condition and energy channel use in lake trout (Salvelinus namaycush) and seasonal variation in food‐chain length in the Lake of Two Rivers, Ontario, Canada. Bayesian mixing models using carbon isotope values (δ13C) of essential amino acids showed that individuals utilising multiple energy channels had higher gonad mass, suggesting that dietary diversity has reproductive fitness benefits.Amino acid nitrogen isotope (δ15N) analysis revealed seasonal variation in food chain length, with extension of the food web during winter and spring and truncation of the food web during summer. Although these findings illustrate increased omnivory of lake trout during summer, there was no clear evidence that seasonal changes in food chain length had an impact on trout condition.These findings provide new insight into potential relationships between food web structure, energy flow and consumer condition in freshwater ecosystems, thereby revealing potential responses of species to shifting ecosystem states under intensifying climate change at high latitudes. [ABSTRACT FROM AUTHOR]

The phenomenon of homing by salmonid fishes to natal sites for breeding is well established, but the chemicals in rivers that they learn as juveniles and identify as adults are not fully determined. Dissolved free amino acid (DFAA) profiles have been hypothesized to allow salmonids to distinguish their natal river from others nearby. To evaluate this hypothesis, we sampled DFAAs in spring and autumn (when juveniles learn and adults return, respectively) from 3 rivers in New England, USA, that support the landlocked Atlantic Salmon, Salmo salar Linnaeus, 1758. We used 3 approaches to determine the level of consistency between seasons and difference between rivers that would be needed for DFAA to support imprinting and subsequent homing for reproduction: hierarchical cluster analysis, statistical difference tests, and equivalence tests. DFAAs were not detected in the water column of the study rivers, but sediment porewater samples yielded DFAAs at measurable concentrations. Hierarchical cluster analysis, difference testing, and equivalence testing all indicated that some combinations of sediment porewater DFAA concentrations differed among rivers and were similar between spring and autumn within a river. Specifically, equivalence tests revealed subsets of sediment porewater DFAAs that were seasonally equivalent within each river, and none of the seasonally equivalent DFAAs were common among all 3 rivers (i.e., each river had a unique DFAA profile). However, exceptions detected in the cluster analysis and equivalence testing raise questions regarding the extent to which DFAAs might be sufficient for salmon imprinting and homing. Thus, DFAAs may fulfill some of the hypothesized prerequisites as salmon imprinting and homing odor cues, but our lack of understanding of salmon discriminatory abilities and limited DFAA data preclude definitive conclusions about the sufficiency of DFAAs alone as homing cues. [ABSTRACT FROM AUTHOR]

The native Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri; YCT) of Yellowstone Lake, Yellowstone National Park, Wyoming, USA, presents a metapopulation that inhabits an ecologically simple, uniquely pristine, and highly protected environment. Recent investigation of a key YCT spawning stock's 1977–1992 dynamics, before predation from an illegally introduced and growing lake trout (Salvelinus namaycush) population measurably affecting YCT population dynamics, showed the new climate regime that began in the 1970s largely explained declining YCT spawning runs, although the concurrent and growing predation effect soon became dominant. The present study's time series analyses of annual (1969–2010) gillnet catches from 11 lake locations showed the metapopulation's age-5 YCT abundance rapidly declined to low levels 7 years after the spawning stock's (median YCT age, 5 years) similar decline. Since the 1980s, the Yellowstone region has experienced increasing drought that has reduced wetlands and associated tributaries used by the lake's spawning YCT. While small tributaries were first to become unsuitable for YCT spawning and early rearing, the spawning stock's tributary was of moderate size and its population persisted later. Two conditions are necessary for YCT metapopulation recovery to historically high levels: (1) climatic conditions must return to those prevalent around 1980 and (2) the lake trout population must be reduced to its mid-1990s levels. The most recent time of YCT metapopulation abundance and broadest spatial extent occurred under those simultaneous conditions. Similarities between the metapopulation's structure and dynamics and those reported for metapopulations of anadromous salmonids are described, including adverse effects of ongoing climate change. [ABSTRACT FROM AUTHOR]

Lakes of the Bécancour River Basin (BRB), located in the former asbestos mining region of Thetford Mines (southern Quebec, Canada), are heavily exposed to pollution. A previous paleolimnological investigation revealed that Trout Lake underwent severe eutrophication following the drainage of former Black Lake for mining purposes in the 1950s. Questions remained regarding the causes and level of degradation of other BRB lakes located upstream (Lake Bécancour) and further downstream (lakes William and Joseph), which also suffer from eutrophication. This paper presents diatom analyses performed on sediment cores from these lakes, along with complementary carbon (C) and nitrogen (N) data, that fill this knowledge gap. It also provides new insights into the natural limnological history of the Thetford Mines region. Low planktonic to benthic diatom ratios in oldest Lake Bécancour sediments revealed lower water levels during the Holocene Thermal Maximum (ca. 9000–5500 cal years BP). They later increased while diatom-inferred water pH decreased in response to cooler and wetter climatic conditions. Lake William responded more strongly than other BRB lakes to anthropogenic settlement activities in its watershed, with the emergence of Aulacoseira subarctica marking its transition to a mesotrophic state at about 1885 CE. Similar to Trout Lake, the drainage of Black Lake further enhanced the eutrophication of lakes William and Joseph, as illustrated by increases in sediment δ13C, decreases in C/N ratios, and proliferation of nutrient-dependent species such as Cyclostephanos invisitatus/makarovae. Our study highlights long-lasting and dramatic effects of such hydrological interventions on multi-lake systems. [ABSTRACT FROM AUTHOR]