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Hybridisation is a complex process that begins with mating. Females have more to lose with each hybrid fertilisation than males, so they should avoid it. Even if females choose con‐specific males as preferred mates, they often cannot control which additional males release sperm during spawning. Polyandry is ubiquitous and may result in hetero‐specific sperm competition between males of different species. In such cases, cryptic female choice (the ability to bias paternity towards certain males under sperm competition) is the last line of defence to prevent hybridisation of her eggs and is highly adaptive if it enables con‐specific sperm preference. Such seems to be the case with the hybridisation of Atlantic salmon (Salmo salar) and brown trout (S. trutta) in their native Europe. Under hetero‐specific sperm competition, hybrid fertilisations in these fish are reported to be reduced via ovarian fluid‐mediated cryptic female choice. It is not known, however, whether the strength of this mechanism is dependent on reinforcement and thus, the historical sympatry/allopatry of hybridising populations. Brown trout are one of the world's worst invasive species. Ecological impacts arise through competition with other species (e.g. Galaxids in the southern hemisphere, Oncorhynchus in western North America). Eastern North America contains native salmonids that evolved in the absence of brown trout but have gametes that are compatible. The 140 ‐year‐old brown trout invasion of Newfoundland is ground zero to study these potential interactions. Their relatively low spread rate across the island may be the result of inherent poor productivity, but data suggest it could also be a function of hybridisation with native Atlantic salmon and brook char (Salvelinus fontinalis). [ABSTRACT FROM AUTHOR]

This study was important to improve proper biosecurity measures and controlling the spread of Aeromonas to prevent future outbreaks. This research sought to determine whether virulent Aeromans species were present in morbid rainbow trout, their resistance and their genetic relatedness. A total number of 542 tissue lesion specimens were collected from gill, liver, heart and kidneys in morbid domesticated fish in Duhok province, Iraq. The gyrB DNA sequence analysis was used to determine the species classification. Drug susceptibility testing was conducted for all isolated strains using disc diffusion technique. The genotyping analysis was carried out using enterobacterial repetitive intergenic consensus-polymerase chain reaction. Thirty-four isolates were found and they were classified into three species (Aeromonas veronii, Aeromonas sorbia, and Aeromonas allosaccharophila), where A. veronii stand as one of the most prevalent species. The most frequently affected organ by Aeromonas was the gills among four different organs. The detection frequencies of the virulence genes aerolysin, outer membrane protein, glycerophospholipid-cholesterol acyltransferase, elastase, flagella, serine protease, cytotonic heat-labile, and hemolysin were 100%, 100%, 79.41%, 64.70%, 76.47%, 67.64%, 70.58%, and 41.17, respectively. None of the strains possessed all of the virulence markers. All isolates were completely resistant to ceftazidime, amoxicillin and doxycycline. All isolates were found to be multi-drug-resistant. Regardless of the nearest geographic source area of samples and the same Aeromonas species, there was a high genetic diversity. The results of this study could help farmers and researchers make informed decisions about measures of biosecurity and proper therapeutic drugs to apply to prevent current outbreaks and prevent them from recurring again. [ABSTRACT FROM AUTHOR]

The preservation of genetic variation is fundamental in biodiversity conservation, yet its importance for population viability remains contentious. Mixed‐source reintroductions, where individuals are translocated into a single vacant habitat from multiple genetically divergent and often depauperate populations, provide an opportunity to evaluate how genetic variation and hybridization influence individual and relative population fitness. Population genetic theory predicts that individuals with higher genetic variation and hybrids among populations should have higher fitness. We tested these two hypotheses by analyzing individual and population‐scale data for westslope cutthroat trout (Oncorhynchus clarkii lewisi) in four mixed‐source reintroductions. We observed more hybrid and fewer nonhybrid offspring than expected across four independent mixed‐source reintroductions. We also found clear evidence that heterozygosity influenced individual reproductive and relative population fitness. Overall, we found a strong, positive relationship between genetic variation, hybridization, and transplant fitness, emphasizing the importance of genetic variation and population mixing in conservation. [ABSTRACT FROM AUTHOR]

Juvenile Pacific salmon (Oncorhynchus spp.) use estuary habitats to varying degrees with some species and populations thought to rely heavily on these areas for early growth. In the Fraser River, British Columbia, there are 18 distinct conservation units of Chinook salmon (O. tshawytscha), and all but one is of conservation concern. Our study compares the outmigration timing, size, and habitat use of juvenile Chinook salmon in the Fraser River estuary. Over 5 years (2016–2020), we captured 6493 juvenile Chinook salmon, with 3318 sampled for stock identification. Fraser River Chinook salmon extensively used estuary habitats, but patterns varied considerably by population. Juvenile Chinook salmon from the Lower Fraser River were most abundant and present the longest, arriving the smallest in late March and early April, and captured until July. South Thompson ocean-type Chinook salmon entered the estuary later, starting to arrive in late May or early June and remaining present until mid-August. Overall, juvenile Chinook salmon varied considerably in their estuary use across populations. Understanding this variation can inform differences in productivity and guide recovery actions. [ABSTRACT FROM AUTHOR]

Salmonids undertake long and strenuous migrations that require aerobic endurance and anaerobic burst swimming. Anthropogenic activity and natural disasters can make already challenging migrations even more difficult. This reinvigorates a central question: what is the maximum swimming capacity of adult salmon species across environmental conditions? We synthesized literature on adult salmon swim performance (Oncorhynchus spp. and Salmo salar) to unfold what is known about how biological (sex and body size) and physical (temperature) factors affect swimming in adult salmon. Maximum anaerobic swimming—bursting, jumping, and leaping—are among the least studied swim performances in adult salmonids. Commonly, swim performance has been measured using swim flumes, but evidently adult salmon can swim faster in the wild than this set-up can capture. We show that larger fish can outswim smaller ones, and that thermal sensitivity of swimming differs inter-and -intra-specifically. Unresolved are how anaerobic swimming differs across temperatures, between males and females and across maturity states. This information can be used to inform exercise physiology research and future management and mitigation actions necessary to conserve these iconic, economically valuable species. [ABSTRACT FROM AUTHOR]

Pacific pink salmon (Oncorhynchus gorbuscha) were deliberately introduced to rivers surrounding the White Sea and has spread to Norway and several other countries surrounding the North Atlantic Ocean. In August 2021, a female pink salmon displaying pale gills and abnormal behaviour was captured in River Lakselva in Northern Norway and later submitted to the Norwegian Veterinary Institute (NVI) for post‐mortem examination. Histological examination of organ samples revealed structures indicative of systemic ichthyophoniasis, caused by Ichthyophonus sp. The parasites appeared to be especially abundant in the heart and skeletal musculature, and local tissue responses were assessed to be absent or very mild. Sequences of the ribosomal 18S rRNA and the mitochondrial cytochrome oxidase 1 (CO1) genes confirmed the diagnosis and identified the pathogen as Ichthyophonus sp. The CO1 sequence further established that the isolate from pink salmon was most similar to sequences of Ichthyophonus sp. from Atlantic salmon, Salmo salar, from the Atlantic Ocean on the east coast of the US and from Atlantic herring, Clupea harengus, from Iceland. We here report the first detection of Ichthyophonus sp. in pink salmon in the North Atlantic Ocean. [ABSTRACT FROM AUTHOR]

The presence of dams on the Columbia River (CR) has reduced Sockeye Salmon (Oncorhynchus nerka|sćwin) numbers to a fraction of their historic numbers. The Syilx Okanagan Nation Alliance (SONA) has led voices of concern regarding the impacts of diminishing Sockeye Salmon numbers on the ecosystem health of the sq,awsitkʷ|Okanagan River (s|OR), a tributary of the CR. In the early 2000s efforts commenced to rehabilitate the s|OR Sockeye population. These efforts have seen the population rise from a running average of 40,000 to 200,000. However, the contemporary spawning capacity of the s|OR is unknown, and this metric is critical to guide the conservation and restoration of this keystone species. We employed a spawning capacity model relating fish length and gravel size to spawning potential. We collected substrate data at 49 sites throughout our 37‐km study area, and used fish length and egg numbers from n = 269 Sockeye Salmon to establish s|OR specific fecundity metrics. The model estimated the s|OR has the capacity to support 147,687 (±33,346) Sockeye spawners, in its current condition. Whilst our model has limitations, we suggest a minimum escapement of 147,687 Sockeye spawners ought to be passed to the s|OR per annum. These fish will have broad ecosystem benefits and will be culturally beneficial to SONA peoples. Finally, continued restoration of s|OR habitat should enhance opportunities for Sockeye Salmon to continue their remarkable recovery. [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]

Objective: Hybridization between native and nonnative fishes represents a global threat to biodiversity. Understanding how hybridization changes in response to management actions is critical to evaluating the efficacy of conservation efforts. Methods: We quantified changes in levels of hybridization between Yellowstone Cutthroat Trout Oncorhynchus virginalis bouvieri and Rainbow Trout Oncorhynchus mykiss in the South Fork Snake River watershed, where a multipronged approach has been implemented to protect the evolutionary distinctiveness of one of the last remaining large‐river populations of Yellowstone Cutthroat Trout. Result: Over a 20‐year period, we observed an increase in the number of sample reaches without hybrids in the South Fork Snake River watershed; however, contrasting patterns were noted in main‐stem and tributary reaches. Through time, hybrid abundance increased at main‐stem reaches of the South Fork Snake River below Palisades Dam but decreased in tributaries. Efforts to reduce hybridization in spawning tributaries, including both suppression and selective passage weirs, were effective at preventing the expansion of hybridization in resident and migratory populations. Multimodel inference was used to understand factors affecting levels of hybridization, and year, sampling reach, and the interaction thereof was identified as the best‐fit model but explained only a small percentage of the overall variation, suggesting that other factors not captured in our model were driving patterns in hybridization. Conclusion: Changes in hybridization in the South Fork Snake River watershed are likely the result of multiple processes, namely management actions to reduce Rainbow Trout and hybrids in tributaries, as well as demographic changes in Rainbow Trout in the main‐stem river below Palisades Dam. Our results suggest that Yellowstone Cutthroat Trout populations in the South Fork Snake River watershed have not experienced widespread interspecific hybridization with Rainbow Trout but that proactive management will be necessary to ensure long‐term conservation. Impact statementOver the past 20 years, hybridization between introduced Rainbow Trout and native Yellowstone Cutthroat Trout in the South Fork Snake River watershed has declined in spawning tributaries because of management actions but increased in select main‐stem river reaches because of Rainbow Trout population growth. [ABSTRACT FROM AUTHOR]

We evaluated the utility of three male‐specific molecular markers, sdY, sdY 227U and OtY2m, in two wild populations of masu salmon (Oncorhynchus masou) in southwestern Hokkaido, Japan. Male‐specific fragments amplified in all phenotypic males across the markers. Phenotypic females were genetically identified as males using sdY (57%) and sdY 227U (4%), but no phenotypic females were identified as males using OtY2m. We conclude that OtY2m was the most reliable sex marker, followed closely by sdY 227U, among those tested in our study populations. Additional research is warranted to test the applicability of these markers in other populations and Oncorhynchus species. [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]

Over the last 10 years, the spawning population of invasive pink salmon (Oncorhynchus gorbuscha) has increased in the river systems in northern Norway to a level that is causing concern about their impact on endemic fauna and ecosystem processes. The scale of transfer of pink salmon carcasses into the terrestrial ecosystem is likely to be a key determinant of terrestrial impact. Bears (Ursus sp.) are responsible for most such transfers in North America but are rare in Norway. The white‐tailed eagle (Haliaeetus albicilla) is common however, and a candidate to be a main cause of such transfers. To evaluate this hypothesis, data on the abundance of white‐tailed eagles and pink salmon were collected along the river Skallelv in northern Norway in 2021, a year the pink salmon spawned in the river, and in 2022, a year no pink salmon spawned in the river. The abundance of white‐tailed eagles along the river was much higher the year pink salmon spawned in the river. Furthermore, white‐tailed eagles were observed aggregating and catching pink salmon where and when pink salmon were present at the spawning and post‐spawning stages. Based on our observations, we suggest that the white‐tailed eagle is the main species involved in the transport of pink salmon from the river into the riparian zone in northern Norway and that other scavengers, in particular the red fox (Vulpes vulpes) and common raven (Corvus corax), play an important role in transporting pink salmon carcasses from the riparian zone to the wider terrestrial ecosystem. [ABSTRACT FROM AUTHOR]

For almost 200 years, the taxonomy of cutthroat trout (Oncorhynchus clarkii), a salmonid native to Western North America, has been in flux as ichthyologists and fisheries biologists have tried to describe the diversity within these fishes. Starting in the 1950s, Robert Behnke reexamined the cutthroat trout and identified 14 subspecies based on morphological traits, Pleistocene events, and modern geographic ranges. His designations became instrumental in recognizing and preserving the remaining diversity of cutthroat trout. Over time, molecular techniques (i.e. karyotypes, allozymes, mitochondrial DNA, SNPs, and microsatellite arrays) have largely reinforced Behnke's phylogenies, but have also revealed that some relationships are consistently weakly supported. To further resolve these relationships, we generated de novo transcriptomes for nine cutthroat subspecies, as well as a Bear River Bonneville form and two Colorado River lineages (blue and green). We present phylogenies of these subspecies generated from multiple sets of orthologous genes extracted from our transcriptomes. We confirm many of the relationships identified in previous morphological and molecular studies, as well as discuss the importance of significant differences apparent in our phylogenies from these studies within a geological perspective. Specific findings include three distinct clades: (1) Bear River Bonneville form and Yellowstone cutthroat trout; (2) Bonneville cutthroat trout (n = 2); and (3) Greenback and Rio Grande cutthroat trout. We also identify potential gene transfer between Bonneville cutthroat trout and a population of Colorado River green lineage cutthroat trout. Using these findings, it appears that additional groups warrant species‐level consideration if other recent species elevations are retained. [ABSTRACT FROM AUTHOR]

Pacific salmon Oncorhynchus spp. hatcheries in British Columbia (BC) have been the major activity of the Salmonid Enhancement Program since 1977. However, the present diminished abundance of Pacific salmon and loss of fisheries in BC called for a review of hatchery effectiveness. The Pacific Salmon Foundation conducted a review resulting in 15 reports publicly available on the Pacific Salmon Foundation marine science website. The review includes evaluations of effectiveness of hatchery production, evidence of interactions with natural salmon populations (including rebuilding natural populations with hatchery‐produced salmon), and future opportunities via genomics. Beyond the estimation of juvenile production and contribution to catches and spawning escapements, the ability to assess effectiveness was limited. The assessments of interactions provided a few examples of positive outcomes, but most were negative. The lack of comparative assessments in purely wild populations seriously limits interpretations of hatchery–wild interactions. Future evaluations of the hatchery effectiveness require more quantitative and integrated assessments of hatchery and wild salmon, open data sharing for public review or research, and related studies could benefit from application of new genomic technologies. This paper presents recommendations to address these needs and calls for regularized reviews of the Salmonid Enhancement Program and its net value to sustainability of BC's Pacific salmon. [ABSTRACT FROM AUTHOR]

The relationships between body size and density of salmonids in natural streams have been well studied, and density‐dependent growth (somatic growth) is interpreted as the principal mechanism responsible for the relationship. Moreover, the body size–density relationship is known to vary over ontogeny. However, the relationship has been studied mostly by experimental procedures (e.g. stocking fish into streams), and knowledge about the relationship in natural populations (e.g. no human‐induced input of fish) is still inadequate. In this study, we compared the relationships between body size and density of young‐of‐the‐year (YOY) masu salmon, Oncorhynchus masou, between different ontogenetic stages, that is, fry (spring) and juveniles (autumn). We used the monitoring data of YOY body sizes and densities across three study sites collected from Horokashubuto stream in Hokkaido, northern Japan, from 2014 to 2022. In the juvenile stage, YOY body size correlated negatively with increasing density. This pattern is typical of the density dependence of salmonid growth demonstrated in many previous studies. In the fry stage, however, YOY body size correlated positively with increasing density. This study suggested that the patterns of density dependence vary as functions of the ontogenetic stages of salmonids. [ABSTRACT FROM AUTHOR]

N‐(1,3‐dimethylbutyl)‐N′‐phenyl‐p‐phenylenediamine‐quinone (6PPD‐quinone) is a widespread contaminant of emerging concern resulting from oxidation of 6PPD, which is an antidegradant substance added to tires. The recent identification of 6PPD‐quinone as the cause of acute mortality in coho salmon has quickly motivated studies on 6PPD‐quinone toxicity to other species. Subsequent findings have shown that 6PPD‐quinone toxicity is highly species specific. Closely related species can differ widely in response to 6PPD‐quinone from extremely sensitive to tolerant. Hence toxicity testing is currently the only way to establish whether a species exhibits 6PPD‐quinone toxicity. We investigated the acute toxicity of 6PPD‐quinone in pink salmon alevins (sac fry). This species has is the only Pacific salmon that so far has not been tested for 6PPD‐quinone sensitivity. Fish were exposed in static water in eight treatments with initial concentrations ranging from 0.1 to 12.8 μg/L. Fish were observed for 48 h, and changes in concentrations of 6PPD‐quinone were monitored throughout the experiment. No mortalities or substantial changes in behavior were recorded. Environ Toxicol Chem 2024;43:1332–1338. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC. [ABSTRACT FROM AUTHOR]

Methods for forecasting the stocks of pacific salmon in Khabarovsk Region are considered. Errors of the forecasts based on theoretical parent-offspring models are associated with variability in weather conditions on the spawning grounds. The progeny dynamics is modeled much better when weather factors are assimilated into the single- factor models. The errors reduction would be expected for multifactor models, assuming that they will be based on reliable data on the number of parents and offspring, though this condition seems questionable in the face of a growing shortage of qualified observers and modelers.

Conditions determining the vector of ontogenetic development and factors influencing the sex ratio for various life-history forms of pacific salmon (gen. Oncorhynchus) are discussed on the basis of cited data.

BackgroundConsiderable resources are spent to track fish movement in marine environments, often with the intent of estimating behavior, distribution, and abundance. Resulting data from these monitoring efforts, including tagging studies and genetic sampling, often can be siloed. For Pacific salmon in the Northeast Pacific Ocean, predominant data sources for fish monitoring are coded wire tags (CWTs) and genetic stock identification (GSI). Despite their complementary strengths and weaknesses in coverage and information content, the two data streams rarely have been integrated to inform Pacific salmon biology and management. Joint, or integrated, models can combine and contextualize multiple data sources in a single statistical framework to produce more robust estimates of fish populations.MethodsWe introduce and fit a comprehensive joint model that integrates data from CWT recoveries and GSI sampling to inform the marine life history of Chinook salmon stocks at spatial and temporal scales relevant to ongoing fisheries management efforts. In a departure from similar models based primarily on CWT recoveries, modeled stocks in the new framework encompass both hatchery- and natural-origin fish. We specifically model the spatial distribution and marine abundance of four distinct stocks with spawning locations in California and southern Oregon, one of which is listed under the U.S. Endangered Species Act.ResultsUsing the joint model, we generated the most comprehensive estimates of marine distribution to date for all modeled Chinook salmon stocks, including historically data poor and low abundance stocks. Estimated marine distributions from the joint model were broadly similar to estimates from a simpler, CWT-only model but did suggest some differences in distribution in select seasons. Model output also included novel stock-, year-, and season-specific estimates of marine abundance. We observed and partially addressed several challenges in model convergence with the use of supplemental data sources and model constraints; similar difficulties are not unexpected with integrated modeling. We identify several options for improved data collection that could address issues in convergence and increase confidence in model estimates of abundance. We expect these model advances and results provide management-relevant biological insights, with the potential to inform future mixed-stock fisheries management efforts, as well as a foundation for more expansive and comprehensive analyses to follow.

The pink salmon Oncorhynchus gorbuscha (Teleostei, Salmonidae) is a North Pacific species. It arrived in Northern Europe through freshwater migration each odd year since 2017, creating management issues. In this study, we sequenced a complete mitochondrial genome for a pink salmon caught in the Bresle River (France) with a museum voucher. The sequence is 16,695 bp in length and is similar to previous mitog)enomes published within this genus. Length heteroplasmy on the control region for this species was detected, as described in previous molecular studies. We tested several primers commonly used for metabarcoding studies on four markers (12S, 16S, COI and Cytb), and all allow the discrimination of this species. This mitogenome is a reference for molecular identification, for instance in like environmental DNA studies. [ABSTRACT FROM AUTHOR]

Hatchery fish and their offspring (including hatchery‐wild hybrids) have lower reproductive success than wild fish. Thus, the straying of hatchery fish may negatively impact wild populations, depending on the number of wild salmon returning and hatchery strays. We investigated the straying status of hatchery‐origin pink salmon (Oncorhynchus gorbuscha), which have a higher straying rate than other salmonids, in an unstocked river at the Shiretoko World Natural Heritage Site, Japan. The hatchery strays accounted for 40.0% and 19.0% of the total samples in 2021 and 2022, respectively. These results indicate that hatchery pink salmon have invaded unstocked rivers and potentially genetically affect wild populations. [ABSTRACT FROM AUTHOR]

In response to ocean heating and hatchery production, pink salmon (Oncorhynchus gorbuscha) returning from the North Pacific Ocean steadily increased after 1975, leading to concerns about their influence on food webs and competition with other species. Using measurements of distance between scale annuli of 24 584 individual sockeye salmon (O. nerka), we examined growth during their 2 or 3 years at sea from 1977 to 2015 for eight populations in Alaska. We found significant, negative autocorrelations at 1 lag year in annual growth of sockeye salmon, with a consistent pattern of lower growth in odd years, i.e. opposite to the biennial pattern of pink salmon abundance. Peak pink salmon abundances reduced growth of sockeye salmon from 7 to 14% during the second year in the ocean compared with growth when pink salmon abundance was low, while third-year growth was reduced up to 17%. The overall effect of pink salmon abundance on sockeye growth was over two times greater than the effect of sockeye salmon abundance. Production hatcheries and ocean heating contribute to the competitive dominance of pink salmon, underscoring the need to consider this unintended anthropogenic effect on the growth and productivity of sockeye salmon throughout the North Pacific. [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]

The impressive †Oncorhynchus rastrosus of the Pacific Northwest's Miocene and Pliocene eras was the largest salmonid ever to live. It sported a hypertrophied premaxilla with a pair of enlarged teeth which the original describers reconstructed as projecting ventrally into the mouth, leading them to assign the species to "Smilodonichthys," a genus now in synonymy. Through CT reconstruction of the holotype and newly collected specimens, we demonstrate that the famed teeth projected laterally like tusks, not ventrally like sabers or fangs. We also expand the original description to characterize sexual dimorphism in mature, breeding individuals. Male and female †Oncorhynchus rastrosus differ in the form of the vomer, rostro-dermethmoid-supraethmoid, and dentary, much as do other extant species of Oncorhynchus. Male specimens possess a more elongate vomer than do females, and female vomers have concave ventral surfaces and prominent median dorsal keels. The dentary of females has no evidence of a kype, though some specimens of †O. rastrosus have a non-uniform density mesial to the tooth bed, which we interpret as a male kype. Unlike extant Oncorhynchus, male and female †O. rastrosus do not differ in premaxilla shape. Because male and females possess hypertrophied premaxillae and lateral premaxillary spikes, the former common name "Sabertoothed Salmon" no longer reflects our understanding of the species' morphology. Accordingly, we redub †O. rastrosus the Spike-Toothed Salmon and postulate that its spikes were multifunctional, serving as defense against predators, in agonism against conspecifics, and as a practical aid to nest construction. [ABSTRACT FROM AUTHOR]

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Nematodes that parasitize salmonids are found in both seawater and freshwater. Unlike seawater species such as those in family Anisakidae, freshwater species have not been well studied. In particular, the influences of these nematodes on the body condition of salmonids remain unclear. We studied the effects of Salmonema cf. ephemeridarum on the body condition of masu salmon Oncorhynchus masou. We found a positive relationship between the number of parasites and fish fork length. In contrast, we found a negative relationship between the body condition (condition factor) of fish and the number of parasites. These results suggest that nematode infection could affect host energy reserves for future growth. [ABSTRACT FROM AUTHOR]

This study investigated the effects of Trachyspermum copticum essential oil nanoemulsion (TCEO-NE) coating on chemical, microbial and sensory changes of Oncorhynchus aguabonita fillets during storage at 4 ℃. The components of T. copticum EO were identified using gas chromatography-mass spectrometry. TCEO-NE was prepared by ultrasonic method and its properties were determined. Fresh O. aguabonita fillets were immersed in TCEO-NE and stored at 4 ℃. Chemical (pH, TVB-N, peroxide and TBARS), microbiological (total aerobic mesophilic bacteria, psychrotrophs, Enterobacteriaceae, lactic acid bacteria and Staphylococcus aureus) and sensory analyses (color, odor and taste) of fish fillets were evaluated on days 0, 2, 4, 6, 8 and 11. Thymol, limonene, and alpha-terpinene were the major compounds in T. copticum EO. The droplet size of TCEO-NE was 127.6 nm and PDI was 0.210. The control group exceeded the peroxide limit on day 6, while TCEO-NE 500.00 and 666.66 did so on day 8. The TBARS value in fish fillets was 0.57 mg MDA/kg on day zero which reached 4.76 mg MDA/kg in the control group and 2.90 mg MDA/kg in TCEO-NE 666.66 after 11 days at 4 ℃. Aerobic mesophilic count in the control group exceeded the permissible level on day 6 and, in TCEO-NE 500.00 and 666.66, on day 8, therefore the shelf life of fish fillets was improved by two days. On the grounds of the favorable properties of TCEO-NE and its positive effects on chemical, microbial and sensory changes in fish fillets, it can be used as a natural food additive. [ABSTRACT FROM AUTHOR]

Reintroduction is an important tool for the recovery of imperiled species. For threatened Pacific salmonids (Oncorhynchus spp.) species, hatchery‐origin (HOR) individuals from a nearby source are often used to reestablish populations in vacant, historically occupied habitat. However, this approach is challenged by the relatively low reproductive success that HOR Pacific salmonids experience when they spawn in the wild, relative to their natural‐origin (NOR) counterparts. In this study, we used genetic parentage analysis to compare the reproductive success of three groups of adult Chinook salmon (Oncorhynchus tshawytscha) reintroduced above Cougar Dam on the South Fork McKenzie River, Oregon: HOR Chinook salmon from an integrated stock; first‐generation, wild‐born descendants (hereafter F1s) of Chinook salmon produced at the same hatchery; and NOR Chinook salmon that are presumed to have been produced below the dam, on the mainstem McKenzie River, or elsewhere and volitionally entered a trap below Cougar Dam. We found that F1s produced nearly as many adult offspring as NORs, and 1.8‐fold more adult offspring than HORs. This result suggests that, for the South Fork McKenzie reintroduction program, a single generation in the wild increases fitness for the descendants of HOR Chinook salmon. Although these results are encouraging, care must be taken before extrapolating our results to other systems. [ABSTRACT FROM AUTHOR]

The development and application of approaches to detect and quantify environmental DNA (eDNA) have potential to improve our understanding of the distribution, abundance, and health of Atlantic Salmon Salmo salar and Pacific salmon Oncorhynchus spp. Here, we review 61 articles focusing on eDNA applications pertaining to salmon occupying natural habitat and aquaculture facilities in the context of advances, opportunities, and challenges. Given recent advances, eDNA now serves as a useful tool for detecting Atlantic Salmon and Pacific salmon and understanding threats to the health of fish and their habitats. Opportunities exist to apply sensitive and minimally invasive eDNA approaches to detect fish and assess fish habitat, assess range expansions of salmon and salmon pathogens, and detect invasive species that may threaten salmon health and abundance. Near real‐time eDNA detection and quantification approaches to inform fisheries management may be on the horizon. Challenges limiting the widespread application of eDNA approaches for informing salmon management include accounting for the many factors affecting detection and quantification of eDNA, limits of data for deriving inference, and expense. Through continued development and refinement, eDNA approaches are anticipated to become increasingly available to, and utilized by, managers of Atlantic Salmon and Pacific salmon fisheries. [ABSTRACT FROM AUTHOR]

Male chum salmon display a courtship behaviour involving an actively "quivering" against female, which is essential for female orgasm. Because generally male prefer larger female, we determined whether the elements of quivering such as the number, the amplitude, and the distance are affected by female body size. Additionally, quivering changes towards oviposition; therefore, we also identified the essential time for the male to succeed in his courtship behaviour. We conducted two experiments to measure quivering parameters: (i) measuring the number and the amplitude of quivering by the acceleration data logger and (ii) measuring the distance between sexes during quivering by observation of recorded video. From the first experiment, there was no significant relationship between the ratio of female‐to‐male body lengths and the number and the amplitude of quivering, while an important relationship between the body length ratio and the distance between sexes from the second experiment. Therefore, we think the distance is more significant than the number or vigour of quivering for male choice, and males quiver more closely to a larger female to increase reproductive success. In addition, when the body length ratio of the sexes is 1.01, the distance during quivering is the shortest. So, there is a possibility that the reproduction of chum salmon is based on size‐assortative mating. And both experiments also showed that male quiver passively as spawning approaches. We suggest that males may not quiver actively for smooth mating as females spawning approaches. [ABSTRACT FROM AUTHOR]

The Mediterranean region stands as a prominent biodiversity hotspot for the full spectrum of its habitats. However, it ranks among the most extensively invaded regions globally. This characterization aptly applies to the Balkan Peninsula, an expanse renowned for its elevated levels of biodiversity and endemism concerning freshwater fishes. Among the most frequently introduced fauna in the Balkans is the rainbow trout Oncorhynchus mykiss (Walbaum, 1792). Among the regions with very limited information available concerning the species' distribution in Greece is Mount Olympus. The current study reports the first record of several Oncorhynchus individuals from upstream of Enipeas River. The specimens in view are the first ichthyofauna record for the region. Also, this manuscript discusses the potential introduction vectors. Two different methodologies were used. The first was the use of citizen-science and the second was the use of underwater cameras and systematic surveys at Enipeas River. The estimated depth at sampling stations ranged from 0.5 to 2 m. Trout individuals were found in all sampling stations, estimated at 7–10 cm in length. [ABSTRACT FROM AUTHOR]

In adult fish, neurogenesis occurs in many areas of the brain, including the cerebellum, with the ratio of newly formed cells relative to the total number of brain cells being several orders of magnitude greater than in mammals. Our study aimed to compare the expressions of aromatase B (AroB), glutamine synthetase (GS), and cystathionine-beta-synthase (CBS) in the cerebellum of intact juvenile chum salmon, Oncorhynchus keta. To identify the dynamics that determine the involvement of AroB, GS, and CBS in the cellular mechanisms of regeneration, we performed a comprehensive assessment of the expressions of these molecular markers during a long-term primary traumatic brain injury (TBI) and after a repeated acute TBI to the cerebellum of O. keta juveniles. As a result, in intact juveniles, weak or moderate expressions of AroB, GS, and CBS were detected in four cell types, including cells of the neuroepithelial type, migrating, and differentiated cells (graphic abstract, A). At 90 days post injury, local hypercellular areas were found in the molecular layer containing moderately labeled AroB+, GS+, and CBS+ cells of the neuroepithelial type and larger AroB+, GS+, and CBS+ cells (possibly analogous to the reactive glia of mammals); patterns of cells migration and neovascularization were also observed. A repeated TBI caused the number of AroB+, GS+, and CBS+ cells to further increase; an increased intensity of immunolabeling was recorded from all cell types (graphic abstract, C). Thus, the results of this study provide a better understanding of adult neurogenesis in teleost fishes, which is expected to clarify the issue of the reactivation of adult neurogenesis in mammalian species. [ABSTRACT FROM AUTHOR]

The recent, rapid spreading of non‐native pink salmon Oncorhynchus gorbuscha in the North Atlantic area has raised concerns about their possible negative impacts on native salmonid species. Potential interactions include competition for food resources during the short freshwater phase of juvenile O. gorbuscha, but little is known about their feeding behavior in the newly occupied North Atlantic rivers. Using stable isotope and stomach content analyses, patterns of freshwater feeding of non‐native O. gorbuscha fry were studied in a large Fennoscandian river, the Teno, that discharges to the Barents Sea. Changes in stable isotope values (δ13C, δ15N, δ34S) and stomach contents from the period of emergence (April to mid‐May) to estuarine entry (late May/June) were examined and provided both temporally integrated and short‐term indicators of freshwater feeding dependency. In addition, the occurrence of juvenile O. gorbuscha and changes in their length and weight during their emergence/migration period were investigated. Juvenile O. gorbuscha were at the spawning grounds from April through to mid‐May with abundance peaking in mid‐May. Fish moved to the estuary by late May and their abundance decreased toward June, and their body size increased concurrently. Stomach analyses indicated no feeding activity in April–early May in the spawning areas, but the stomach fullness indices increased markedly in fish sampled in the estuary in May and June. The most important prey items in stomachs were Chironomidae and Ephemeroptera larvae. Significant changes in all analysed stable isotopes were detected among sample periods, with a peak in mid‐May and June showing significantly lower values than other sample periods. A change from the higher values reflective of parental marine feeding to the lower values reflective of freshwater feeding indicated active in‐river feeding by juveniles during the study period. The documented active freshwater feeding of non‐native juvenile O. gorbuscha suggests potential resource competition with native fluvial fishes, particularly salmonids. [ABSTRACT FROM AUTHOR]

Wildlife must increasingly balance trade‐offs between the need to access important foods and the mortality risks associated with human‐dominated landscapes. Human disturbance can profoundly influence wildlife behavior, but managers know little about the relationship between disturbance–behavior dynamics and associated consequences for foraging. We address this gap by empirically investigating the consequences of human activity on a keystone predator–prey interaction in a region with limited but varied industrial disturbance. Using stable isotope data from 226 hair samples of grizzly bears (Ursus arctos horribilis) collected from 1995 to 2014 across 22 salmon‐bearing watersheds (88,000 km2) in British Columbia, Canada, we examined how human activity influenced their consumption of spawning salmon (Oncorhynchus spp.), a fitness‐related food. Accounting for the abundance of salmon and other foods, salmon consumption strongly decreased (up to 59% for females) with increasing human disturbance (as measured by the human footprint index) in riparian zones of salmon‐bearing rivers. Declines in salmon consumption occurred with disturbance even in watersheds with low footprints. In a region currently among the least influenced by industrial activity, intensification of disturbance in river valleys is predicted to increasingly decouple bears from salmon, possibly driving associated reductions in population productivity and provisioning of salmon nutrients to terrestrial ecosystems. Accordingly, we draw on our results to make landscape‐scale and access‐related management recommendations beyond current streamside protection buffers. This work illustrates the interaction between habitat modification and food security for wildlife, highlighting the potential for unacknowledged interactions and cumulative effects in increasingly modified landscapes. [ABSTRACT FROM AUTHOR]

Aquatic systems are warming and exceeding upper thermal limits (UTLs) for many fish species, yet understanding how they inform resilience to climate change is challenging. Using Pacific salmon (Oncorhynchus spp.) as a model, we conducted a systematic review involving 168 papers investigating UTL in five species. We found considerable variation in UTL among species, within species and across life stages; largely attributed to methodological approaches (e.g. CTmax/UILT, Aerobic/Cardiac Scope, Thermal Migration Barriers, Rearing Mortality, Thermal Preference/Avoidance). Given that each method has strengths and weaknesses owing to logistics, time scale and ecological realism, we offer a new framework for assessing vulnerability to warming that stresses the importance of considering UTL metrics in the context of intended use (i.e. the development of management guidelines, projections of future persistence and survival) and what aspect (physiological or behavioural) of thermal response a metric investigates. Comparing studies with identical UTL approaches revealed that within species, UTL was higher for populations historically encountering higher temperatures—suggesting local thermal adaptation. Within populations, UTL differed across the lifecycle, being highest in fry and lowest in embryo and migrating adults, but this was not universal. For spawning Pacific salmon, UTL has not been assessed and few studies have examined estuarine and marine stages. Ultimately, this data gap limits the life‐history thermal perspectives that can be drawn and may indicate a broader gap for all fishes, given that Pacific salmon are among the most well‐studied species. Our framework illustrates the inherent and methodological inconsistencies in UTL and offers a guide for how thermal limits can best be used to assess the warming tolerance and vulnerability of fishes. [ABSTRACT FROM AUTHOR]

Genetic monitoring of Pacific salmon in the Columbia River basin provides crucial information to fisheries managers that is otherwise challenging to obtain using traditional methods. Monitoring programs such as genetic stock identification (GSI) and parentage‐based tagging (PBT) involve genotyping tens of thousands of individuals annually. Although rare, these large sample collections inevitably include misidentified species, which exhibit low genotyping success on species‐specific Genotyping‐in‐Thousands by sequencing (GT‐seq) panels. For laboratories involved in large‐scale genotyping efforts, diagnosing non‐target species and reassigning them to the appropriate monitoring program can be costly and time‐consuming. To address this problem, we identified 19 primer pairs that exhibit consistent cross‐species amplification among salmonids and contain 51 species informative variants. These genetic markers reliably discriminate among 11 salmonid species and two subspecies of Cutthroat Trout and have been included in species‐specific GT‐seq panels for Chinook Salmon, Coho Salmon, Sockeye Salmon, and Rainbow Trout commonly used for Pacific salmon genetic monitoring. The majority of species‐informative amplicons (16) were newly identified from the four existing GT‐seq panels, thus demonstrating a low‐cost approach to species identification when using targeted sequencing methods. A species‐calling script was developed that is tailored for routine GT‐seq genotyping pipelines and automates the identification of non‐target species. Following extensive testing with empirical and simulated data, we demonstrated that the genetic markers and accompanying script accurately identified species and are robust to missing genotypic data and low‐frequency, shared polymorphisms among species. Finally, we used these tools to identify Coho Salmon incidentally caught in the Columbia River Chinook Salmon sport fishery and used PBT to determine their hatchery of origin. These molecular and computing resources provide a valuable tool for Pacific salmon conservation in the Columbia River basin and demonstrate a cost‐effective approach to species identification for genetic monitoring programs. [ABSTRACT FROM AUTHOR]

Increased flood magnitude and frequency due to climate change can reduce the population productivity of organisms such as amphibian and fish species in river and stream systems; therefore, conserving habitats that maintain high productivity under such conditions is crucial. Here, we examined the relationship between the freshwater productivity of anadromous salmonids (measured as fry migrating to the ocean per spawner) and catchment hydrogeomorphology and identified the characteristics of rivers and streams that are prone to flood disturbance.We surveyed the spawner abundance and number of fry of pink salmon (Oncorhynchus gorbuscha) and measured environmental factors, including the average catchment slope and stream power, as characteristics of hydrogeomorphology, in 10 streams in the Shiretoko Peninsula, northern Japan. We then used generalised linear mixed models to predict the freshwater productivity of pink salmon populations in each catchment across the study region under current and future climatic conditions.The productivity of pink salmon in the study region differed among the sampled catchments and was negatively affected by the average catchment slope, stream power, and maximum daily precipitation averaged over the catchment. Namely, flood disturbance reduced the freshwater productivity of pink salmon, and salmon productivity in individual catchments was explained by catchment hydrogeomorphology. The predicted future productivity with increased precipitation was also lower than the current productivity.Our approach can be applied to other salmonids that have similar spawning behaviour to pink salmon. Highly productive catchments under the future climatic conditions predicted by the present study should be prioritised for conservation to ensure a sustainable salmonid population. [ABSTRACT FROM AUTHOR]

The value of estuaries as nursery habitat for juvenile anadromous salmon is likely variable across estuaries and species. Here, we compiled published empirical data on juvenile salmon estuarine growth and residency. We aimed to quantify the range and variability of these aspects for five species of Pacific salmon across estuaries, methodologies, and life histories. The majority of studies focused on Chinook and coho salmon, largely from their southern range. While there is some evidence of higher growth in wild-origin fish relative to hatchery-origin fish, the wide range of metrics employed made identification of trends among life histories challenging, and unification of reporting could strengthen future research. Different salmon life histories exhibited different residencies, with natural-origin subyearling coho exhibiting the longest mean residency (∼3 months) and 1+ sockeye salmon exhibiting the shortest (3.7 days). Across life histories, hatchery fish exhibited much shorter estuary residencies than wild fish. Collectively, our review highlights key patterns in salmon estuary ecology, identifies knowledge gaps, and lays the foundation for future studies to quantify the importance of estuaries for specific salmon. [ABSTRACT FROM AUTHOR]

Objective: Efforts to ameliorate the negative effects of diversion dams on aquatic species of concern are important in rivers where water withdrawal supports agricultural economies, and they are likely to become increasingly important with impending climate change. A multi‐year study was conducted to evaluate the survival consequences of diversion dam passage for juvenile Chinook Salmon Oncorhynchus tshawytscha and steelhead O. mykiss in the highly managed Yakima River, Washington. Canal entrainment and passage were evaluated at four diversion dams in the lower Yakima River where seaward‐migrating juvenile salmon and steelhead either pass dams via spill bays or enter canals with downstream fish‐screening facilities designed to collect entrained fish and return them to the main‐stem river. Methods: We used acoustic telemetry to monitor the downstream migration behavior of 4403 juvenile Chinook Salmon and steelhead during 2018–2021. Result: Percent entrainment into canals was substantial (6–59%) at three of the four diversion dams studied, and entrainment probability was positively associated with the proportion of streamflow diverted into canals. Survival probability estimates for groups of tagged fish that were entrained into canals were lower than survival probability estimates for tagged fish that passed through spill bays on the dams. Absolute differences in survival probabilities between routes ranged from 0.099 to 0.369, demonstrating that canal entrainment reduced the survival of outmigrating juvenile Chinook Salmon and steelhead. We also found that entrainment resulted in migration delays, which could further affect survival because fish are increasingly exposed to predation and decreased water quality as water temperature increases throughout the migration season. Conclusion: Canal entrainment resulted in increased mortality and travel time through the study area, highlighting the need to limit the entrainment of juvenile salmon and steelhead at diversion dams in rivers where salmon recovery is important. Impact statementRivers are often managed to benefit humans, but science can provide insights that can be used to reduce negative impacts on native aquatic species. In this study, we found that canal entrainment resulted in decreased survival of juvenile Chinook Salmon and steelhead, providing actionable information for improving diversion dam operations to aid salmon recovery efforts. [ABSTRACT FROM AUTHOR]

Anadromous salmonid species vary in their use of open ocean, coastal, and inland marine waters. To better understand this diversity in behavior and habitat use, 120 coastal cutthroat trout, Oncorhynchus clarkii clarkii, were caught in southern Puget Sound, Washington, USA, tagged with acoustic transmitters, and 95 were detected by a network of receiver stations. Despite sufficient time to reach other parts of the Salish Sea where many receivers operated, none was detected beyond southern Puget Sound, indicating localized movements. Within southern Puget Sound, fish were detected at 34 of 127 receivers in marine sites throughout the year but especially in spring and fall. Most detection events (between first and last detections at a given receiver) were brief (60.5% were ≤ 2 h and 76.8% ≤ 6 h), indicating movement along the shoreline. However, 823 events (12.8%) exceeded 12 h and 222 (3.4%) exceeded 24 h at a receiver, indicating longer occupancy at certain sites and by certain individuals. The detections also indicated that cutthroat trout were active throughout the 24-h period, but they moved more often at night and less often in other periods than would occur by chance, and they moved more often on ebbing and flooding tides and less often at slack periods. Fish with pressure-sensitive transmitters were almost always (97.3% of records) within 2.5 m of the surface and 76.8% between 1 – 2 m, despite deeper and shallower waters nearby where they could have been detected. The data provide new insights into the behavior of this species, whose marine ecology has not been extensively studied, and differs markedly from the region's other native salmonids. [ABSTRACT FROM AUTHOR]

Diversity of life strategies of pacific salmon (gen. Oncorhynchus) is reviewed on the basis of published data. Cases of polycyclicity and changes in life strategy during the ontogenesis are described.

Pinniped predation on commercially and ecologically important prey has been a source of conflict for centuries. In the Salish Sea, harbor seals (Phoca vitulina) are suspected of impeding the recovery of culturally and ecologically critical Pacific salmon (Oncorhynchus spp.). In Fall 2020, a novel deterrent called Targeted Acoustic Startle Technology (TAST) was deployed at Whatcom Creek to deter harbor seals from preying on fall runs of hatchery chum (O. keta) and Chinook (O. tshawytscha) salmon in Bellingham, Washington, USA. Field observations were conducted in 2020 to compare the presence and foraging success of individual harbor seals across sound exposure (TAST-on) and control (TAST-off) conditions. Observations conducted the previous (2019) and following (2021) years were used to compare the effects observed in 2020 to two control years. Using photo-identification, individual seals were associated with foraging successes across all 3 years of the study. Generalized linear mixed models showed a significant 45.6% reduction in the duration (min) individuals remained at the creek with TAST on, and a significant 43.8% reduction in the overall foraging success of individuals. However, the observed effect of TAST varied across individual seals. Seals that were observed regularly within one season were more likely to return the year after, regardless of TAST treatment. Generalized linear models showed interannual variation in the number of seals present and salmon consumed. However, the effect of TAST in 2020 was greater than the observed variation across years. Our analyses suggest TAST can be an effective tool for managing pinniped predation, although alternate strategies such as deploying TAST longer-term and using multi-unit setups to increase coverage could help strengthen its effects. Future studies should further examine the individual variability found in this study. [ABSTRACT FROM AUTHOR]

Simple Summary: Masu salmon (Oncorhynchus masou), a commercially important fish endemic to the North Pacific Ocean, received national second-level protected animal status in China in 2021. Employing gastrointestinal tract environmental DNA (GITeDNA) metabarcoding and traditional morphology, this study explored the diet of fluviatile Masu salmon. The results showed a diverse prey composition, with larger fish exhibiting a significant preference for terrestrial invertebrates. The fluviatile Masu salmon acts as a crucial link between aquatic and terrestrial food webs, emphasizing the need for conservation efforts in riparian habitats. This study recommends combining GITeDNA metabarcoding and morphological observation for a comprehensive understanding of fish diet diversity. Masu salmon, Oncorhynchus masou (Brevoort 1856), a commercially important fish species endemic to the North Pacific Ocean, attained national second-level protected animal status in China in 2021. Despite this recognition, knowledge about the trophic ecology of this fish remains limited. This study investigated the diet diversity of fluviatile Masu salmon in the Mijiang River, China, utilizing the gastrointestinal tract environmental DNA (GITeDNA) metabarcoding and morphological identification. The results revealed a diverse prey composition, ranging from terrestrial and aquatic invertebrates to small fishes. The fluviatile Masu salmon in general consumed noteworthily more aquatic prey than terrestrial prey. There were much more prey taxa and a higher diet diversity detected by GITeDNA metabarcoding than by morphological identification. GITeDNA metabarcoding showed that larger and older Masu salmon consumed significantly more terrestrial insects than aquatic prey species did, with 7366 verses 5012 sequences in the group of ≥20 cm, 9098 verses 4743 sequences in the group of ≥100 g and 11,540 verses 729 sequences in the group of age 3+. GITeDNA metabarcoding also showed size- and age-related diet diversity, indicating that the dietary niche breadth and trophic diversity of larger and older Masu salmon increased with food resources expanding to more terrestrial prey. Terrestrial invertebrates of riparian habitats play a vital role in the diet of fluviatile Masu salmon, especially larger individuals, highlighting their importance in connecting aquatic and terrestrial food webs. Conservation plans should prioritize the protection and restoration of riparian habitats. This study advocates the combined use of GITeDNA metabarcoding and morphological observation for a comprehensive understanding of fish diet diversity. [ABSTRACT FROM AUTHOR]

As Pacific salmon (Oncorhynchus spp.) decline across much of their range, it is imperative to further develop minimally invasive tools to quantify population abundance. One such advancement, trans‐generational genetic mark–recapture (tGMR), uses parentage analysis to estimate the size of wild populations. Our study examined the precision and accuracy of tGMR through a comparison to a traditional mark–recapture estimate for Chilkat River Chinook salmon (O. tshawytscha) in Southeast Alaska. We examined how adult sampling location and timing impact tGMR by comparing estimates derived using samples collected in the lower river mainstem to those using samples obtained in upriver spawning tributaries. Results indicated that tGMR estimates using a representative sample of mainstem adults were most concordant with, and 3% more precise than, the traditional mark–recapture estimate for this stock. Importantly, the timing and location of adult sampling were found to impact abundance estimates, depending on what proportion of the population dies or moves to unsampled areas between downriver and upriver sampling events. Additionally, we identified potential sources of bias in tGMR arising from violations of key assumptions using a novel individual‐based modeling framework, parameterized with empirical values from the Chilkat River. Simulations demonstrated that increased reproductive success and sampling selectivity of older, larger individuals, introduced negative bias into tGMR estimates. Our individual‐based model offers a customizable and accessible method to identify and quantify these biases in tGMR applications (https://github.com/swrosenbaum/tGMR%5fsimulations). We underscore the critical role of system‐specific sampling design considerations in ensuring the precision and accuracy of tGMR projects. This study validates tGMR as a potentially useful tool for improved population enumeration in semelparous species. [ABSTRACT FROM AUTHOR]

Anadromous Oncorhynchus mykiss (steelhead) express diverse migratory behaviors and life-history strategies. Adult migration timing diversity within steelhead is often categorized into divergent early- and late-migration phenotypes. On the Rogue River in Oregon, adult steelhead return from the ocean during distinct early-summer, late-summer, and winter runs. Additionally, some juvenile steelhead briefly return to freshwater as "half pounders," before completing their oceanic growth phase and returning to freshwater again to spawn. Using markers from a Genotyping-in-Thousands by sequencing amplicon panel, we describe a genetic axis that discriminates between Rogue River steelhead that express early- vs. late-migration phenotypes. Then we examine whether late-summer run adults carry early- or late-migration associated alleles and assign half pounders as early or late migrators. Both late-summer run adults and half pounders bear highly heterozygous genotypes and recombined haplotypes at the greb1l–rock1 region on chromosome 28 associated with adult migration timing, suggestive of ongoing gene flow among individuals with early- and late-migration haplotypes. Our classification suggests that half pounders are a mixed assemblage of offspring from all three adult runs. We discuss the utility of markers within the greb1l–rock1 region to predict phenotypes and highlight the need to validate candidate gene-trait associations across lineages before applying them for management. Finally, we consider the implications of our results on the maintenance of life-history diversity within steelhead. [ABSTRACT FROM AUTHOR]

In mid‐May 2022, pink salmon Oncorhynchus gorbuscha smolts were caught in the rivers Botnsá, Grímsá, and Langá in Iceland. This observation provides the first evidence of successful spawning and the completion of the freshwater phase of the life cycle in Icelandic rivers. It is the most western record of O. gorbuscha smolts in Europe, further west than Russia, Norway, and the UK. Smolts originating from Iceland potentially support the recruitment of this species in the North Atlantic and may lead to the establishment of a self‐sustaining population in Iceland. [ABSTRACT FROM AUTHOR]

Chum salmon (Oncorhynchus keta) exhibit a remarkable ability to adapt to changes in salinity during their life cycle. However, the fertilized egg stages are sensitive to salinity, affecting ontogeny and hatching. This study investigated the effect of salinity (0, 1, 3, and 5 PSU) on the survival of two developmental stages (<1 day after fertilization and <1 day after the eyed-egg stage) of fertilized eggs. Based on the experimental results, we assessed the spawning ground environment using the in situ salinity data of the Namdae River from 1997 to 2002, where the largest number of salmon in Korea migrate to spawn. Survival of the <1-day-old fertilized eggs decreased sharply at 3 PSU or more, and all eggs died at 5 PSU. Hatching of the eyed-egg stage occurred under all environmental conditions. After 2010, the salinity of the layer of water in contact with the sediment in the lower reaches of the river increased (>6.9 PSU) with the frequency of high waves. Overall, the function of the lower river in spawning and hatching is weakening. This study enhances our understanding of the effects of climate change, including increased wave activity, on salmon spawning grounds. [ABSTRACT FROM AUTHOR]

Recruitment failure is a major threat to freshwater mussel (Order Unionida) populations worldwide. Assessments of the recruitment status and determining the bottleneck factors of mussel recruitment are crucial for preventing future declines in mussel populations. In this study, we investigated the recruitment status (i.e., size structure and the proportion of juveniles within a population) of the endangered freshwater pearl mussel Margaritifera laevis in 22 rivers, represented by two reaches at one study site, in eastern Hokkaido, northern Japan. We also quantified the density of the host fish Oncorhynchus masou masou and examined the relationship between the proportion of juveniles and host-fish density to assess host-limitation threats to M. laevis recruitment. Our assessments showed that 13 (59.1%) sites had no signs of recent recruitment within 10 years, with a low mean proportion of juveniles [mean: 0.02 (range 0.00–0.09) fraction], indicating that these populations are threatened by sustained recruitment failure. The proportion of juveniles was positively associated with host-fish density, suggesting that host-fish limitation could be a bottleneck factor for M. laevis recruitment. These results highlight the urgent need for prompt conservation measures, including the enhancement of host-fish availability, to sustain M. laevis populations in the study region. [ABSTRACT FROM AUTHOR]

Pink salmon, Oncorhynchus gorbuscha (Walbaum, 1792), a native of the North Pacific Ocean, is an invasive fish species rapidly expanding in the North Atlantic, the adjacent waters of the Arctic Ocean, and rivers in their watersheds. We investigated the species composition of parasites and the structure of parasite communities of the introduced pink salmon in Kandalaksha Bay of the White Sea in 2021. The parasites were identified using morphological and genetic methods. A total of 14 parasite species were recorded. The core species of the parasitic fauna of the introduced pink salmon in the Kandalaksha Bay were Clistobothrium sp., ʽDiphyllobothriumʼ spp., Scolex polymorphus Rudolphi, 1819 (plerocercoids), Brachyphallus crenatus (Rudolphi, 1802), Derogenes varicus (Müller, 1784), Lecithaster salmonis Yamaguti, 1934 (adults), Anisakis simplex (Rudolphi, 1809) sensu stricto (juvenile III), and Hysterothylacium aduncum (Rudolphi, 1802) (juveniles III and IV). We provided the first evidence that the Pacific genetic lineage of A. simplex sensu stricto occurs in European marine waters. In 2021, there was a noticeable increase in the mean abundance of the third-stage juveniles of Anisakis in the White Sea pink salmon compared to the period of 1990s-early 2000s. We conclude that the pink salmon occupies the same trophic niche in the new region of its introduction as in the native range. [ABSTRACT FROM AUTHOR]