Coho Salmon Research Articles

Apical and mechanistic effects of 6PPD-quinone on different life-stages of the fathead minnow (Pimephales promelas)

N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine-quinone (6PPD-quinone) is an emerging contaminant of concern that is generated through the environmental oxidation of the rubber tire anti-degradant 6PPD. Since the initial report of 6PPD-quinone being the cause of urban runoff mortality syndrome of Coho salmon, numerous species have been identified as either sensitive or insensitive to acute lethality caused by 6PPD-quinone. In sensitive species, acute lethality might be caused by uncoupling of mitochondrial respiration in gills. However, little is known about effects of 6PPD-quinone on insensitive species. Here we demonstrate that embryos of fathead minnows (Pimephales promelas) are insensitive to exposure to concentrations as great as 39.97 μg/L for 168 h, and adult fathead minnows are insensitive to exposure to concentrations as great as 9.4 μg/L for 96 h. A multi-omics approach using a targeted transcriptomics array, (EcoToxChips), and proton nuclear magnetic resonance (1H NMR) was used to assess responses of the transcriptomes and metabolomes of gills and livers from adult fathead minnows exposed to 6PPD-quinone for 96 h to begin to identify sublethal effects of 6PPD-quinone. There was little agreement between results of the EcoToxChip and metabolomics analyses, likely because genes present on the EcoToxChip were not representative of pathways suggested to be perturbed by metabolomic analysis. Changes in abundances of transcripts and metabolites in livers and gills suggest that disruption of one‑carbon metabolism and induction of oxidative stress might be occurring in gills and livers, but that tissues differ in their sensitivity or responsiveness to 6PPD-quinone. Overall, several pathways impacted by 6PPD-quinone were identified as candidates for future studies of potential sublethal effects of this chemical.

Environmental concentrations of tire rubber-derived 6PPD-quinone alter CNS function in zebrafish larvae

N-(1,3-Dimethylbutyl)-N′-phenyl-p-phenylenediamine quinone (6PPD-quinone) is a degradation product of 6PPD, an antioxidant widely used in rubber tires. 6PPD-quinone enters aquatic ecosystems through urban stormwater runoff and has been identified as the chemical behind the urban runoff mortality syndrome in coho salmon. However, the available data suggest that the acute effects of 6PPD-quinone are restricted to a few salmonid species and that the environmental levels of this chemical should be safe for most fish. In this study, larvae of a “tolerant” fish species, Danio rerio, were exposed to three environmental concentrations of 6PPD-quinone for only 24 h, and the effects on exploratory behavior, escape response, nonassociative learning (habituation), neurotransmitter profile, wake/sleep cycle, circadian rhythm, heart rate and oxygen consumption rate were analyzed. Exposure to the two lowest concentrations of 6PPD-quinone resulted in altered exploratory behavior and habituation, an effect consistent with some of the observed changes in the neurotransmitter profile, including increased levels of acetylcholine, norepinephrine, epinephrine and serotonin. Moreover, exposure to the highest concentration tested altered the wake/sleep cycle and the expression of per1a, per3 and cry3a, circadian clock genes involved in the negative feedback loop. Finally, a positive chronotropic effect of 6PPD-quinone was observed in the hearts of the exposed fish. The results of this study emphasize the need for further studies analyzing the effects of 6PPD-quinone in “tolerant” fish species.

Causal analysis of escapement of farmed salmonids in southern Chile

Escaped salmonids are considered among the most serious threats to the aquatic environment. One hundred and nine escape incidents were reported in Chile from 2004-2021, representing some 8.53 million escaped salmonids. Of this total, 5.73 million were Atlantic salmon Salmo salar (67.2%), 0.83 million coho salmon Oncorhynchus kisutch (9.8%), and 1.96 million were rainbow trout O. mykiss (23.0%). It is estimated that 70.1% of the escaped salmonids were recorded in the Los Lagos Region, 23% in the Aysén Region, and 4.6% in the Magallanes Region. In total, 80.5% of the escapes were recorded from seawater facilities, while freshwater units accounted for 19.5%. The highest percentage of escaped salmon recorded in seawater over 2004-2021 was 1.71% of harvested salmonids in 2013. Some 39.5% of the escapes in 2015-2021 were attributed to rupturing of net cages, mainly due to adverse climatic conditions. The additional regulations introduced in 2020 by the Chilean authority, has helped to minimize the escape of farmed salmonids. As a consequence, just one escape event was reported in 2021, corresponding to 3.85% of the total number of escapees recorded from 2004-2021.

Title: detection of juvenile coho salmon (Oncorhynchus kisutch) indicates widespread successful reproduction and different life-history strategies in the extreme south of Patagonia

Title: detection of juvenile coho salmon (Oncorhynchus kisutch) indicates widespread successful reproduction and different life-history strategies in the extreme south of Patagonia

Immune Transcriptional Response in Head Kidney Primary Cell Cultures Isolated from the Three Most Important Species in Chilean Salmonids Aquaculture.

Fish cell culture is a common in vitro tool for studies in different fields such as virology, toxicology, pathology and immunology of fish. Fish cell cultures are a promising help to study how to diagnose and control relevant viral and intracellular bacterial infections in aquaculture. They can also be used for developing vaccines and immunostimulants, especially with the ethical demand aiming to reduce and replace the number of fish used in research. This study aimed to isolate head kidney primary cell cultures from three Chilean salmonids: Salmo salar, Oncorhynchus kisutch, and Oncorhynchus mykiss, and characterize the response to bacterial and viral stimuli by evaluating various markers of the innate and adaptive immune response. Specifically, the primary cell cultures of the head kidney from the three salmonids studied were cultured and exposed to two substances that mimic molecular patterns of different pathogens, i.e., Lipopolysaccharide (LPS) (bacterial) and Polyinosinic: polycytidylic acid (POLY I:C). Subsequently, we determined the mRNA expression profiles of the TLR-1, TLR-8, IgM, TLR-5, and MHC II genes. Head kidney primary cell cultures from the three species grown in vitro responded differently to POLY I:C and LPS. This is the first study to demonstrate and characterize the expression of immune genes in head kidney primary cell culture isolated from three salmonid species. It also indicates their potential role in developing immune responses as defense response agents and targets of immunoregulatory factors.

Rejection of Lepeophtheirus salmonis driven in part by chitin sensing is not impacted by seawater acclimitization in Coho salmon (Oncorhynchus kisutch)

There is tremendous variation in life-history strategies among anadromous salmonids. Species that enter the ocean environment at small sizes (< 20 g) are likely under more physiological pressure from pathogens; however, little data is available on responses at these early stages. With this in mind, we performed salmon louse challenges with Coho salmon either immediately after seawater entry (SW; ca. 10 g) or after 30 days in SW (ca. 20 g). Irrespective of size or time in SW, parasites were rapidly rejected by the host, with > 90% of all parasites lost by 16 days post-infection (dpi). Rejection was concomitant with host epithelial granulomatous infiltrations that initially targeted the embedded frontal filament (4 dpi) and the entire parasite by 10 dpi. Illumina sequencing, followed by functional enrichment analysis, revealed a concerted defense response in the fin within 1 dpi that included multiple innate and adaptive immunity components. Strikingly, early indications of an allergic-type inflammatory response were associated with chitin sensing pathways orchestrated by early overexpression of the IgE-receptor, fcer1g. Additionally, there was profound overexpression of several classes of c-type lectin receptors, including dectin-2, mincle, and dc-sign at 1 dpi onward. These profiles and upregulation of cellular effector markers were corroborated by histopathological evaluation, revealing the simultaneous presence of mast cell/eosinophilic granular cells, sacciform cells, macrophages/histiocytes, and granulocytes in fin. At 10 dpi and concurrent with parasite expulsion, there was evidence of immunoregulation in addition to tissue remodelling pathways. At 16 dpi, the response was effectively abrogated. Simultaneous profiling of the parasite transcriptome revealed early induction of chitin metabolism and immunomodulation, toxin production and ECM degradation; however, after 7 dpi, these were replaced with overexpression of stress and immune defense genes. These data present the first evidence for Coho salmon demonstrating chitin- and sugar moiety-sensing as key drivers of salmon louse rejection.

Effect of replacing a portion of fish meal with unfermented and/or fermented soybean meal on the growth performance, antioxidant and immunity capacity and related gene expression in juvenile Coho Salmon, Oncorhynchus kisutch

AbstractThis study investigated the effect of replacing a portion of fish meal with unfermented and/or fermented soybean meal in juvenile coho salmon (Oncorhynchus kisutch). Four groups of juveniles (initial weight 110.31 ± 3.12 g) in triplicate were fed for 90 days on four different iso‐nitrogen and iso‐lipid diets. The G0 diet comprised 28% fish meal protein (control group). In the other three diets (G1, G2, and G3), 10% of the fish meal protein was replaced with unfermented and/or fermented soybean meal protein. The main results were: Compared with G0, the WGR, SGR, CF; the contents of GLU, TP, ALB, T‐CHO, and T‐AOC, the activities of AKP, CAT, SOD, GST, GSH‐PX, O2·‐CR, OH·‐CR, and GSH and the expressions of SOD, GSH‐PX, GST, CAT, and Nrf2 genes of juvenile coho salmon in G3 were significantly (p &lt; 0.05) increased, However, the FCR, HSI, the content of MDA, and the expressions of TNF‐α and IL‐6 genes of juvenile coho salmon in G3 were significantly (p &lt; 0.05) decreased. There was no significant effect (p &gt; 0.05) of diet on their survival rate (SR). In conclusion, the fermented soybean meal in the juvenile coho salmon diet can replace 10% of the fish meal protein.

Salmonid Pituitary Cells as a Test System for Identifying Endocrine-Disrupting Compounds.

The pituitary gland is a central regulator of reproduction, producing two gonadotropins, follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh), which regulate gonadal development, sex steroid synthesis, and gamete maturation. The present study sought to optimize an in vitro test system using pituitary cells isolated from previtellogenic female coho salmon and rainbow trout, focusing on fshb and lhb subunit gene expression. Initially, we optimized culture conditions for duration and benefits of culturing with and without addition of endogenous sex steroids (17β-estradiol [E2] or 11-ketotestosterone) or gonadotropin-releasing hormone (GnRH). The results suggest that culturing with and without E2 was valuable because it could mimic the (+) feedback effects on Lh that are observed from in vivo studies. After optimizing assay conditions, a suite of 12 contaminants and other hormones was evaluated for their effects on fshb and lhb gene expression. Each chemical was tested at four to five different concentrations up to solubility limitations in cell culture media. The results indicate that more chemicals alter lhb synthesis than fshb. The more potent chemicals were estrogens (E2 and 17α-ethynylestradiol) and the aromatizable androgen testosterone, which induced lhb. The estrogen antagonists 4-OH-tamoxifen and prochloraz decreased the E2-stimulated expression of lhb. Among several selective serotonin reuptake inhibitors tested, the sertraline metabolite norsertraline was notable for both increasing fshb synthesis and decreasing the E2 stimulation of lhb. These results indicate that diverse types of chemicals can alter gonadotropin production in fish. Furthermore, we have shown that pituitary cell culture is useful for screening chemicals with potential endocrine-disrupting activity and can support the development of quantitative adverse outcome pathways in fish. Environ Toxicol Chem 2023;42:1730-1742. © 2023 SETAC.

Dam removal enables diverse juvenile life histories to emerge in threatened salmonids repopulating a heterogeneous landscape

Human stressors block, eliminate, and simplify habitat mosaics, eroding landscapes’ life history diversity and thus biological resilience. One goal of restoration is to alleviate human stressors that suppress life history diversity, but life history responses to these efforts are still coming into focus. Here, we report life history diversity emerging in threatened salmonids (Oncorhynchus spp.) repopulating the recently undammed Elwha River (WA, United States) in adjacent but environmentally distinct tributaries. The ~20 km tributaries entered the Elwha River &amp;lt;1 km apart, but one had a colder stream temperature regime and swifter waters due to its high, snow-dominated elevation and steep valley gradient (~3%), while the other had a warmer stream temperature regime and slower waters because it drained a lake, was at lower elevation, and had a lower stream gradient (~1.5%). Following the 2012 removal of Elwha Dam, the tributaries’ salmonids generally became more abundant and expressed diverse life histories within and among species. The warmer, low-gradient tributary produced more age-1+ coho salmon while the colder, steeper tributary produced a notably high abundance of steelhead smolts in 2020. Additionally, salmonids exiting the warmer tributary were older and possibly larger for their age class, emigrated ~25 days earlier, and included age-0 Chinook salmon that were larger. Also, assemblage composition varied among years, with the most abundant species shifting between Chinook salmon and coho salmon, while steelhead abundances generally increased but were patchy. These patterns are consistent with a newly accessible, heterogeneous landscape generating life history diversity against the backdrop of patchy recruitment as salmonids—some with considerable hatchery-origin ancestry—repopulate an extirpated landscape. Overall, dam removal appears to have promoted life history diversity, which may bolster resilience during an era of rapid environmental change and portend positive outcomes for upcoming dam removals with similar goals.

High‐throughput genotyping in estimating genetic resources and detecting pathogens in aquaculture

High‐throughput genotyping in estimating genetic resources and detecting pathogens in aquaculture

Dynamics of juvenile salmon and forage fishes in nearshore kelp forests

Abstract North‐east Pacific juvenile salmon and forage fishes, including the endangered salmon species Chinook (Oncorhynchus tshawytscha), coho (Oncorhynchus kisutch), chum (Oncorhynchus keta), and pink (Oncorhynchus gorbuscha), and the forage fishes species of Pacific herring (Clupea pallasi), surf smelt (Hypomesus pretiosus), and sand lance (Ammodytes hexapterus), utilize kelp environments. Many details of the kelp forest ecosystem function for these fishes are lacking. Kelp forests, salmon, and forage fishes are declining precipitously as the global climate shifts and developments along coastal shorelines expand. It is therefore essential to understand kelp forest function for these species. Analysis of 7 years of snorkelling survey videos indicates that both forage fishes and salmon use kelp forests throughout the outmigration season, and that their interactions occur in small and large groups, primarily along the outer edge of kelp beds. Over the course of outmigration, juvenile Chinook and coho salmon encounter sand lance first, followed by smelt and herring. The majority of interactions are intermingling, in which a subset proceed to predation, primarily on herring. It is important to develop and implement specific fishery and habitat conservation measures to preserve and restore these functions. Long‐term research has shown that intact, conserved nearshore habitats function better ecologically than restored habitats, and individual kelp forests can function differently for forage fishes and salmon. Therefore, conservation plans should be developed to conserve wild kelp forests which are documented to provide ecosystem function for salmon and forage fishes, by protecting them from various development impacts, including dredging, filling, and water‐quality decline. Coastal restoration is often only successful when ecosystem‐limiting factors that result in a loss of habitat are resolved. Kelp forest restoration, therefore, should be prioritized for regions of known high historical kelp forest importance, with restoration actions focused on correcting the limiting factors that caused loss and/or degradation.

Implications of Metrics and Methodology for Juvenile Salmonid Monitoring in Western Oregon Streams

We attempted to determine whether electrofishing removal estimates or single-pass snorkeling was a more reliable method for Oregon Department of Fish and Wildlife (ODFW) monitoring of juvenile coho salmon (Oncorhynchus kisutch) and steelhead (O. mykiss) abundance and occupancy trends. Based on 1997 to 2000 data, we assumed abundance estimates from the method that tracked more closely with parental abundance would better approximate true juvenile abundance. Parental abundance from spawning ground surveys and juvenile abundance metrics unique to each method were estimated from 2000 to 2004 and 2007 to 2008. Parental abundance did not explain the variation in juvenile abundance from either method (r2 < 0.22), invalidating our assumption, but results had relevance for snorkel surveys used in ODFW monitoring. For both species, correlations between density (fish m2 -1) and abundance (quantity, based on fish km-1) estimates from snorkeling were weak (r < 0.379), but correlations between abundance estimates from both methods were strong (r > 0.846), implying abundance was more appropriate than density for ODFW monitoring. Neither method could sample all habitats, and annually variable proportions of coho salmon (15 to 47%) and steelhead (0 to 24%) abundance estimates obtained by electrofishing were in pools too shallow to meet the ODFW depth criterion for snorkeling. This resulted in lowering the criterion to ≥ 20 cm in 2010. The lower criterion, relative to original, has not shown differences in trends, but 30% more pools have been sampled, resulting in 23% higher abundance estimates with 10% proportionately smaller confidence intervals. These changes improved ODFW monitoring and related management decisions.

Rapid Assessment of Fish Freshness for Multiple Supply-Chain Nodes Using Multi-Mode Spectroscopy and Fusion-Based Artificial Intelligence.

This study is directed towards developing a fast, non-destructive, and easy-to-use handheld multimode spectroscopic system for fish quality assessment. We apply data fusion of visible near infra-red (VIS-NIR) and short wave infra-red (SWIR) reflectance and fluorescence (FL) spectroscopy data features to classify fish from fresh to spoiled condition. Farmed Atlantic and wild coho and chinook salmon and sablefish fillets were measured. Three hundred measurement points on each of four fillets were taken every two days over 14 days for a total of 8400 measurements for each spectral mode. Multiple machine learning techniques including principal component analysis, self-organized maps, linear and quadratic discriminant analyses, k-nearest neighbors, random forest, support vector machine, and linear regression, as well as ensemble and majority voting methods, were used to explore spectroscopy data measured on fillets and to train classification models to predict freshness. Our results show that multi-mode spectroscopy achieves 95% accuracy, improving the accuracies of the FL, VIS-NIR and SWIR single-mode spectroscopies by 26, 10 and 9%, respectively. We conclude that multi-mode spectroscopy and data fusion analysis has the potential to accurately assess freshness and predict shelf life for fish fillets and recommend this study be expanded to a larger number of species in the future.

Presence of 6PPD-quinone in runoff water samples from Norway using a new LC–MS/MS method

The chemical 6PPD-quinone is highly toxic to some fish species of the Oncorhynchus and Salvelinus genera and is the oxidation product of the common car tire additive 6PPD. We present a new sample preparation method that involves liquid-liquid extraction of water samples followed by silica-based solid phase extraction prior to LC–MS/MS analysis. The new sample preparation method showed good analyte recovery from spiked water samples (78%–91%) and a low ion suppression effect, surpassing previously published methods. This new method was successfully validated, achieving a limit of quantification of 5 ng/L and estimated expanded measurement uncertainty of 18.6%. In a proof-of-concept study, the method was applied to several water samples from various sources in Southern Norway. These were runoff samples from tunnel washing, from a tunnel runoff treatment plant and downstream of the plant drain. In addition, two water samples from puddles were included: one was run-off from an artificial soccer turf field and one from a puddle on a country road. The results of the analyses revealed that the concentration of 6PPD-quinone was above the LC50 reported for coho salmon (Oncorhynchus kisutch) in all samples except the samples from and downstream of the treatment plant. The highest measured concentration was 258 ng/L, which is the 2.7-fold of the reported LC50 in coho salmon (95 ng/L). Our initial data emphasize the need for more comprehensive environmental monitoring of 6PPD-quinone as well as toxicological studies in aquatic organisms.

Piscine orthoreovirus 3a detected from farmed coho salmon with jaundice syndrome displays positive selection and polymorphisms in S1 and M2 viral segment

Piscine orthoreovirus (PRV) is associated with heart and skeletal muscle inflammation (HSMI) and circulatory disorders in salmonid species. In Chile, PRV affects the coho salmon industry most, as this virus correlates with both HSMI disease and jaundice syndrome. However, the causality of the latter has not been proven. In this study, we characterize nucleotide codifying and amino acid sequences of the PRV-3 detected in coho salmon with clinical signs and macroscopic evidence of jaundice syndrome in the seawater phase. The results support the diversification of the PRV-3 into two variants, PRV-3a and PRV-3b. The PRV-3a has been predominantly detected in coho salmon in the last five years. In addition, the new molecular sequences are characterized by polymorphisms exclusive to Chile in the σ3, p13 proteins encoded by the S1 segment and the μ1 protein encoded by the M2 segment. Additionally, we detected positive selection within the nucleotide sequence corresponding to the σ3 outer capsid protein. So far, PRV-3a has not been reported in other hosts in Chile, such as Atlantic salmon or rainbow trout. These findings highlight the diversification of PRV-3 in Chile and demonstrate a strong host-pathogen relationship which warrants further research to establish a link between PRV and jaundice syndrome in coho salmon.

Who spawns where? Temperature, elevation, and discharge differentially affect the distribution of breeding by six Pacific salmonids within a large river basin

Within the geographic range of salmonid fishes, many apparently suitable rivers and streams are used for reproduction by some species but not others. This is widely known but seldom addressed, as studies often examine factors determining the distribution of one or only a few species. We examined physical factors associated with the spawning distribution of six native Pacific salmonids (pink, chum, coho, and Chinook salmon, bull trout, and steelhead) in the Skagit River basin, Washington. Annual mean temperature and catchment elevation had the strongest association with spawning assemblage distribution, but stream length, annual discharge, seasonal hydrology, and land use were also influential. Some species (e.g., pink and Chinook salmon) were more closely associated with each other and with common variables than others, and bull trout were the most distinctive. For interpretation, we investigated the roles of adult body size, timing of spawning, and duration of juvenile rearing, but none of these factors explained groupings in the data. Interspecific differences in habitat association remained, suggesting fundamental constraints on species distributions with implications for conservation and restoration.

Establishing an In Vitro Model to Assess the Toxicity of 6PPD-Quinone and Other Tire Wear Transformation Products

The tire wear transformation product 6PPD-quinone (6PPDQ) has been implicated as the causative factor for broad scale mortality events for coho salmon in the Pacific Northwest. Highly variable sensitivity to 6PPDQ in closely related salmonids complicates efforts to evaluate the broader toxicological impacts to aquatic ecosystems. Our goals were to (1) validate the large range of in vivo species sensitivities reported for coho, Chinook, and sockeye salmon and (2) develop an in vitro platform for assessing 6PPDQ toxicity. In vivo studies confirmed the acute sensitivity of juvenile coho (12 h LC50 = 80.4 ng/L) and demonstrated that sockeye salmon were not vulnerable to mortality. Chinook salmon were sensitive to 6PPDQ mortality at initial concentrations >25 μg/L, ∼10-fold greater than reported environmental measurements. In vitro, the coho salmon cell line CSE-119 was acutely sensitive to 6PPDQ (metabolic EC50 = 7.9 μg/L, cytotoxicity EC50 = 6.1 μg/L). Analogous Chinook (CHSE-214) and sockeye salmon (SSE-5) cell lines were nonresponsive in both assays, and rainbow trout RTG-2 cells began showing metabolic effects at 68 μg/L (EC5). Recreation of species-specific 6PPDQ sensitivity in vitro implicates conserved modes of action in CSE-119 that could be utilized for mechanistic studies of 6PPDQ toxicity and screening of other PPD transformation products.

Evaluation of stormwater microbiomes for the potential biodegradation of tire wear particle contaminants

Stormwater is a major source of many contaminants of emerging concern, which can be toxic to both aquatic and terrestrial organisms. This project aimed to identify novel biodegraders of toxic tire wear particle (TWP) contaminants associated with coho salmon mortality. This study has (i) characterized the prokaryotic communities of stormwater in both urban and rural settings; (ii) evaluated the ability of stormwater isolates to degrade two model TWP contaminants, hexa(methoxymethyl)melamine and 1,3-diphenylguanidine; and (iii) evaluated the toxicological impact of these model contaminants on the growth of six model bacteria. Rural stormwater possessed a diverse microbiome dominated by Oxalobacteraceae, Microbacteriaceae, Cellulomonadaceae, and Pseudomonadaceae taxa, while urban stormwater showed much less microbial diversity overall. Additionally, multiple stormwater isolates appeared capable of using model TWP contaminants as their sole carbon source. Each model contaminant was also found to alter growth patterns of model environmental bacteria including, with 1,3-DPG appearing more acutely toxic at high concentrations. This study identified several stormwater isolates that have the potential to be used as a sustainable solution to stormwater quality management.

An environmental resistance model to inform the biogeography of aquatic invasions in complex stream networks

AbstractAimFreshwater invasions are a global conservation issue. Emerging tools for biogeographical analyses can provide critical information for their effective management and monitoring. Here, we propose a method to assess the distribution of environmental resistance of stream ecosystems to biological invasions by coupling multi‐stage habitat potential models for non‐native species.LocationAndean Patagonia (Chile and Argentina).TaxaNorth American beaver (Castor canadensis), Chinook salmon (Oncorhynchus tshawytscha) and coho salmon (O. kisutch).MethodsEnvironmental resistance to invasive species was mapped throughout a large region of Patagonia by stacking multi‐stage habitat relationships for each target species and assessing the complementation between critical habitats at multiple scales. We generated an environmental model of stream networks derived from high‐resolution topographic and climatic data representing 15,406 drainage basins (&gt;1 km2) covering an area of 369,791 km2. We quantified the intrinsic potential of stream‐reaches (100‐m and 1000‐m) to sustain high‐quality habitats and assessed habitat complementation (i.e., abundance and proximity) at the sub‐basin scale as a proxy for environmental resistance.ResultsOur model revealed high heterogeneity in the distribution of environmental resistance to invasions throughout the study region, providing case‐specific insights for the research and management of invaders.Main ConclusionsEnvironmental resistance modelling is a novel method to study the biogeography of riverine invasions. Our approach is compatible with additional sources of information about species and the environment and shows versatility to diverse invasion scenarios and data sources. This method can be useful in prioritising research and management of incipient and spreading invasions, especially for large and data‐poor regions.

Parvicapsula pseudobranchicola in the northeast Pacific Ocean is rare in farmed Atlantic salmon Salmo salar despite widespread occurrence and pathology in wild Pacific salmon Oncorhynchus spp.

BackgroundInfection with the myxozoan parasite Parvicapsula pseudobranchicola causes disease in wild and farmed salmonids in Norway. In the northeast Pacific Ocean, the parasite has been reported in Pacific salmon Oncorhynchus spp. without evidence of disease. The objectives of the present study were to confirm the identity of P. pseudobranchicola in the Pacific, document its host and geographic ranges, and describe associated pathological changes.MethodsOcean-entry year wild pink salmon Oncorhynchus gorbuscha, chum salmon O. keta, Chinook salmon O. tshawytscha, coho salmon O. kisutch and sockeye salmon O. nerka were collected in summer and autumn surveys near Vancouver Island (VI) and from a winter survey in the Gulf of Alaska. Samples were also obtained from farmed Atlantic salmon Salmo salar and Chinook salmon near VI. Samples were analysed by qPCR and histology using conventional staining or in situ hybridisation. Parasite sequence was obtained from small subunit ribosomal RNA gene (SSU rDNA).ResultsIdentical 1525 base-pair SSU rDNA sequences from infected pink salmon, chum salmon and Chinook salmon shared 99.93% identity with a P. pseudobranchicola sequence from Norwegian Atlantic salmon. In autumn surveys, the prevalence was greatest in chum salmon (91.8%) and pink salmon (85.9%) and less so in Chinook salmon (68.8%) and sockeye salmon (8.3%). In farmed salmon, the prevalence was zero in Atlantic salmon (n = 967) and 41% in Chinook salmon (n = 118). Infections were preferentially sited in pseudobranch and visualised by in situ hybridisation. Heavy parasite burdens in all species of Pacific salmon were inconsistently associated with focal granulomatous pseudobranchitis.ConclusionsIn the northeast Pacific, widespread occurrence of P. pseudobranchicola in Pacific salmon together with its absence or sporadic occurrence in farmed Atlantic salmon differs from its epidemiology in Norway, despite similar pathological development in the pseudobranch. Consequences of the infections to the health of wild Pacific salmon, identity of the invertebrate host and the distribution and abundance of infective actinospores are unknown and remain high priorities for research.Graphical