Fish move among complementary habitats to meet life history requirements, but their movements are often restricted by anthropogenic barriers. Programs to restore aquatic habitat connectivity via barrier remediation constitute a multi-billion-dollar investment, while intentional fragmentation through installation of conservation barriers remains an important management tool to protect native trout populations. Often, such programs for salmonids have focused on cold headwaters. However, a recent emphasis on the growth potential of warmer downstream reaches during non-summer seasons has ignited renewed interest in better understanding the benefits of spatial and temporal thermal heterogeneity in watersheds. Here, we use two heuristic case studies to illustrate how temperature data and growth models can more fully inform conservation decisions about barriers. One example explores the growth-related costs of protecting native Columbia River Redband Trout Oncorhynchus mykiss gairdneri from an invasion of Smallmouth Bass Micropterus dolomieu; a conservation barrier that intentionally fragments the watershed may reduce annual growth potential up to 6.4%. The other example illustrates how strategically replacing impassible road culverts may increase annual growth potential for midwestern Brook Trout Salvelinus fontinalis by 2.3% to 5.9%, based on the remediation strategy. We focus discussion on applying annual growth potential in habitats typically considered thermally unsuitable in summer to inform aquatic connectivity decisions that may enhance growth, and, therefore, fecundity and population resiliency, in coldwater fish. Our case studies demonstrate the application of readily available stream temperature data in combination with published growth models to better understand the implications of connectivity decisions for growth potential among complementary thermal habitats throughout a watershed. With this work, we provide a framework to formally account for growth potential in barrier management strategies and discuss the broader relevance and considerations for its use.

Human activities and climate change threaten coldwater organisms in freshwater ecosystems by causing rivers and streams to warm, increasing the intensity and frequency of warm temperature events, and reducing thermal heterogeneity. Cold-water refuges are discrete patches of relatively cool water that are used by coldwater organisms for thermal relief and short-term survival. Globally, cohesive management approaches are needed that consider interlinked physical, biological, and social factors of cold-water refuges. We review current understanding of cold-water refuges, identify gaps between science and management, and evaluate policies aimed at protecting thermally sensitive species. Existing policies include designating cold-water habitats, restricting fishing during warm periods, and implementing threshold temperature standards or guidelines. However, these policies are rare and uncoordinated across spatial scales and often do not consider input from Indigenous peoples. We propose that cold-water refuges be managed as distinct operational landscape units, which provide a social and ecological context that is relevant at the watershed scale. These operational landscape units provide the foundation for an integrated framework that links science and management by (1) mapping and characterizing cold-water refuges to prioritize management and conservation actions, (2) leveraging existing and new policies, (3) improving coordination across jurisdictions, and (4) implementing adaptive management practices across scales. Our findings show that while there are many opportunities for scientific advancement, the current state of the sciences is sufficient to inform policy and management. Our proposed framework provides a path forward for managing and protecting cold-water refuges using existing and new policies to protect coldwater organisms in the face of global change.

Hatcheries have long produced salmonids for fisheries and mitigation, though their widespread use is increasingly controversial because of potential impacts to wild salmonids. We conducted a global literature search of peer-reviewed publications (1970– 2021) evaluating how hatchery salmonids affected wild salmonids, developed a publicly available database, and synthesized results. Two hundred six publications met our search criteria, with 83% reporting adverse/minimally adverse effects on wild sal-monids. Adverse genetic effects on diversity were most common, followed by effects on productivity and abundance via ecological and genetic processes. Few publications (3%) reported beneficial hatchery effects on wild salmonids, nearly all from intensive recovery programs used to bolster highly depleted wild populations. Our review suggests hatcheries commonly have adverse impacts on wild salmonids in freshwater and marine environments. Future research on less studied effects— such as epigenetics— could improve knowledge and management of the full extent of hatchery impacts.

Lahontan cutthroat trout (Oncorhynchus clarkii henshawi) and Paiute cutthroat trout (Oncorhynchus clarkii seleniris) are rare taxa of conservation concern. Their generally low population densities and remote distribution make traditional sampling difficult, but they primarily exist in small streams, a habitat type where environmental DNA (eDNA) sampling has proven extremely effective. We developed a single eDNA assay that allows sensitive detection of both taxa, which occur separate from each other in different regions of the Lahontan Basin. Performance of the assay was evaluated using most lineages of Lahontan cutthroat trout and Paiute cutthroat trout as well as tissues of other cutthroat trout and rainbow trout that have been introduced within the Lahontan Basin. The assay amplified DNA only from the target taxa, and only from eDNA samples collected where those taxa were known to be present. This assay should enhance the ability of managers to more precisely and efficiently describe the distribution of these taxa.

The current extinction crisis requires effective assessment and monitoring tools. Genetic approaches are appealing given the relative ease of field sampling required to estimate genetic diversity characteristics assumed related to population size, evolutionary potential, and extinction risk, and to evaluate hybridization with non-native species simultaneously. However, linkages between population genetic metrics of diversity from survey-style field collections and demographic estimates of population size and extinction risk are still in need of empirical examples, especially for remotely distributed species of conservation concern where the approach might be most beneficial. We capitalized on an exceptional opportunity to evaluate congruence between genetic diversity metrics and demographic-based estimates of abundance and extinction risk from a comprehensive Multiple Population Viability Analysis (MPVA) in a threatened fish, the Lahontan cutthroat trout (LCT). We sequenced non-native trout reference samples and recently collected and archived tissue samples of most remaining populations of LCT (N = 60) and estimated common genetic assessment metrics, predicting minimal hybridization with non-native trout, low diversity, and declining diversity over time. We further hypothesized genetic metrics would correlate positively with MPVA-estimated abundance and negatively with extinction probability. We uncovered several instances of hybridization that pointed to immediate management needs. After removing hybridized individuals, cautious interpretation of low effective population sizes (2-63) suggested reduced evolutionary potential for many LCT populations. Other genetic metrics did not decline over time nor correlate with MPVA-based estimates of harmonic mean abundance or 30-year extinction probability. Our results demonstrate benefits of genetic monitoring for efficiently detecting hybridization and, though genetic results were disconnected from demographic assessment of conservation status, they suggest reduced evolutionary potential and likely a higher conservation risk than currently recognized for this threatened fish. We emphasize that genetic information provides essential complementary insight, in addition to demographic information, for evaluating species status.

The recently promulgated Navigable Waters Protection Rule eliminates federal protection for ephemeral streams previously covered by the Clean Water Act in the USA, but the regulatory agencies assert that map limitations preclude any quantification of the scope and extent of affected surface waters. However, ephemeral stream channels, which only flow in direct response to precipitation events and are important contributors to water quality and aquatic ecosystems, have been the subject of extensive mapping efforts that do allow for basic estimation of the effects of the new rule. We use the National Hydrography Dataset and a simple headwater stream model derived from digital elevation data to conduct a rapid assessment of ephemeral stream extent. We estimate that, by length, 43 to 56% of conterminous USA stream channels are ephemeral and that ½ of these features will no longer be protected under the new rule.

Many species of conservation interest exist solely or largely in isolated populations. Ideally, prioritization of management actions among such populations would be guided by quantitative estimates of extinction risk, but conventional methods of demographic population viability analysis (PVA) model each population separately and require temporally extensive datasets that are rarely available in practice. We introduce a general class of statistical PVA that can be applied to many populations at once, which we term multiple population viability analysis or MPVA. The approach combines models of abundance at multiple spatial locations with temporal models of population dynamics, effectively borrowing information from more data-rich populations to inform inferences for data-poor populations. Covariates are used to explain population variability in space and time. Using Bayesian analysis, we illustrate the method with a dataset of Lahontan cutthroat trout (Oncorhynchus clarkii henshawi) observations that previously had been analyzed with conventional PVA. We find that MPVA predictions are similar in bias and higher in precision than predictions from simple PVA models that treat each population individually; moreover, the use of covariates in MPVA allows for predictions in minimally-sampled and unsampled populations. The basic MPVA model can be extended in multiple ways, such as by linking to a sampling and observation model to provide a full accounting of uncertainty. We conclude that the approach has great potential to expand the use of PVA for species that exist in multiple, isolated populations.

The removal or remediation of thousands of culverts at road-stream crossings to restore connectivity is a major conservation investment in aquatic systems in North America. Effectiveness monitoring is necessary to confirm that passage has been restored for the species of interest and to justify project costs. We compared the performance of (1) recapture of batch-marked fish by backpack electrofishing, (2) recapture of PIT-tagged fish by electrofishing, (3) detection of PIT-tagged fish by a mobile antenna, and (4) detection of PIT-tagged fish at stationary antennas for verifying upstream passage of native Westslope Cutthroat Trout Oncorhynchus clarkii lewisi (WCT) and nonnative Brook Trout Salvelinus fontinalis at remediated culverts in four Rocky Mountain streams. Generally, detection probability at stationary antennas was higher (range = 0.74-0.97) than capture by electrofishing (range = 0.24-0.77) or detection by the mobile antenna (range = 0.47-0.66). All four methods confirmed upstream passage by trout that were originally marked or tagged below the culvert, although overall recapture rates were low (<= 20%). During summer and early fall, the continuously sampling stationary antennas detected more than twice as many PIT-tagged trout moving upstream through the culvert than either the mobile antenna or the electrofisher. Upstream movement by PIT-tagged trout was first detected by stationary antennas 1-10 d after tagging. For all methods, upstream passage was most frequently detected for fish that were marked or tagged in the 100-m reach adjacent to the culvert. The relative cost of the four mark-recapture methods to evaluate upstream passage of age-1 and older WCT was compared with the cost of "sib-split," a genetic method based on pedigree analysis, which was used previously to evaluate passage of age-0 WCT in the study streams. Stationary antennas, the mobile antenna, and sib-split were comparatively expensive for a single-year study because of PIT equipment and laboratory costs, respectively, and electrofishing was less than half the cost.

Discovering genetic markers associated with phenotypic or ecological characteristics can improve our understanding of adaptation and guide conservation of key evolutionary traits. The Lahontan cutthroat trout (Oncorhynchus clarkii henshawi) of the northern Great Basin Desert, USA, demonstrated exceptional tolerance to high temperatures in the desert lakes where it resided historically. This trait is central to a conservation hatchery effort to protect the genetic legacy of the nearly extinct lake ecotype. We genotyped full-sibling families from this conservation broodstock and samples from the only two remaining, thermally distinct, native lake populations at 4,644 new single nucleotide polymorphisms (SNPs). Family-based genome-wide association testing of the broodstock identified nine and 26 SNPs associated with thermal tolerance (p p < 0.1), measured in a previous thermal challenge experiment. Genes near the associated SNPs had complex functions related to immunity, growth, metabolism and ion homeostasis. Principal component analysis using the thermotolerance-related SNPs showed unexpected divergence between the conservation broodstock and the native lake populations at these loci. F-ST outlier tests on the native lake populations identified 18 loci shared between two or more of the tests, with two SNPs identified by all three tests (p < 0.01); none overlapped with loci identified by association testing in the broodstock. A recent history of isolation and the complex genetic and demographic backgrounds of Lahontan cutthroat trout probably limited our ability to find shared thermal tolerance loci. Our study extends the still relatively rare application of genomic tools testing for markers associated with important phenotypic or environmental characteristics in species of conservation concern.

Using Mark-Resight Surveys to Investigate Sightability during Snorkel Surveys Matthew Amick and Curtis Roth Pacific States Marine Fisheries Commission, Idaho Department of Fish and Game Presenter: Matthew Amick, matthew.amick@idfg.idaho.gov Snorkel surveys are the most broadly used technique by the Idaho Department of Fish and Game to monitor salmonid populations. Snorkel surveys allow managers to monitor salmonid populations when other techniques (e.g., electrofishing) are not feasible. One of the key assumptions of a snorkel survey is that you are able to visibly see all the fish present in the transect during sampling. However, environmental conditions can affect the ability of snorkels to see the fish during surveys. Therefore, in 2007 the Idaho Department of Fish and Game began conducting mark-resight surveys coupled with habitat surveys to investigate how habitat factors influence sightability. Mark-resight surveys were conducted by angling juvenile steelhead Oncorhynchus mykiss and then measuring them for total length, clipping the upper caudal fin, and then returning the fish to the stream in the same area where they were caught. The following day the angled transect was snorkeled to enumerate the number of marked and unmarked fish in the transect. Additionally, snorkel surveys were conducted for 50 m upstream and 50 m downstream of the angled transect to account for fish moving in and out of the transect overnight. In total, 127 surveys were conducted between 2007 and 2014 throughout the Salmon and Clearwater River Basins. Regression analysis was then used to evaluate the relationship between habitat factors and sightability. Results of this study will provide managers with a greater understanding of the influence of habitat factors on the sightability of juvenile steelhead.

Population viability analysis (PVA) uses concepts from theoretical ecology to provide a powerful tool for quantitative estimates of population dynamics and extinction risks. However, conventional statistical PVA requires long-term data from every population of interest, whereas many species of concern exist in multiple isolated populations that are only monitored occasionally. We present a hierarchical multi-population viability analysis model that increases inference power from sparse data by sharing information among populations to assess extinction risks while accounting for incomplete detection and sampling biases with explicit observation and sampling sub-models. We present a case study in which we customized this model for historical population monitoring data (1985-2015) from federally threatened Lahontan cutthroat trout populations in the Great Basin, USA. Data were counts of fish captured during backpack electrofishing surveys from locations associated with 155 isolated populations. Some surveys (25%) included multi-pass removal sampling, which provided valuable information about capture efficiency. GIS and remote sensing were used to estimate August stream temperatures, peak flows, and riparian vegetation condition in each population each year. Field data were used to derive an annual index of nonnative trout densities. Results indicated that population growth rates were higher in colder streams and that nonnative trout reduced carrying capacities of native trout. Extinction risks increased with more environmental stochasticity and were also related to population extent, water temperatures, and nonnative densities. We developed a graphical user interface to interact with the fitted model results and to simulate future habitat scenarios and management actions to assess their influence on extinction risks in each population. Hierarchical multi-population viability analysis bridges the gap between site-level field observations and population-level processes, making effective use of existing datasets to support management decisions with robust estimates of population dynamics, extinction risks, and uncertainties.

Lahontan Cutthroat Trout (LCT) Oncorhynchus clarkii henshawi and Paiute Cutthroat Trout (PCT) O. c. selernis are found in the Lahontan hydrographic basin of northern Nevada, northeastern California, and southeastern. Oregon and together form the Lahontan Basin evolutionary lineage of Cutthroat Trout O. clarkii. The Alvord Cutthroat Trout O. c. ssp. native to the Alvord Lake subbasin in the northwestern Lahontan Basin was also part of this lineage but went extinct due to Rainbow Trout O. mykiss introgression in the mid-20th century. Both LCT and PCT are federally listed as threatened under the U.S. Endangered Species Act. Given its historic distribution in a single small stream and both phenotypic and genetic distinctiveness, PCT is currently recognized as a separate evolutionarily significant unit (ESU). For LCT, three ESUs are identified based upon meristic, morphological, ecological, and genetic data. These putative LCT ESUs separate lacustrine forms in the western Lahontan Basin (Truckee, Carson, and Walker River basins) from largely fluvial forms in the eastern Lahontan Basin (Humboldt and Reese River basins) and northwestern Lahontan Basin (Quinn River, Coyote Lake, and Summit Lake basins). The more recent recognition of a much longer evolutionary history of Cutthroat Trout and several influential genetic papers identifying previously unrecognized diversity within Cutthroat Trout have prompted a need to re-evaluate the overall taxonomy of this species. Here, we review earlier literature and draw on new information from recent studies to delineate uniquely identifiable evolutionary units within the Lahontan Basin lineage of Cutthroat Trout. Though in several cases various anthropogenic and natural influences have made definitive conclusions difficult, based on this collective information and the goal of conserving potentially important genetic, evolutionary, and life history diversity, we propose recognition of six uniquely identifiable evolutionary units within the Lahontan Cutthroat Trout lineage: (1) Paiute Cutthroat Trout upper East Carson River; (2) western Lahontan Basin Truckee, Walker, and Carson rivers together with Summit Lake; (3) northwestern Lahontan Basin Quinn River; (4) eastern Lahontan Basin Humboldt and Reese rivers; (5) Lake Alvord basin Virgin-Thousand and Trout Creek drainages; and (6) Coyote Lake basin Willow and Whitehorse rivers.

The Paiute Cutthroat Trout (PCT) Oncorhynchus clarkii seleniris is classified as a subspecies within the greater Cutthroat Trout O. clarkii ssp. complex and is federally listed as threatened under the Endangered Species Act. However, genetic studies to date have revealed very little genetic differentiation between the PCT and its closest relative, the Lahontan Cutthroat Trout (LCT) O. clarkii henshawi. These results casted doubt on whether the PCT is a genetically distinct subspecies or merely a phenotypic variant of the LCT. Here, we present a genomic analysis of Cutthroat Trout subspecies and populations to resolve the genetic and phylogenetic relationship between PCT and LCT. Our results demonstrate substantial genetic structure and differentiation between PCT and LCT populations. In contrast to current thinking, our phylogenetic reconstructions show the PCT to be a distinct evolutionary lineage that diverged from LCT before the LCT differentiated into its current populations (i.e., rather than PCT divergence due to geographic isolation from an LCT population in the Carson River). We conclude that the PCT is genetically distinct from the LCT.

Freshwater fishes living in streams and rivers can be affected strongly by isolation, which causes a disproportionate degree of fragmentation in such dendritic systems. Isolation disrupts important ecological and migratory processes as well as the ability to access refuge habitats during disturbances. The restoration of habitat connectivity, then, should be a productive strategy for improving the resiliency of freshwater fish populations, but the local and broader ecological benefits of barrier removal are still poorly understood. We report on a longterm, spatially intensive effort to monitor the responses of inland trout to stream habitat reconnection at a watershed scale, using both demographic and genetic techniques. Individual-based genetic assignment uncovered clear evidence of movement into the primary tributary of interest, which had been blocked by an assumedly complete barrier, but the source population generating this movement varied over time. A linear mixed-effect model suggested trout densities in this target stream increased threefold and the population sustained more large migratory-sized individuals after habitat reconnection. Densities overall did not respond to fire or variable spring flows which occurred during the course of the study, but a negative parameter estimate for "year" suggested a possible decline in densities over the study period ( although 95% CIs barely overlapped with 0). Population genetic metrics showed no change in population differentiation or metrics of genetic diversity in most cases, except for a significant decline in allelic richness in the target population. The effective biological reconnection documented in our study should improve population resiliency in the future, but we discuss the benefits of spatially intensive monitoring at a watershed scale and combined inference from both demographic and genetic metrics to uncover unexpectedly complex fish responses to habitat reconnection.

Climate change is contributing to the severity and rate of stream degradation by changing the timing of peak flows, altering flow regimes, creating more frequent and intense disturbances, and increasing stream temperatures. Herein we describe three case studies of trout stream adaptation that address existing and climate-driven causes of degradation through habitat restoration. The case studies vary in geography and complexity, but all include restoration efforts intended to address multiple causes of stream degradation and improve the resilience of these streams to floods, droughts, and wildfires. Four elements of successful climate adaptation projects emerge: (1) habitat assessments that help drive project location and design, (2) projects that directly address climate change impacts and increase habitat resilience, (3) projects that combine to achieve watershed-scale impacts, and (4) projects that include sufficient monitoring to determine their effectiveness. We describe solutions to common challenges in conducting climate change adaptation, including how to balance scientific assessments with opportunities when choosing projects, how smaller projects can be aggregated to achieve watershed-scale benefits, and how citizen science efforts can augment monitoring programs. El cambio climatico esta contribuyendo a incrementar la severidad y la tasa de degradacion de los rios a traves de la alteracion en la estacionalidad del flujo maximo, modificacion del regimen de flujos, generacion de perturbaciones mas frecuentes e intensas e incremento de la temperatura de los rios. Aqui se describen tres casos de estudio de la adaptacion de rios en donde habita la trucha, en los que se abordan las causas de la degradacion que son provocadas por el cambio climatico, mediante la restauracion del habitat. Los casos de estudio varian en cuanto a ubicacion geografica y complejidad, pero en todos se contemplan esfuerzos de restauracion enfocados a abordar multiples causas de degradacion de rios y mejoramiento de la resiliencia de estos ante inundaciones, sequias e incendios naturales. Se consideraron cuatro elementos para lograr una adaptacion exitosa al cambio climatico: 1) evaluaciones del habitat que ayuden a disenar y establecer donde llevar a cabo los proyectos; 2) proyectos que aborden directamente los impactos del cambio climatico y el incremento en la resiliencia del habitat; 3) proyectos que, al combinarse, logren resultados a nivel de cuenca hidrologica; y 4) proyectos que incluyan un monitoreo suficiente como para que se pueda determinar su efectividad. Tambien se describen soluciones a los clasicos retos que implica la adaptacion al cambio climatico, incluyendo como encontrar un balance entre evaluaciones cientificas y eleccion de proyectos, como se pueden integrar varios proyectos pequenos para conseguir beneficios a escala de cuenca y como se puede incrementar el monitoreo mediante esfuerzos ciudadanos.

We contrasted various genetic analyses to evaluate their utility and constraints for detecting movement of cutthroat trout (Oncorhynchus clarkii) through restored culverts in different field settings: population-level metrics of genetic variability (heterozygosity and allelic richness); Bayesian clustering and assignment of individual genotypes from age 1+ fish; and a novel “sib-split” approach, where movement patterns are extracted from the spatial distribution of young-of-year (YOY) full-sibling groups inferred via pedigree reconstruction. Family structure greatly influenced population-level and individual clustering results in our small headwater populations, even though field sampling was implemented to avoid siblings. Sib-split, which uses family structure to detect movement, uncovered passage of YOY just weeks after emergence. When retrospectively applied to older individuals, it proved essential in interpreting clustering patterns and captured passage in several families of 1- and 2-year-olds. Where family structuring may negatively affect genetic analyses or, alternatively, be prominent enough to allow application of sib-split is difficult to predict a priori; we discuss benefits and limitations of all approaches under different ecological, spatial, and management scenarios.

Accelerating climate change and other cumulative stressors create an urgent need to understand the influence of environmental variation and landscape features on the connectivity and vulnerability of freshwater species. Here, we introduce a novel modeling framework for aquatic systems that integrates spatially explicit, individual-based, demographic and genetic (demogenetic) assessments with environmental variables. To show its potential utility, we simulated a hypothetical network of 19 migratory riverine populations (e.g., salmonids) using a riverscape connectivity and demogenetic model (CDFISH). We assessed how stream resistance to movement (a function of water temperature, fluvial distance, and physical barriers) might influence demogenetic connectivity, and hence, population vulnerability. We present demographic metrics (abundance, immigration, and change in abundance) and genetic metrics (diversity, differentiation, and change in differentiation), and combine them into a single vulnerability index for identifying populations at risk of extirpation. We considered four realistic scenarios that illustrate the relative sensitivity of these metrics for early detection of reduced connectivity: (1) maximum resistance due to high water temperatures throughout the network, (2) minimum resistance due to low water temperatures throughout the network, (3) increased resistance at a tributary junction caused by a partial barrier, and (4) complete isolation of a tributary, leaving resident individuals only. We then applied this demogenetic framework using empirical data for a bull trout (Salvelinus confluentus) metapopulation in the upper Flathead River system, Canada and USA, to assess how current and predicted future stream warming may influence population vulnerability. Results suggest that warmer water temperatures and associated barriers to movement (e.g., low flows, dewatering) are predicted to fragment suitable habitat for migratory salmonids, resulting in the loss of genetic diversity and reduced numbers in certain vulnerable populations. This demogenetic simulation framework, which is illustrated in a web-based interactive mapping prototype, should be useful for evaluating population vulnerability in a wide variety of dendritic and fragmented riverscapes, helping to guide conservation and management efforts for freshwater species.

Forecasts of species distributions under future climates are inherently uncertain, but there have been few attempts to describe this uncertainty comprehensively in a probabilistic manner. We developed a Monte Carlo approach that accounts for uncertainty within generalized linear regression models (parameter uncertainty and residual error), uncertainty among competing models (model uncertainty), and uncertainty in future climate conditions (climate uncertainty) to produce site-specific frequency distributions of occurrence probabilities across a species' range. We illustrated the method by forecasting suitable habitat for bull trout (Salvelinus confluentus) in the Interior Columbia River Basin, USA, under recent and projected 2040s and 2080s climate conditions. The 95% interval of total suitable habitat under recent conditions was estimated at 30.1-42.5 thousand km; this was predicted to decline to 0.5-7.9 thousand km by the 2080s. Projections for the 2080s showed that the great majority of stream segments would be unsuitable with high certainty, regardless of the climate data set or bull trout model employed. The largest contributor to uncertainty in total suitable habitat was climate uncertainty, followed by parameter uncertainty and model uncertainty. Our approach makes it possible to calculate a full distribution of possible outcomes for a species, and permits ready graphical display of uncertainty for individual locations and of total habitat.

Fish have been translocated throughout the world, and introductions often have been executed repeatedly and have used mixtures of different strains from the native range. This history might have contributed to their invasive potential by allowing introduced and invading populations to circumvent expected reductions in genetic diversity from founder effects in a scenario termed the genetic paradox of invasions. We characterize patterns of genetic diversity in nonnative Brook Trout Salvelinus fontinalis, which have been introduced across the western United States for over a century but have also invaded broadly and pose a primary threat to native trout. We analyzed 155 coding gene single nucleotide polymorphisms (SNPs) in 34 nonnative Brook Trout populations sampled across eight large river systems as well as samples from the only four hatchery strains with documented use in Idaho. We uncovered similar within-population genetic diversity and large effective population sizes in naturalized populations compared with hatchery samples. Naturalized populations also showed substantial genetic structuring (maximum pairwise F-ST= 0.23) across and even within watersheds and indicated suggestions of admixture in certain regions. Assignment probabilities confirmed two main hatcheries as the origin of most fish collected in the field; however, the four hatcheries were excluded as being the origin for 8% of individuals, mirroring results from clustering analyses and suggesting the influence of an additional unsampled hatchery source or sources. Simulated admixtures of hatchery samples produced genetic patterns similar to those observed in field samples, further supporting an influence of multiple historic hatchery stocks on the contemporary genetic structure of Brook Trout in Idaho. Our study highlights the potential contribution of historic hatchery and introduction practices in creating genetically variable and structured naturalized Brook Trout populations across Idaho, which may have allowed these fish to defy the genetic paradox early on in their nonnative history and set the stage for successful establishment and subsequent invasion. Received September 4, 2012; accepted March 31, 2013