Westslope Cutthroat Trout Populations Research Articles

Evaluating the effects of selective passage of migratory Westslope Cutthroat Trout on nonnative admixture

AbstractObjectiveHybridization with nonnative Rainbow Trout Oncorhynchus mykiss is a primary threat to the persistence of Westslope Cutthroat Trout O. clarkii lewisi. Managers concerned with conserving migratory populations of Westslope Cutthroat Trout in the presence of Rainbow Trout often face the predicament of tolerating the spread of hybridization, intentionally isolating Westslope Cutthroat Trout populations, or suppressing Rainbow Trout source populations.MethodsSelective passage of migratory Westslope Cutthroat Trout above existing barriers is a management approach with the potential to limit hybridization, while minimizing the population impacts of barriers in populations with preexisting low levels of hybridization. We took a before‐after–control‐impact approach to evaluate a phenotype‐based selective passage management program for migratory Westslope Cutthroat Trout in the Jocko River, Montana.ResultOf the 364 genotyped individuals selectively passed upstream of a barrier, 82% had a proportion of Rainbow Trout admixture < 0.01 and 98% had < 0.10. Over 14 years (2005–2019), there was no significant increase in hybridization at sites (n = 12) upstream of the barrier, but metrics increased within some of our control (no barrier, n = 8) sites. This increase was greatest at a site just downstream from the barrier, suggesting that blocked fish might have spawned opportunistically in this adjacent tributary.ConclusionWe demonstrate that phenotypic traits were useful in implementing a selective Westslope Cutthroat Trout passage program that allowed managers to promote the migratory life history without increasing nonnative admixture in this watershed.

Genetic variation in westslope cutthroat trout reveals that widespread genetic rescue is warranted

Although human fragmentation of freshwater habitats is ubiquitous, the genetic consequences of isolation and a roadmap to address them are poorly documented for most fishes. This is unfortunate, because translocation for genetic rescue could help mitigate problems. We used genetic data (32 SNPs) from 203 populations of westslope cutthroat trout (Oncorhynchus clarkii lewisi) to (1) document the effect of fragmentation on genetic variation and population structure, (2) identify candidate populations for genetic rescue, and (3) quantify the potential benefits of strategic translocation efforts. Human-isolated populations had substantially lower genetic variation and elevated genetic differentiation, indicating that many populations are strongly influenced by random genetic drift. Based on simple criteria, 23 populations were candidates for genetic rescue, which represented a majority (51%) of suitable populations in one major region (Missouri drainage). Population genetic theory suggests that translocation of a small number of individuals (∼5 adults) from nearby populations could dramatically increase heterozygosity by up to 58% (average across populations). This effort provides a clear template for future conservation of westslope cutthroat trout, while simultaneously highlighting the potential need for similar efforts in many freshwater species.

Ultrasound imaging identifies life history variation in resident Cutthroat Trout.

Human activities that fragment fish habitat have isolated inland salmonid populations. This isolation is associated with loss of migratory life histories and declines in population density and abundance. Isolated populations exhibiting only resident life histories may be more likely to persist if individuals can increase lifetime reproductive success by maturing at smaller sizes or earlier ages. Therefore, accurate estimates of age and size at maturity across resident salmonid populations would improve estimates of population viability. Commonly used methods for assessing maturity such as dissection, endoscopy and hormone analysis are invasive and may disturb vulnerable populations. Ultrasound imaging is a non-invasive method that has been used to measure reproductive status across fish taxa. However, little research has assessed the accuracy of ultrasound for determining maturation status of small-bodied fish, or reproductive potential early in a species' reproductive cycle. To address these knowledge gaps, we tested whether ultrasound imaging could be used to identify maturing female Westslope Cutthroat Trout (Oncorhynchus clarkii lewisi). Our methods were accurate at identifying maturing females reared in a hatchery setting up to eight months prior to spawning, with error rates ≤ 4.0%; accuracy was greater for larger fish. We also imaged fish in a field setting to examine variation in the size of maturing females among six wild, resident populations of Westslope Cutthroat Trout in western Montana. The median size of maturing females varied significantly across populations. We observed oocyte development in females as small as 109 mm, which is smaller than previously documented for this species. Methods tested in this study will allow researchers and managers to collect information on reproductive status of small-bodied salmonids without disrupting fish during the breeding season. This information can help elucidate life history traits that promote persistence of isolated salmonid populations.

Hybridization alters growth and migratory life-history expression of native trout.

Human‐mediated hybridization threatens many native species, but the effects of introgressive hybridization on life‐history expression are rarely quantified, especially in vertebrates. We quantified the effects of non‐native rainbow trout admixture on important life‐history traits including growth and partial migration behavior in three populations of westslope cutthroat trout over five years. Rainbow trout admixture was associated with increased summer growth rates in all populations and decreased spring growth rates in two populations with cooler spring temperatures. These results indicate that non‐native admixture may increase growth under warmer conditions, but cutthroat trout have higher growth rates during cooler periods. Non‐native admixture consistently increased expression of migratory behavior, suggesting that there is a genomic basis for life‐history differences between these species. Our results show that effects of interspecific hybridization on fitness traits can be the product of genotype‐by‐environment interactions even when there are minor differences in environmental optima between hybridizing species. These results also indicate that while environmentally mediated traits like growth may play a role in population‐level consequences of admixture, strong genetic influences on migratory life‐history differences between these species likely explains the continued spread of non‐native hybridization at the landscape‐level, despite selection against hybrids at the population‐level.

Distribution and Abundance of Westslope Cutthroat Trout in Relation to Habitat Characteristics at Multiple Spatial Scales

AbstractThe distribution and relative abundance of Westslope Cutthroat Trout (WCT) Oncorhynchus clarkii lewisi in relation to habitat characteristics remain unknown across large portions of the species’ range. The goals of this research were to provide a foundational understanding of WCT distribution and relative abundance related to habitat characteristics in tributaries of the St. Maries River, Idaho—a highly altered watershed. The basin drains an area of approximately 1,863 km2 and has a longitudinal elevation difference of about 207 m. Backpack electrofishing and habitat assessments were conducted at 68 reaches in 35 different tributaries of the St. Maries River in 2017 and 2018. Habitat was measured at small (reach‐level) and large (watershed‐level) scales. A total of 652 WCT was sampled from 52 of 68 total reaches. Habitat characteristics varied by age‐class, but most WCT were estimated to be age 0 and age 1. Logistic regression models indicated that the presence of age‐0 WCT was positively related to stream gradient and elevation, but negatively related to water temperature, road density, fine substrate, stream depth, and the presence of Brook Trout (BKT) Salvelinus fontinalis. The relative abundance of age‐0 WCT was positively associated with road density and inversely related to wetted width, canopy cover, and elevation. The presence of age‐1 and older (age‐1+) WCT was positively related to gradient, canopy cover, and elevation, but negatively associated with road density, temperature, stream depth, and the presence of BKT. Relative abundance of age‐1+WCT was positively associated with gradient, large substrate, canopy cover, and road density. Conversely, the relative abundance of age‐1+WCT was inversely related to wetted width and elevation. This research indicates that WCT populations can persist in response to altered landscapes when suitable habitat exists. However, unmitigated threats, such as nonnative species competition (e.g., BKT), hybridization with Rainbow Trout O. mykiss, habitat loss, and habitat fragmentation, pose persistent complications to WCT abundance in locations where populations appear robust but their actual abundance is unknown.

Distinct Phenotypes of Native Cutthroat Trout Emerge under a Molecular Model of Lineage Distributions

AbstractRecent molecular investigations using contemporary and century‐old museum specimens questioned the traditional four‐subspecies taxonomic arrangement of Cutthroat Trout Oncorhynchus clarkii in the southern Rocky Mountains and revealed six lineages, including two that are likely extinct. We examined extant lineage specimens to determine whether morpho‐meristic taxonomic approaches better classified Cutthroat Trout under (1) the traditional Geographic Model, which recognizes different subspecies east and west of the Continental Divide and in the Rio Grande basin; or (2) the Molecular Model, which uses genotypes to assign populations to four lineages. Classification success of the Molecular Model was higher than that of the Geographic Model, whether comparisons involved single‐trait, principal components, or discriminant function analyses. Native east slope South Platte River trout (putative Greenback Cutthroat Trout O. clarkii stomias) were distinct and correctly classified, as were 83% of Rio Grande Cutthroat Trout O. clarkii virginalis populations. In all, 100% of the Blue Lineage populations of Colorado River Cutthroat Trout O. clarkii pleuriticus (putative west slope native of the White, Yampa, Green, and downstream Colorado River drainages) and 71% of the Green Lineage populations of Colorado River Cutthroat Trout (native to west slope Gunnison and Dolores River drainages and Colorado River headwaters) were correctly classified (89% overall) under the Molecular Model. Green Lineage misclassifications were mainly from morphologically and genetically distinct populations located east of the Continental Divide, whose native status remains unknown. In contrast, only 63% of those east slope and west slope Cutthroat Trout populations were correctly classified under the Geographic Model. Cohesion of distinct phenotypes and genotypes of present‐day native Cutthroat Trout lineages was remarkable given widespread and massive early stocking of various lineages outside of their native ranges. Strong congruence of morphological and molecular patterns demonstrated the power of joint morphological and molecular analyses. We encourage management that preserves diversity of these rare Cutthroat Trout lineages that evolved in concert with their environment.

Climate, Demography, and Zoogeography Predict Introgression Thresholds in Salmonid Hybrid Zones in Rocky Mountain Streams.

Among the many threats posed by invasions of nonnative species is introgressive hybridization, which can lead to the genomic extinction of native taxa. This phenomenon is regarded as common and perhaps inevitable among native cutthroat trout and introduced rainbow trout in western North America, despite that these taxa naturally co-occur in some locations. We conducted a synthetic analysis of 13,315 genotyped fish from 558 sites by building logistic regression models using data from geospatial stream databases and from 12 published studies of hybridization to assess whether environmental covariates could explain levels of introgression between westslope cutthroat trout and rainbow trout in the U.S. northern Rocky Mountains. A consensus model performed well (AUC, 0.78–0.86; classification success, 72–82%; 10-fold cross validation, 70–82%) and predicted that rainbow trout introgression was significantly associated with warmer water temperatures, larger streams, proximity to warmer habitats and to recent sources of rainbow trout propagules, presence within the historical range of rainbow trout, and locations further east. Assuming that water temperatures will continue to rise in response to climate change and that levels of introgression outside the historical range of rainbow trout will equilibrate with those inside that range, we applied six scenarios across a 55,234-km stream network that forecast 9.5–74.7% declines in the amount of habitat occupied by westslope cutthroat trout populations of conservation value, but not the wholesale loss of such populations. We conclude that introgression between these taxa is predictably related to environmental conditions, many of which can be manipulated to foster largely genetically intact populations of westslope cutthroat trout and help managers prioritize conservation activities.

Genetic Status and Conservation of Westslope Cutthroat Trout in Glacier National Park

AbstractInvasive hybridization is one of the greatest threats to the persistence of Westslope Cutthroat Trout Oncorhynchus clarkii lewisi. Large protected areas, where nonhybridized populations are interconnected and express historical life history and genetic diversity, provide some of the last ecological and evolutionary strongholds for conserving this species. Here, we describe the genetic status and distribution of Westslope Cutthroat Trout throughout Glacier National Park, Montana. Admixture between Westslope Cutthroat Trout and introduced Rainbow Trout O. mykiss and Yellowstone Cutthroat Trout O. clarkii bouvieri was estimated by genotyping 1,622 fish collected at 115 sites distributed throughout the Columbia, Missouri, and South Saskatchewan River drainages. Currently, Westslope Cutthroat Trout occupy an estimated 1,465 km of stream habitat and 45 lakes (9,218 ha) in Glacier National Park. There was no evidence of introgression in samples from 32 sites along 587 km of stream length (40% of the stream kilometers currently occupied) and 17 lakes (2,555 ha; 46% of the lake area currently occupied). However, nearly all (97%) of the streams and lakes that were occupied by nonhybridized populations occurred in the Columbia River basin. Based on genetic status (nonnative genetic admixture ≤ 10%), 36 Westslope Cutthroat Trout populations occupying 821 km of stream and 5,482 ha of lakes were identified as “conservation populations.” Most of the conservation populations (N = 27; 736 km of stream habitat) occurred in the Columbia River basin, whereas only a few geographically restricted populations were found in the South Saskatchewan River (N = 7; 55 km) and Missouri River (N = 2; 30 km) basins. Westslope Cutthroat Trout appear to be at imminent risk of genomic extinction in the South Saskatchewan and Missouri River basins, whereas populations in the Columbia River basin are widely distributed and conservation efforts are actively addressing threats from hybridization and other stressors. A diverse set of pro‐active management approaches will be required to conserve, protect, and restore Westslope Cutthroat Trout populations in Glacier National Park throughout the 21st century.Received September 10, 2015; accepted March 30, 2016 Published online August 12, 2016

Influences of Summer Water Temperatures on the Movement, Distribution, and Resource Use of Fluvial Westslope Cutthroat Trout in the South Fork Clearwater River Basin

AbstractAlthough many Westslope Cutthroat Trout Oncorhynchus clarkii lewisi populations in Idaho are robust and stable, population densities in some systems remain below management objectives. In many of those systems, such as in the South Fork Clearwater River (SFCR) system, environmental conditions (e.g., summer temperatures) are hypothesized to limit populations of Westslope Cutthroat Trout. Radiotelemetry and snorkeling methods were used to describe seasonal movement patterns, distribution, and habitat use of Westslope Cutthroat Trout in the SFCR during the summers of 2013 and 2014. Sixty‐six radio transmitters were surgically implanted into Westslope Cutthroat Trout (170–405 mm TL) from May 30–June 25, 2013, and June 20–July 6, 2014. Sedentary and mobile summer movement patterns by Westslope Cutthroat Trout were observed in the SFCR. Westslope Cutthroat Trout were generally absent from the lower SFCR. In the upper region of the SFCR, fish generally moved from the main‐stem SFCR into tributaries as water temperatures increased during the summer. Fish remained in the middle region of the SFCR where water temperatures were cooler than in the upper or lower regions of the SFCR. A spatially explicit water temperature model indicated that the upper and lower regions of the SFCR exceeded thermal tolerance levels of Westslope Cutthroat Trout throughout the summer. During snorkeling, 23 Westslope Cutthroat Trout were observed in 13 sites along the SFCR and at low density (mean ± SD, 0.0003 ± 0.0001 fish/m2). The distribution of fish observed during snorkeling was consistent with the distribution of radio‐tagged fish in the SFCR during the summer. Anthropogenic activities (i.e., grazing, mining, road construction, and timber harvest) in the SFCR basin likely altered the natural flow dynamics and temperature regime and thereby limited stream habitat in the SFCR system for Westslope Cutthroat Trout.Received June 15, 2015; accepted December 21, 2015 Published online May 16, 2016

Trends in Abundance and the Influence of Bioclimatic Factors on Westslope Cutthroat Trout in Idaho

Abstract Westslope cutthroat trout Oncorhynchus clarkii lewisi is arguably the most widely distributed subspecies of cutthroat trout in western North America, but there has been a general lack of information on the species’ trends in abundance across the landscape. We estimated westslope cutthroat trout population growth rates using 17 data sets (including data from screw traps, daytime snorkeling, and angler creel) averaging 19 y of record that indexed abundance across Idaho. We also examined several bioclimatic indices to help understand what factors might be influencing trout abundance. Lastly, we evaluated observation error in the data sets, which is important because high observation error leads to spuriously inflated error bounds around estimates of population growth rates, reducing the ability to detect changes in population growth. Trend data showed that abundance for westslope cutthroat trout was increasing for 10 data sets, declining for 2 data sets, and stable for 5 data sets (where the confidence intervals spanned zero). Bioclimatic indices generally explained a low amount of variation in westslope cutthroat trout abundance through time, but abundance of Chinook salmon Oncorhynchus tshawytscha was significantly related to trend most often; however, the influence of Chinook salmon on westslope cutthroat trout abundance was equally likely to be positive or negative. Most (58%) data sets were estimated to have zero or minimal observation error; high observation error was most prevalent in screw-trap data sets. The results of our study suggest that most westslope cutthroat trout populations in Idaho are currently stable or increasing in abundance, but the factors affecting population trends are poorly understood.

Trophic Ecology of Nonnative Northern Pike and their Effect on Conservation of Native Westslope Cutthroat Trout

AbstractWestslope Cutthroat Trout Oncorhynchus clarkii lewisi in Coeur d'Alene Lake, Idaho, have declined in recent years; predation by Northern Pike Esox lucius, a nonnative sport fish, is thought to be a causative mechanism. The goal of this study was to describe the seasonal food habits of Northern Pike and determine their influence on Westslope Cutthroat Trout in Coeur d'Alene Lake by using a bioenergetics modeling approach. Fish were sampled monthly from March 2012 to May 2013 using pulsed‐DC electrofishing and experimental gillnetting in four bays. Northern Pike catch rates from electrofishing were generally low but increased slightly each season and were highest in the southern portion of the lake; catch rates from gillnetting were approximately 50% higher during the two spring sampling periods compared with the summer and fall. Seasonal growth and food habits of 695 Northern Pike (TL = 16.2–108.0 cm; weight = 24–9,628 g) were analyzed. Diets primarily consisted of kokanee O. nerka, Westslope Cutthroat Trout, and Yellow Perch Perca flavescens. Results of a bioenergetics model estimated that Westslope Cutthroat Trout represented approximately 2–30% of the biomass consumed by age‐1–4 Northern Pike. Total Westslope Cutthroat Trout biomass consumed by Northern Pike (2008–2011 year‐classes) across all seasons sampled was estimated to be 1,231 kg (95% CI = 723–2,396 kg), and the total number consumed was 5,641 (95% CI = 3,311–10,979). The highest occurrence of Westslope Cutthroat Trout in Northern Pike diets was observed during spring. Thus, reducing Northern Pike predation on Westslope Cutthroat Trout would be one tool worth considering for conserving Westslope Cutthroat Trout populations in Coeur d'Alene Lake.Received March 7, 2014; accepted September 22, 2014

Quantifying the Effectiveness of Conservation Measures to Control the Spread of Anthropogenic Hybridization in Stream Salmonids: a Climate Adaptation Case Study

AbstractQuantifying the effectiveness of management actions to mitigate the effects of changing climatic conditions (i.e., climate adaptation) can be difficult, yet critical for conservation. We used population genetic data from 1984 to 2011 to assess the degree to which ambient climatic conditions and targeted suppression of sources of nonnative Rainbow Trout Oncorhynchus mykiss have influenced the spread of introgressive hybridization in native populations of Westslope Cutthroat Trout O. clarkii lewisi. We found rapid expansion in the spatial distribution and proportion of nonnative genetic admixture in hybridized populations from 1984 to 2004, but minimal change since 2004. The spread of hybridization was negatively correlated with the number of streamflow events in May that exceeded the 75th percentile of historic flows (r = −0.98) and positively correlated with August stream temperatures (r = 0.89). Concomitantly, suppression data showed a 60% decline in catch per unit effort for fish with a high proportion of Rainbow Trout admixture, rendering some uncertainty as to the relative strength of factors controlling the spread of hybridization. Our results illustrate the importance of initiating management actions to mitigate the potential effects of climate change, even where data describing the effectiveness of such actions are initially limited but the risks are severe.Received November 22, 2013; accepted March 1, 2014

Juvenile Movement among Different Populations of Cutthroat Trout Introduced as Embryos to Vacant Habitat

AbstractTranslocations are frequently used to increase the abundance and range of endangered fishes. One factor likely to affect the outcome of translocations is fish movement. We introduced embryos from five Westslope Cutthroat Trout Oncorhynchus clarkii lewisi populations (both hatchery and wild) at five different locations within a fishless watershed. We then examined the movement of age‐1 and age‐2 fish and looked for differences in movement distance among source populations and among introduction sites; we also examined the interactions among age, population, and introduction site. At age 1, most individuals (90.9%) remained within 1,000 m their introduction sites. By age 2, the majority of individuals (58.3%) still remained within 1,000 m of their introduction site, but considerably more individuals had moved downstream, some more than 6,000 m from their introduction site. We observed a significant interaction between age and source population (F 4, 1077 = 15.45, P < 0.0001) as well as between age and introduction site (F 41, 1077 = 11.39, P < 0.0008), so we presented results in the context of these interactions. Within age‐groups, we observed differences in movement behavior among source populations and among donor populations of Westslope Cutthroat Trout. We discuss these findings in light of previous research on juvenile salmonid movement.Received April 20, 2012; accepted June 3, 2013

Development and evaluation of 200 novel SNP assays for population genetic studies of westslope cutthroat trout and genetic identification of related taxa

DNA sequence data were collected and screened for single nucleotide polymorphisms (SNPs) in westslope cutthroat trout (Oncorhynchus clarki lewisi) and also for substitutions that could be used to genetically discriminate rainbow trout (O. mykiss) and cutthroat trout, as well as several cutthroat trout subspecies. In total, 260 expressed sequence tag-derived loci were sequenced and allelic discrimination genotyping assays developed from 217 of the variable sites. Another 50 putative SNPs in westslope cutthroat trout were identified by restriction-site-associated DNA sequencing, and seven of these were developed into assays. Twelve O. mykiss SNP assays that were variable within westslope cutthroat trout and 12 previously published SNP assays were also included in downstream testing. A total of 241 assays were tested on six westslope cutthroat trout populations (N = 32 per population), as well as collections of four other cutthroat trout subspecies and a population of rainbow trout. All assays were evaluated for reliability and deviation from Hardy-Weinberg and linkage equilibria. Poorly performing and duplicate assays were removed from the data set, and the remaining 200 assays were used in tests of population differentiation. The remaining markers easily distinguished the various subspecies tested, as evidenced by mean G(ST) of 0.74. A smaller subset of the markers (N = 86; average G(ST) = 0.40) was useful for distinguishing the six populations of westslope cutthroat trout. This study increases by an order of magnitude the number of genetic markers available for the study of westslope cutthroat trout and closely related taxa and includes many markers in genes (developed from ESTs).

RAD sequencing yields a high success rate for westslope cutthroat and rainbow trout species‐diagnostic SNP assays

Hybridization with introduced rainbow trout threatens most native westslope cutthroat trout populations. Understanding the genetic effects of hybridization and introgression requires a large set of high-throughput, diagnostic genetic markers to inform conservation and management. Recently, we identified several thousand candidate single-nucleotide polymorphism (SNP) markers based on RAD sequencing of 11 westslope cutthroat trout and 13 rainbow trout individuals. Here, we used flanking sequence for 56 of these candidate SNP markers to design high-throughput genotyping assays. We validated the assays on a total of 92 individuals from 22 populations and seven hatchery strains. Forty-six assays (82%) amplified consistently and allowed easy identification of westslope cutthroat and rainbow trout alleles as well as heterozygote controls. The 46 SNPs will provide high power for early detection of population admixture and improved identification of hybrid and nonhybridized individuals. This technique shows promise as a very low-cost, reliable and relatively rapid method for developing and testing SNP markers for nonmodel organisms with limited genomic resources.

Genetic variation in westslope cutthroat trout Oncorhynchus clarkii lewisi: implications for conservation

Twenty-five populations of westslope cutthroat trout from throughout their native range were genotyped at 20 microsatellite loci to describe the genetic structure of westslope cutthroat trout. The most genetic diversity (heterozygosity, allelic richness, and private alleles) existed in populations from the Snake River drainage, while populations from the Missouri River drainage had the least. Neighbor-joining trees grouped populations according to major river drainages. A great amount of genetic differentiation was present among and within all drainages. Based on Nei’s D S , populations in the Snake River were the most differentiated, while populations in the Missouri River were the least. This pattern of differentiation is consistent with a history of sequential founding events through which westslope cutthroat trout may have experienced a genetic bottleneck as they colonized each river basin from the Snake to the Clark Fork to the Missouri river. These data should serve as a starting point for a discussion on management units and possible distinct population segments. Given the current threats to the persistence of westslope cutthroat trout, and the substantial genetic differentiation between populations, these topics warrant attention.

ASSESSING THE IMPACTS OF RIVER REGULATION ON NATIVE BULL TROUT (SALVELINUS CONFLUENTUS) AND WESTSLOPE CUTTHROAT TROUT (ONCORHYNCHUS CLARKII LEWISI) HABITATS IN THE UPPER FLATHEAD RIVER, MONTANA, USA

ABSTRACTHungry Horse Dam on the South Fork Flathead River, Montana, USA, has modified the natural flow regimen for power generation, flood risk management and flow augmentation for anadromous fish recovery in the Columbia River. Concern over the detrimental effects of dam operations on native resident fishes prompted research to quantify the impacts of alternative flow management strategies on threatened bull trout (Salvelinus confluentus) and westslope cutthroat trout (Oncorhynchus clarkii lewisi) habitats. Seasonal and life‐stage specific habitat suitability criteria were combined with a two‐dimensional hydrodynamic habitat model to assess discharge effects on usable habitats. Telemetry data used to construct seasonal habitat suitability curves revealed that subadult (fish that emigrated from natal streams to the river system) bull trout move to shallow, low‐velocity shoreline areas at night, which are most sensitive to flow fluctuations. Habitat time series analyses comparing the natural flow regimen (predam, 1929–1952) with five postdam flow management strategies (1953–2008) show that the natural flow conditions optimize the critical bull trout habitats and that the current strategy best resembles the natural flow conditions of all postdam periods. Late summer flow augmentation for anadromous fish recovery, however, produces higher discharges than predam conditions, which reduces the availability of usable habitat during this critical growing season. Our results suggest that past flow management policies that created sporadic streamflow fluctuations were likely detrimental to resident salmonids and that natural flow management strategies will likely improve the chances of protecting key ecosystem processes and help to maintain and restore threatened bull trout and westslope cutthroat trout populations in the upper Columbia River Basin. Copyright © 2011 John Wiley & Sons, Ltd.

Levels of Introgression in Westslope Cutthroat Trout Populations Nine Years after Changes to Rainbow Trout Stocking Programs in Southeastern British Columbia

AbstractIntrogressive hybridization (introgression) between native westslope cutthroat trout Oncorhynchus clarkii lewisi and introduced rainbow trout O. mykiss in the upper Kootenay River, British Columbia, appears to be recent and is partly attributed to rainbow trout stocking in Koocanusa Reservoir starting in 1988. In 1998, rainbow trout stocking was stopped or replaced with the stocking of triploids throughout the watershed. The goal of this research was to determine the effect of the change in stocking practices on the level of introgression between westslope cutthroat trout and rainbow trout. We monitored 14 sites from 1999 to 2007 using four co‐dominant, diagnostic nuclear markers. Sample sites were categorized as (1) open if no fish migration barriers existed between the site and the reservoir or (2) closed if migration barriers were present between the site and the reservoir. We classified fish as pure westslope cutthroat trout if no rainbow trout alleles were detected; fish were classified as hybrids if they had at least one rainbow trout allele present. We also classified fish into parental and hybrid types based on their multilocus genotypes. Open sites had more hybrids (15.8%) than closed sites (4.8%; χ2 = 23.38, df = 1, P < 0.0001). Westslope cutthroat trout backcrosses were the most common hybrid genotype at both open and closed sites. At open sites near the reservoir, introgression levels stayed relatively high (20–30%) and pure rainbow trout and rainbow trout backcross individuals were common. Introgression significantly increased at two of three sites surveyed at intermediate distances upstream from the reservoir (i.e., 27–87 km). The new stocking program does not appear to have reduced introgression or prevented the spread of hybrid individuals in the upper Kootenay River, and more active management strategies will be required to prevent further introgression and loss of unique westslope cutthroat trout populations.

Use of an Annular Chamber for Testing Thermal Preference of Westslope Cutthroat Trout and Rainbow Trout

ABSTRACT Remaining populations of westslope cutthroat trout (Oncorhynchus clarkii iewisi) in western North America are primarily confined to cold headwaters whereas nonnative rainbow trout (Oncorhynchus mykiss) predominate in warmer, lower elevation stream sections historically occupied by westslope cutthroat trout. We tested whether differing thermal preferences could account for the spatial segregation observed in the field. Thermal preferences of age-1 westslope cutthroat trout and rainbow trout (125 to 150 mm total length) were assessed in the laboratory using a modified annular preference chamber at acclimation temperatures of 10, 12, 14, and 16°C. Final preferred temperature of westslope cutthroat trout (14.9°C) was similar to that of rainbow trout (143°C) when tested in a thermal gradient of 11–17°C. The high degree of overlap in thermal preference indicates the two species have similar thermal niches and a high potential for competition. We suggest several modifications to the annular preference chamber to improve performance in future studies.

Comparative Thermal Requirements of Westslope Cutthroat Trout and Rainbow Trout: Implications for Species Interactions and Development of Thermal Protection Standards

AbstractWater temperature appears to play a key role in determining population persistence of westslope cutthroat trout Oncorhynchus clarkii lewisi, but specific thermal performance and survival criteria have not been defined. We used the acclimated chronic exposure laboratory method to determine upper thermal tolerances and growth optima of westslope cutthroat trout and rainbow trout O. mykiss, a potential nonnative competitor that occupies much of the former range of westslope cutthroat trout. Rainbow trout had a distinct survival advantage over westslope cutthroat trout at water temperatures above 20°C. The ultimate upper incipient lethal temperature of rainbow trout (24.3°C; 95% confidence interval [CI] = 24.0–24.7°C) was 4.7°C higher than that of westslope cutthroat trout (19.6°C; 95% CI = 19.1–19.9°C). In contrast, both species had similar growth rates and optimum growth temperatures (westslope cutthroat trout: 13.6°C; rainbow trout: 13.1°C) over the temperature range of 8–20°C, although rainbow trout grew over a wider range and at higher temperatures than did westslope cutthroat trout. The rainbow trout's higher upper temperature tolerance and greater growth capacity at warmer temperatures may account for the species' displacement of westslope cutthroat trout at lower elevations. Our results indicate that maximum daily temperatures near the optimum growth temperature of 13–15°C would ensure suitable thermal habitat for westslope cutthroat trout populations. The low upper temperature tolerance and optimum growth temperature of westslope cutthroat trout relative to those of other salmonids suggest that this subspecies may be particularly susceptible to stream temperature increases associated with global warming and anthropogenic habitat disturbance.