Cutthroat Trout Oncorhynchus Clarkii Research Articles

Predicting Persistence of Rio Grande Cutthroat Trout Populations in an Uncertain Future

AbstractThe Rio Grande Cutthroat Trout Oncorhynchus clarkii virginalis (RGCT) occupies just 12% of its ancestral range. As the southernmost subspecies of Cutthroat Trout, we expect a warming climate to bring additional stressors to RGCT populations, such as increased stream temperatures, reduced streamflows, and increased incidence of wildfire. We developed a Bayesian network (BN) model using site‐specific data, empirical research, and expert knowledge to estimate the probability of persistence for each of the 121 remaining RGCT conservation populations and to rank the severity of the threats they face. These inputs quantified the genetic risks (e.g., inbreeding risk and hybridization risk), population demographics (disease risk, habitat suitability, and survival), and probability of stochastic disturbances (stream drying risk and wildfire risk) in an uncertain future. We also created stream temperature and base flow discharge models coupled with regionally downscaled climate projections to predict future abiotic conditions at short‐term (2040s) and long‐term (2080s) time horizons. In the absence of active management, we predicted a decrease in the average probability of population persistence from 0.53 (current) to 0.31 (2040s) and 0.26 (2080s). Only 11% of these populations were predicted to have a greater than 75% chance of persisting to the 2080s. Threat of invasion by nonnative trout had the strongest effect on population persistence. Of the 78 populations that are already invaded or lacking complete barriers, 60% were estimated to be extirpated by 2080 and the remainder averaged only a 10% chance of persistence. In contrast, the effects of increased stream temperatures were predicted to affect the future persistence of only 9% of the 121 RGCT populations remaining, as most have been restricted to high‐elevation habitats that are cold enough to buffer against some stream warming. Our BN model provides a framework for evaluating threats and will be useful to guide management actions that are likely to provide the most benefit for long‐term conservation.

Comparison of Methods to Verify Upstream Passage by Trout at Remediated Culverts in Four Rocky Mountain Streams

AbstractThe 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.

Salmon species identification via direct DNA sequencing of single amplicons

For Pacific salmonids, a key fisheries forensics and management goal is to identify individuals to specific species and potentially specific populations or geographic regions within species. Genetic variation can be used for both species and individual identification, and variation at single nucleotide polymorphisms (SNPs) within two specific DNA amplicons was used to distinguish among coho salmon (Oncorhynchus kisutch), Chinook salmon (Oncorhynchus tshawytscha), sockeye salmon (Oncorhynchus nerka), chum salmon (Oncorhynchus keta), pink salmon (Oncorhynchus gorbuscha), masu salmon (Oncorhynchus masou), steelhead/rainbow trout (Oncorhynchus mykiss), cutthroat trout (Oncorhynchus clarkii), and Atlantic salmon (Salmo salar). A total of 2510 individuals was evaluated from a Pacific rim distribution of populations for sockeye salmon, chum salmon, and pink salmon, whereas masu salmon samples were genotyped only from Japanese populations. Coho salmon, Chinook salmon, steehead trout, cutthroat trout, and Atlantic salmon samples were genotyped only from North American populations. All individual salmonids genotyped were assigned with 100% accuracy to their respective species via diagnostic homozygous SNP genotypes for each species except for cutthroat trout, where no diagnostic SNP genotypes were observed in the two amplicons evaluated.

Population connectivity of adfluvial and stream-resident Lahontan cutthroat trout: implications for resilience, management, and restoration

Population connectivity between resident and migratory cutthroat trout (Oncorhynchus clarkii ssp.) is understudied, but has implications for population viability and management. We examined evidence for stream residency, studied the spatial patterns of stream use by adfluvial and stream-resident trout, and measured migration rates with changing stream depth for Lahontan cutthroat trout (Oncorhynchus clarkii henshawi) in the Summit Lake Basin, Nevada (USA). Passive integrated transponder technology and a fry trap were used to track fish movements and identify the distribution of resident and adfluvial trout. Stream residents were distributed throughout the network. Adfluvial spawners concentrated in lower reaches, but also migrated up to 12.9 km. Adfluvial juveniles migrated to the lake from lower reaches and from upstream of adfluvial spawners. High stream depths coincided with more adfluvial juveniles migrating to the lake and more adfluvial spawners moving into the stream, which led to more accessing the upper watershed. This work shows that connectivity is central to adfluvial–resident Lahontan cutthrout trout population dynamics and may lead to increased probability of persistence — a characteristic of these isolated, threatened trout populations.

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.

Effects of Air Exposure During Simulated Catch‐and‐Release Angling on Survival and Fitness of Yellowstone Cutthroat Trout

AbstractConcerns have been raised regarding the practice of exposing fish to air during catch‐and‐release (C&R) angling. The purpose of this study was to evaluate the effects of air exposure on short‐ and long‐term survival and progeny production of Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri. Prespawn adults were sampled at a weir during upstream migration in 2016 and 2017, exposed to a simulated angling event of 102 s, and then exposed to air for a randomly selected duration of 0, 30, or 60 s. An additional control group was added during 2017 in which fish were not exposed to simulated angling or air. In total, 1,519 fish were sampled in 2016, and 744 fish were sampled in 2017. Additionally, age‐0 fish (2016: n = 2,924; 2017: n = 1,492) were collected to evaluate the effects of air exposure on the production of progeny. No effect of angling itself or of angling and air exposure was observed on short‐term (≤60 d posttreatment) or long‐term (>1 year posttreatment) survival of adults, with one exception. During 2016, fish that had been air exposed for 60 s had a statistically higher short‐term survival rate than fish that received no air exposure. Air exposure had no effect on the proportion of fish that successfully spawned. Regression analysis revealed that neither angling nor air exposure affected progeny production. Considering that much of the literature, as well as this study, reports little to no influence of air exposure on salmonid mortality or reproductive success, it seems highly unlikely that air exposure of less than 60 s during C&R angling would have negative population‐level effects.

Performance of Diploid and Triploid Westslope Cutthroat Trout Fry Stocked into Idaho Alpine Lakes

AbstractAnglers value alpine lakes as providing a scenic, backcountry fishing experience that is seldom found in other fisheries. However, trout introduced into alpine lakes often pose a risk to native salmonids in downstream habitats by establishing source populations in headwater locations. Although the stocking of sterile triploid trout can reduce this risk, poststocking performance should be equivalent to that of diploid fish before fisheries managers and anglers consider triploid stocking to be ideal. In this study, three different mechanical pressure treatments were tested to create mixed‐sex triploid Westslope Cutthroat Trout Oncorhynchus clarkii lewisi; at 65,500.19 kPa (9,500 psi) applied for 5 min at 300 Celsius‐minutes after fertilization, triploid induction rates were 100%, and egg survival to eye‐up was only 8% lower than that for diploids. Fry were stocked into 51 alpine lakes (25 with triploids and 26 with diploids), and fish were sampled 3 years later. Gill‐net catch rates, growth, and condition did not differ between stocked diploid and triploid fish. General linear models revealed that test fish were larger and in better condition in lakes with fewer total fish (indicating a density‐dependent influence on growth and condition) and were also in better condition in lower‐elevation lakes containing a higher percentage of shallow habitat. Ploidy level was not included in the most plausible model for any of the response variables. Our results indicate that triploid Westslope Cutthroat Trout can easily be created by using pressure treatment of fertilized eggs and appear to be a suitable alternative to diploids in alpine lake stocking programs. Our study suggests that the use of triploid Westslope Cutthroat Trout in alpine lakes similar to those we studied would prevent hybridization with native salmonids in adjacent habitats without reducing the abundance, size, or condition of fish available to anglers in the lakes where they are stocked.

Impacts of Nonnative Brown Trout on Yellowstone Cutthroat Trout in a Tributary Stream

AbstractNonnative trout are a considerable threat to native salmonids, yet our understanding of the mechanisms behind interspecific interactions remains limited. We evaluated the impacts of nonnative Brown Trout Salmo salar on a population of Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri in Montana. We contrasted diets, growth, and survival of Yellowstone Cutthroat Trout occurring in allopatry (i.e., where no Brown Trout were present) with individuals sympatric (i.e., co‐occurring) with nonnative Brown Trout. We assessed summer and autumn diets using gastric lavage methods and survival and growth using mark–recapture analyses. Overlap in diets at sites where Yellowstone Cutthroat Trout were sympatric with Brown Trout was high during July (Horn's index: H = 0.94) and October (H = 0.83). In the presence of Brown Trout, Yellowstone Cutthroat Trout growth rates were significantly lower for juvenile (<175 mm) length and adult (≥175 mm) length and mass than in allopatric sites. Allopatric Yellowstone Cutthroat Trout survival was greater across size‐classes; the most pronounced difference was in the age‐2 size‐class (125–175 mm). Together, these results in concert with observed changes in length‐frequency data, indicating a considerable lack of Yellowstone Cutthroat Trout recruitment where they are sympatric with Brown Trout, suggest the negative implications of Brown Trout are notable.

Using forensic geochemistry via fish otoliths to investigate an illegal fish introduction

Illegal fish introductions create some of the most challenging problems for resource managers because of their potential to harm existing recreational fisheries and their impact on species of conservation concern. Determining the origin of a suspected illegal fish introduction can aid managers in preventing the colonization and subsequent ecosystem impacts of introduced species. In this study, we used forensic geochemistry via fish otoliths to investigate an illegal walleye (Sander vitreus) introduction in Swan Lake, Montana, which provides critical habitat for threatened bull trout (Salvelinus confluentus) and native westslope cutthroat trout (Oncorhynchus clarkii). Core to edge geochemical profiles of 87Sr/86Sr and Sr/Ca ratios in the walleye otoliths revealed that these fish had been introduced to Swan Lake within the past growing season, and their geochemical signature matched that of walleye sampled from Lake Helena, Montana, located 309 road kilometres away. This research highlights application of a tool fisheries managers can use to identify the natal waterbody source of illegally introduced fish.

Size, age, growth and site fidelity of anadromous cutthroat trout Oncorhynchus clarkii clarkii in the Salish Sea.

Pacific salmon Oncorhynchus spp. have been the focus of scientific research for over a century, but anadromous trout in this genus, in particular anadromous coastal cutthroat trout Oncorhynchus clarkii clarkii, have been neglected. Oncorhynchus clarkii clarkii occupy a diverse range of habitats including fresh water, brackish estuaries and marine water, but have a relatively small home range making them ideal for studies of behaviour and movements during ocean residency. In 2015, we sampled O. c. clarkii monthly along a small stretch of beach (47.08° N, 122.98° W) in Eld Inlet, south Puget Sound, Washington using a beach seine. We collected tissue for genetic tagging and stock identification and scales for aging from 427 O. c. clarkii, ranging in size from 118 to 478 mm fork length. Additionally, we enumerated redds in natal streams of those fish tagged to describe inter-habitat movement patterns and investigate site fidelity of juvenile and adult O. c. clarkii in the marine environment. Consistent with other anadromous salmonids, O. c. clarkii captured at our study beach exhibited rapid growth rates, particularly in spring following dispersal into the marine environment (mean ± SD = 0.61 ± 0.29 mm-d ). Genetic tag data revealed that while O. c. clarkii undergo inter-estuarine migrations, O. c. clarkii of all life stages exhibited site fidelity in the marine environment. Twenty-one percent (64/305) of sampled O. c. clarkii were recaptured at least once during the course of the study while multiple fish (n = 3) were recaptured up to five times. These results suggest that O. c. clarkii occupying south Puget Sound reside in or regularly return to a small geographic area in the nearshore environment for much of their life and therefore may be particularly vulnerable to anthropogenic disturbance (development, angling, etc.).

Age, growth and reproductive biology of threatened westslope cutthroat trout Oncorhynchus clarkii lewisi inhabiting small mountain streams.

Age, growth and reproductive characteristics of six westslope cutthroat trout Oncorhynchus clarkii lewisi populations were studied in the south-western Rocky Mountains in Alberta, Canada. Sagittal otoliths were collected from 605 fish ranging in size from 36 to 250 mm (fork length). The maximum detected age (13 years for females and 12 for males) was higher than has been reported previously in published literature; but no significant differences in age distributions between males and females were found. Length growth rates, estimated using the von Bertalanffy growth function, showed that males and females had similar growth rates. Sex ratios varied between 0.3 to 2.0 females per male. Age and size at 50% maturity were greater for females than males (4.9 years and 139 mm v. 3.7 years and 125 mm). Mature females were yearly spawners with highly variable fecundities (mean ± S.D. = 223 ± 94) and their ovaries contained both developed and undeveloped eggs. Across all populations, mean instantaneous mortality rate (Z) was estimated as 0.555, annual survival rates for 0-1 year-old fish were 3.2 and 57.4% for older fish. An altitudinal distribution gradient was observed, with older fish occupying upper stream reaches. This study provides a comprehensive evaluation of biological characteristics and structure of O. c. lewisi populations inhabiting small, mountain streams and should provide useful basic information for management policies of this threatened species in eastern drainage of the Canadian Rocky Mountains.

Effects of Climate‐Related Stream Factors on Patterns of Individual Summer Growth of Cutthroat Trout

AbstractColdwater fishes are sensitive to abiotic and biotic stream factors, which can be influenced by climate. Distributions of inland salmonids in North America have declined significantly, with many of the current strongholds located in small headwater systems that may serve as important refugia as climate change progresses. We investigated the effects of discharge, stream temperature, trout biomass, and food availability on summer growth of Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri, a species of concern with significant ecological value. Individual size, stream discharge, sample section biomass, and temperature were all associated with growth, but had differing effects on energy allocation. Stream discharge had a positive relationship with growth rates in length and mass; greater rates of prey delivery at higher discharges probably enabled trout to accumulate reserve tissues in addition to structural growth. Temperature effects were positive but not significant, and support in growth models was limited, likely due to the cold thermal regimes of the study area. The strength of the discharge effect on growth suggests that climate adaptation strategies for coldwater fishes that focus solely on thermal characteristics may be misleading and highlights the importance of considering multiple factors, including hydrologic regimes, in conservation planning.

A Portfolio Framework for Prioritizing Conservation Efforts for Yellowstone Cutthroat Trout Populations

Managing and conserving native taxa are becoming increasingly challenging because of mounting threats and limited resources, predicating the need for frameworks to prioritize conservation actions. We integrated attributes of population persistence, genetic status, threats from nonnative species, and threats from climatic shifts to prioritize conservation actions for Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri. We used the individual attributes to rank populations and provide a framework for identifying the benefits of individual conservation actions. The majority of extant populations (57%) had a high probability (>0.75) of persistence, but nearly 70% of populations were either slightly hybridized or sympatric with nonnative species, and 44% of extant populations occupied habitat with low climatic resilience. Overall, we found that 36% of populations ranked as high (>0.75) conservation priority, and these populations primarily occupied large, relatively high‐elevation habitats. The prioritization framework provides a platform for identifying and ranking actions with the greatest conservation effectiveness.

Elimination of Myxobolus cerebralis in Placer Creek, a Native Cutthroat Trout Stream in Colorado

Placer Creek, a tributary of Sangre de Cristo Creek in Colorado's San Luis Valley, supported an allopatric core conservation population of native Rio Grande Cutthroat Trout Oncorhynchus clarkii virginalis during much of the 20th century. After the failure of gabion barriers in the late 1990s, Brook Trout Salvelinus fontinalis infected with Myxobolus cerebralis invaded from Sangre de Cristo Creek. By 2005, whirling disease (WD) and competition from Brook Trout reduced Rio Grande Cutthroat Trout numbers to less than 10% of the total trout population. New barriers were constructed in 2006 and the stream was treated with rotenone in 2007 and 2009 to eliminate all fish prior to the reintroduction of Rio Grande Cutthroat Trout. Results of WD research studies in Montana, California, and Colorado indicated it might be possible to break the life cycle of the parasite in some situations. Our management interventions included (1) reducing the fish population in the stream to zero for approximately 14months, (2) introducing lineage V and VI Tubifex tubifex worms, which are not susceptible to M. cerebralis, and (3) eliminating a small off-channel pond that provided optimal habitat that sustained a localized high-density population of lineage III T. tubifex, the oligochaete host susceptible to M. cerebralis. Electrofishing during the fall of 2009 and spring of 2010 indicated the drainage was devoid of fish. Fry, juvenile, and adult Rio Grande Cutthroat Trout were stocked in September and October of 2010 and 2011. Approximately 975,000 lineage V and VI T. tubifex were introduced into Placer Creek between 2010 and 2012 as possible oligochaete competitors for the lineage III worms. The off-channel pond was filled in, and the surface was reseeded in April 2012. No evidence of M. cerebralis infection was detected among more than 280 Rio Grande Cutthroat Trout tested between July 2012 and July 2016, indicating the parasite had been eradicated from the Placer Creek basin upstream of the barriers.

Connections and containers: Using genetic data to understand how watershed evolution and human activities influence cutthroat trout biogeography.

Species with large geographic distributions often exhibit complex patterns of diversity that can be further complicated by human activities. Cutthroat trout (Oncorhynchus clarkii) are one of the most widely distributed freshwater fish species in western North America exhibiting substantial phenotypic and genetic variability; however, fish stocking practices have translocated populations outside of their native range and may have obscured intraspecific boundaries. This study focuses on cutthroat trout populations representing three distinct evolutionary clades that are found intermixed within a contact zone between the Bonneville and upper Snake River watersheds in the western United States. We used mitochondrial and microsatellite genetic data, as well as historical stocking records, to evaluate whether populations of cutthroat trout in the contact zone are native or are introduced. We found significant genetic differentiation and fine-scale genetic population structure that was organized primarily by watershed boundaries. While we detected increased genetic diversity in some areas in close proximity to the greatest number of stocking events, the highly organized population structure both within and between areas of the contact zone indicates that the populations are native to the watersheds. Intermixing of distinct evolutionary lineages of cutthroat trout appears to be the result of historical connections between paleodrainages. Our analyses provide a context for understanding how genetic data can be used to assess the status of populations as native or introduced.

Trophic Ecology of Lahontan Cutthroat Trout: Historical Predator–Prey Interaction Supports Native Apex Predator in a Unique Desert Lake

AbstractPyramid Lake, Nevada, remains one of the last strongholds for lacustrine‐strain Lahontan Cutthroat Trout Oncorhynchus clarkii henshawi (LCT) following historical declines throughout their native range. Historically the apex predator in this aquatic ecosystem, LCT in Pyramid Lake are now entirely dependent on hatchery programs and are potentially threatened by the introduction of a nonnative piscivore. The purpose of this study was to identify strong food web interactions and potential trophic limitations to the LCT population. Our specific objectives were to estimate seasonal foraging patterns, trophic position (TP), trophic niche space, diet composition, and seasonal depth distributions of LCT relative to prey fish. Additionally, we evaluated the potential for competition between LCT and nonnative, piscivorous Sacramento Perch Archoplites interruptus by examining diet overlap and trophic niche space. Our results indicate that LCT are apex predators, with TP (large [>400 mm TL] LCT: TP [mean ± 1.96·SE] = 4.30 ± 0.04) and diet composition indicating high rates of piscivory (average annual rate of piscivory = 77.1% for large LCT) throughout the year. Small (200–400 mm TL) LCT exhibited weak dietary overlap (Schoener's index = 0.55) and large LCT exhibited strong overlap (Schoener's index = 0.72) with Sacramento Perch. These trends were present despite seasonal changes in distribution and catch of Tui Chub Siphateles bicolor, the primary prey of LCT. Analysis of isotopic niche space also indicated strong overlap between the trophic niche of large LCT (89%) and that of Sacramento Perch; however, overall catch rates of Sacramento Perch were low. Despite the presence of a potential nonnative competitor, LCT demonstrate high rates of piscivory and are not negatively influenced by Sacramento Perch. Overall, these results demonstrate how trophic niche ecology can be used to evaluate key food web interactions in anthropogenically altered ecosystems.

Proactive Rainbow Trout Suppression Reduces Threat of Hybridization in the Upper Snake River Basin

AbstractPreserving remaining nonhybridized populations Cutthroat Trout Oncorhynchus clarkii is a conservation priority often requiring management action. Although proactive programs for Rainbow Trout O. mykiss and hybrid suppression offer a flexible tool, particularly in large interconnected river basins, this management approach is used less frequently than alternatives such as barriers and piscicides. We describe the results of a targeted Rainbow Trout hybrid suppression program spanning 15 years in the upper Snake River, Wyoming, a core stronghold for Yellowstone Cutthroat Trout O. clarkii bouvieri. Initially, Rainbow Trout hybrids were relatively common in the Gros Ventre River, a major tributary to the Snake River. Between 2002 and 2016, 926 individuals of Rainbow Trout ancestry were removed from the Gros Ventre River. Relative abundance of Rainbow Trout hybrids decreased over this time, while the Yellowstone Cutthroat Trout population increased. Temporal genetic data collected in 2007–2008 and again in 2014 demonstrate that the overall proportion Rainbow Trout admixture and the proportion of hybrids in a sample both significantly decreased in the Gros Ventre River and did not increase elsewhere in the Snake River basin. In conclusion, proactive Rainbow Trout suppression appears to have reduced the threat of Rainbow Trout hybridization in this river basin and helped protect an interconnected metapopulation that has a highly diverse life history and genetic variation important for long‐term persistence.

Modeling the fish community population dynamics and forecasting the eradication success of an exotic fish from an alpine stream

Management actions aimed at eradicating exotic fish species from riverine ecosystems can be better informed by forecasting abilities of mechanistic models. We illustrate this point with an example of the Logan River, Utah, originally populated with endemic cutthroat trout (Oncorhynchus clarkii utah), which compete with exotic brown trout (Salmo trutta). The coexistence equilibrium was disrupted by a large scale, experimental removal of the exotic species in 2009–2011 (on average, 8.2% of the stock each year), followed by an increase in the density of the native species. We built a spatially-explicit, reaction-diffusion model encompassing four key processes: population growth in heterogeneous habitat, competition, dispersal, and a management action. We calibrated the model with detailed long-term monitoring data (2001–2016) collected along the 35.4-km long river main channel. Our model, although simple, did a remarkable job reproducing the system steady state prior to the management action. Insights gained from the model independent predictions are consistent with available knowledge and indicate that the exotic species is more competitive; however, the native species still occupies more favorable habitat upstream. Dynamic runs of the model also recreated the observed increase of the native species following the management action. The model can simulate two possible distinct long-term outcomes: recovery or eradication of the exotic species. The processing of available knowledge using Bayesian methods allowed us to conclude that the chance for eradication of the invader was low at the beginning of the experimental removal (0.7% in 2009) and increased (20.5% in 2016) by using more recent monitoring data. We show that accessible mathematical and numerical tools can provide highly informative insights for managers (e.g., outcome of their conservation actions), identify knowledge gaps, and provide testable theory for researchers.

Effects of Temperature and Spatial Scale on Rio Grande Cutthroat Trout Growth and Abundance

Diversity in habitat and life-history strategies promote a species' long-term persistence. However, life-history strategies are most commonly studied at broad spatial and temporal scales. We applied longevity growth models and closed N-mixture models to examine within- versus between stream variability in life-history characteristics of Rio Grande Cutthroat Trout in northern New Mexico streams. We developed a von Bertalanffy growth model and a closed N-mixture model in a hierarchical Bayesian framework to examine the importance of fine-scale variability in temperature and density-dependence on growth and abundance. The model indicated that accumulation of degree days likely positively influenced instantaneous growth rates and, to a lesser extent, negatively affected asymptotic body length. A nonlinear response of abundance to temperature was also observed, suggesting that Cutthroat Trout productivity along the temperature continuum was affected by physiological limitations (e.g., optimal growth temperatures). Parameter variability was greatest at the segment level for asymptotic size and abundance, but greatest at the stream level for the rate at which asymptotic size is reached. In total, the results suggest that fine-scale habitat heterogeneity (i.e., temperature) may play important roles in the continued persistence of Rio Grande Cutthroat Trout. Management actions should, therefore, consider the role of fine-scale processes for improving the likelihood of future population persistence.

Mercury and Selenium in Twelve Cutthroat Trout Tissues from High‐Elevation Colorado Lakes, USA: Intraspecific and Interspecific Comparisons

AbstractTotal mercury (THg) and selenium (Se) concentrations were measured in 12 internal and external tissues of three different wild Cutthroat Trout Oncorhynchus clarkii populations occurring in pristine, high‐elevation lakes of Colorado, USA, within watersheds devoid of past and present mining activities. We have not found any comparable baseline data sets. The primary objective was to show how THg and Se bioaccumulate differentially among 12 tissues from Cutthroat Trout occurring in widely distant wilderness areas. A second objective was to compare THg and Se tissue levels in Cutthroat Trout from the pristine habitats to Cutthroat Trout populations elsewhere and to Brown Trout Salmo trutta from urbanized environments. The baseline tissue data from this investigation could be used to gauge other intraspecific and interspecific data sets. Results showed that both THg and Se concentrations were generally highest in the detoxifying organ tissues (liver, kidney, and spleen) and lowest in external tissues (skin and gill). In all three populations, THg levels were highest in kidney and second highest in spleen, while Se was highest in the liver. For 10 of 12 tissues, mean concentrations of THg were lower in Brown Trout from an urban setting than in the Cutthroat Trout from the pristine sites; testes and skin were the exceptional tissues. Threshold effect tissue concentrations of THg for muscle or ovary/eggs were not evident. None of the 34 Cutthroat Trout showed U.S. Environmental Protection Agency Se exceedances for ovary/eggs or muscle tissue. To show the relationship between THg and Se in each tissue, we calculated mean Se : Hg molar ratios, which were lowest in epaxial muscle in all three populations (range = 5.74–30.58), while kidney Se : Hg ratios were also in the low range (17.88–42.63). All tissues had a ratio greater than 1. In spite of atmospheric source‐inputs of both organometalloids to these pristine sites, the trout tissues appear to show no indication of deleterious trends.