Native Cutthroat Trout Research Articles

Stream Restoration Effects on Habitat and Abundance of Native Cutthroat Trout

ABSTRACTRestoration practitioners spend more than $1 billion each year restoring US rivers and streams but commit comparatively few resources to evaluating project effectiveness. Meanwhile, monitoring and disseminating the outcomes of restoration projects remains our best opportunity to learn from past successes and failures and to, ultimately, improve the cost effectiveness of restoration. We monitored the physical and biological outcomes of a stream restoration project in the Rocky Mountains, the goals of which were to improve habitat for and productivity of native Colorado River Cutthroat Trout (CRCT) and the scope of which included three contiguous reaches under different restoration treatments. Moreover, we evaluated the efficacy of the restoration project relative to its stated goals and objectives. To test for restoration effects on physical and biological indicators we coupled a before‐after, control‐impact (BACI) study design with (generalized) linear mixed models. Over the course of 7 years, we detected restoration‐related increases in floodplain connectivity, streambank stability, and riparian shrub cover, as well as decreases in summer stream temperature. Despite measured improvements in native trout habitat, we detected no evidence of a restoration effect on density or biomass of age‐1 and older CRCT. Although our study was somewhat limited in scope, our findings contribute toward a relatively small body of work on monitoring and effectiveness of river restoration.

Local environments, not invasive hybridization, influence cardiac performance of native trout under acute thermal stress.

Climate-induced expansion of invasive hybridization (breeding between invasive and native species) poses a significant threat to the persistence of many native species worldwide. In the northern U.S. Rocky Mountains, hybridization between native cutthroat trout and non-native rainbow trout has increased in recent decades due, in part, to climate-driven increases in water temperature. It has been postulated that invasive hybridization may enhance physiological tolerance to climate-induced thermal stress because laboratory studies indicate that rainbow trout have a higher thermal tolerance than cutthroat trout. Here, we assessed whether invasive hybridization improves cardiac performance response to acute water temperature stress of native wild trout populations. We collected trout from four streams with a wide range of non-native admixture among individuals and with different temperature and streamflow regimes in the upper Flathead River drainage, USA. We measured individual cardiac performance (maximum heart rate, "MaxHR", and temperature at arrhythmia, "ArrTemp") during laboratory trials with increasing water temperatures (10-28°C). Across the study populations, we observed substantial variation in cardiac performance of individual trout when exposed to thermal stress. Notably, we found significant differences in the cardiac response to thermal regimes among native cutthroat trout populations, suggesting the importance of genotype-by-environment interactions in shaping the physiological performance of native cutthroat trout. However, rainbow trout admixture had no significant effect on cardiac performance (MaxHR and ArrTemp) within any of the three populations. Our results indicate that invasive hybridization with a warmer-adapted species does not enhance the cardiac performance of native trout under warming conditions. Maintaining numerous populations across thermally and hydrologically diverse stream environments will be crucial for native trout to adapt and persist in a warming climate.

Saving Walker Lake: A Pleistocene Terminal Lake in the Anthropocene

Abstract: Just to the east of the Sierra Nevada mountain range a series of Pleistocene terminal lakes are present in the Great Basin of the intermountain West between the Rockies and the Sierra. Among them is Walker Lake, fed by the branches of the Walker River that flow eastward from their snow-melt sources in the Sierra. The terminal lakes have no natural outlets. Water is lost from the lakes' surfaces by evaporation during the hot summer months in the high desert. Since the arrival of Euro-Americans in the nineteenth century, the diversion of water for agricultural, industrial, and domestic uses from the Walker River has reduced the level of the lake to a point where it no longer supports a once-thriving fishery of native cutthroat trout. In the past, western Nevada Paiute peoples relied on the lake's fish as a food source. In the twentieth century the inland anadromous trout drew sports fishing enthusiasts. Climate change has caused additional strains on the water supply for the lake to the point where many have declared Walker Lake a dead lake with only shrinking shorelines and increased salinization in its future. In response, Nevada's U.S. senator Harry Reid, members of the Walker River Indian Reservation, sport fishing organizations, as well as environmental organizations pushed for legislation from Congress to promote efforts to save the lake in the 1990s. These efforts have produced some promising results.

Dietary overlap and selectivity among mountain steppe river fish in the United States and Mongolia.

Lotic systems in mountain regions have historically provided secure habitat for native fish populations because of their relative isolation from human settlement and lack of upstream disturbances. However, rivers of mountain ecoregions are currently experiencing heightened levels of disturbance due to the introduction of nonnative species impacting endemic fishes in these areas. We compared the fish assemblages and diets of mountain steppe fishes of the stocked rivers in Wyoming with rivers in northern Mongolia where stocking is absent. Using gut content analysis, we quantified the selectivity and diets of fishes collected in these systems. Nonnative species had more generalist diets with lower levels of selectivity than most native species and native species had high levels of dietary specificity and selectivity. High abundances of nonnative species and high levels of dietary overlaps in our Wyoming sites is a cause of concern for native Cutthroat Trout and overall system stability. In contrast, fish assemblages characterizing Mongolia mountain steppe rivers were composed of only native species with diverse diets and higher selectivity values, suggesting low probability for interspecific competition.

Climate change impacts on native cutthroat trout habitat in Colorado streams

AbstractHeadwater streams support vital aquatic habitat yet are vulnerable to changing climate due to their high elevation and small size. Coldwater fish are especially sensitive to the altered streamflow and water temperature regimes during summer low flow periods. Though previous studies have provided insights on how changes in climate and alterations in stream discharge may affect habitat availability for various native cutthroat trout species, suitable physical habitats have not been evaluated under future climate projections for the threatened Greenback Cutthroat Trout (GBCT) native to headwater regions of Colorado, USA. Thus, this study used field data collected from selected headwater streams across the current distribution of GBCT to construct one‐dimensional hydraulic models to evaluate streamflow and physical habitat under four future climate projections. Results illustrate reductions in both predicted streamflow and physical habitat for all future climate projections across study sites. The projected mean summer streamflow shows greater decline (−52% on average) compared to the projected decline in mean August flow (−21% on average). Moreover, sites located at a relative higher elevation with larger substrate and steeper slope were projected to experience more reductions in physical habitat due to streamflow reductions. Specifically, streams with step‐pool morphologies may experience grater changes in available habitat compared to pool‐riffle streams. Future climate change studies related to coldwater fish that examine spatial variation in flow alteration could provide novel data to complement the existing literature on the thermal characteristics. Tailoring reintroduction and management efforts for GBCT to the individual headwater stream with adequate on‐site monitoring could provide a more holistic conservation approach.

High dispersal rates in hybrids drive expansion of maladaptive hybridization

Hybridization between native and invasive species, a major cause of biodiversity loss, can spread rapidly even when hybrids have reduced fitness. This paradox suggests that hybrids have greater dispersal rates than non-hybridized individuals, yet this mechanism has not been empirically tested in animal populations. Here, we test if non-native genetic introgression increases reproductive dispersal using a human-mediated hybrid zone between native cutthroat trout (<i>Oncorhynchus clarkii</i>) and invasive rainbow trout (<i>Oncorhynchus mykiss</i>) in a large and connected river system. We quantified the propensity for individuals to migrate from natal rearing habitats (migrate), reproduce in non-natal habitats (stray), and the joint probability of dispersal as a function of genetic ancestry. Hybrid trout with predominantly non-native rainbow trout ancestry were more likely to migrate as juveniles and to stray as adults. Overall, hybrids with greater than 50% rainbow trout ancestry were 5.7 times more likely to disperse than native or hybrid trout with small amounts of rainbow trout ancestry. Our results show a genetic basis for increased dispersal in hybrids that is likely contributing to the rapid expansion of invasive hybridization between these species. Management actions that decrease the probability of hybrid dispersal may mitigate the harmful effects of invasive hybridization on native biodiversity.

Hybridization decreases native cutthroat trout reproductive fitness.

Examining natural selection in wild populations is challenging, but crucial to understanding many ecological and evolutionary processes. Additionally, in hybridizing populations, natural selection may be an important determinant of the eventual outcome of hybridization. We characterized several components of relative fitness in hybridizing populations of Yellowstone cutthroat trout and rainbow trout in an effort to better understand the prolonged persistence of both parental species despite predictions of extirpation. Thousands of genomic loci enabled precise quantification of hybrid status in adult and subsequent juvenile generations; a subset of those data also identified parent-offspring relationships. We used linear models and simulations to assess the effects of ancestry on reproductive output and mate choice decisions. We found a relatively low number of late-stage (F3+) hybrids and an excess of F2 juveniles relative to the adult generation in one location, which suggests the presence of hybrid breakdown decreasing the fitness of F2+ hybrids later in life. Assessments of reproductive output showed that Yellowstone cutthroat trout are more likely to successfully reproduce and produce slightly more offspring than their rainbow trout and hybrid counterparts. Mate choice appeared to be largely random, though we did find statistical support for slight female preference for males of similar ancestry. Together, these results show that native Yellowstone cutthroat trout are able to outperform rainbow trout in terms of reproduction and suggest that management action to exclude rainbow trout from spawning locations may bolster the now-rare Yellowstone cutthroat trout.

Evaluating a Permeable‐Style Fish Barrier to Prevent the Upstream Recolonization of Nonnative Trout during an Eradication Project

AbstractIn the western United States, the introduction of nonnative Brook Trout Salvelinus fontinalis has had deleterious effects on native Cutthroat Trout Oncorhynchus clarkii subspecies, and often their populations must be controlled or removed if Cutthroat Trout are to persist. Man‐made fish migration barriers are integral components of stream restoration projects, and typical designs are intended to be permanent and to funnel water over a drop and onto a splash pad. The water elevation drop and the splash pad, which prevents a jumping pool, are mechanisms intended to prevent fish from navigating upstream past the structure. In this study, we installed a temporary, permeable‐style barrier that was designed to have water flow onto and through a horizontal, expanded steel panel. When fish attempted to move upstream, they would follow the water flow and end up underneath the barrier, with no opportunity to move further. The permeable barrier proved effective in preventing the upstream movement of Brook Trout, and additional advantages to this barrier style included straightforward permitting, low cost, ease of installation, and quick removal. The main disadvantage was the periodic cleaning (average of 7 trips/year) that was required to maintain its intended function. The permeable barrier separated stream reaches for 5 years, allowing biologists to (1) eradicate Brook Trout upstream through multiple‐pass backpack electrofishing while preserving native Yellowstone Cutthroat Trout O. clarkii bouvieri and (2) apply a successful piscicide treatment below the barrier.

Diet overlap among non-native trout species and native cutthroat Trout (Oncorhynchus clarkii) in two U.S. ecoregions.

The invasion of freshwater ecosystems by non‐native species can constitute a significant threat to native species and ecosystem health. Non‐native trouts have long been stocked in areas where native trouts occur and have negatively impacted native trouts through predation, competition, and hybridization. This study encompassed two seasons of sampling efforts across two ecoregions of the western United States: The Great Basin in summer 2016 and the Yellowstone River Basin in summer 2017. We found significant dietary overlaps among native and non‐native trouts within the Great Basin and Yellowstone River Basin ecoregions. Three orders of invertebrates (Ephemeroptera, Trichoptera, and Diptera) composed the majority of stomach contents and were responsible for driving the observed patterns. Great Basin trout had higher body conditions (k), and non‐native Great Basin trout had higher gut fullness values than Yellowstone River Basin trout, indicating a possible limitation of food in the Yellowstone River Basin. Native fishes were the least abundant and had the lowest body condition in each ecoregion. These findings may indicate a negative impact on native trouts by non‐native trouts. We recommend additional monitoring of native and non‐native trout diets, regular invertebrate surveys to identify the availability of diet items, and reconsidering stocking efforts that can result in overlap of non‐native fishes with native cutthroat trout.

Diets of Longnose Sucker in Yellowstone Lake, Yellowstone National Park, USA

Invasive species introduction and expansion is the second greatest threat to global biodiversity decline after habitat degradation. Introduced in the 1930s, the benthivorous Longnose Sucker (Catostomus catostomus) became established in Yellowstone Lake, Wyoming, USA, and used tributary streams for spawning. With this introduction, concerns were raised regarding their possible competition for food resources with native adfluvial Yellowstone Cutthroat Trout (Oncorhynchus clarkii bouvieri). Additionally, insufficient literature exists on Longnose Sucker feeding habits throughout their range, and there has been no comprehensive study of Longnose Sucker diet in Yellowstone Lake. The need exists for understanding the community ecology and food web dynamics in Yellowstone Lake, especially as non-native Lake Trout (Salvelinus namaycush) have caused declines in Yellowstone Cutthroat Trout through predation. The objectives of this study were to examine possible size-specific shifts in feeding habits, evaluate feeding strategy, and compare historical and contemporary diet data of Longnose Suckers in Yellowstone Lake. Diet data collected during summer of 2018 were analyzed by length-class to test for size-specific diet shifts. As Longnose Sucker length increased, copepods (Diacyclops bicuspidatus thomasi, Leptodiaptomus ashlandi or Hesperodiaptomus shoshone) decreased in proportion by weight. In contrast, dipterans (Chironomidae) and amphipods (Hyalella spp. or Gammarus spp.) varied in proportion by weight in the diet across length classes. We assessed the feeding strategy by evaluating the relationship between prey-specific abundance and percent frequency of occurrence. This assessment indicates that Longnose Suckers have a heterogeneous diet and generalized feeding strategy as all prey items had a prey-specific abundance value of <50%. Diet composition differed significantly between historical and contemporary samples, likely related to the differences in sampling locations and possibly due to a Lake Trout-induced trophic cascade. This study established the diet composition and feeding habits of Longnose Suckers residing in Yellowstone Lake, thus, expanding our knowledge of Longnose Sucker feeding patterns and ecology.

Leveraging public harvest to reduce invasive hybridization in Yellowstone National Park: field identification and harvest of cutthroat\xa0\xd7\xa0rainbow trout hybrids

Leveraging public harvest can be a cost-effective invasive species management tool, but target taxa must be correctly identified and removed at rates that achieve biological objectives. We explored the potential role of recreational anglers to curtail expanding hybridization between invasive rainbow trout (Oncorhynchus mykiss; RT) and native Yellowstone cutthroat trout (O. clarkii bouvieri; YCT) in the Lamar River watershed in Yellowstone National Park. We sought to (1) develop a hybrid identification key that could be used by anglers and (2) estimate angler participation, catch, and potential exploitation rates. We assessed seven morphological features of trout in the field (n = 251, 15 locations) and collected fin clips to estimate RT ancestry proportion using genetic analysis. An identification key was built using recursive partitioning to objectively distinguish YCT from RT and hybrids. A single-choice dichotomous key (white pelvic fin tip present/absent) correctly classified 93% of fish as native (YCT) or containing RT ancestry (RT or hybrid). Success increased to 97% when a second criterion was added (head spot count ≥ 6). Using angler surveys (2013–2017), we estimated that 10,000 anglers catch 50,000 trout annually. In a popular road-accessible area, most trout are probably caught and released ~ 5 times each year. The combination of high angler participation, substantial annual catch, and an accurate and easy to use identification method indicate that leveraging public harvest is a promising management tool. Invasive hybridization is a global conservation issue threatening many native taxa; this case study highlights some factors for resource managers to consider prior to implementing public harvest regulations and the benefits of standardized keys to distinguish hybrids in the field.

Killing One Trout to Save Another: A Hegemonic Political Ecology with Its Biopolitical Basis in Yellowstone’s Native Fish Conservation Plan

Yellowstone National Park implements a native fish conservation plan to control translocated trout species competing with native cutthroat trout. Under the plan, millions of lake trout are removed from Yellowstone Lake and park streams are poisoned to eradicate other translocated trout species. Killing trout introduced by officials decades earlier is a significant reversal in fisheries management. This article employs a biopolitical analysis of data derived qualitatively from semistructured interviews, relevant documents, and participant observation in the field. Results reveal a diversity of sometimes antagonistic stakeholders subjecting themselves to the practice of killing formerly revered fish based on truth claims circulating around the crisis-based conservation of native fish populations and ecosystem health. Stakeholders support logics of killing in relation to their varied engagements with fish, leading to a multiplicity of biopolitical motivations endorsing nativism in Yellowstone. From this multiplicity emerges a conservation hegemon where power over life and death is enacted in many corners of the biosocial collective but trends toward dominant knowledge and practice vested in the National Park Service. Theorizing killing for conservation as a hegemonic political ecology reconceptualizes the place of species outside of problematic dichotomies like native and nonnative. Instead, species are either hegemonic or counterhegemonic based on their lively positions relative to the conservation hegemon, leading to more honestly articulated motivations behind resource management goals associated with the practice of killing for conservation.

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.

A mechanistic assessment of seasonal microhabitat selection by drift-feeding rainbow trout Oncorhynchus mykiss in a Southwestern headwater stream

The positioning of fishes within a riverscape is dependent on the proximity of complementary habitats. In this study, foraging and non-foraging habitat were quantified monthly over an entire year for a rainbow trout (Oncorhynchus mykiss) population in an isolated, headwater stream in southcentral New Mexico. The stream follows a seasonal thermal and hydrologic pattern typical for a Southwestern stream and was deemed suitable for re-introduction of the native and close relative, Rio Grande cutthroat trout (O. clarkii virginalis). However, uncertainty associated with limited habitat needed to be resolved if repatriation of the native fish was to be successful. Habitat was evaluated using resource selection functions with a mechanistic drift-foraging model to explain trout distributions. Macroinvertebrate drift was strongly season- and temperature-dependent (lower in winter and spring, higher in summer and fall). Models identified stream depth as the most limiting factor for habitat selection across seasons and size-classes. Additionally, positions closer to cover were selected during the winter by smaller size-classes (0, 1, 2), while net energy intake was important during the spring for most size-classes (0, 1, 2, 3). Drift-foraging models identified that 81% of observed trout selected positions that could meet maintenance levels throughout the year. Moreover, 40% of selected habitats could sustain maximum growth. Stream positions occupied by rainbow trout were more energetically profitable than random sites regardless of season or size-class. Larger size-classes (3, 4+) were energetically more limited throughout the year than were smaller size-classes. This research suggests that habitat in the form of deep pools is of paramount importance for rainbow trout or native cutthroat trout.

Legacy introductions and climatic variation explain spatiotemporal patterns of invasive hybridization in a native trout.

Hybridization between invasive and native species, a significant threat to worldwide biodiversity, is predicted to increase due to climate-induced expansions of invasive species. Long-term research and monitoring are crucial for understanding the ecological and evolutionary processes that modulate the effects of invasive species. Using a large, multidecade genetics dataset (N=582 sites, 12,878 individuals) with high-resolution climate predictions and extensive stocking records, we evaluate the spatiotemporal dynamics of hybridization between native cutthroat trout and invasive rainbow trout, the world's most widely introduced invasive fish, across the Northern Rocky Mountains of the United States. Historical effects of stocking and contemporary patterns of climatic variation were strongly related to the spread of hybridization across space and time. The probability of occurrence, extent of, and temporal changes in hybridization increased at sites in close proximity to historical stocking locations with greater rainbow trout propagule pressure, warmer water temperatures, and lower spring precipitation. Although locations with warmer water temperatures were more prone to hybridization, cold sites were not protected from invasion; 58% of hybridized sites had cold mean summer water temperatures (<11°C). Despite cessation of stocking over 40years ago, hybridization increased over time at half (50%) of the locations with long-term data, the vast majority of which (74%) were initially nonhybridized, emphasizing the chronic, negative impacts of human-mediated hybridization. These results show that effects of climate change on biodiversity must be analyzed in the context of historical human impacts that set ecological and evolutionary trajectories.

Vive la résistance: genome-wide selection against introduced alleles in invasive hybrid zones.

Evolutionary and ecological consequences of hybridization between native and invasive species are notoriously complicated because patterns of selection acting on non-native alleles can vary throughout the genome and across environments. Rapid advances in genomics now make it feasible to assess locus-specific and genome-wide patterns of natural selection acting on invasive introgression within and among natural populations occupying diverse environments. We quantified genome-wide patterns of admixture across multiple independent hybrid zones of native westslope cutthroat trout and invasive rainbow trout, the world's most widely introduced fish, by genotyping 339 individuals from 21 populations using 9380 species-diagnostic loci. A significantly greater proportion of the genome appeared to be under selection favouring native cutthroat trout (rather than rainbow trout), and this pattern was pervasive across the genome (detected on most chromosomes). Furthermore, selection against invasive alleles was consistent across populations and environments, even in those where rainbow trout were predicted to have a selective advantage (warm environments). These data corroborate field studies showing that hybrids between these species have lower fitness than the native taxa, and show that these fitness differences are due to selection favouring many native genes distributed widely throughout the genome.

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.

Patterns of hybridization among cutthroat trout and rainbow trout in northern Rocky Mountain streams.

Introgressive hybridization between native and introduced species is a growing conservation concern. For native cutthroat trout and introduced rainbow trout in western North America, this process is thought to lead to the formation of hybrid swarms and the loss of monophyletic evolutionary lineages. Previous studies of this phenomenon, however, indicated that hybrid swarms were rare except when native and introduced forms of cutthroat trout co-occurred. We used a panel of 86 diagnostic, single nucleotide polymorphisms to evaluate the genetic composition of 3865 fish captured in 188 locations on 129 streams distributed across western Montana and northern Idaho. Although introgression was common and only 37% of the sites were occupied solely by parental westslope cutthroat trout, levels of hybridization were generally low. Of the 188 sites sampled, 73% contained ≤5% rainbow trout alleles and 58% had ≤1% rainbow trout alleles. Overall, 72% of specimens were nonadmixed westslope cutthroat trout, and an additional 3.5% were nonadmixed rainbow trout. Samples from seven sites met our criteria for hybrid swarms, that is, an absence of nonadmixed individuals and a random distribution of alleles within the sample; most (6/7) were associated with introgression by Yellowstone cutthroat trout. In streams with multiple sites, upstream locations exhibited less introgression than downstream locations. We conclude that although the widespread introduction of nonnative trout within the historical range of westslope cutthroat trout has increased the incidence of introgression, sites containing nonadmixed populations of this taxon are common and broadly distributed.

Introduced lake trout alter nitrogen cycling beyond Yellowstone Lake

Introduced predators can have large effects on the ecosystem in which they were introduced, but how much these effects extend to other ecosystems beyond the invaded one is less known. We compared how lake trout (Salvelinus namaycush) affected nutrient cycling in an invaded and adjacent ecosystem in Yellowstone National Park, Wyoming, USA. Introduced lake trout in Yellowstone Lake caused the native Yellowstone cutthroat trout (Oncoryhynchus clarkii bouvieri) population to decline. Native cutthroat trout are a dominant animal in the lake and may alter nutrient cycling in both Yellowstone Lake where they reside and in tributary streams used for spawning. We estimated changes in nutrient transport and nutrient uptake in both Yellowstone Lake and Clear Creek, a spawning stream, before and after the invasion of lake trout. Annual area‐specific excretion fluxes from cutthroat trout were nine times higher in Clear Creek compared to Yellowstone Lake when cutthroat trout were abundant. However, fluxes within the lake and stream were similar after cutthroat trout declined. In Yellowstone Lake, zooplankton excretion supplied 86% of ammonium (NH4+) that was taken up, but cutthroat trout only supplied &lt;0.3% after the introduction of lake trout. Conversely, NH4+ excreted by cutthroat trout was likely a major flux in Clear Creek, because NH4+ fluxes from cutthroat trout exceeded watershed export of NH4+ in years when &gt;3000 cutthroat trout spawned. Furthermore, NH4+ excretion fluxes from spawning cutthroat trout in Clear Creek supplied up to 6.1% of the NH4+ demanded by microbes after the introduction of lake trout. However, based on modeled past NH4+ uptake, we estimated that up to 60% of NH4+ excreted by spawning cutthroat trout may have been taken up by stream microbes when cutthroat trout were abundant. Therefore, transported NH4+ from spawning cutthroat trout was likely an integral part of N cycling in tributary streams in the past. By comparing the effects of declining cutthroat trout on two ecosystems, we show that lake trout had a larger effect on N cycling within an adjacent stream ecosystem than the invaded lake ecosystem itself, because the migratory behavior of cutthroat trout concentrated them in spawning streams increasing their effect.

Effects of Hybridization between Nonnative Rainbow Trout and Native Westslope Cutthroat Trout on Fitness‐Related Traits

AbstractHybridization between introduced and native fauna is a risk to native species and may threaten the long‐term persistence of numerous taxa. Rainbow Trout Oncorhynchus mykiss has been one of the most widely introduced species around the globe and often hybridizes with native Cutthroat Trout O. clarkii in the Rocky Mountains. Previous work has shown that hybridization negatively affects reproductive success, but identification of the traits contributing to that reduction has been elusive. In this study, we used a combination of field and laboratory techniques to assess how hybridization with Rainbow Trout affects seven traits during several stages of Westslope Cutthroat Trout development: embryonic survival, ova size, ova energy concentration, sperm motility, juvenile weight, juvenile survival, and burst swimming endurance. Rainbow Trout admixture was correlated with an increase in embryonic survival and ova energy concentration but with a decrease in juvenile weight and burst swimming endurance. These correlations differed from previously observed patterns of reproductive success and likely do not explain the declines in reproductive success associated with admixture. Future investigation of additional, unstudied traits and the use of different environments may shed light on the traits responsible for reproductive success in admixed Cutthroat Trout.