Cutthroat Trout Research Articles

Targeting Aggregations of Telemetered Lake Trout to Increase Gillnetting Suppression Efficacy

AbstractConserving Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri by suppressing invasive Lake Trout Salvelinus namaycush in Yellowstone Lake is a high priority for Yellowstone National Park resource managers. Here, we tested whether targeting telemetered Lake Trout could increase the efficacy of Lake Trout suppression by gill netting. Mobile acoustic tracking surveys were performed to identify aggregations of tagged Lake Trout in summer (June–August) 2017. Lake Trout aggregations were relayed daily to suppression crews by phone, radio, or text and a printed map. Suppression crews set 30 large‐mesh gill nets targeting telemetered Lake Trout aggregations (target treatment) and 124 large‐mesh gill nets not targeting telemetered aggregations (nontarget treatment). Mean loge(CPUE) was higher for the target treatment (0.37; 95% credible interval [CRI] = 0.08–0.65) than for the nontarget treatment (−0.37; 95% CRI = −0.51 to −0.21). Mean of the target treatment was higher than the mean of the nontarget treatment for over 99% of the 1,000 draws from the joint posterior distribution. Because of telemetry costs, mean CPUE per US$10,000 spent was similar between the target treatment (0.20; 95% CRI = 0.15–0.26) and the nontarget treatment (0.15; 95% CRI = 0.13–0.17). Telemetry is an effective strategy for improving Lake Trout CPUE, which corresponds to an increased efficiency in the Lake Trout suppression program.

The influence of multiyear drought and associated reduction in tributary connectivity on an adfluvial fish species

AbstractMultiyear drought is projected to increase in frequency and duration in arid and semiarid regions across the world, threatening native species and ecosystem function. The effects of multiyear drought are often exacerbated by human water use, which manifest in reduced lake elevation, reduced stream discharge, and disruption in hydrologic connectivity for aquatic species. Here, we demonstrate that drought‐driven decreasing lake levels reduce the connectivity between tributaries and lakes by creating an elevation‐explicit tributary distance map. We combined long‐term fish catch data and a lake elevation time‐series with our elevation‐explicit tributary distance map to test whether population demographics are related to drought‐driven changes in tributary connectivity. We surveyed the littoral zone of Bear Lake, Utah and Idaho, USA, from full pool to a depth of >12 m, totalling 94.86 km2 surveyed. As lake elevation decreased from full pool to the lowest historical elevation, tributary channel distance increased by 3.5 km. Bear Lake Bonneville Cutthroat Trout (Oncorhynchus clarkii utah) catch per unit effort was strongly negatively correlated with tributary channel distance, though individual growth rates were not influenced by drought metrics. We predicted the response of age 0 to age 10 cutthroat under high and low cobble availability, estimating up to a 61% decline in cutthroat CPUE when tributary channel distance is at its maximum historical value. Our study provided a unique opportunity to identify quantitative linkages between climate‐driven changes in habitat connectivity and an ecologically important endemic sportfish, expanding our understanding of potential pathways through which climate change may affect lentic ecosystems and fishes.

Anadromous Coastal Cutthroat Trout (Oncorhynchus clarkii clarkii) as a Host for Argulus pugettensis (Crustacea, Branchiura): Parasite Prevalence, Intensity and Distribution

Coastal cutthroat trout [Oncorhynchus clarkii clarkii (Richardson, 1836)] from the marine waters of Puget Sound, WA, was documented as a new host for the ectoparasite Argulus pugettensis (Dana, 1852). The prevalence of A. pugettensis was 66% (49 of 74) on cutthroat trout and 0% (0 of 55) on coho salmon [O. kisutch (Walbaum, 1792)] collected during the winter of 2017/2018. Infestations occurred most frequently on the dorsal surface, with intensities ranging from 1 to 26 argulids per fish (mean intensity 3.94 ± 4.93 S.D.). In contrast, the prevalence of the common salmon louse [Lepeophtheirus salmonis (Krøyer, 1837)] was 72% for cutthroat trout and 31% for coho salmon. Relative to other native salmonids, little is known regarding the status, ecology and threats for coastal cutthroat trout. New information reported here is a first step in understanding the relationship between this wild, native trout and infestations by parasitic sea lice and should be followed by future studies aimed to identify population level consequences.

Combined effects of early snowmelt and climate warming on mountain lake temperatures and fish energetics

ABSTRACT Mountain regions are experiencing some of the highest air temperature increases and ice cover decreases. However, few studies have examined the effects of climate warming and earlier snowmelt on mountain lake thermal characteristics and energetic implications for fish. We assessed potential climate-induced thermal changes and energetic consequences for cutthroat trout (Oncorhynchus clarkii spp.) and brook trout (Salvelinus fontinalis) in the Southern Rocky Mountains, United States. We found that summer growing degree days increased by an average 21 percent with 2°C air warming and 43 percent with 5°C air warming. But earlier snowmelt increased growing degree days by an average 48 percent. The average maintenance ration with 2°C and 5°C warming increased respectively by 13.8 and 21.9 percent for cutthroat trout and 23.8 and 37.4 percent for brook trout. The average increase in food required with earlier snowmelt was 43.4 percent for cutthroat trout and 52.3 percent for brook trout. Thus, earlier snowmelt can have a greater effect on fish energy requirements than a 5°C rise in air temperatures. Snowmelt recession together with a 5°C air temperature rise could more than double food requirements for fish to maintain constant body weight. If lake productivity increases with these climatic changes, then trout growth could improve; otherwise, energetic demands may result in lower fish growth.

Warmer temperatures interact with salinity to weaken physiological facilitation to stress in freshwater fishes.

Management of stressors requires an understanding of how multiple stressors interact, how different species respond to those interactions and the underlying mechanisms driving observed patterns in species’ responses. Salinization and rising temperatures are two pertinent stressors predicted to intensify in freshwater ecosystems, posing concern for how susceptible organisms achieve and maintain homeostasis (i.e. allostasis). Here, glucocorticoid hormones (e.g. cortisol), responsible for mobilizing energy (e.g. glucose) to relevant physiological processes for the duration of stressors, are liable to vary in response to the duration and severity of salinization and temperature rises. With field and laboratory studies, we evaluated how both salinity and temperature influence basal and stress-reactive cortisol and glucose levels in age 1+ mottled sculpin (Cottus bairdii), mountain sucker (Catostomus platyrhynchus) and Colorado River cutthroat trout (Oncorhynchus clarki pleuriticus). We found that temperature generally had the greatest effect on cortisol and glucose concentrations and the effect of salinity was often temperature dependent. We also found that when individuals were chronically exposed to higher salinities, baseline concentrations of cortisol and glucose usually declined as salinity increased. Reductions in baseline concentrations facilitated stronger stress reactivity for cortisol and glucose when exposed to additional stressors, which weakened as temperatures increased. Controlled temperatures near the species’ thermal maxima became the overriding factor regulating fish physiology, resulting in inhibitory responses. With projected increases in freshwater salinization and temperatures, efforts to reduce the negative effects of increasing temperatures (i.e. increased refuge habitats and riparian cover) could moderate the inhibitory effects of temperature-dependent effects of salinization for freshwater fishes.

Dynamic Habitat Disturbance and Ecological Resilience (DyHDER): modeling population responses to habitat condition

AbstractUnderstanding how populations respond to spatially heterogeneous habitat disturbance is as critical to conservation as it is challenging. Here, we present a new, free, and open‐source metapopulation model: Dynamic Habitat Disturbance and Ecological Resilience (DyHDER), which incorporates subpopulation habitat condition and connectivity into a population viability analysis framework. Modeling temporally dynamic and spatially explicit habitat disturbance of varying magnitude and duration is accomplished through the use of habitat time‐series data and a mechanistic approach to adjusting subpopulation vital rates. Additionally, DyHDERuses a probabilistic dispersal model driven by site‐specific habitat suitability, density dependence, and directionally dependent connectivity. In the first application of DyHDER, we explore how fragmentation and projected climate change are predicted to impact a well‐studied Bonneville cutthroat trout metapopulation in the Logan River (Utah,USA). The DyHDERmodel predicts which subpopulations are most susceptible to disturbance, as well as the potential interactions between stressors. Further, the model predicts how populations may be expected to redistribute following disturbance. This information is valuable to conservationists and managers faced with protecting populations of conservation concern across landscapes undergoing changing disturbance regimes. The DyHDERmodel provides a valuable and generalizable new tool to explore metapopulation resilience to spatially and temporally dynamic stressors for a diverse range of taxa and ecosystems.

Threats to at‐risk salmonids of the Canadian Rocky Mountain region

AbstractTrout and charr, members of the salmonid family, have high conservation value but are also susceptible to anthropogenic threats in part due to the specificity of their habitat requirements. Understanding historical and future threats facing these species is necessary to promote their recovery. Of freshwater trout and charr in the Canadian Rocky Mountain region, westslope cutthroat trout (Oncorhynchus clarkii lewisi), bull trout (Salvelinus confluentus; a charr species) and Athabasca rainbow trout (Oncorhynchus mykiss) are of conservation concern. And indeed, range contractions and declining populations are evident throughout much of their ranges. Range contraction was most evident in the southern Alberta designatable unit (DU) of westslope cutthroat trout. Diminished populations were also evident in the downstream watersheds of the Alberta bull trout range, and throughout the Athabasca rainbow trout range. We assessed historical and future threats to evaluate the relative importance of individual threats to each DU and compare their impact among species. Individual threats fall into the broad categories of angling, non‐native species and genes, habitat loss and alteration, and climate change. Severity of each threat varies by DU and reflects the interaction between species’ biology and the location of the DU. Severity of threats facing each DU has changed over time, reflecting extirpation of native populations, changes in management and industry best practices, expansion of non‐native species and progressing climate change. The overall threat impact for each DU indicates a high probability of substantial and continuing declines and calls for immediate action.

Organic Pellet Decomposition Induces Mortality of Lake Trout Embryos in Yellowstone Lake

AbstractYellowstone Lake is the site of actions to suppress invasive Lake Trout Salvelinus namaycush and restore native Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri and natural ecosystem function. Although gill netting is effective (Lake Trout λ ≤ 0.6 from 2012 through 2018), the effort costs more than US$2 million annually and only targets Lake Trout age 2 and older. To increase suppression efficiency, we developed an alternative method using organic (soy and wheat) pellets to increase mortality of Lake Trout embryos on spawning sites. Decomposition of pellets during two in situ experiments caused dissolved oxygen (DO) concentrations to temporarily decline to lethal levels (<3.4 mg/L) within days of application. Embryo mortalities during the first exposure period (days 1–6 following application) were high at all treatment levels (1.75–28.0 kg/m2) at the substrate surface and within interstices 20 cm below the surface, varying from 97 ± 1.8% (mean ± SE) to 100 ± 0.0%, but may have been enhanced by a handling effect (exposure to sunlight). Embryo mortalities during the second exposure period (days 11–22) were highest 20 cm below the surface, varying from 78 ± 9.7% to 100 ± 0.0%. Almost all (98 ± 3.1%) Lake Trout embryos died after exposure to DO < 3.4 mg/L for >200 h during the second period. Pellets caused lethal DO for several weeks below the substrate surface, despite largely dissolving and dissipating from the surface of treated areas by day 39. Broad‐scale application of pellets at 1.75 kg/m2 following the spawning period in autumn may reduce Lake Trout recruitment and enhance population suppression because the area of 14 verified spawning sites is only 11.4 ha (0.03% of lake surface area). Pellet application may be useful in other similar systems as part of an integrated pest management approach targeting multiple life stages of invasive freshwater fish.

Life history structure of westslope cutthroat trout: Inferences from otolith microchemistry

Life history diversity is important for population stability and is dependent on connectivity to habitat that supports all life stages and life history strategies for a species. Westslope Cutthroat Trout Oncorhynchus clarkii lewisi (WCT) exhibit plasticity in life history strategies in response to environmental variability, but fisheries managers have been challenged with evaluating the life history structure of WCT populations. The goals of this research were to use strontium isotopes (i.e., 87Sr/86Sr) derived from ambient water and sagittal otoliths to assess spatial variability and describe the life history structure of WCT. Water samples (n = 49) and WCT (n = 571) sagittal otoliths were collected throughout the Coeur d’Alene Lake basin in Idaho and analyzed for Sr isotopes. Model-based discriminant function analysis was used to assign WCT to natal tributaries and to infer maternal origins. Life history structure was inferred from maternal signatures and indicated that fluvial (68% of all fish), resident (27%), and adfluvial (5%) life history strategies were present. Connectivity in lotic systems and from lotic to lentic environments supports WCT life history diversity and contributes to a broad distribution of the species.

Return of a Giant: Coordinated Conservation Leads to the First Wild Reproduction of Lahontan Cutthroat Trout in the Truckee River in Nearly a Century

Many freshwater fish populations have been greatly reduced, with particular loss of migratory fishes. Recovering depleted populations is challenging as threats are often plentiful and complex, especially in arid environments where demands for water resources are high. Here, we describe how a collaborative, multifaceted approach has spurred natural reproduction—a major step towards Lahontan Cutthroat Trout Oncorhynchus clarkii henshawi (LCT) recovery in Pyramid Lake and the Truckee River, Nevada, once home to one of largest freshwater salmonids in North America. The factors limiting LCT were immense, including habitat fragmentation, degradation, and non‐native species attributes common in the declines of native salmonids. Yet for the first time in over 80 years and each year since 2014, adfluvial LCT have spawned in the lower Truckee River, resulting in the production of tens of thousands of young‐of‐year. The progress and positive trajectory towards recovery were driven by a holistic view of the Truckee River watershed beginning in the early 1990's that envisioned bringing numerous conservation building blocks together to expedite the conservation and recovery for the listed fishes of Pyramid Lake. Although additional challenges remain, the LCT recovery program in the Truckee River basin provides a template for the conservation of imperiled fishes.

A mechanistic understanding of ecological responses to land‐use change in headwater streams

AbstractAnthropogenic activities, such as oil and natural gas development (ONGD), have significantly altered the landscape. It is often challenging to identify the mechanistic processes underlying ecological responses to land‐use change (LUC). In aquatic ecosystems, alterations to habitat and food availability and water quality associated with increased LUC are key mechanistic pathways that deserve management consideration. We used structural equation modeling to evaluate how LUC associated with ONGD could influence macroinvertebrate and fish across 40 sites in six headwater streams in the Wyoming Range of the Upper Green River Basin, Wyoming. The most important mechanistic pathway varied, but responses were frequently driven by a direct effect of LUC or related to changes in food availability and water quality. Habitat complexity was the least important mechanistic pathway in our models. Our results also highlight that responses may reflect an organism's degree of habitat or resource specialization and/or sensitivity to changes in water quality. Habitat pathways were more important for habitat specialists (e.g., Mottled Sculpin, Cottus bairdii), food pathways were more important for food specialists (e.g., Colorado River Cutthroat Trout, Oncorhynchus clarki pleuriticus; Mountain Sucker, Catostomus platyrhynchus), and sensitivity to increased salinity was important for intolerant species (e.g., O. clarki, C. bairdii, and predatory macroinvertebrates). Continued identification of the specific mechanisms underlying species’ responses to increased LUC will aid in the conservation of ecologically and economically important species.

Swimming Performance of Rainbow Trout and Westslope Cutthroat Trout in an Open-Channel Flume

AbstractWe used an open-channel flume to characterize the swimming performance of Rainbow Trout Oncorhynchus mykiss and Westslope Cutthroat Trout Oncorhynchus clarki lewisi ranging nominally in fork length from 15 to 30 cm. With an open-channel flume, we observed volitional swim performance of wild-caught Rainbow Trout and Westslope Cutthroat Trout; the fish were not coerced, prodded, or spooked into action. We also observed the maximum short-duration swim speed of the fish, providing important effective leap or velocity challenge information for the design of intentional barriers. We conducted the experiment with a consistently low water velocity challenge and characterized swim speeds by using weighted least-squares regression, revealing no evidence of a difference in swim speeds between the two species. We estimated the overall average swim speed for Rainbow Trout to be 0.84 m/s (SE = 0.02), with a 95% confidence interval of 0.79–0.89 m/s, and that for Westslope Cutthroat Trout to be 0.84 m/s (SE = 0.03), with a 95% confidence interval of 0.78–0.90 m/s. The maximum swim speeds observed were 2.72 m/s for Rainbow Trout and 3.55 m/s for Westslope Cutthroat Trout. The project results provide new information on the swimming ability of wild Rainbow Trout and Westslope Cutthroat Trout that can be used to improve fish passage or barrier design.

Compensatory growth offsets poor condition in native trout populations

Abstract Compensatory growth—when individuals in poor condition grow rapidly to catch up to conspecifics—may be a mechanism that allows individuals to tolerate stressful environmental conditions, both abiotic and biotic. This phenomenon has been documented fairly widely in laboratory and field experiments, but evidence for compensatory growth in the wild is scarce. Cutthroat trout (Oncorhynchus clarkii subsp.) are cold‐water specialists that inhabit montane streams in western North America where seasonal conditions can be harsh and growth rates vary greatly among seasons. Understanding if individuals compensate for periods of reduced growth and body condition will improve understanding of the requirements of fish throughout their life‐cycle and across freshwater habitats. We quantified compensatory growth of juvenile cutthroat trout using extensive mark–recapture data from 11 stream populations (1,125 individuals) and two subspecies inhabiting a wide range of ecological settings in the northern Rocky Mountains, U.S.A. Our objectives were to determine how growth was linked across seasons and whether individuals behaviourally compensated for depressed body condition via emigration. Fish in relatively poor condition consistently demonstrated compensatory growth in mass during subsequent seasons. In contrast, fish in relatively better condition responded with positive growth in length during the summer signalling these fish may be better suited to headwater environments; no compensatory growth in length was found during the winter. Furthermore, there was no evidence that individual condition mediated migration tendencies of fish to seek more favourable habitat. Across a wide range of environmental conditions, we found consistent empirical support for compensatory growth in mass in the wild. A critical next step is to quantify how changing abiotic and biotic conditions influence the ability of stream fishes to compensate for locally or seasonally challenging conditions, thereby affecting long‐term resiliency, viability, and adaptation in the face of changing environmental conditions.

Application of multiple-population viability analysis to evaluate species recovery alternatives.

Population viability analysis (PVA) is a powerful conservation tool, but it remains impractical for many species, particularly species with multiple, broadly distributed populations for which collecting suitable data can be challenging. A recently developed method of multiple-population viability analysis (MPVA), however, addresses many limitations of traditional PVA. We built on previous development of MPVA for Lahontan cutthroat trout (LCT) (Oncorhynchus clarkii henshawi), a species listed under the U.S. Endangered Species Act, that is distributed broadly across habitat fragments in the Great Basin (U.S.A.). We simulated potential management scenarios and assessed their effects on population sizes and extinction risks in 211 streams, where LCT exist or may be reintroduced. Conservation populations (those managed for recovery) tended to have lower extinction risks than nonconservation populations (mean = 19.8% vs. 52.7%), but not always. Active management or reprioritization may be warranted in some cases. Eliminating non-native trout had a strong positive effect on overall carrying capacities for LCT populations but often did not translate into lower extinction risks unless simulations also reduced associated stochasticity (to the mean for populations without non-native trout). Sixty fish or 5-10 fish/km was the minimum reintroduction number and density, respectively, that provided near-maximum reintroduction success. This modeling framework provided crucial insights and empirical justification for conservation planning and specific adaptive management actions for this threatened species. More broadly, MPVA is applicable to a wide range of species exhibiting geographic rarity and limited availability of abundance data and greatly extends the potential use of empirical PVA for conservation assessment and planning.

Effects of Air Exposure on Survival of Yellowstone Cutthroat Trout Angled from a Stream with Warm Water Temperatures

AbstractWe evaluated the effects of air exposure on Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri caught and released in a cold-water stream with elevated water temperatures (i.e., > 14°C) in southeastern Idaho. Anglers caught fish in a 2.3-km section of Fall Creek, Idaho, during August 2018. Sampled fish remained underwater while we measured and then tagged them with T-bar anchor tags. We exposed fish to air for 0, 30, or 60 s and then released them at the point of capture. We continuously monitored temperature during the study period. Water temperatures during the study varied from 10.0 to 19.7°C and averaged 14.9°C (SE = 0.08). In total, anglers caught 161 Yellowstone Cutthroat Trout over 10 d. Of those fish, we did not expose 54 to air; we exposed 54 to air for 30 s, and 53 for 60 s. We used electrofishing to recapture tagged fish and estimate relative survival. Relative survival was highest for fish exposed to air for 60 s (0.40 [SE = 0.25]) followed by 0 s (0.35 [SE = 0.25]) and 30 s (0.30 [SE = 0.27]), but differences were not statistically significant. Results from this study are consistent with other air-exposure studies suggesting that air exposure of 60 s or less is not likely a concern in Yellowstone Cutthroat Trout fisheries. Releasing fish as quickly as possible is always encouraged, but management regulations restricting air exposure seem unnecessary given the collective body of field-based research on air exposure. Nevertheless, similar studies on other systems and species are warranted.

Response of juvenile salmonids to large wood placement in Columbia River tributaries

Placement of large wood is a common stream restoration technique in western North America and increasingly in other parts of the world. Considerable information exists on response of anadromous salmonids in small (< 15 m bankfull width) coastal streams of western North America, but limited information exists on anadromous fish response to wood placement in larger streams or in the more arid interior Columbia River Basin. An extensive post-treatment design was used to sample 29 large wood placement projects to determine their physical and biological effectiveness. We sampled paired treatment and control reaches that were approximately 20 times longer than bankfull width and quantified fish abundance and habitat attributes during summer. Proportion of pool area, number of pools, large wood (LW), and pool forming large wood were significantly higher in paired treatment than control reaches. Juvenile Chinook salmon (Oncorhynchus tshawytscha), steelhead (O. mykiss) coho salmon (O. kisutch), and cutthroat trout (O. clarkii) abundances were significantly higher in treatment than control reaches, but no significant responses were detected for mountain whitefish (Prosopium williamsoni) or dace (Rhinichthys spp.). Chinook and coho responses were positively correlated with LW and pool area suggesting wood placement produced reach-scale increases of juvenile salmonid abundance.

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.

Variable hybridization outcomes in trout are predicted by historical fish stocking and environmental context.

Hybridization can profoundly affect the genomic composition and phenotypes of closely related species, and provides an opportunity to identify mechanisms that maintain reproductive isolation between species. Recent evidence suggests that hybridization outcomes within a species pair can vary across locations. However, we still do not know how variable outcomes of hybridization are across geographic replicates, and what mechanisms drive that variation. In this study, we described hybridization outcomes across 27 locations in the North Fork Shoshone River basin (Wyoming, USA) where native Yellowstone cutthroat trout and introduced rainbow trout co-occur. We used genomic data and hierarchical Bayesian models to precisely identify ancestry of hybrid individuals. Hybridization outcomes varied across locations. In some locations, only rainbow trout and advanced backcrossed hybrids towards rainbow trout were present, while trout in other locations had a broader range of ancestry, including both parental species and first-generation hybrids. Later-generation intermediate hybrids were rare relative to backcrossed hybrids and rainbow trout individuals. Using an individual-based simulation, we found that outcomes of hybridization in the North Fork Shoshone River basin deviate substantially from what we would expect under null expectations of random mating and no selection against hybrids. Since this deviation implies that some mechanisms of reproductive isolation function to maintain parental taxa and a diversity of hybrid types, we then modelled hybridization outcomes as a function of environmental variables and stocking history that are likely to affect prezygotic barriers to hybridization. Variables associated with history of fish stocking were the strongest predictors of hybridization outcomes, followed by environmental variables that might affect overlap in spawning time and location.

Similar individual-level responses to stressors have different population-level consequences among closely related species of trout

In this paper, we applied an individual-based model to study the population-level impacts of sub-lethal stressors affecting the metabolic pathways of three closely related trout species: Oncorhynchus mykiss (rainbow trout, RT), Salmo trutta (brown trout, BT) and Oncorhynchus calrki stomias (greenback cutthroat trout, GCT). Both RT and BT are well-studied species, and the former is widely used as a standard cold-water test species. These species are known to outcompete GCT, which is listed as threatened under the US Endangered Species Act. Our goal was to understand the extent to which stressor effects, which are often measured at the individual level, on taxonomically-related (i.e., surrogate) species can be informative of impacts on population dynamics in species that cannot be tested (e.g., listed species). When comparing stressor effects among species, we found that individual-level responses to each stressor were qualitatively comparable. Individual lengths and number of eggs decreased by similar percentages with respect to baseline, even if small quantitative differences were present depending on the physiological mode of action of the stressor. Individual-level effects in GCT were slightly greater when ingestion efficiency decreased, whereas effects in GCT and RT were greater when maintenance costs increased, and effects in BT were slightly greater when costs of growth increased. In contrast, results at the population level differed markedly among species with GCT the most impacted by sub-lethal stress effects on individual metabolism. Our findings suggest that using non-listed species to assess the risks of stressors to listed species populations may be misleading, even if the species are closely related and show similar individual-level responses. Mechanistic population models that incorporate species life history and ecology can improve inter-species extrapolation of stressor effects.

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.