Nonnative Brook Trout Salvelinus Fontinalis Research Articles

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.

Evaluation of management factors affecting the relative success of a brook trout eradication program using YY male fish and electrofishing suppression

Removal of non-native fish populations can be crucial to the conservation of native species, but often presents a complex challenge for managers. The goal of Trojan Y chromosome (TYC) programs is to skew the non-native sex ratio until only males remain, leading to eradication. We present results from a simulation model used to explore effects of alternative management approaches on an in-progress mechanical removal and TYC program to eradicate non-native brook trout (Salvelinus fontinalis). Simulation results indicated that stocking fingerling YY males (∼137 mm) was more effective than stocking catchable-sized YY males (∼230 mm), although questions about intercohort competition warrant further investigation. Increasing the proportion of mature fingerling YY males reduced treatment time by increasing the number of YY male spawners and increasing density-dependent mortality on young, mature wild brook trout. Maximizing the spatial distribution of YY male releases may be crucial to program success but is also dependent upon immediate dispersal movements. Principles derived from our results can be broadly applied to the management of other aquatic invaded systems using TYC programs to eradicate non-native species.

Within-stream phenotypic divergence in head shape of brown trout associated with invasive brook trout

Abstract Competition with a non-native species can lead to morphological changes in native organisms induced by phenotypic plasticity, and by selection against individuals that do not adjust their morphology to the novel selection pressure. The morphological changes in native organisms are often associated with rapid behavioural responses to competition with the invader. However, knowledge of the interaction between the behaviour and morphology of native organisms competing with a non-native species remains scarce. Here, we investigated the effect of competition with non-native brook trout Salvelinus fontinalis on head shape of native brown trout Salmo trutta in a stream system where changes in diet and territorial behaviour of sympatric brown trout have previously been demonstrated. We found that sympatric brown trout had smaller eyes, shorter lower jaws and more terminal mouth than allopatric conspecifics. These differences in head shape were highly repeatable over a period of 12 months. Apparent survival indicated that the selection on head shape of brown trout was weaker in the sympatric than in the allopatric stretch of the stream. The results suggest that these changes reinforce divergences of foraging strategies between the allopatric and sympatric brown trout, which can negatively affect their population dynamics and trophic function in the food-web.

Utilizing environmental DNA for fish eradication effectiveness monitoring in streams

Introductions of non-native fish have the potential to cause substantial economic and ecological losses. In the western United States, non-native brook trout (Salvelinus fontinalis) are widely established and pose a threat to the persistence of bull trout (Salvelinus confluentus), a native char listed as threatened under the US Endangered Species Act, due to competition and hybridization. With brook trout identified as a factor limiting the recovery of ESA-listed bull trout in the Malheur Watershed of eastern Oregon, managers are pursuing brook trout eradication efforts. Key to the eradication efforts is reliable monitoring to ensure success. Traditional monitoring practices have included mechanical sampling methods such as seining, netting, and electrofishing, all of which are labor intensive and limited in reliability. Environmental DNA (eDNA) is genetic material naturally shed by organisms that can be found in bulk environmental samples without isolating individual organisms. Sampling for eDNA can often be done with less time and expense than traditional methods, making it ideal for effectiveness monitoring following fish eradication efforts. This study placed a single fish in a live-car where the species was not otherwise present and used eDNA sampling to infer the likelihood of detection downstream. Results suggest that a single fish can be detected reliably up to 500 m downstream when multiple samples are taken. This eDNA sampling method provides managers with the ability to conduct eradication effectiveness monitoring reliably and rapidly and may also be used to detect new fish invasions as well as rare or cryptic species.

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.

Electrofishing, Snorkel Spearing, and Piscicide Eradicate Brook Trout From a Small, Isolated Bull Trout Population

Abstract In 1992, a sampling crew from a multiagency group discovered a population of Bull Trout Salvelinus confluentus estimated at approximately 50 individuals in Threemile Creek, Oregon threatened with competition and hybridization with nonnative Brook Trout Salvelinus fontinalis. The group implemented Brook Trout and Bull Trout × Brook Trout hybrid removal using multiple techniques to conserve this population of Bull Trout in the Klamath River basin. From 1996 to 2000, backpack electrofishing and night snorkel spearing were used upstream of a culvert barrier to eradicate Brook Trout and hybrid trout from a 3.59-km section of Threemile Creek over 101 total days of effort (274 person-days of effort). Night snorkel spearing removed a lower percentage (7%) of target fish relative to electrofishing (93%), although this technique eliminated the risk of electrofishing injury to Bull Trout. Concurrent with the reduction and subsequent removal of Brook Trout, Bull Trout observations increased over 15-fold from 19 in 1996 to 299 in 2012 and Bull Trout distribution more than doubled, from 1.48 to 3.43 km during that same time. Immediately downstream in a separate 2.3-km section, the group eradicated Brook Trout using backpack electrofishing and rotenone application upstream of two man-made barriers from 2004 to 2010 over 43 total days of effort (150 person-days of effort). By 2016, Bull Trout had expanded their distribution throughout the upper 3.59-km section of Threemile Creek. Effort expended to eradicate Brook Trout was similar to successful eradication in other streams with comparable habitat characteristics and similar treatment length (< 3.0 km). However, similar removal attempts have not been successful in streams with comparable habitat characteristics that had longer treatment length (> 7.0 km). Application of piscicide may be more cost effective than manual techniques to remove Brook Trout when there is no risk to native fish of conservation concern, where target species are allopatric, and when piscicide use is publicly accepted. Brook Trout eradication proved valuable in providing Bull Trout the opportunity for range expansion and increased abundance necessary for recovery in the Klamath River basin.

Simulated Effects of YY‐Male Stocking and Manual Suppression for Eradicating Nonnative Brook Trout Populations

AbstractEradication of nonnative Brook Trout Salvelinus fontinalis populations is difficult to achieve with standard techniques, such as electrofishing removal or piscicides; new approaches are needed. A novel concept is to stock “supermale” hatchery fish with wild conspecifics. Supermales (MYY) have two Y‐chromosomes, resulting in offspring that are all males; over time, successful supermale reproduction could eradicate the wild population. We constructed an age‐structured stochastic model to investigate the effects of manually suppressing wild fish and stocking MYY fingerlings on the long‐term viability of hypothetical nonnative Brook Trout populations. In streams, an annual stocking rate of supermales equivalent to 50% of wild age‐0 Brook Trout density combined with an annual selective suppression rate equivalent to 50% of wild Brook Trout density resulted in a time to extirpation of only 2–4 years if supermale fitness was equivalent to wild male fitness. However, time to extirpation in streams was 5–15 years if supermale fitness was 80% lower than wild male fitness. In alpine lakes, higher supermale stocking rates and nonselective gillnetting were required to eradicate Brook Trout populations. If supermales were assumed to be as fit as wild males, however, any supermale stocking rate greater than 49% in alpine lakes or 60% in streams achieved eradication in 10 years or less, regardless of the suppression rate. Because manual suppression and the stocking of MYY fingerlings can readily be conducted at the levels assumed in our simulations, use of such an integrated pest management (IPM) approach could extirpate undesirable Brook Trout populations within reasonably short periods of time. Given the recent successful development of an MYY Brook Trout broodstock capable of producing large numbers of MYY fingerlings and given the positive results of the present simulations for both streams and alpine lakes, field testing of MYY stocking is warranted within an IPM program that includes manual suppression for eradicating undesirable Brook Trout populations.Received October 14, 2016; accepted June 10, 2017 Published online August 22, 2017

Nonnative Trout Invasions Combined with Climate Change Threaten Persistence of Isolated Cutthroat Trout Populations in the Southern Rocky Mountains

AbstractEffective conservation of Cutthroat Trout Oncorhynchus clarkii lineages native to the Rocky Mountains will require estimating effects of multiple stressors and directing management toward the most important ones. Recent analyses have focused on the direct and indirect effects of a changing climate on contemporary ranges, which are much reduced from historic ranges owing to past habitat loss and nonnative trout invasions. However, nonnative trout continue to invade Cutthroat Trout populations in the southern Rocky Mountains. Despite management to isolate and protect these native populations, nonnatives still surmount barriers or are illegally stocked above them. We used data on the incidence of invasions by nonnative Brook Trout (BT) Salvelinus fontinalis and the rate of their invasion upstream to simulate effects on a set of 309 conservation populations of Colorado River Cutthroat Trout (CRCT) O. c. pleuriticus isolated in headwater stream fragments. A previously developed Bayesian network model was used to compare direct and indirect effects of climate change (CC) alone on population persistence versus the added effects of BT invasions. Although CC alone is predicted to extirpate only one CRCT population by 2080, BT invasions and CC together are predicted to completely extirpate 122 populations (39% of the total) if managers do not intervene. Another 113 populations (37%) will be at risk of extirpation after CC and invasions, primarily owing to stochastic risks in short stream fragments that are similar under CC alone. Overall, invasions and CC will reduce the number of stream fragments that are long enough to buffer CRCT populations against negative genetic consequences and stochastic disturbances by 48, a decrease of 38% compared to CC alone. High priorities are (1) research to estimate how CC and human factors alter the incidence and rate of BT invasions and (2) management to prevent new illegal introductions, repair inadequate barriers, and monitor and address new invasions.Received March 21, 2016; accepted November 9, 2016 Published online February 27, 2017

Thermal Regimes, Nonnative Trout, and Their Influences on Native Bull Trout in the Upper Klamath River Basin, Oregon

AbstractThe occurrence of fish species may be strongly influenced by a stream's thermal regime (magnitude, frequency, variation, and timing). For instance, magnitude and frequency provide information about sublethal temperatures, variability in temperature can affect behavioral thermoregulation and bioenergetics, and timing of thermal events may cue life history events, such as spawning and migration. We explored the relationship between thermal regimes and the occurrences of native Bull Trout Salvelinus confluentus and nonnative Brook Trout Salvelinus fontinalis and Brown Trout Salmo trutta across 87 sites in the upper Klamath River basin, Oregon. Our objectives were to associate descriptors of the thermal regime with trout occurrence, predict the probability of Bull Trout occurrence, and estimate upper thermal tolerances of the trout species. We found that each species was associated with a different suite of thermal regime descriptors. Bull Trout were present at sites that were cooler, had fewer high‐temperature events, had less variability, and took longer to warm. Brook Trout were also observed at cooler sites with fewer high‐temperature events, but the sites were more variable and Brook Trout occurrence was not associated with a timing descriptor. In contrast, Brown Trout were present at sites that were warmer and reached higher temperatures faster, but they were not associated with frequency or variability descriptors. Among the descriptors considered, magnitude (specifically June degree‐days) was the most important in predicting the probability of Bull Trout occurrence, and model predictions were strengthened by including Brook Trout occurrence. Last, all three trout species exhibited contrasting patterns of tolerating longer exposures to lower temperatures. Tolerance limits for Bull Trout were lower than those for Brook Trout and Brown Trout, with contrasts especially evident for thermal maxima. Our results confirm the value of exploring a suite of thermal regime descriptors for understanding the distribution and occurrence of fishes. Moreover, these descriptors and their relationships to fish should be considered with future changes in land use, water use, or climate.Received March 4, 2016; accepted July 27, 2016 Published online October 11, 2016

Nonchemical Eradication of an Introduced Trout from a Headwater Complex in Banff National Park, Canada

AbstractNonnative salmonids have been intentionally introduced in North America for over a century and are now implicated in the decline of native aquatic species. Current management objectives for some national parks are to remove invasive species, where feasible. We evaluated the use of mechanical methods (gill nets and electrofishing) to eliminate a naturalized population of nonnative Brook Trout Salvelinus fontinalis in a subalpine headwater basin. We required a total of 1,383 continuous net nights/ha (net nights = number of nets × number of nights) over 5 years to completely eradicate 1,527 Brook Trout from both Lower (9.7 ha; 6 m maximum depth) and Middle (23.1 ha; 25 m maximum depth) Devon Lakes, Alberta, Canada. Electrofishing along the shoreline of both lakes resulted in the capture of 301 Brook Trout over 4 years. We required a total of 52.1 h/km of electrofishing over 5 years to completely remove 3,288 Brook Trout from the upper 4.5 km of the Clearwater River. We verified the absence of fish in the lakes with 1,558 net nights/ha of sampling over 4 years and in the river with 13.6 h/km over 2 years. This project confirms that mechanical methods are a viable option for removing introduced fish, even in relatively large (up to 20 ha) and deep (up to 25 m) mountain lakes. We surmised that our success (relative to that of projects described in the literature) was due to (1) the relatively simple alpine habitat, in which river and lake margins were clear of macrophytes, suspended sediment, and overhanging riparian vegetation, and (2) the sheer magnitude of our effort.

Evaluating the Ability of Tiger Muskellunge to Eradicate Brook Trout in Idaho Alpine Lakes

AbstractIn western North America, nonnative Brook Trout Salvelinus fontinalis in alpine lakes threaten the persistence of native trout and often offer limited sport fishing opportunity as they are prone to stunting. Stocking tiger muskellunge (Northern Pike Esox lucius × Muskellunge E. masquinongy), which are reproductively sterile, may be an option to eradicate Brook Trout in some alpine lakes. We used floating gill nets to survey 17 alpine lake Brook Trout populations, then stocked 13 lakes with tiger muskellunge, with four additional lakes serving as controls. Tiger muskellunge were stocked at a mean TL of 317 mm and a density of 40 fish/ha. Brook Trout were resampled for 4 or 5 years after stocking to evaluate changes in Brook Trout TL and CPUE (fish/net‐night). Declines in CPUE were substantial for both treatment and control lakes but were significantly greater in treatment lakes. Mean Brook Trout CPUE in treatment lakes declined from 23.1 fish/net‐night to 2.3 fish/net‐night 5 years after stocking tiger muskellunge, whereas in control lakes, CPUE declined from 25.5 fish/net‐night to 7.8 fish/net‐night 5 years later. Complete eradication appeared to occur in two lakes within 2 years, and in two more lakes by year 5. In lakes where tiger muskellunge were stocked, the proportion of Brook Trout ≥250 mm TL in the catch increased significantly in years 1, 2, and 4 after stocking (compared with prestocking data), whereas no increase occurred in control lakes. Tiger muskellunge were most successful in reducing Brook Trout CPUE in lakes with no inlets or outlets, while elevation and lake area may also have played a role. Our results suggest tiger muskellunge can improve the size structure and potentially eradicate Brook Trout populations from some alpine lakes. However, we recommend combining any tiger muskellunge stocking with other conventional removal methods to increase the likelihood of successful eradication.Received October 27, 2014; accepted March 22, 2015

Factors Influencing Successful Eradication of Nonnative Brook Trout from Four Small Rocky Mountain Streams Using Electrofishing

AbstractWe successfully eradicated nonnative Brook Trout Salvelinus fontinalis by electrofishing from 2.4‐ to 3.0‐km treatment reaches of four Rocky Mountain streams in Montana to conserve sympatric populations of native Westslope Cutthroat Trout Oncorhynchus clarkii lewisi. At least 6, and as many as 14, removal treatments of two to four electrofishing passes per treatment were required to successfully eradicate Brook Trout from these treatment reaches. We increased success by modifying our treatment efforts during this study from single annual treatments to several treatments a year to take advantage of autumn spawning and winter aggregating behavior. Eradication by electrofishing cost US$3,500 to $5,500 per kilometer where no riparian vegetation or woody debris clearing was necessary, increasing to $8,000 to $9,000 per kilometer where clearing was needed. Treatment costs without stream clearing were similar to costs of eradication using piscicides. Eradication by electrofishing may be preferable where native fish occur in sympatry with nonnative fish in smaller streams (base flow wetted widths <3.0 m) because native fish can be salvaged during removal efforts and because electrofishing may be more acceptable to the public than use of piscicides.Received December 16,2013; accepted June 9,2014

Use of an Ecosystem‐Based Model to Evaluate Alternative Conservation Strategies for Juvenile Chinook Salmon in a Headwater Stream

AbstractDeclining abundance of Chinook Salmon Oncorhynchus tshawytscha across the Pacific Northwest is an issue of great concern ecologically, culturally, and economically. Growth during the first summer is vitally important for juvenile Chinook Salmon, as it influences not only life history decisions (to smolt or not to smolt) but also subsequent river and ocean survival. Using Ecopath with Ecosim, we developed a food web model for a representative stream in the Salmon River basin, Idaho, to evaluate potential species‐specific and food web effects of three management strategies: (1) adding salmon carcasses or carcass analogs to promote primary production and detrital availability that were lost due to declining salmon returns; (2) removal of nonnative Brook Trout Salvelinus fontinalis, which are competitors with and predators on juvenile Chinook Salmon; and (3) stocking hatchery Chinook Salmon into streams to supplement wild production. Overall, juvenile Chinook Salmon responded strongly to increases in basal resources. Removal of Brook Trout had little effect on potential production for juvenile Chinook Salmon, but the responses of sculpins Cottus spp. were strong, primarily due to the sculpins’ high degree of dietary overlap with and predation by Brook Trout. Supplementation with hatchery‐origin juveniles depressed the production of wild juvenile Chinook Salmon, especially at the densities commonly applied to streams in this region. Our results suggest that efforts to enhance basal resources are likely to be the most effective in promoting the production of juvenile Chinook Salmon and nearly all food web groups considered in our model system. Removal of nonnative Brook Trout is unlikely to substantially affect salmon but could have a disproportionately large effect on nongame species, which are generally overlooked in single‐species management approaches.Received October 2, 2013; accepted March 24, 2014

Comparing competitive ability and associated metabolic traits between a resident and migratory population of bull trout against a non-native species

Juvenile bull trout Salvelinus confluentus from two geographically and ecologically distinct popu- lations were compared with regard to their ability to compete with non-native brook troutSalvelinus fontinalis in an artificial stream, and with respect to their rates of oxygen consumption. Bull trout collected from a migra- tory population foraged more successfully against brook trout competitors than those from a resident population, capturing more of a limited amount of food items presented. The migratory population was also more ag- gressive (measured by the number of nips, chases and lateral threat displays) against brook trout competitors than the resident population. Bull trout from the migra- tory population had a higher oxygen consumption rate (203 mg O2 kg · hr -1 ) in the field than similar sized fish from the resident population (183 mg O2kg · hr -1 ). These results suggest native bull trout have population-level variation in competitive ability against a non-native spe- cies and such competitive ability is positively associated with metabolism and migratory life history.

Coding Gene Single Nucleotide Polymorphism Population Genetics of Nonnative Brook Trout: The Ghost of Introductions Past

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

Do Beaver Dams Impede the Movement of Trout?

Dams created by North American beavers Castor canadensis (hereafter, “beavers”) have numerous effects on stream habitat use by trout. Many of these changes to the stream are seen as positive, and many stream restoration projects seek either to reintroduce beavers or to mimic the habitat that they create. The extent to which beaver dams act as movement barriers to salmonids and whether successful dam passage differs among species are topics of frequent speculation and warrant further research. We investigated beaver dam passage by three trout species in two northern Utah streams. We captured 1,375 trout above and below 21 beaver dams and fitted them with PIT tags to establish whether fish passed the dams and to identify downstream and upstream passage; 187 individual trout were observed to make 481 passes of the 21 beaver dams. Native Bonneville Cutthroat Trout Oncorhynchus clarkii utah passed dams more frequently than nonnative Brown Trout Salmo trutta and nonnative Brook Trout Salvelinus fontinalis. We determined that spawn timing affected seasonal changes in dam passage for each species. Physical characteristics of dams, such as height and upstream location, affected the passage of each species. Movement behaviors of each trout species were also evaluated to help explain the observed patterns of dam passage. Our results suggest that beaver dams are not acting as movement barriers for Bonneville Cutthroat Trout or Brook Trout but may be impeding the movements of invasive Brown Trout.

Are invasive and native trout functionally equivalent predators? Results and lessons from a field experiment

Non-native brook trout (Salvelinus fontinalis) have replaced native cutthroat trout (Oncorhynchus clarkii) in streams across the western USA. Brook trout have been assumed to be functionally equivalent to cutthroat trout, which could argue against extensive control of this species by government agencies. This assumption was assessed by comparing the diets of the two trout species and their effects on benthic invertebrates in a large-scale field experiment. Within a Rocky Mountain stream where brook trout had already become established, trout species and density were manipulated in 20 fenced sections to create four treatments: cutthroat trout at a density natural for un-invaded streams, brook trout at two densities, natural for invaded streams or reduced to match the natural density of cutthroat trout, and areas depleted of most trout. By the end of the experiment, populations of brook trout at natural density consumed seven times more benthic invertebrates than cutthroat trout. Brook trout also consumed more Apatania caddisflies and Heterlimnius riffle beetles per capita than cutthroat trout. However, the greater consumption did not cause detectable reductions in benthic invertebrate densities. A large-scale experiment of trout removal like this may be inherently limited in its ability to detect effects on benthic invertebrate densities. However, this and several companion studies indicate that stream ecosystems would not function in the same way if cutthroat trout are replaced by brook trout. Sustaining these functions, as well as biodiversity and socioeconomic values, are important reasons to conserve native cutthroat trout. Copyright © 2012 John Wiley & Sons, Ltd.

Effect of Brook Trout Removal from a Spawning Stream on an Adfluvial Population of Lahontan Cutthroat Trout

AbstractIndependence Lake (Nevada and Sierra counties, California) harbors the only extant native population of Lahontan cutthroat trout Oncorhynchus clarkii henshawi in the Truckee River system and one of two extant adfluvial populations in the Lahontan basin. The persistence of this population has been precarious for more than 50 years, with spawning runs consisting of only 30–150 fish. It is assumed that this population was much larger prior to the introduction of nonnative brook trout Salvelinus fontinalis. Brook trout overlap with cutthroat trout in upper Independence Creek, where the cutthroat trout spawn and their resulting progeny emigrate to Independence Lake. In 2005, we began removing brook trout from upper Independence Creek using electrofishers and monitored the cutthroat trout population. Stomach analysis of captured brook trout revealed cutthroat trout fry, and cutthroat trout fry survival increased significantly from 4% to 12% with brook trout removal. Prior to brook trout removal, the only Lahontan cutthroat trout progeny emigrating to Independence Lake were fry; with brook trout removal, juveniles were found entering the lake. In 2010, 237 potential spawners passed a prefabricated weir upstream of Independence Lake. Although the results of this study suggest that brook trout removal from upper Independence Creek has had a positive influence on the population dynamics of Independence Lake Lahontan cutthroat trout, additional years of removal are needed to assess the ultimate effect this action will have upon the cutthroat trout population.Received August 26, 2011; accepted February 29, 2012

Do Nonnative Salmonines Exhibit Greater Density and Production than the Natives They Replace? A Comparison of Nonnative Brook Trout with Native Cutthroat Trout

AbstractThe replacement of native species by nonnative species has been widely documented. In aquatic ecosystems, nonnative trout often replace native trout species to which they are closely related. For example, nonnative brook trout Salvelinus fontinalis have replaced native cutthroat trout Oncorhynchus clarkii throughout western North America. We hypothesized that when brook trout replace cutthroat trout, they exhibit greater density, biomass, and annual production than were achieved by the cutthroat trout before replacement. We tested this hypothesis in five paired streams in the Teton River drainage, Idaho, over a 4‐year period. Streams were paired based on similar habitat characteristics and the presence of brook trout or cutthroat trout as the dominant species. Fish were surveyed annually by means of multipass depletion along representative stream reaches. Consistent with our hypothesis, brook trout exhibited 2.4 times greater density, 1.7 times greater biomass, and 2.5 times higher annual production than cutthroat trout. Because age‐0 fish were difficult to capture during electrofishing, we also analyzed the data without this age‐class. When age‐0 fish were excluded, brook trout exhibited 1.3, 1.6, and 2.1 times greater density, biomass, and annual production, respectively, than cutthrout trout. These results are consistent with the hypothesis that nonnative salmonines exhibit greater density and production than the native fishes they replace, though the generality of this finding remains to be tested. The differences that we observed not only testify to the success of brook trout as an introduced species in western North America but also point to likely changes in ecosystem function that may accompany their replacement of native cutthroat trout.

When Eradication is not an Option: Modeling Strategies for Electrofishing Suppression of Nonnative Brook Trout to Foster Persistence of Sympatric Native Cutthroat Trout in Small Streams

AbstractSubspecies of inland cutthroat trout Oncorhynchus clarkii occupy a fraction of their historic ranges, and displacement by nonnative brook trout Salvelinus fontinalis is among the greatest threats to existing populations in small, headwater streams. Electrofishing is often used to suppress brook trout and enhance cutthroat trout populations, but these efforts are labor intensive and costly. To help managers more effectively plan and implement brook trout control programs, we used survival estimates from a field experiment to construct matrix population models for both species and linked the models by making the vital rates of young cutthroat trout a function of brook trout density to represent the effect of invasion. We then explored the response of cutthroat trout populations (growth rate [λ] and probability of persistence for 50 years) to brook trout suppression across various levels of electrofishing frequency, sampling intensity, capture efficiency, and brook trout immigration rate. Cutthroat trout (λ = 1.10) and brook trout (λ = 1.20) both had moderate to high values of λ when biotic interactions were weak or absent, but the cutthroat trout λ declined markedly (λ = 0.61) when biotic interactions with brook trout were strong. Model results supported the hypothesis that rapid displacement of cutthroat trout is facilitated by the brook trout's inherent demographic advantage and biotic effects on young cutthroat trout. Where annual brook trout suppression was not possible, the temporal distribution of suppression events within a systematic control design strongly influenced the response of cutthroat trout. The absolute response and cost effectiveness of suppression efforts were influenced particularly by the brook trout immigration rate and also by removal efficiency. Multiple consecutive years of suppression (≥3 years) benefited cutthroat trout, but to be effective such suppression should not be interrupted for more than two consecutive years once initiated. Eradication would be ideal, but the maintenance control of brook trout and the management of cutthroat trout in sympatry with brook trout will probably remain among the viable management options for the foreseeable future.