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Lake trout Salvelinus namaycush (top panel) and bull trout Salvelinus confluentus (bottom panel) from Quartz Lake, Glacier National Park, Montana, in 2006.
Source publication
The establishment of nonnative lake trout Salvelinus namaycush in lakes containing lacustrine-adfluvial bull trout Salvelinus confluentus often results in a precipitous decline in bull trout abundance. The exact mechanism for the decline is unknown, but one hypothesis is related to competitive exclusion for prey resources. We had the rare opportuni...
Contexts in source publication
Context 1
... trout Salvelinus namaycush (Figure 1) introduc- tions and invasions have been implicated in declines of bull trout Salvelinus confluentus (Figure 1), a federally threatened species, native to numerous lakes in the Intermountain West (Donald and Alger 1993;Fredenberg 2002;Martinez et al. 2009). Lake trout were widely introduced outside their native range beginning in the early 1900s, particularly in the Intermountain West, to create commercial fisheries or to provide anglers with opportunities to catch an additional top-level predator (Crossman 1995;Martinez et al. 2009). Lake trout distribution continues to expand throughout the west- ern United States due to invasions of interconnected waterways and illegal introductions ( Martinez et al. 2009). Unfortunately, range expansion of lake trout has often led to the declines in native salmonid populations in many western North American lakes (Fredenberg 2002;Koel et al. 2005), and altered trophic dynamics in aquatic and terrestrial ecosystems (Spencer et al. 1991;Koel et al. 2005;Tronstad 2008;Ellis et al. ...
Context 2
... trout Salvelinus namaycush (Figure 1) introduc- tions and invasions have been implicated in declines of bull trout Salvelinus confluentus (Figure 1), a federally threatened species, native to numerous lakes in the Intermountain West (Donald and Alger 1993;Fredenberg 2002;Martinez et al. 2009). Lake trout were widely introduced outside their native range beginning in the early 1900s, particularly in the Intermountain West, to create commercial fisheries or to provide anglers with opportunities to catch an additional top-level predator (Crossman 1995;Martinez et al. 2009). Lake trout distribution continues to expand throughout the west- ern United States due to invasions of interconnected waterways and illegal introductions ( Martinez et al. 2009). Unfortunately, range expansion of lake trout has often led to the declines in native salmonid populations in many western North American lakes (Fredenberg 2002;Koel et al. 2005), and altered trophic dynamics in aquatic and terrestrial ecosystems (Spencer et al. 1991;Koel et al. 2005;Tronstad 2008;Ellis et al. ...
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Citations
... Our results mirror other isotopic studies where lake trout often exhibit low degrees of isotopic overlap with other invertivore salmonid species [85,86]; that is, even when consuming similar diets isotopic overlap between lake trout and cutthroat trout was low [53]. However, lake trout can exhibit high degrees of overlap when compared to piscivorous salmonids [56,87]; even when a diet shift was observed, isotopic overlap was high for lake trout between high-and moderate-density states. From our decadal comparison of diet and stable isotope similarity and overlap, we observed a clear signal that invasive piscivorous lake trout exhibited diet plasticity as the predator and prey populations responded to 24 years of suppression. ...
Diet plasticity is a common behavior exhibited by piscivores to sustain predator biomass when preferred prey biomass is reduced. Invasive piscivore diet plasticity could complicate suppression success; thus, understanding invasive predator consumption is insightful to meeting conservation targets. Here, we determine if diet plasticity exists in an invasive apex piscivore and whether plasticity could influence native species recovery benchmarks and invasive species suppression goals. We compared diet and stable isotope signatures of invasive lake trout and native Yellowstone cutthroat trout (cutthroat trout) from Yellowstone Lake, Wyoming, U.S.A. as a function of no, low-, moderate-, and high-lake trout density states. Lake trout exhibited plasticity in relation to their density; consumption of cutthroat trout decreased 5-fold (diet proportion from 0.89 to 0.18) from low- to high-density state. During the high-density state, lake trout switched to amphipods, which were also consumed by cutthroat trout, resulting in high diet overlap (Schoener's index value, D = 0.68) between the species. As suppression reduced lake trout densities (moderate-density state), more cutthroat trout were consumed (proportion of cutthroat trout = 0.42), and diet overlap was released between the species (D = 0.30). A shift in lake trout δ13C signatures from the high- to the moderate-density state also corroborated increased consumption of cutthroat trout and lake trout diet plasticity. Observed declines in lake trout are not commensurate with expected cutthroat trout recovery due to lake trout diet plasticity. The abundance of the native species in need of conservation may take longer to recover due to the diet plasticity of the invasive species. The changes observed in diet, diet overlap, and isotopes associated with predator suppression provides more insight into conservation and suppression dynamics than using predator and prey biomass alone. By understanding these dynamics, we can better prepare conservation programs for potential feedbacks caused by invasive species suppression.
... The Lake Trout Salvelinus namaycush is one species that has been widely introduced outside of its native range due to its ability to support valuable commercial and recreational fisheries (Healey 1978;Crossman 1995;Eshenroder et al. 1995;Mackenzie-Grieve and Post 2005). Unfortunately, the introduction and establishment of invasive Lake Trout populations have contributed to declines in abundance of native salmonid populations through competition, predation, or both (Donald and Alger 1993;Fredenberg 2002;Koel et al. 2005;Guy et al. 2011;Cox et al. 2013;Fredenberg et al. 2017). For example, invasive Lake Trout contributed to the collapse of the native Bull Trout Salvelinus confluentus population in Flathead Lake, Montana (Beauchamp et al. 2006;Hansen et al. 2016), and to declines in abundance of native Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri in Yellowstone Lake, Wyoming (Koel et al. , 2020a. ...
Expansion of an invasive Lake Trout Salvelinus namaycush population in Swan Lake, Montana threatens a core area population of Bull Trout Salvelinus confluentus. Given the recent development of novel suppression methods, such as use of carcass‐analog pellets to cause high‐mortality of embryos, there was a need to quantify spawning season aggregation sites, site use, and spawning habitat for Lake Trout in Swan Lake. Acoustic tags were implanted in 85 Lake Trout during the summer of 2018 and 2019. Nightly tracking efforts during autumn in both years resulted in 1,744 relocations for 49 individual Lake Trout. Kernel‐density analysis was used to evaluate Lake Trout aggregation sites identifying 10 distinct spawning sites. All spawning sites were located in the littoral zone along areas of steep bathymetric relief and composed 48% of total relocations during both spawning seasons. In 2019, side‐scan sonar imaging was used to classify and quantify the total area of spawning substrate, which composed 12.8% of the total surface area estimated for spawning sites 1, 6, and 9, and 11.4% for aggregation sites 2, 3, 4, 5, 7, 8, and 10. Simultaneous treatment of all spawning sites would require 205,709 ± 86 kg of carcass‐analog pellet material, resulting in 370.4 ± 0.2 kg of phosphorous and 7,487.9 ± 3.1 kg of nitrogen inputs to Swan Lake. Thus, pellet treatment would increase the Carlson's trophic state index (TSI) values from 20.8 to 27.7 for total phosphorous, and from 22.1 to 26.2 for total nitrogen. Based on a TSI threshold of < 40 for an oligotrophic lake, the use of carcass‐analog pellets could be feasible for supplementing the gillnetting suppression of Lake Trout in Swan Lake.
... Given that an ontogenetic diet shift could obfuscate other dietary analyses, a logistic regression analysis was used to test the relationship between Neosho bass TL and type of prey consumed. The ontogenetic diet shift was estimated as the length at which crayfish or fish were predicted to occur in ≥50% of stomachs (Guy et al., 2011;. ...
Despite documented negative effects on aquatic ecosystems, management agencies commonly receive requests to stock non-native rainbow trout Oncorhynchus mykiss(Walbaum). We used a multinomial N-mixture model and a suite of diet analyses to evaluate effects of large (265–530 mm) rainbow trout on reach-scale abundance and feeding ecology of an important endemic, keystone species, Neosho bass Micropterus velox (Hubbs and Bailey). We evaluated potential changes by seasonally sampling multiple reaches of an Ozark Highlands stream before and after the introduction of
10,800 rainbow trout. Results suggest that rainbow trout altered the distribution of Neosho bass, as indicated by a negative relationship between rainbow trout density and Neosho bass abundance. Trophically, rainbow trout and Neosho bass <101 mm were equivalent, but rainbow trout diets significantly overlapped with larger Neosho bass. High rainbow trout abundance in the spring post-stocking period changed Neosho bass diets by altering predator–prey dynamics with crayfish. Controlling for fish size revealed Neosho bass consumed larger crayfish than rainbow trout, although rainbow trout and Neosho bass between 200 and 299 mm consumed equal-sized crayfish. Outcomes of rainbow trout stockings are likely density- and size-dependent, but caution is warranted for introductions in warmwater streams.
... Additionally, the Lake Trout population was doubling every 1.6 years and was projected to reach 131,000 adults by 2010 without management intervention (Hansen et al. 2008). With those conditions, Lake Trout posed a threat to federally threatened Bull Trout S. confluentus through predation and competition (Fredenberg 2002;Martinez et al. 2009;Guy et al. 2011). To protect Bull Trout and restore kokanee, IDFG initiated a two-part predator removal program in 2006 aimed at collapsing the Lake Trout population and reducing Rainbow Trout predation. ...
Annual Idaho Department of Fish and Game report of fishery research activities for Lake Pend Oreille Fishery Recovery Project
... Populations of bull trout (Salvelinus confluentus), one of the most threatened cold-water fishes in North America, have dramatically declined in most lakes where lake trout have been introduced or invaded (16). Bull trout and lake trout are apex predators and share similar feeding strategies, diets, and morphologies, making competition and predation likely between these species (19). Despite this major conservation threat, no studies have evaluated the impacts of lake trout invasion across entire food webs supporting native bull trout. ...
Significance
Invasive species are a leading cause of biodiversity loss, yet the mechanisms by which invaders progressively disrupt ecosystems and native species remain unclear. Using an extensive isotope dataset across multiple ecosystems and stages of invasion, we show that an invasive fish predator (lake trout) disrupted food webs by forcing native fishes to feed on suboptimal food sources in different habitats, resulting in the functional extirpation of the native predator, threatened bull trout. Our results provide insights into the magnitude, direction, and timing of food web disruption from invasive species and will be important for predicting ecosystem consequences of species invasions.
... Given that the existence of an ontogenetic diet shift could potentially obfuscate other dietary analyses, a logistic regression using binary response data was used to test the relationship between Redspot Chub total length (TL) and crayfish consumption. The potential ontogenetic diet shift to crayfish predation was determined as the length at which crayfish were predicted to occur in !50% of Redspot Chub stomachs (Guy et al., 2011;Schmitt et al., 2018). ...
Redspot Chub (Nocomis asper) are a keystone species in Ozark Highland streams of the Arkansas River drainage because their unique mound-building reproductive behavior facilitates recruitment of nest associates. Data contributing to an increased understanding of Redspot Chub natural history is required for the conservation of this species as it is listed as a species of concern throughout most its range. The findings presented within thoroughly describe Redspot Chub feeding ecology by documenting an ontogenetic diet shift, patterns of seasonal resource use, important prey taxa, estimates of trophic position and dietary niche breadth, as well as documenting feeding strategies at population and individual levels. Diet collections were conducted seasonally in 2018 and 2019 at four sites on Spavinaw Creek in Arkansas and Oklahoma. Logistic regression provided evidence for a previously undescribed ontogenetic diet shift, which split Redspot Chub into two functionally different species from a dietary perspective. Redspot Chub ! 162 mm total length (TL, RSCB) occupied a significantly higher trophic level than Redspot Chub , 162 mm TL (RSCS). In terms of caloric contribution, Ringed Crayfish (Faxonius neglectus neglectus) nearly accounted for the entire diet of RSCB, regardless of season. This specialization led to a much lower overall dietary niche breadth when compared to RSCS. Furthermore, Amundsen plots clearly depicted RSCB population specialization on crayfish. Conversely, Trichoptera were the most important prey across seasons for RSCS. Coleoptera, Ringed Crayfish, and Ephemeroptera ranked second in importance at different times seasonally. RSCS also possessed a relatively narrow niche width, but Amundsen plots exhibited a pattern where seasonally the two most important prey types were specialized on by approximately half the population, while smaller proportions of the population specialized on less commonly used prey resources. Species' diets reflect an integration of numerous ecological components; therefore, our aim was to provide a greater understanding of Redspot Chub feeding ecology that may be useful for informing future conservation assessments and management decisions.
... Climate change may also promote the spread and impact of nonnative trout on bull trout populations (Rieman et al. 2007;Ruesch et al. 2012;Eby et al. 2014). Brook trout (Salvelinus fontinalis) and lake trout (Salvelinus namaycush) have been identified as considerable threats to bull trout populations through predation (Martinez et al. 2009;Hansen et al. 2010;Fredenberg 2014), competition (Guy et al. 2011;Warnock and Rasmussen 2014), and nonintrogressive hybridization resulting in gametic wastage (Leary et al. 1993;Kanda et al. 2002;DeHaan et al. 2010). Recently, there has been growing concern regarding the effects of introduced brown trout (Salmo trutta) on bull trout populations in western Montana, USA, particularly in light of recent evidence suggesting expansion of brown trout into historical bull trout habitat in response to warming stream temperatures (Al-Chokhachy et al. 2016). ...
Executive Summary Changing climate and introduced species are placing an increasing number of species at risk of extinction. Increasing extinction risk is increasing calls to protect species by relocating, or translocating, them to locations with more favorable biotic or climatic conditions. Managed relocation, or assisted migration, of species entails risks to both the conservation target organisms being moved as well as the recipient ecosystems into which they are moved. Recognizing this risk, calls have been made for practitioners interested in considering a managed relocation project to engage in a serious risk assessment prior to advancing a project. We engaged a team of researchers and resource managers to create risk assessment protocols that could be used by natural resource managers within U.S. National Parks, or elsewhere, to help inform a decision of whether the risks involved in managed relocation are warranted. These protocols facilitate evaluation of the ecological risk of species managed relocation as part of planning and decision making. This is not a policy document. It neither introduces new policy, nor serves to interpret or resolve current policies regarding managed relocation (or assisted migration) as a natural resource management strategy. We assembled a team of five university researchers and ten federal resource management researchers and staff to develop a practical management-oriented risk assessment strategy. We jointly agreed to a set of principles to guide this managed relocation risk assessment strategy. This protocol and accompanying spreadsheet would be used to help a decision-maker structure a decision process but would not strive to provide a formulaic decision output. Identifying, evaluating, and managing risk is a subjective decision that is the responsibility of the decision authority. We began by defining the scope of this work to include moving populations or species for the purpose of conserving the target populations or species that are threatened by climate or invasive species. We also included species movements for the purpose of retaining some critical ecosystem function. We did not include management actions such as planned ecosystem re-alignment for climate change or other kinds of translocations associated with ecosystem manipulation (e.g., habitat restoration), although these protocols may be useful for some of those management actions with minor modification. We adopted the premise that risk decisions are inherently subjective and that different aspects of risk (e.g. the risk of a moved species introducing a novel pathogen to an ecosystem, the risk of unwanted evolution in the moved species) are non-additive. Hence, our strategy is designed to encourage managers to think broadly and comprehensively about risk in order to make the best possible decision given alternate opposing risks (i.e. the risk of extinction versus the risk of causing unintended harm to other species and ecosystems in the process of trying to save a species). We identified six major areas of risk, with a total of seventeen sub-categories. These are: • Risks of no managed relocation action. Risk of: o no action on the target o no action on the recipient ecosystem • Risks of managed relocation action to the target. Risks of: o action on the translocated individuals o target source population extirpation through diminished numbers o reduced ecological functioning of the source ecosystem o causing undesired evolution in the target • Risks of action on non-targets in the recipient ecosystem. Risks of: o target transmitting novel disease or associated pest o negative competitive interactions on non-target populations o predation, herbivory, or allelopathic effects on non-target populations o driving undesirable evolution in non-target species • Risks of action on higher order attributes of the recipient ecosystem. Risks of: o indirect and negative impacts on ecosystem structure o changing ecosystem function • Risks associated with invasion. Risks of: o invasion within the intended recipient ecosystem o invasion beyond recipient ecosystem o irreversibility of the managed relocation action • Risks associated with socio-economic values. Risks to: o culturally or economically important species o valued ecosystem services For each risk category we provide guidance on risk scoring. Risk scoring is comprised of a risk rank category (low, moderate, high, very high) and a confidence score (low, medium, high). Confidence is a combined attribute of the strength of evidence and the agreement of that evidence. The protocols are presented in an accompanying Excel spreadsheet that uses a graphical tool to allow users to visualize a composite of risk and confidence. We are adamant about not summing across risk categories. Instead, we provide a graphing tool that summarizes risk within categories. We suggest that users could find risks posed by a proposed action to be acceptable if: 1) Confidence scores are sufficient that managers feel confident that the risk assessment is informative; 2) There is no single risk category that is so high and so important as to make the project unacceptably risky; and 3) The general distribution of risk is not so high as to exceed some level of expectation that one of many potential problems could arise and lead to decision regret. We frame this risk assessment within the context of other critical questions that need to be answered in order to proceed toward strategic planning for a managed relocation action. These include justifying ecological need, assessing technical feasibility, cost, management priority and social acceptability. If all these criteria are met, then these same protocols can be used in a multi-criteria assessment to compare across different strategic plans for managed relocation (e.g. relocation location, relocation numbers, source and husbandry of relocated individuals). We provide brief guidance on how that may be completed with no presumption of final decision determination. Finally, in the process of developing these risk scoring protocols, we tested them on a suite of four case studies (bull trout, Karner blue butterfly, giant sequoia and Pitcher’s thistle). These are provided in this document as examples of the logic and process that we outline for assessing risk. These were, however, done without broad consultation and should be taken not as definitive risk assessments of managed relocation for these species, but as examples of how one might use our strategy for an assessment of ecological risk associated with managed relocation.
... The lack of a seasonal movement pattern for bull trout in Williston Reservoir, may be a function of interspecific competition with lake trout. When bull trout occur in sympatry with lake trout, lake trout often establish themselves as the dominant pelagic piscivore (Donald & Alger, 1993;Ferguson, Taper, Guy, Syslo, & Tonn, 2012;Guy et al., 2011). Accordingly, further research into diet, space-use and niche overlap between these two species in our study system, may help to explain the observed inter-specific differences in entrainment vulnerability. ...
Potadromous salmonids that reside in hydropower reservoirs often have a high recreational and conservation value. However, the potential seasonal turbine entrainment vulnerability patterns of potadromous salmonids are not well understood. Here, we use acoustic telemetry to test the hypothesis that adults of two species of the Salvelinus genus (bull trout and lake trout) differ in their seasonal patterns of entrainment and entrainment vulnerability over a 2‐year period. Our results show that while both species were entrained at similarly low annual rates (~1%), these two salmonids differed in their patterns of forebay residency and proximity, with implications for entrainment risk. Bull trout occupied the forebay at low rates across all seasons, with no clear seasonal pattern of forebay proximity. In contrast, lake trout displayed a strongly seasonal pattern of entrainment vulnerability with a distinct movement away from the forebay during the summer, and a large increase in forebay proximity and use in the winter and spring. These findings provide a novel species‐specific demonstration of the potential entrainment vulnerability of lake trout. The seasonal patterns of entrainment vulnerability seen in previous bull trout studies, where bull trout occupied top pelagic predator niches, were not replicated in our study where bull trout occur in sympatry with another top pelagic predator. These findings, which indicate that species composition plays an important role determining entrainment vulnerability, have important implications for the conservation of indigenous lake trout and bull trout populations, and together highlight the need for a site‐specific approach to entrainment quantification.
... The Lake Trout Salvelinus namaycush is an important commercial and recreational fish that has been widely introduced in North America (Crossman 1995). Through predation, competition, or both, invasive Lake Trout have caused the decline of salmonid populations throughout the intermountain western United States (Martinez et al. 2009;Guy et al. 2011;Syslo et al. 2013;Fredenberg et al. 2017). For example, the introduction of invasive Lake Trout into Lake Tahoe led to the extirpation of native Lahontan Cutthroat Trout Oncorhynchus clarkii henshawi (Crossman 1995). ...
Conserving 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.
... Additionally, the Lake Trout population was doubling every 1.6 years and was projected to reach 131,000 adults by 2010 without management intervention (Hansen et al. 2008). With those conditions, Lake Trout posed a threat to federally-threatened Bull Trout S. confluentus through predation and competition (Fredenberg 2002;Martinez et al. 2009;Guy et al. 2011). To protect Bull Trout and restore kokanee, the Idaho Department of Fish & Game (IDFG) started a two-part predator removal program in 2006 aimed at collapsing the Lake Trout population and reducing Rainbow Trout predation. ...
Two year progress report for IDFG's Lake Pend Oreille Fishery Recovery Project. Covers research related to kokanee monitoring, invasive species (Lake Trout, Walleye) removal evaluation, Lake Trout monitoring, Rainbow Trout growth evaluations, and Walleye monitoring.
