Hayley Corrine Glassic

Invasive species are managed to mitigate ecosystem effects; however, unintended consequences from suppression may result. Investigating effects of management actions on lower trophic levels and nutrient dynamics can help avoid unintended consequences. Fish carcasses were deposited in the littoral zone in Yellowstone Lake, Wyoming, U.S.A. during autumn to suppress embryos of invasive trout at spawning sites. We studied the degree to which adding carcasses to the littoral spawning sites indirectly benefited invasive trout by altering food resources. We used carcasses as a natural isotopic tracer to estimate the degree to which carcasses were integrated into the food web. We compared basal resource and amphipod δ15N and δ13C signatures at reference and treatment sites. We also estimated the contribution of carcasses to amphipod diet, and the contribution of amphipods to invasive and native trout diets using stable isotope mixing models. Isotopic signatures of basal resources did not differ between treatment and reference sites. Amphipod isotopic signatures changed after treatment, but < 3% of amphipod diet was carcass. Amphipods contributed < 24% of estimated diet for invasive trout, but > 55% of estimated diet for native trout. δ13C values of amphipods and phytoplankton increased at reference and treatment sites after autumn turnover suggesting carcass material was distributed throughout the lake. Unused carcasses from gillnetting discarded in the profundal zone since 1995 appeared to influence nutrient dynamics more than those added to the littoral zone. We demonstrate that investigating food web dynamics simultaneously with suppression to prevent unintended consequences can aid in developing effective management plans that minimally alter ecosystems.

Understanding the relationship between breeding adults and juvenile recruitment is fundamental to conserving at-risk species. Knowing whether recruitment is depensatory or compensatory at low adult abundances can aid in developing conservation strategies and recovery goals. However, this relationship requires long-term monitoring of two life stages, which can be costly and uncommon, especially for non-commercially harvested fish species, such as Bull Trout ( Salvelinus confluentus). Listed as threatened in the United States and Canada, Bull Trout have declined throughout their range due to anthropogenic disturbances. Although spawner–recruitment relationships can create realistic recovery goals, this relationship is poorly characterized for Bull Trout. We used redd counts and juvenile densities from 11 Bull Trout populations to (1) establish spawner-recruitment dynamics, (2) assess environmental influences on productivity, and (3) derive recovery benchmarks. Bull Trout exhibited high recruitment productivity at low adult abundances and clear recruitment limitations at high abundances. Recruitment productivity varied across populations but was not associated with environmental covariates. We highlight strong compensatory capacity at low adult abundances, indicating possible recovery if limiting factors beyond spawning and rearing habitats are addressed.

Riverscapes are the integration of terrestrial and aquatic systems from headwaters to estuaries that provide habitat and ecosystem benefits when in good health. However, current riverscape degradation is pervasive, impairing the function and resulting benefits of these systems. Healthy riverscapes are adaptive and some can ‘heal’ after disturbance with minimal to no human assistance. As riverscape health is threatened, a need exists to address current degradation and understand the potential for riverscape restoration—concisely communicating what comprises healthy riverscapes is essential to direct limited resources and increase efficacy of restoration and conservation efforts. Healthy riverscapes have (i) space to interact within their valley bottom; (ii) natural flow, sediment, and vegetation regimes appropriate to the biophysical setting and river type; and (iii) structural forcing to support diversity and that creates varied residence times for water, sediment, and vegetation. These three principles are grounded in interdisciplinary science and lessons from riverscape scientists and restoration practitioners across the world. Understanding the context, anthropogenic influences, boundary conditions, and legacy effects influencing riverscapes is essential for the appropriate application of these principles in pursuit of achieving riverscape health. Emphasizing a holistic, biogeomorphic view of riverscapes through these principles can guide policies, restoration actions, and monitoring frameworks that ensure that riverscapes remain capable of accommodating and adjusting to disturbances while continuing to support biodiversity and human benefits. This article is categorized under: Water and Life > Conservation, Management, and Awareness

Objective: Long-term monitoring programs typically require standardization to evaluate changes in fish populations over time. However, incorporating new methods into long-term monitoring programs may become increasingly important as climate change affects sampling conditions. In the upper Yellowstone River, Montana, biologists have monitored trout abundance using closed-population, Lincoln–Petersen mark–recapture methods since 1978. In the past two decades, changes in the hydrologic regime have prevented monitoring from occurring annually. Methods: We investigated the feasibility of individually marking trout and using the POPAN formulation of the Jolly–Seber model to estimate the abundance and survival of three trout species in the upper Yellowstone River as an alternative to historical monitoring methods. When using this modeling framework, annual sampling can be conducted in a reduced time period, allowing monitoring to be adapted to changing environmental conditions. Results: Analysis of a 3-year empirical data set provided estimates of abundance and survival for most sites and species. Simulations indicated that POPAN models produced unbiased estimates of demographic parameters when detection probability was 0.20 or greater, and the precision of estimates increased as detection probability and apparent survival increased. Conclusions: Using POPAN models to analyze mark–recapture data from individually marked fish may provide a feasible alternative for monitoring trout populations in the upper Yellowstone River. Although further evaluation of monitoring methods as data collection continues is warranted, open-population models show promise as a useful tool for monitoring the trout fishery in the Yellowstone River and in rivers across Montana.

Understanding how invasive species affect the stability and function of ecosystems is critical for conservation. Here, we quantified the effect of an actively suppressed invasive species on the Yellowstone Lake ecosystem using a food web energetics approach. We compared energy flux, functional state, and stability of four food web states: a pre‐invasion network and three post‐invasion networks undergoing active invasive species suppression, namely, initial invasion, expansion, and decline. Invasion caused ≥ 25% change (±) in energy flux for most consumers, and total flux increased twofold post‐invasion. Flux to the species of conservation concern, Yellowstone cutthroat trout (Oncorhynchus virginalis bouvieri), was 2.8 times less post‐invasion versus pre‐invasion, whereas invasive lake trout (Salvelinus namaycush) flux was up to 17.3 times higher compared to the initial invasion network. The dominant functional state and food web stability did not change post‐invasion, likely due to introduction of a generalist predator and the stabilizing effect of suppression. Lake trout invasion in Yellowstone Lake caused large changes to energy flux, shifting dominant fluxes away from the species of conservation concern, despite not changing functional state or stability. We demonstrate that changes in energy flux may signal invasions in ecosystems, but functional state or stability may not necessarily reflect the magnitude of invasion influences. For invaded fish communities, a better understanding of how the invasive species control the food web beyond just the direct influence on prey can be achieved by investigating energy flux, functional state, and food web stability. Furthermore, evaluating the effect of suppression beyond the invasive species can demonstrate the far‐reaching value of suppression management actions for conservation.

Objective: Long-term standardized monitoring programs are fundamental to assessing how fish populations respond to anthropogenic stressors. Standardized monitoring programs may need to adopt new methods to adapt to rapid environmental changes that are associated with a changing climate. In the upper Yellowstone River, Montana, biologists have used a standardized, mark–recapture monitoring protocol to annually estimate the abundance of trout since 1978 to assess population status and trends. However, within the past two decades, climate change has caused changes in discharge timing that have prevented standardized monitoring from occurring annually. Methods: We investigated the feasibility of using two analytical methods, N-mixture models and mean capture probability, for estimating the abundance of three trout species in the upper Yellowstone River using the historical long-term data set; these methods allow abundance to be estimated when a mark–recapture estimate cannot be obtained due to hydrologic conditions. Result: When compared with abundance estimates from mark–recapture methods, N-mixture models most often resulted in negatively biased abundance estimates, whereas mean capture probability analyses resulted in positively biased abundance estimates. Additionally, N-mixture models produced negatively biased estimates when tested against true abundance values from simulated data sets. The bias in the N-mixture model estimates was caused by poor model fit and variation in capture probability. The bias in the mean capture probability estimates was caused by heterogeneity in capture probability, likely caused by variable environmental conditions, which were not accounted for in the models. Conclusion: N-mixture models and mean capture probability are not viable alternatives for estimating abundance in the upper Yellowstone River. Thus, exploring additional adaptations to sampling methodologies and analytical approaches, including models that require individually marked fish, will be valuable for this system. Climate change will undoubtedly necessitate changes to standardized sampling methods throughout the world; thus, developing alternative sampling and analytical methods will be important for maintaining the utility of long-term data sets.

Prioritizing restoration opportunities effectively across entire riverscape networks (i.e., riverine landscape including floodplain and stream channel networks) can be difficult when relying on in‐channel, reach‐scale monitoring data, or watershed‐level summaries that fail to capture riverscape heterogeneity and the information necessary to implement restoration actions. Leveraging remote sensing and geospatial tools to develop spatially continuous information across nested hierarchical scales may support increased understanding of local riverscape reaches in their broader network context. Using riparian (vegetation) and geomorphic (elevation) indicators to assess status of riverscape health, along with a measure of restoration capacity (valley bottom area), could be adapted to fit specific management goals related to riverscape restoration. Frameworks using remotely sensed vegetation and elevation data to prioritize restoration continuously across riverscapes at restoration‐relevant, reach‐scales may uphold the ecosystem services provided by riverscapes. By incorporating local knowledge and identifying caveats for using these datasets, continuous inferences can be applied at network scales (watershed to regional extent and reach‐scale resolution) to prioritize restoration over a wide variety of ecoregions.This article is categorized under: Water and Life > Conservation, Management, and Awareness Water and Life > Methods Water and Life > Stresses and Pressures on Ecosystems

In Yellowstone Lake, Wyoming, the largest inland population of nonhybridized Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri, hereafter Cutthroat Trout, declined throughout the 2000s because of predation from invasive Lake Trout Salvelinus namaycush, drought, and whirling disease Myxobolus cerebralis. To maintain ecosystem function and conserve Cutthroat Trout, a Lake Trout gill netting suppression program was established in 1995, decreasing Lake Trout abundance and biomass. Yet, the response of Cutthroat Trout to varying Lake Trout suppression levels, collectively with the influence of disease and climate, is unknown. We developed an ecosystem model (calibrated to historical data) to forecast (2020–2050) whether Cutthroat Trout would achieve recovery benchmarks given disease, varying suppression effort, and climate change. Lake Trout suppression influenced Cutthroat Trout recovery; current suppression effort levels resulted in Cutthroat Trout recovering from historical lows in the early 2000s. However, Cutthroat Trout did not achieve conservation benchmarks when incorporating the influence of disease and climate. Therefore, the National Park Service intends to incorporate age-specific abundance, spawner biomass, or both in conservation benchmarks to provide better indication of how management actions and environmental conditions influence Cutthroat Trout. Our results illustrate how complex interactions within an ecosystem must be simultaneously considered to establish and achieve realistic benchmarks for species of conservation concern.

The decomposition of vertebrates in lake ecosystems has been largely understudied despite being a vital part of ecosystem processes. Invasive lake trout (Salvelinus namaycush) invaded Yellowstone Lake and caused a decline in the native Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri) population. To restore Yellowstone cutthroat trout, lake trout were suppressed by gillnetting annually since 1995 and has continued to present, with most carcasses deposited in the profundal zone (>70 m). As a part of suppression management, a fraction of carcasses from gillnetting were ground and placed on littoral spawning sites (causing lake trout embryo mortality via hypoxia). We conducted experiments (2018 and 2019) to determine how carcass state (i.e., whole vs. ground) and location of deposition (i.e., profundal or littoral) affected decomposition rates. Whole carcasses in the depths of Yellowstone Lake decomposed nine times slower (rate of decay, k = −0.0075 day⁻¹; 95% CI = −0.0063–−0.0089) than ground carcasses in the littoral zone (k = −0.0679 day⁻¹; 95% CI = −0.0590–−0.0768). Whole carcasses had a half-life of 91 days while ground carcasses had a half-life of 10 days. We showed that carcass state and location cause a differential decomposition for lake trout carcasses in Yellowstone Lake. Understanding carcass persistence in lakes can inform the management of suppression-produced carcasses in large lakes and provide insight into potential effects of carcass deposition from other sources, such as spawning events or fish kills, on nutrient cycling.

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.