Browsing by Author "Johnson, Brett, advisor"
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Item Open Access Arctic Char Salvelinus alpinus can enhance fisheries in reservoirs with trophic constraints(Colorado State University. Libraries, 2014) Olsen, Devin M., author; Johnson, Brett, advisor; Boone, Randall, committee member; Myrick, Christopher, committee member; Lepak, Jesse, committee memberThe 20th century was a period of rapid reservoir construction in the western United States. Initially, many of these reservoirs hosted productive recreational fisheries for introduced salmonids, but then waned from oligotrophication, dam operations, and the effects of introduced opossum shrimp Mysis diluviana. Managers have sought alternative fish species that could withstand these trophic constraints. In 1990 the state of Colorado introduced Arctic Char Salvelinus alpinus into Dillon Reservoir hoping they would prey on Mysis and produce a valuable "boutique fishery". My study investigated the outcomes of this introduction. I found that the introduction resulted in a reproducing population, creating one of the only public fisheries for Arctic Char in the lower 48 states of the USA, and the southernmost population in the world. Arctic Char diet was composed primarily of Mysis shrimp, and their growth was among the fastest of lacustrine populations worldwide. While bioenergetics simulations showed that approximately 3-6 times as many Arctic Char would need to be stocked annually to effectively control Mysis shrimp, Arctic Char did channel energy formerly sequestered in Mysis into desirable recreational fish biomass. Despite this desirable ecosystem service, the stocking program is paradoxical. In an era when nonnative species comprise a primary threat to aquatic biodiversity, condoning new introductions is concerning. However, in many human-dominated environments such as reservoirs, exotic fishes already comprise the majority of species. Fishery managers are left with the problem of choosing relatively innocuous strategies that can still provide recreational benefits in systems plagued by a variety of anthropogenic stressors.Item Open Access Management strategies for multi-use recreational fisheries: coexistence of kokanee and trophy lake trout in western waters(Colorado State University. Libraries, 2013) Pate, William M., author; Johnson, Brett, advisor; Kendall, William, committee member; Lepak, Jesse, committee member; Teel, Tara, committee memberKokanee Oncorhynchus nerka are stocked in coldwater reservoirs throughout the western United States for sport fishing and they are a popular fish for both managers and anglers alike. Lake trout Salvelinus namaycush have also been introduced to many western reservoirs, partly because they can attain relatively large sizes (> 30 kg). These piscivores require a large, high quality forage base to sustain high growth rates, and kokanee can fulfill this requirement. However, where lake trout and kokanee co-occur, lake trout often grow in size and numbers and their consumptive demand increases beyond the capacity of the kokanee population to support. Consequently, kokanee abundance declines followed by precipitous declines in lake trout growth and body condition. My work focused on finding a management strategy that could produce sustainable fisheries for both in Blue Mesa Reservoir, Colorado, where lake trout appear to have diminished kokanee abundance. In 2009, managers began a lake trout removal program in an attempt to achieve the primary management goal of a sustainable, hatchery-dependent kokanee population and the secondary goal to provide a trophy lake trout fishery. I developed an age-structured kokanee population model using estimates of natural mortality, harvest, and predation from lake trout with a fixed annual stocking quota of kokanee fry. Age-specific estimates of natural and fishing mortality were estimated using an allometric model and creel survey, respectively. I then determined lake trout consumptive demand on the kokanee population with a bioenergetics model by estimating lake trout abundance, growth rates, diet, and energy densities of predator and prey species. Then alternative management scenarios to reduce lake trout consumptive demand were evaluated using the Fishery Analysis and Modeling Simulator parameterized for the Blue Mesa Reservoir lake trout population. After estimating the current level of lake trout removal (μ = 0.231 for age-4 through age-9), it was incrementally increased to determine the level that allowed for a stable kokanee population (μ = 0.381). The simulations suggested that removal of lake trout must be intensified if kokanee and fast-growing lake trout are to persist in Blue Mesa Reservoir, Colorado.Item Open Access Origins and movements of invasive piscivores determined from otolith isotopic markers(Colorado State University. Libraries, 2011) Wolff, Brian, author; Johnson, Brett, advisor; Clements, William, committee member; Sanford, William, committee memberI examined the utility of the strontium isotope (87Sr/86Sr) ratio in fish otoliths to determine the origins and movements of invasive piscivores in the Upper Colorado River Basin (UCRB). My goal was to determine if invasive piscivores entered riverine habitat of endangered fishes in the UCRB by escaping from reservoirs, and if so, determine their likely origins. Fishes were collected from 14 reservoirs and rivers directly downstream. My objectives were to examine the distinctiveness of 87Sr/86Sr ratios from fishes in different reservoirs, and temporal stability and interspecies variability of 87Sr/86Sr ratios of fishes within reservoirs. Variance components analysis and model selection using an information theoretic approach were used to rank relative importance of reservoir location, species and year on 87Sr/86Sr ratios. My results showed that in most cases 87Sr/86Sr ratios were unique across reservoirs, overlapped among species in a given reservoir, and were temporally stable across years. I was able to identify the likely reservoir of origin of river-caught fish, and I was able to determine, in some cases, the year of escapement. Overall, my results demonstrate the utility of 87Sr/86Sr ratios for tracking fish movement and origin in river-reservoir systems and provide important insights into processes that affect escapement risk such as dam operations, weather conditions, fish behavior and fish physiology. Analysis of stable isotopes hydrogen (δ2H), carbon (δ13C), and oxygen (δ18O) were performed on a sub-set of otoliths from the 87Sr/86Sr ratio analysis to determine if additional markers aid in discriminating between groups, particularly when 87Sr/86Sr ratios are similar among different locations. A canonical discriminant function analysis was used to visually differentiate different locations using natural isotopes. These results showed that δ13C and δ18O provide very useful separation among different groups, particularly when in conjunction with 87Sr/86Sr ratios.Item Open Access Seasonal trophic niche dynamics of mottled sculpin and juvenile brown trout in a regulated Rocky Mountain river(Colorado State University. Libraries, 2023) Platis, Nitsa, author; Kanno, Yoichiro, advisor; Johnson, Brett, advisor; Kampf, Stephanie, committee memberIn temperate ecosystems, resource availability fluctuates seasonally due to changes in environmental conditions and productivity throughout the year. Intra- and inter-specific trophic niche overlap under resource limitation is a measure of competitive interactions and influences species coexistence and community dynamics, but patterns of this overlap are highly variable among regions and communities. In this study, we analyzed stomach content samples and stable isotope signatures to evaluate seasonal trophic niche dynamics of mottled sculpin Cottus bairdii (80-130 mm total length) and juvenile brown trout Salmo trutta (80-200 mm) in the Blue River, Colorado, USA, where aquatic production varies seasonally and has been reduced due to upstream dams. Prey biomass (i.e., benthic macroinvertebrates) peaked in spring and declined through summer and autumn. Stomach content and stable isotope analysis results revealed diet composition of mottled sculpin and brown trout shifted seasonally in response to changes in resource availability. In autumn, both species exhibited the highest frequency of empty stomachs and expanded population trophic niches due to increased inter-individual diet variation despite decreased individual trophic niche breadth. Interspecific trophic niche overlap was relatively high across all seasons, but the lowest degree of overlap occurred in autumn of both years. Isotopic analysis revealed similar trends of wider isotopic niches and reduced overlap in autumn compared to spring. Taken together, our data indicate that seasonal variation impacts individual and population-level trophic niche dynamics, as well as interspecific niche overlap between mottled sculpin and juvenile brown trout. These two species occur sympatrically in many coldwater streams throughout North America, and our findings indicate trophic segregation under resource limitation may serve as a mechanism that facilitates their coexistence.Item Open Access The effects of climate change on high elevation lake ecosystems(Colorado State University. Libraries, 2019) Christianson, Kyle R., author; Johnson, Brett, advisor; Hooten, Mevin, committee member; Denning, Scott, committee member; Myrick, Christopher, committee memberHigh elevation lakes are an important class of the world's fresh water. Nearly 10% of all lakes globally reside above 2,100 m ASL and almost half of the world's population relies on water from high elevation regions. Also, these lakes provide important cool water habitat refugia for aquatic biota. However, high elevation areas are sensitive to changes in climate and are changing faster than other regions. Likewise, secondary effects of a changing climate like drought, forest fire, and eutrophication threaten lake habitats, exacerbating changes from air warming. Despite the importance of high elevation lakes and their increased threat from climate change, little is known about high elevation lakes and their vulnerability to these threats. The goal of my dissertation was first (Chapter 1) to determine historic changes in lake surface temperatures for a set of high elevation lakes in the Southern Rocky Mountains, USA (SRM). Then, I determined potential future changes to thermal stratification (Chapter 2) and the length of the open water season (Chapter 3) for a subset of lakes in the Rawah Wilderness Area (RWA) within the SRM. For these future predictions, I estimated alterations in lake surface and bottom temperatures from multiple stressors, as well as how these changes may affect aquatic habitat for native and nonnative fish species that reside in the region. Although historic lake temperature trend analyses are numerous, remote lakes, including many high elevation lakes, are typically underrepresented due to limited availability of long-term datasets. In Chapter 1, I developed a Bayesian modeling technique to analyze sparse data from high elevation lakes that allowed me to estimate lake surface warming across a large region (SRM). The analysis allowed for inclusion of lakes with few repeated measurements, and observations made prior to 1980 when more intensive lake monitoring began. I accumulated the largest dataset of high elevation lake surface temperatures globally analyzed to date. Data from 590 high elevation lakes in the Southern Rocky Mountains showed a 0.13°C decade-1 increase in surface temperatures and a 14% increase in seasonal degree days since 1955. Like surface temperature trends, many studies have also examined the effects of climate warming on lake thermal stratification, but few have addressed environmental changes concomitant with climate change, such as alterations in water clarity and lake inflow. Although air temperature rise is a predominant factor linked to lake thermal characteristics, climate-driven changes at watershed scales can substantially alter lake clarity and inflow, exacerbating the effects of future air warming on lake thermal conditions. In Chapter 2, I employed the mechanistic General Lake Model (GLM) to simulate future thermal conditions of typical mountain lakes of the western United States. I found that after air temperature, alterations in inflow had the largest effect on lake thermal conditions, changes in wind had the least effect, and large lakes experienced more than double the increase in lake stability than small lakes. Assuming air temperature rise alone, summer stability of mountain lakes of the western United States was predicted to increase by 15-23% at +2°C air temperatures, and by 39-62% at +5°C air temperatures. When accounting for associated changes in clarity and inflow, lake stability was predicted to increase by 208% with +2°C air warming and 318% at +5°C air warming. Finally, the open water duration at high elevations is increasing at a higher rate than at lower elevations. Earlier snowmelt, resulting in decreased ice cover duration, is having a proportionally higher effect on mountain lakes than other regions. But the effect early melt and increased air temperatures have on mountain lake thermal characteristics and implications for fish is unclear. Mountain lakes exhibit a variety of thermal conditions, altering metabolic requirements for ectotherms. In Chapter 3, I coupled GLM with a fish bioenergetics model to assess potential thermal changes and energetic consequences for native Cutthroat Trout (Oncorhynchus clarkii spp.) and nonnative but present Brook Trout (Salvelinus fontinalis) in a continuously mixed polymictic and seasonally stratified dimictic mountain lake during early and nominal snowpack melt in the SRM. I found that early snowmelt alone had a larger consumptive demand for all species than an air temperature increase of 2°C, but combined these environmental changes are most effective. Early melt coupled with 5°C air warming could more than double the food requirements for Cutthroat Trout and Brook Trout. Ultimately, food availability may dictate the future success of fish in mountain regions. My dissertation research expanded the current knowledge of high elevation lake thermal conditions, developed a novel method to utilize sparse datasets, and provided valuable holistic insight to potential future changes in lake thermal structure and habitat suitability for fish while accounting for localized and watershed scale consequences of climate change.Item Open Access Triploid walleye: a new frontier for nonnative predator management in the West(Colorado State University. Libraries, 2022) Farrell, Collin J., author; Johnson, Brett, advisor; Myrick, Christopher, advisor; Hansen, Adam, committee member; Angeloni, Lisa, committee memberTo view the abstract, please see the full text of the document.