Smallmouth Bass Micropterus dolomieu are highly invasive in rivers of the Upper Colorado River Basin (UCRB) and are considered a substantial threat to native fish recovery. Smallmouth Bass can negatively impact native fishes through predation and could compete for potentially limiting resources. Quantification of diets is a crucial step to estimate predatory and competitive threats of nonnative fishes, but predicting when and where potential deleterious effects may occur is challenged by spatial and temporal variation in feeding habits and resource availability (Pintor and Byers 2015). Moreover, variation in abundance and physiological rates of nonnative fishes across environmental gradients can drive fluctuations in consumption, particularly if predatory efficiency changes under certain conditions (e.g., turbidity, temperature, etc.; Ward and Vaage 2019; Hedden et al. 2021). In addition, assessing diets over broad spatial and temporal scales that encompass gradients of environmental conditions allows for identification of potential 'hot spots' of predatory impacts on native fishes. Currently, intensive and expensive bass removal efforts occur throughout the UCRB in the Colorado, Green, White, and Yampa rivers. For instance, in 2021, researchers conducted ~2,800 hours of electrofishing and removed nearly 38,000 Smallmouth Bass during targeted removal efforts; yet Smallmouth Bass catch rates were the highest since removal began (Donald Tuttle, Colorado State University, personal communication). Removal crews catch all age classes of Smallmouth Bass including juveniles, sub-adults, and adults. Captured bass <100 mm TL are typically counted, and larger bass are measured for length and weight before being euthanized. However, despite this intensive effort, there is little understanding of the potential predatory and competitive threat posed by these fish. To date, diet analysis has only been conducted in the Yampa River, where rates of predation on fish varied widely, presumably due to differences in availability (Johnson et al. 2008). Age and growth data, which are important for understanding population dynamics, are also limited across the UCRB. A more comprehensive assessment of Smallmouth Bass diet in Utah could help identify areas with high incidence of native fish predation or potential competition (if coupled with surveys of availability) to help prioritize intensive removal efforts and aid in conservation of native fishes. All current removal efforts in the White River are done via cataraft electrofisihing and must be done in spring due to flow limitations. Therefore, there is concern that no removal efforts occur later in the summer and early fall. This presents an important data gap and management uncertainty, as YOY Smallmouth Bass in particular may be abundant during this time period. This proposed work would take advantage of existing broad-scale removal efforts and use removed bass to expand our knowledge on the feeding ecology and population dynamics of this highly invasive species. In addition, we will sample and remove Smallmouth Bass during summer time periods that are currently not part of removal efforts. Understanding spatial variability of the impacts of nonnative species on native species can help focus limited funding and personnel time to areas where impacts are greatest. In the White River, while Smallmouth Bass abundance and size structure varies annually depending on year-class strength of years past (and potentially escapement from the reservoir), there is a consistent gradient in abundance and size structure from Taylor Draw Dam to the confluence with the Green River (i.e., Smith, Breen et al. 2016-2021). For example, in 2021, CPUE was 28.3 bass/hr and fish ≥200 mm TL made up 31% of total catch in reaches between Taylor Draw Dam and the Colorado-Utah border. Catch rates on the Utah side were similar (23.5 bass/hr), but fish ≥200 mm TL made up only 8% of total catch. Because rates of piscivory often increase with predator size (e.g., Hedden et al. 2022), the predatory impacts on native fishes might be much higher in the upper reaches of the White River despite similar catch rates throughout. Alternatively, predation rates could be higher in the lower White River in Utah, because it is suspected to be a nursery area for native fishes. However, if competition for potentially limiting resources exists between juvenile Smallmouth Bass and juvenile native fishes, these impacts might be strongest in the lower reaches of the White River. Whereas the White River serves as a good example of how an increased understanding of bass diet and population structure could identify biotic interaction "hotspots", these same ideas could apply more broadly to rivers across the UCRB that still contain native fishes. Quantifying spatial variability in fish diets is a crucial step in understanding potential consequences of fish invasions, and could help guide management efforts. For example, we could identify critical time periods of local areas for Smallmouth Bass removal, and others that are less important. In addition, electroshocking removal efforts could have deleterious effects on native fishes, which are also shocked. Frazer et al. 2017 demonstrated that imperiled Flannelmouth and Bluehead suckers that were shocked and handled demonstrated a post handling flight response; most suckers of both species emigrated from a tributary to the UCRB (Coal Creek) within 48 h after being captured while suckers that were detected only via arrays (not shocked and handled) remained resident for 10--12 d and likely spawned. This important finding suggests electroshocking native fishes should be kept at the minimum possible effort. The findings of this proposed project could help prioritize Smallmouth Bass removal efforts to the most important time periods and will provide critical information on Smallmouth Bass age-size distribution and densities in summer as well as potential removal techniques for this time period.

1) Quantify the diet and trophic niches of Smallmouth Bass (already being removed) and juvenile native fishes in the White River, an important tributary of the Upper Colorado River Basin, to determine predation rates and potential for competition 2) Quantify prey resource availability to place fish diets in context of non-native fish predation on native fishes and the potential for competition 3) Sample and remove Smallmouth Bass in summer months to determine age-size distribution and densities as well as exploring potential removal techniques for this time period, which is currently not part of the Removal Program.

The White River is approximately 195 miles (314 km) long, in the U.S. states of Colorado and Utah and is a tributary of the Green River (which flows into the Colorado River). It flows into Utah in the NE corner of the state, has a contemporary median summer base flow of 309 ft/s. This project is focused on the lower White River between Taylor Draw Dam (river mile [RM] 104.3) and the Green River confluence (RM 0). The lower White River is considered one of the few remaining functioning riverine ecosystems in the entire upper Colorado River basin and is thus of high conservation concern (Anderson et al. 2019; Pennock et al. 2021a, 2021b). The White River is the only major tributary of the Green River that retains a relatively natural flow regime with complex habitat required for all life-stages of native fishes.

The White River Management Plan (Pennock et al 2021) is intended to help guide conservation, restoration and management of the lower White River over the next several decades and is also developed as an adaptive management plan to facilitate learning. The recommended conservation and restoration actions are intended to maintain and enhance native riparian vegetation and instream habitat for native desert fishes including federally endangered Colorado Pikeminnow (Ptychocheilus lucius), federally endangered Razorback Sucker (Xyrauchen texanus), Speckled Dace (Rhinichthys osculus), Bluehead Sucker (Catostomus discobolus), Flannelmouth Sucker (C. latipinnis), and Roundtail Chub (Gila robusta). The objectives of this proposed work fits within this broad goal and also the Plan objective to: "Conserve necessary and sufficient habitat to allow for thriving native fish, vegetation communities, and riparian dependent animal species."robusta).

Not applicable

The project will result in greater availability of high water quality habitat for native fishes with less predatory and competitory impacts from invasive smallmouth bass.

All permits are already in place as part of the Upper Colorado River Recovery program workplans. We will follow national Institutional Animal Care and Use Committee (IACUC) guidelines and will be required to apply for an IACUC permit via Utah State University. UDWR has the necessary permits for handling fishes as part of the existing and on-going removal efforts. USU would need to be added to the Colorado permit. This project will cause no additional time or effort for UDWR or USFWS removal crews.

Extensive Smallmouth Bass removal efforts are already underway by UDWR each year in the White River in Utah as part of the Upper Colorado River Endangered Fish Recovery Program. The White River also provides an excellent location for this research, as it is one of only two tributaries that still retain a mostly natural spring snowmelt flood-driven hydrograph. Consequently, complex habitat needed by fish remains, thus removing one other additional common limiting factor (degraded habitat; Pennock et al. 2022) and allowing us to concentrate on the non-native interactions. In 2022 and 2023, UDWR sampled the White River for removal efforts in one 3-day trip from Big Trujillo to Bonanza Bridge, one 3-day trip from Bonanza Bridge to Enron, and one 5-day trip from Big Trujillo to Enron. The USFWS and CPW also sample fish in this reach, offering up additional possibilities for samples at alternative times. The USU graduate student will accompany the removal crews and collect euthanized SMB at each sampling event and by existing ~5 mile reach (Smith et al al. 2021, see Table 1). In addition, USU will make up to 5 additional sampling/removal trips during summer and early fall base flows. We will work with UDWR and explore a variety of low flow removal efforts including canoe electroshocking, backpack electroshocking, seining, and perhaps a creative new pack-raft technique. During these trips, we will move much more slowly than the typical removal trips and be sure to target low flow habitats potentially preferred by YOY SMB. For each reach, we will retain up to 50 samples from each life stage (juvenile, sub-adult, and adult) as well as 10 (lethal) samples of predominant juvenile native fishes. Stomachs will be removed by the USU graduate student in the field and preserved in ethanol, and flesh samples for isotopes will be collected and air dried. A non-lethal tissue sample will be taken from a subsample of native fish from a fleshy fin (e.g., dorsal fin) for isotopic analyses. All sample fish will be weighed and measured. To inform isotopic analyses, on several trips during peak productivity, which typically occurs during base flows mid- to late summer, we will sample predominant potential prey items from the electroshocking catch for fish, using a D frame kick net for invertebrates, from rock scrubs for periphyton, and direct subsamples of submerged macrophytes. The graduate student will make sure all relevant data are collected to quantify relative prey availability. Back in the laboratory, the graduate student and trained undergraduate technicians will analyze the diets. We will identify diet items to the lowest taxonomic group possible. We will weigh prey fish individually, and for invertebrate prey, we will obtain blot-dry wet weight en masse to the nearest 0.01 g for each taxonomic group. To determine potential and interspecific competition we will calculate diet overlap using Schoener's index (Schoener 1970). The single value α is diet overlap from 0 (no overlap) to 1 (complete overlap), and values greater than 0.6 are indicative of significant diet overlap (Schoener 1970). We will utilize carbon (δ13C) and nitrogen (δ15N) stable isotopes to further explore a time-integrated representation of predator diets and assess potential interspecific competition. In addition, isotopes overcome some of the uncertainty associated with diet data caused by empty stomachs. Samples will be dried for 48 h at 70 °C, ground into a homogenized powder, and placed into pre-weighed tin capsules. Isotope samples will be processed at the Utah State University Stable Isotope Lab for analysis of δ13C and δ15N, and percent composition of both carbon and nitrogen. Standard quality control protocols, including multiple blind quality control samples of similar, known composition, will be used. Isotopic signatures are reported in δ-notation (Vander Zanden and Rasmussen 1999; Eq. 2). The standard for δ13C is PeeDee belemnite, and the standard for δ15N is atmospheric nitrogen; these standards insure a precision of ± 0.3‰. To calculate trophic position of fishes we will use Vander Zanden and Rasmussen (1999; Eq. 3). We will assume primary consumers have a trophic position of 2 and a δ15N trophic fractionation value of 3.4 ± 1.1‰ (Minagawa and Wada 1984). We will use a two source baseline correction with mean δ15N value of a dominant grazer. To evaluate trophic overlap of Smallmouth Bass with other native fishes, we will use SIBER (Stable Isotope Bayesian Ellipses; Jackson et al. 2011) in R to estimate pairwise overlap of bivariate (δ13C and δ15N) niche regions between species or size class. The SIBER model produces ellipses around the centroid that include ± 1 SD. We will calculate the percent species overlap at the intersection of ellipse areas. Additionally, as a measure of carbon source use within lakes, we will calculate percent aquatic versus terrestrial contribution using a two-source mixing model (Vander Zanden and Vadeboncoeur 2002; Eq. 4). For trophic position comparisons between species and life stages, we will use Welch's t test of δ15N values in R, because this test does not assume equal variances among our variable sample sizes. To compare across all size classes, we will use a one-way ANOVA. For this analysis, we will confirm homogeneity of variance and normal residuals, using α ≤ 0.05 to determine significance, and Tukey's test to determine pairwise differences. Field equipment (electroshocking raft and fish sampling equipment) will be largely available from UDWR as part of the removal effort. In addition, USU owns both a canoe electroshocker, many seines, and backpack electroshockers. We will supply our own raft to carry camping gear and additional field supplies (which we already own). Laboratory equipment for diet and isotope analyses are already available at the Budy Fish Ecology Lab.

Please see above under Methods.

Utah Division of Wildlife Resources US Geological Survey - UCFWRU US Fish and Wildlife Service Upper Colorado River Endangered Fish Recovery Program (please also see letter of support)

The results of this project will guide future removal efforts towards a goal of being most effective at SMB removal and at minimizing SMB impacts on native fishes.

* Currently, intensive and expensive bass removal efforts occur throughout the UCRB in the Colorado, Green, White, and Yampa rivers. A more comprehensive, range-wide assessment of Smallmouth Bass diet in the UCRB in the White River could help identify areas with high incidence of native fish predation or potential competition (if coupled with surveys of availability) to help prioritize intensive removal efforts and aid conservation of native fishes. This proposed work would take advantage of existing broad-scale removal efforts and use removed bass to expand our knowledge on the feeding ecology and population dynamics of this highly invasive species across the Upper Colorado River Basin. * Quantifying spatial variability in fish diets is a crucial step in understanding potential consequences of fish invasions, and could help guide management efforts. Understanding spatial variability of the impacts of nonnative species on native species can help focus limited funding and personnel time to areas where impacts are greatest. For example, we could identify critical time periods of local areas for Smallmouth bass removal, and others that are less important. * In addition, electroshocking removal efforts could have deleterious effects on native fishes, which are also shocked. The findings of this proposed project could help prioritize Smallmouth bass removal efforts to only the most important time periods. * Finally, there is management uncertainty about the lack of removal efforts during base flows and interest in determining if there are removal techniques that could be effective during this time period, in particular for YOY SMB