Introduced Sport Fish and Fish Conservation in a Novel Food Web: Evidence of Predatory Impact

This study addressed a fundamental question in applied ecology and conservation; what is the predatory impact of introduced piscivorous sport fish on imperiled native fish populations? More specifically, which of many introduced species and size-classes represent the greatest threats and should be targeted for control? In order to explore this important question, an integrated analysis of stable isotopes, quantified observed diet analysis, and stable isotope mass-balance models were used to quantify trophic interactions. These tools were used to construct food web models that were then compared to draw inferences regarding the relative contribution of prey fish, including rare native fish, to the diet of introduced sport fish. The stable isotope-derived food web illustrated a slight decoupling in energy flow between a pelagic and a benthic-littoral sub-web. The quantified diet analysis suggested piscivory was low overall, and that the introduced sport fish assemblage relied heavily on zooplankton and aquatic insect prey. The integrated stable isotope and quantified diet analysis demonstrated that the consumption of prey fish, particularly pelagic prey fish, was typically underestimated using stomach content analyses. From the evaluation, comparison, and integration of food web models, I suggest that substantial predation was occurring on the early life stages of Utah Lake fishes, including native fishes, and it was not being observed using stomach content analysis. My comparative modeling demonstrated that introduced sport fish are an impediment to native fish conservation and identified the small size-class of white bass as the most immediate threat

Compensatory Recruitment, Dynamic Habitat, and Selective Gear Present Challenges to Large-Scale Invasive Species Control

Control of long‐established invasive species to aid threatened native species presents major logistic and economic challenges. Invasive common carp (Cyprinus carpio ) recently accounted for over 90% of the fish biomass in Utah Lake (Utah, USA), driving many undesired changes to ecosystem structure. Carp control efforts have removed \u3e12,000 tons of carp from the system over 10 yr. However, the impact of recent removal efforts on carp population structure and dynamics remains unclear. We develop an integrated age‐structured population model incorporating carp harvest data with independent standardized monitoring data to evaluate population‐level consequences of the removal effort and evaluate future removal scenarios. Specifically, we estimate the age structure, biomass, and population dynamics of carp in Utah Lake and simulate carp population responses to potential future management strategies. The model estimates carp population biomass has decreased to approximately 27.6% of pre‐removal biomass and 23.4% of the greatest estimated biomass. However, carp removal gear is highly selective of older, larger individuals, and current removal efforts have had little impact on young age‐classes. Evidence of recent strong juvenile cohorts of carp suggests a compensatory response to removal efforts that may increase total biomass as these age‐classes mature. Simulations of carp population response to potential alternative harvest approaches demonstrate that the current gear is unlikely to maintain carp biomass below target thresholds even at substantially increased levels of effort due to gear selectivity, compensatory recruitment, and periodic large recruitment events resulting from lake level increases. A hypothetical gear selecting for juvenile carp is predicted to have a meaningful chance of maintaining low carp biomass, particularly if effort is at least tripled over current levels. These simulations illustrate the value of analytically exploring multiple management approaches when conducting adaptive management. Even when historically used methods have been successful at approaching management objectives, their suitability may change as ecosystem state changes. Periodic re‐assessment of ecosystem state and willingness to explore potential alternative approaches is critical to the long‐term success of any management program as socio‐ecological systems and knowledge change

Modeling Vulnerability of Juvenile June Sucker to the Piscivore Community in Utah Lake

Freshwater fish have experienced the highest extinction among all North American vertebrates, with interactions with introduced species among the leading factors of freshwater fish declines. In Utah Lake, effective hatchery stocking efforts have allowed the spawning abundance of threatened June sucker (Chasmistes liorus) to increase substantially over the past decade. Yet, recruitment of wild-spawned individuals remains very limited. A recent survivorship study of June sucker found individuals \u3c 300mm experienced exceptionally poor survival, indicating a recruitment bottleneck persists despite significant recovery efforts. Current recovery efforts of Utah Lake have broadly targeted habitat improvement; However, prior studies have shown that large predation pressures by nonnatives have achieved recruitment bottlenecks in closely related species, suggesting predation could be limiting recovery efforts. Therefore, failure to address these pressures, if present, would result in overall recovery failure. Furthermore, predator management has focused on targeting Invasive Northern Pike (Esox Lucius) for potential impacts on June sucker. However, the impacts of more abundant nonnative piscivores remain largely unspecified. Here, we use a simulation approach based on empirical data to examine the relative predation pressure on June sucker of different lengths from individual piscivore species as well as the piscivore community as a whole. We generated a distribution of the lengths of prey consumed by the piscivore community by (1) randomly selecting a piscivore species based on the observed relative abundance in catch data, (2) randomly selecting a piscivore length based on length-at-age from literature-derived estimates and age composition, and (3) randomly selecting prey lengths based on predator-specific quantile regressions of observed fish prey lengths as a function of predator length. We repeated the simulation 10,000 times to achieve a representative model. We found the overall distribution of prey items consumed was unimodal and right-skewed with a median prey length of 74mm (95% simulation interval: 13 – 238 mm) and a maximum prey length of 535mm. The third quartile (120mm) was roughly the same length as that of June Suckers at their first annulus (111mm), suggesting that most predation on June sucker is occurring during their first year. Abundant mesopredators, Channel catfish, White bass, and Black bullhead drove the vast majority of predation pressure, consuming \u3e 98% of prey items consumed, while high trophic level predators accounted for the remaining \u3c 2%. Together, these results suggest substantial predation pressure on age-0 June sucker, particularly from Channel Catfish and White Bass, may explain the limited recruitment of wild June sucker despite the steadily increasing abundance of spawning adults. Thus, predation on juvenile June Suckers presents a significant barrier to the establishment of a self-sustaining June Sucker population in Utah Lake

Relative Vulnerability of Juvenile June Sucker to White Bass Predation in Utah Lake

Introduced species are one of the leading causes of native fish population declines globally, and predation by introduced species can impact native fish abundance, growth, and survival. In Utah Lake, many nonnative predators have been intentionally introduced over the past century to promote recreational fishing, including White Bass (Morone chrysops), one of the most abundant species in the system. June Suckers (Chasmistes liorus) are endemic to Utah Lake, but have experienced severe declines in abundance due to a combination of historical overexploitation, habitat degradation, and the introduction of nonnative fishes. Previous studies have indicated that first-year survivorship of June Suckers less than 100 mm is effectively zero, but survivorship increases to 50% by 300mm, suggesting the presence of high predation mortality for smaller individuals. Here, we examine the relative vulnerability of juvenile June Suckers to White Bass predation to determine if White Bass could be contributing to the high mortality of June Suckers at sizes less than 300mm. We used White Bass diet data collected via gastric lavage and dissection from 1983 to 2021 to fit quantile regressions of predator length to fish prey length. From these regressions, we determined the distributions of prey fish lengths in diets of White Bass of a given length, across all observed White Bass lengths. We used a catch-curve to estimate White bass relative abundance across all lengths observed, using commercial seine data from 2020 and accounting for gear size-selectivity. We then simulated the length and number of fish prey consumed by all White Bass using the abundance of White Bass at each length and the quantile regressions of prey length to predator length. We found the distribution of the length of prey items consumed to be unimodal and right-skewed with a median prey length of 39.306 mm (95% simulation confidence interval: 14.017 mm – 91.842 mm). The upper limit of the distribution of White Bass simulated prey item lengths was less than the length of June Suckers at first annulus (111 mm), suggesting that White bass predation on June Suckers would be limited to their first year. Given the high densities of White Bass in Utah Lake, predation on larval June Suckers may present a significant barrier to survival past the first annulus and could impede the establishment of a robust naturally reproducing June Sucker population in Utah Lake. Further research is needed to understand the possible impacts of other nonnative predators, such as Walleye (Sander vitreus) and Channel Catfish (Ictalurus punctatus). Our results establish a valuable predator-prey model that management agencies and stakeholders can use to inform effective management plan(s) and potentially mitigate the impact of non-native White Bass on June Sucker recruitment in Utah Lake

Development of a Utah Lake Ecosystem Monitoring Plan: Food Web Ecology, Restoration and Conservation

In order to provide data that reflects changes in the status or trends of ecosystems or ecosystem components, environmental monitoring efforts aim to measure important resources over time. When collected properly, such data can be essential in guiding adaptive management during restoration or conservation implementation. Utah Lake, located in north-central Utah, is a large, eutrophic shallow lake ecosystem that has endured a suite of anthropogenic disturbances over the last 200 years. Among the more disruptive disturbances are the introduction of an exotic fish, Common carp (Cyprinus carpio), and excessive nutrient pollution, which are the primary catalysts for Utah Lakes current eutrophic state. Common carp function as ecosystem engineers as their benthic, omnivorous feeding habits can drive a shift from a clear water, highly vegetated state, to a turbid water state with minimal aquatic vegetation. Nutrient inputs exacerbate the problem by stimulating dense filamentous algae blooms which further limit light penetration beneath the water surface thus ultimately preventing the establishment of submerged aquatic vegetation. As a first step toward restoring the Utah Lake ecosystem to a more natural state, managers have initiated one of the largest biomanipulation (fish removal) efforts known to take place, where the annual goal is to remove five million pounds of carp for six consecutive years. Recognizing the importance of monitoring potential changes due to carp removal, researchers and managers have initiated the development of a Utah Lake ecosystem monitoring plan. The goal of this project is to develop a standardized sampling approach to essential ecosystem parameters, in which sample design and statistical accuracy and precision prove to be as sufficient as possible. Using food web ecology, we identified priority ecosystem parameters for monitoring efforts, and provide hypotheses regarding changes in parameters that may result from carp removal. We evaluated historical and recent data, as well as data collected specifically for this study related to Utah Lake algae and diatoms, water chemistry and quality, zooplankton, macroinvertebrates, fish, and macrophytes. Our results highlighted the importance of considering food web ecology during restoration efforts, and provided a framework that may facilitate adaptive management during one of the largest natural experiments known to be performed

An Energetic Assessment of Predator Consumption in Utah Lake with Consideration of Future Options for June Sucker Recovery

The prolific invasion of nonnative piscivores to Utah Lake is considered a major impediment to recovery of the endangered June sucker. Our overall goal was to estimate predator consumption rates of the nonnative piscivorous fishes that currently dominate Utah Lake and to evaluate their potential impact on June suckers and other prey under current conditions and potential future management options. We synthesized all available information on lake temperature regime and the size, growth, and diet of walleye, catfish, white bass, and crappie as inputs into a bioenergetics analysis of predator consumption across one year, representing current conditions. We extrapolated our estimates of individual predator consumption to a population level using relative CPUE as a surrogate for abundance. Lastly, we compared isotopic-derived trophic position to diet composition, and re-ran a subset of model scenarios accordingly. Dominant predators (e.g., white bass, walleye, and catfish) consumed a high proportion of white bass, perch, carp, and crappie. However, unidentified fish in the diet make up a substantial portion of predator consumption; if even a small portion of the unidentified fish component is suckers, predatory impact on June suckers is likely significant. In addition, our predictions of predator consumption under a low carp abundance scenario (i.e., after removal) suggest that in years of poor recruitment of other predominant prey items (e.g., white bass and crappie), some predators may switch prey and consume a higher concentration of June suckers. Our comparison of isotopic-derived position to diet data indicated that: 1) Utah Lake is characterized by two primary pathways, a more pelagic food web and a more littoral food web, and 2) there appears to be fairly close correspondence between isotope-derived trophic position and diet composition for large top piscivores, and 3) more generalists (e.g., white bass) appear to be eating more fish (and potentially June suckers) than appears in the static diet samples. Lastly, we note that current modeling evaluations of the potential effects of different management scenarios are extremely limited by the lack of abundance information for almost all fishes of Utah Lake. Obtaining reliable estimates of relative abundance should be a top priority for future monitoring