Informing Control Efforts for a Prolific Invasive Species: Characterizing Common Carp Spatio-Temporal Distribution and Evaluating the Impacts of Gear Selectivity in Utah Lake

Management programs that aim to reduce the consequences of invasive species are often challenged by populations that can rapidly recover from removal efforts. Selectivity, the relative impact of harvest on different size classes, can contribute to population recovery when younger fish are not effectively targeted. In Utah Lake, the location of one of the world’s largest freshwater fish control programs, managers have been attempting to control the common carp (Cyprinus carpio, hereafter “carp”) population since 2009 but efforts have been hindered by the use of selective fishing gears. I conducted a lake-wide field study to gain insights into the distribution of juvenile and small adult carp in time and space and to identify fishing gears that can be incorporated in control efforts. I evaluated factors influencing the presence and abundance of juvenile, small adult, and all ages of carp in survey samples, and identified strong temporal trends across years, with carp catch being 125 to 270 times more likely in 2023 than 2021. While the highly variable nature of Utah Lake impacted my study and additional sampling might provide further insights, it is important to assess which age classes are the most critical to capture. To do this, I used a simulation framework that integrates age-based gear selectivity and the widely implemented commercial fisheries metric of maximum sustainable yield (MSY) to evaluate the effect of improving selectivity among younger carp. I found that improving selectivity on younger, but mature, age classes achieved the control program’s biomass target with only 2.5 times maximum historic effort, while further improving juvenile selectivity had minimal benefit. The historic level of fishing effort was below that required to achieve MSY regardless of selectivity scenario, suggesting the control program would be harvesting at a sustainable rate even if gear selectivity were improved. Controlling invasive species becomes much more feasible if an approach that targets all adult age classes can be identified and incorporating sustainable harvest metrics into simulation models of invasive species populations provides a framework for evaluating a harvest control program’s ability to achieve management objectives

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