An Evaluation of the Food Web Dynamics and Predator Prey Interactions in Scofield Reservoir

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Winters, Lisa K.
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Evaluation; Food Web; Dynamics; Predator; Prey; Interactions; Scofield; Reservoir
thesis / dissertation description
Aquatic food webs are comprised of complex spatial and temporal interactions within and among trophic levels. Human manipulations, such as construction of reservoirs, enhance uncertainties in our understanding of aquatic food web structure. Reservoirs are novel ecosystems which contain a unique composition of species. Species introductions into reservoirs may disrupt interactions within an already complex and poorly understood food web.In Scofield Reservoir in central Utah, a rapidly expanding population of Utah chub poses a detrimental threat to the blue-ribbon trout fishery. The Utah Division of Wildlife Resources has recently stocked cutthroat trout and tiger trout (a brown trout x brook trout hybrid) to potentially reduce Utah chub numbers as well as to enhance the sport fishery. I used standard fisheries techniques to describe food web interactions and to assess the potential for these piscivorous trout to control the expanding Utah chub population, as well as to determine sport fish performance and quantify diet overlap amongst top predators. Using bioenergetics modeling, I estimated very high rates of Utah chub consumption by cutthroat trout and tiger trout. These two species collectively display control of Utah chub, as the chub population is no longer exponentially increasing. Cutthroat trout and tiger trout are likely not food limited and convincingly exhibit high performance, as abundance of Utah chub is still extremely high. Both diet and stable isotope analysis showed significant overlap between these top piscivores in the reservoir, but also substantial overlap between cutthroat trout and rainbow trout with respect to Utah chub. Analysis of rainbow trout suggested this species is performing poorly; rainbow trout had extremely low catch rates and never switched to a piscivorous diet at larger sizes. Utah chub and rainbow trout had similar diet compositions, and thus rainbow trout may be competing (poorly) for food and space resources. This study contributes to our understanding of interspecific interactions among these unique assemblages of top predators in this artificial system. This research also provides knowledge to fisheries managers tasked with providing optimal sport fisheries in these artificial and dynamic systems, as well as expanding on our extremely limited knowledge of tiger trout ecology.