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

Reservoirs are artificial, dynamic, and highly manipulated systems, where the appearance of either intentionally or unintentionally introduced species may disrupt interactions within an already complex food web. In some situations, a top predator fish may be stocked as a biological control agent, if these fish-eating predators monopolize on the nuisance and unwelcome prey. Scofield Reservoir, Utah has historically been an extremely popular blue-ribbon fishery. However, a recent decrease in rainbow trout catch and increase in the minnow, Utah chub, have made it difficult to find a balance between providing the public with a trophy sport fishery and maintaining an ecologically sustainable system. In this context, it is important for fisheries managers to understand how these stocked top predators may interact with each other, as well as with their prey, to maximize fish performance and enhance and maintain the sport fishery. I used gill nets to capture fish and collect samples to answer key questions: 1) which of the three trout stocked in the reservoir (cutthroat trout, rainbow trout, or tiger trout) has the greatest potential to reduce Utah chub numbers, 2) what is the relative performance of these three trout, and 3) how do these three top predators interact with each other within this system? To answer these questions, I used energetic modeling to predict how many chub an average trout of each species was consuming. Additionally, I used length-weight relationships to estimate the healthiness of each species and calculated metrics of diet overlap to determine if these fish were food- or space-limited, which may constrain fish performance. Results of my study indicated that Utah chub density in the reservoir was extremely high, and chub made up the majority of the catch. Rainbow trout, in contrast, were caught very infrequently. Additionally, rainbow trout contributed little to the biological control of Utah chub, whereas cutthroat trout and tiger trout populations consumed millions of Utah chub each year, and appeared to be keeping the chub population in check. Diet overlap between cutthroat trout and tiger trout was high due to shared Utah chub prey, but since this prey is in excess, they are likely not competing for resources. Alternatively, rainbow trout relied heavily on aquatic invertebrates and zooplankton. Rainbow trout may be performing poorly in the reservoir because they compete for food or space with the abundant Utah chub. These results will help fisheries professionals manage unique assemblages of sport fish, as well as provide pertinent knowledge to the fields of reservoir ecology and biological control