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'Exploring the potential for biological control of an explosive prey base by a suite of three predatory fishes in a high elevation, western reservoir'
Lisa Winters (lisa.winters@aggiemail.usu.edu), Utah State University Phaedra Budy (phaedra.budy@usu.edu), USGS Cooperative Fish and Wildlife Research Unit, Utah State University; Gary Thiede (gary.thiede@usu.edu), Department of Watershed Sciences, Utah State University

In western reservoirs, the rapid spread of introduced species into an already artificial species assemblage can alter trophic interactions in ways that can be difficult to predict, creating challenges for fisheries management. In Scofield Reservoir, Utah, the unintentional introduction of Utah chub and subsequent potential for a population explosion has prompted managers to shift the stocking program from exclusively rainbow trout, to include tiger trout and Bear Lake cutthroat trout as potential biological controls, as well as to enhance angling opportunities. However, unintended consequences of these introductions are likely, and could include reduced catch-per-unit-effort (CPUE) and condition in preferred sport fishes. We initiated a multi-faceted study consisting of field observations, theoretical comparisons of gape limitation, and bioenergetic simulations, to investigate interspecific interactions between the top predators and infer predator demand versus prey supply. Our results suggest that a substantial portion of chub outgrow the gape limit of trout predators; 16% of chub (those greater than 250 mm in length) are not susceptible to predation. Furthermore, stable isotope and diet analyses demonstrate considerable trophic niche overlap suggesting the potential for competition among these trout species is high. Large cutthroat and tiger trout occupy a top piscivorous trophic niche with both species consuming large amounts of prey fish throughout all age classes. In addition, the trophic niche space of rainbow trout overlaps considerably with Utah chub, with both species relying heavily on invertebrate consumption. Relative weight and Fulton’s condition factor for all three species are all below average (100 and 1, respectively). Moreover, there is a significant decline of rainbow trout catch (CPUE) in recent years, which could potentially be due to competition for shared food resources and or the preferred littoral feeding space. Collectively, this research will aid managers in designing and implementing the best stocking strategy to optimize sport fish growth and survival, control chub, and, in turn, enhance and maintain angler satisfaction.