Winters, L. and P. Budy. 2015. Exploring crowded trophic niche space in a novel reservoir fish assemblage: how many predators is too many? Transactions of the American Fisheries Society. 144:1117-1128. USGS FSP IP-065178.
Species interactions within novel fish assemblages in highly-managed reservoir systems can be difficult to predict and understand. In a high elevation Utah reservoir, USA, the unintentional introduction of Utah Chub Gila atraria and subsequent population expansion prompted managers to shift from stocking exclusively Rainbow Trout Oncorhynchus mykiss to include Tiger Trout Salmo trutta (female) x Salvelinus fontinalis (male) and Cutthroat Trout O. clarkii utah, a novel suite of top predators and potential competitors. We examined the interspecific interactions among Scofield Reservoir, Utah piscivores using a multifaceted approach including gut analyses, stable isotopes, and gape limitation studies to characterize the potential for competition and predation to occur amongst top predators. Our results suggest large Cutthroat Trout and Tiger Trout occupy a top piscivore trophic niche and are more littoral, but because food resources are not limited, competition is unlikely. In contrast, Rainbow Trout occupy an omnivore niche space and are more pelagic. Apparent survival of Rainbow Trout has declined significantly in recent years, potentially due to shared food resources or preferred feeding space with Utah Chub. In contrast, Cutthroat Trout and Tiger Trout are capable of consuming prey up to 50% of their own size, much larger than predicted based on their theoretical gape limit alone. Collectively, this research aids in understanding biotic interactions within a top-heavy and novel fish community and assists towards developing and implementing suitable management strategies to control nuisance species in valuable sport fisheries.