Reservoirs are a prominent landscape feature in the western United States, and these artificial systems are highly managed and manipulated through sport fish stockings. Introductions into these systems can lead to unpredictable interactions amongst species within an already dynamic food web. Therefore, understanding how these fishes may interact with one another, as well as with their prey, is necessary to maximize sport fish performance and enhance the fisheries. In Utah, sport fish performance is of strong interest to the state agencies that manage these valuable fisheries. A collaborative partnership among the Utah Division of Wildlife Resources, Utah State University, and the U.S. Geological Survey, Utah Cooperative Fish and Wildlife Unit is taking a close look at these dynamics in a popular Utah fishing destination ? Scofield Reservoir.

Sport fish have been widely studied and monitored throughout the United States. However, increased concern towards un-intentionally introduced fish which may disrupt the delicately balanced and managed food webs has led to efforts to better evaluate species interactions and the fishery overall. Additionally, there is limited knowledge on efficient and effective control of these nuisance species populations, while still providing the valuable sport fishery anglers desire. The research being conducted through the partnership in Utah aims to understand the factors determining the relative performance of three top predator species in Scofield Reservoir, as well as to quantify each species' potential to act as biological control agents.

Scofield Reservoir is an extremely popular, 2,800-acre, Blue Ribbon (defined as "waters that provide highly-satisfying fishing and outdoor experiences" by Utah Division of Wildlife Resources) rainbow trout fishery located in central Utah. However, in recent years, a decline in rainbow trout catch along with an expanding undesired prey fish population, has led managers to take action in order to continue providing the public with a trophy fishery. In this case, the undesired prey fish is the Utah chub, a regionally-native species known for its fast growth rates and high reproductive potential, characteristics which may lead to competition with trout for food resources. Therefore, the Utah Division of Wildlife Resources has recently changed their fish stocking program to include tiger trout, a sterile brook trout-brown trout hybrid (stocked since 2005), and native Bear Lake cutthroat trout (stocked since 2009) as potential biological control species. These biological control agents are piscivorous (fish eating) fish, hopefully acting as top predators in the system and monopolizing upon the abundant nuisance prey fish. Utah State University's research is intended to determine just how effective these stocking efforts have been, and will be in the future, at controlling the Utah chub population.

Fish performance, a general measure of how healthy fish are, is controlled through many mechanisms and driven by both biotic and abiotic factors. To evaluate the relative performance of these sport fish in Scofield Reservoir, gill nets were used to capture a representative sample of fishes, with length, weight, and diet data collected. Careful scrutiny of this information then provides a snapshot of how these fish are performing, as well as how resources are partitioned based on population size, growth rate, relative return rate, trophic position, and diet overlap. Accordingly, this study has shown that rainbow trout are caught infrequently and at small sizes; less than 1 percent of our gill-net catch consists of rainbow trout, with only a few above a preferred catch size of 20 inches. Additionally, these fish may be performing poorly, because they compete for food with the abundant Utah chub. Cutthroat trout and tiger trout, however, have sizable populations, dominated by large, healthy fish whose diets contain up to 75 percent Utah chub. To this effect, results suggest that management strategies involving rainbow trout stocking may not lead to top trophy fisheries, given their poor performance and likely competition with prey fishes.

Furthermore, with these collected data, models are used to simulate and estimate the Utah chub consumption potential of each of the trout predator species. Current findings show that the average adult cutthroat trout may consume over 40 Utah chub in a single year and the average tiger trout may consume over 60 chub in a year. Scaled up to the population level, the study shows that collectively, cutthroat trout and tiger trout are consuming a significant proportion of the extensive Utah chub population. While there still is a portion of the chub population larger than trout are physically capable of consuming, results suggest that these two trout species may not only be capable of controlling the undesired Utah chub, but additionally are performing well in the reservoir and are enhancing the sport fishery. An exciting aspect of this new fishery has been the multiple state record tiger trout caught from Scofield Reservoir in the last few years! The Utah chub are likely a plentiful and favorable prey source helping these sterile tiger trout grow to large sizes.

These research results provide context for making management decisions by state and federal agencies charged with managing artificial aquatic systems with nuisance prey. Our research also contributes to the exceptionally small knowledge base of tiger trout biology. In addition, the modeling currently underway will ultimately inform how fishes may be used as biological control agents throughout a variety of aquatic systems.

Each month, the ONB features articles from Cooperative Fish and Wildlife Research Units across the country. Working with key cooperators, including WMI, Units are leading exciting, new fish and wildlife research projects that we believe our readers will appreciate reading about.