by Lisa Winters, Utah State University
How do you stop an exploding population of an unwanted fish, quickly and effectively, without ruining the trout fishery so popular with Utah anglers?
This is the basis of my graduate research work as a member of Utah State University’s Fish Ecology Lab. For the past two years, I have made periodic trips to Scofield Reservoir, a 2,815 acre man-made lake on the Price River in the heart of Manti-La Sal National Forest.
With guidance from Professor Phaedra Budy, my advisor in the Department of Watershed Sciences, I’ve spent nearly every day-light hour catching fish, measuring their length and weight, and extracting valuable body parts to determine how fish in the reservoir are performing.
I work with Bear Lake cutthroat trout, rainbow trout, and tiger trout (a brown trout x brook trout hybrid), along with vast numbers of Utah chub. The chub, a minnow species not native to the Colorado River drainage where Scofield is located, is where the problem lies. The introduced fish reproduces often and exhibits exceptionally fast growth. Since the chub’s 2005 appearance in the reservoir, anglers have observed a substantial decrease in the rainbow trout fishery as well as an exponential increase in the Utah chub population. Needless to say, the fishermen are not thrilled.
Tasked with managing Utah’s fishing opportunities, the Utah Division of Wildlife Resources has worked to mitigate the chub problem with the use of ‘biological control agents’. This process involves the use of top predators that consume vast amounts of unwelcome prey. In Scofield Reservoir, the Utah Division of Wildlife Resources has stocked Bear Lake cutthroat trout, a popular native fish of the state, along with tiger trout, a hybrid species known for its fast growth and aggressive nature, as potential biological control agents to curb the chub.
After seven years of efforts, the Utah Division of Wildlife Resources is beginning to see an impact. Working with the agency, I’m trying to determine what and how much the biological control agents are consuming.
With this knowledge, I can simulate the Utah chub consumption potential for each species. My findings show that each individual adult cutthroat or tiger trout may consume 50 chub in a single year. Multiply that by 200,000 of each species in the reservoir, and you can see a lot of Utah chub are being eaten.
Unfortunately, rainbow trout, which once made Scofield a great fishery, have not fared as well. Rainbow trout are not known to be fish-eaters, and, in fact, I’ve found they consume many of the same aquatic organisms as Utah chub.
Thus, the iconic rainbows are disappearing from the lake–they simply can’t compete with the voracious chub.
Our work has broad implications for the many reservoir systems throughout the West. Like Scofield Reservoir, many of these waters have unique and artificial assemblages of fish which make them vulnerable to unwanted invaders capable of altering the species composition. My hope is that, by forming the management decision-making process, I will help find the delicate balance between predators and prey and, for Scofield Reservoir, return it to the thriving blue-ribbon fishery it once was.
——–Part of the July 2014 (PDF) issue of Lateral Lines.