Effects of Brown Trout Removal on Reestablishing Populations of Rainbow Trout

Study area:
Gunnison River, Cache la Poudre River, and Yampa River

Project status:
Completed

Research Objectives

• To determine the effects of brown trout removal on reestablishing populations of rainbow trout.
• To improve rainbow trout retention and survival following reintroduction. ​

Project Description

Rainbow trout populations experienced significant population declines following the establishment of whirling disease to Colorado in the 1980s. Brown trout, however, evolved with the parasite that causes whirling disease in their European home range, so they developed a resistance to the disease. As a result, brown trout populations exhibited increases in many rivers across the state as they filled the open niches left behind by rainbow trout.

Fishery managers stock whirling disease resistant rainbow trout​(known as the Hofer) in many of the state's waters to supplement and recover populations lost to the disease. However, increased numbers of brown trout result in greater predation on and competition with the stocked rainbows, reducing the survival and retention of rainbow trout populations. 

As a result, CPW initiated several experiments on the Gunnison and Cache la Poudre Rivers to examine the effects of brown trout removal on fry and adult rainbow trout retention and survival following reintroduction to these rivers. 

In the Gunnison River (Gunnison Gorge National Conservation Area near Montrose, Colo.), researchers conducted two experiments. Two 1-mile sections of the river were used for the experiments. In both, brown trout fry were removed from one half of the 1-mile study section, and rainbow trout were introduced after the removal. The results from these experiments suggested that rainbow trout fry exhibited higher survival and retention rates in the sections from which brown trout fry had been removed compared to those sections in which brown trout fry had not been removed.  

The largest of the brown trout removal experiments occurred in the Cache la Poudre River, northwest of Fort Collins, Colo. The study, a cooperative research project between Colorado State University and Colorado Parks and Wildlife, was designed to determine if adult brown trout removal and relocation could be used to reestablish wild rainbow trout populations. During the experiments, researchers examined the interactions between brown trout and stocked rainbow trout, including the potential for competition, predation, emigration, survival and growth.

Movement and survival data suggest that brown trout removal and relocation had a positive short-term effect. Rainbow trout displayed higher survival and lower emigration rates in the removal section in the months following the removal. On average, rainbow trout grew 1.5 inches in the two months following their introduction. However, rainbow trout emigration and survival decreased as time went on, suggesting that brown trout removal and relocation did not increase long-term survival or retention of rainbow trout.​

CPW initiated an experiment in the Yampa River in 2017 to determine the effects of large-scale brown trout removal on wild and stocked fingerling and catchable rainbow trout survival. Brown trout removal occurred across four years and over a nearly seven mile stretch of the Yampa River between Stagecoach Reservoir and Lake Catamount. Because this is a closed system between two reservoirs and the removals occurred over a greater distance than in the Cache la Poudre River, brown trout removal appeared to be more effective at reducing the number of brown trout adults in the system, and thereby competition and predation on wild and stocked rainbow trout. Results are pending, but preliminary estimates suggest that both rainbow trout retention and survival increased in the Yampa River as a result of the brown trout removals.

CPW will continue to conduct research on the interactions between brown trout and rainbow trout. Using this research, CPW hopes to reestablish rainbow trout populations in Colorado's rivers. ​

Associated Publications

Fetherman, E. R., D. L. Winkelman, L. L. Bailey, G. J. Schisler, and K. Davies. 2015. Brown trout removal effects on short-term survival and movement of Myxobolus cerebralis­­-resistant rainbow trout. Transactions of the American Fisheries Society 144:610-626.​

Fetherman, E. R., and G. J. Schisler. 2013. Sport Fish Research Studies. Federal Aid Project F394-R12. Federal Aid in Fish Restoration Job Progress Report. Colorado Parks and Wildlife. Aquatic Research Section. Fort Collins, CO. 132.

Read an overview of Colorado's research on whirling disease in the Whirling Disease Fact Sheet.