Understanding Factors Influencing Rainbow Trout Growth in the Colorado River

Science Center Objects

Rainbow trout is a desirable sport fish that has been introduced in many locations around the world. Although introductions of rainbow trout and other nonnative fishes provide recreational fishing opportunities, they also pose threats to native fish populations. The Glen Canyon Dam Adaptive Management Program has tasked scientists and managers with identifying management options that allow rainbow trout to thrive from Glen Canyon Dam downstream to Lees Ferry, while minimizing impacts to downstream populations of native fish, especially the endangered humpback chub. This project aims to identify factors that drive rainbow trout growth through a combination of approaches. A synthesis of tailwaters (river segments just downstream of dams) across the Western US has elucidated larger scale patterns, relating river flows to the size of adult rainbow trout. Ultimately, this project aims to guide adaptive management of rainbow trout, balancing recreational interests within the tailwater with downstream native fish conservation.

Background & Importance

Two USGS researchers on a small boat in the Colorado River sampling aquatic insects with a net.

Sampling aquatic invertebrate drift approximately 15.2 kilometers (~9.5 miles) upstream of Lees Ferry in order to characterize the prey resources available to rainbow trout (March 4, 2016).(Credit: Dallana Garcia-Peña, USGS. Public domain.)

Most rainbow trout residing downstream of Glen Canyon Dam can be found in the approximately 25 kilometer (~15.5 mile) segment of river between the dam and Lees Ferry. Rainbow trout were introduced upstream of Lees Ferry following closure of Glen Canyon Dam and were stocked through 1998. Stocking ceased after it became clear the population had become self-sustaining. Over the last 20 years, there have been a series of boom and busts in the population and the abundance of large trophy sized fish has generally declined. Similar patterns have been documented in many other tailwater (river segments just downstream of dams) fisheries in the Western US and these declines can have significant impacts on local economies. Thus, understanding factors driving both patterns of population size and growth is important to managing the fishery and providing desired recreational opportunities.   

Conditions upstream of Lees Ferry affecting the rainbow trout population can have consequences for native fish populations which inhabit downstream sections of river. A large proportion of the endangered humpback chub population resides in the Little Colorado River and around the confluence with the Colorado River, approximately 100 kilometers (~62 miles) downstream from Lees Ferry. Rainbow trout can disperse from Lees Ferry (and other locations downstream in Marble Canyon) to areas near the Little Colorado River, where they can have negative effects on humpback chub due to competition and predation. Movement of rainbow trout from Lees Ferry to downstream areas is likely related to both population size and conditions for growth. Dam operations, including spring high flow experiments and high summer flow volumes, can increase juvenile rainbow trout recruitment, leading to very high densities of small fish. These fish are supported by a prey base composed almost exclusively of small prey, chiefly midges and blackflies. As these small fish grow, they may outpace the available prey resources, leading to decreased growth and condition (an index of fish plumpness). At high fish densities, when competition for food resources is strong, fish may disperse downstream in search of more favorable growing conditions.

General Methods

This project uses a diversity of approaches to identify drivers of rainbow trout growth both in Lees Ferry and across other tailwaters in the Western US. The first of these approaches is large-scale field monitoring efforts that use capture-recapture techniques to estimate growth and population size of rainbow trout. These efforts involve surveys, where fish are given individual tags, then on subsequent surveys a number of these tagged fish are recaptured.  This sampling scheme allows for direct estimates of growth and population size. These estimate can then be related to factors such as dam operations or the availability and quality of prey resources. 

Ecological models are a second approach used to understand patterns of rainbow trout growth. Drift-foraging bioenergetics models describe the process by which rainbow trout acquire food resources (foraging on invertebrate prey drifting in the current) and how the calories from these food resources are allocated to growth, metabolism, and reproduction (i.e., bioenergetics). Since rainbow trout are cold-blooded, their metabolism is a function of water temperature, which strongly influences growth.  The ecological models being applied to rainbow trout in Lees Ferry incorporate temperature, invertebrate prey availability, and prey size. These approaches can be used to predict how altered temperatures or food resources influence rainbow trout growth.

Lastly, much can be learned about rainbow trout growth by using a comparative approach to look at patterns across tailwaters in the Western US. Information from 29 dams spanning 1-19 years was compiled to evaluate how river flow, fish density, and other physical factors influence rainbow trout recruitment and mean adult length. By examining these patterns across a broad range of tailwater systems, researchers can capture more variation in flow management strategies, leading to more general inferences about the important drivers of rainbow trout growth

Important Results

To date, this project has produced important insights into factors influencing rainbow trout growth. Large-scale field efforts have shown that rainbow trout growth varies considerably across seasons and years, related to both food availability and fish densities. The highest growth rates observed were in areas with lower rainbow trout densities, suggesting a depression of growth rates due to competition for food at higher fish densities. Drift-foraging bioenergetics models have revealed the importance of temperature and attributes of the invertebrate prey, such as overall prey availability and prey size, to rainbow trout growth. Increased temperatures will likely limit rainbow trout growth unless these temperatures produce a concurrent increase in the availability of prey resources. This is due to increased metabolic costs associated with warmer river temperatures and the energy necessary to acquire prey. The synthesis of information across tailwaters has shown the importance of both physical factors related to dam releases and biological factors, such as fish density. Mean adult size of rainbow trout was positively related to increased annual flows and negatively related to catch of conspecifics. Together, these approaches to examine patterns of rainbow trout growth inform resource managers and the Glen Canyon Dam Adaptive Management Program.