Rainbow Trout in the Colorado River, Grand Canyon Active
Rainbow trout are a desirable sport fish that have been introduced in many locations around the world, including the Colorado River. Although introductions of rainbow trout and other nonnative fishes provide recreational fishing opportunities, they can 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 the Lees Ferry Fishery, while minimizing impacts to downstream populations of native fish, especially threatened humpback chub.
Background & Importance
Rainbow trout were introduced to the Colorado River in 1964 as a sport fish below Glen Canyon Dam, following its closure. Most rainbow trout residing downstream of Glen Canyon Dam can be found in the segment of river between the dam and Lees Ferry called Glen Canyon, a 15.5 mile stretch of river (approximately 25 kilometers) below Glen Canyon Dam considered a Blue Ribbon trout fishery.
Rainbow trout were stocked until 1998. Stocking ceased after it became clear the population had become self-sustaining. In the 1990s, increases in rainbow trout abundance may have been caused by changes in how the dam was operated, with less daily variation in flows starting in late 1990 resulting in increased survival of trout eggs and juveniles. 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 since it provides desired recreational opportunities.
Interactions with Humpback Chub
Conditions upstream of Lees Ferry can have consequences for native fish populations which inhabit downstream sections of river. Glen Canyon Dam has impacted native fish species by restricting seasonal variation in flows, decreasing water temperature, and decreasing the amount of sediment in the water. A large proportion of threatened humpback chub, a fish species found only in the Colorado River Basin, resides in the Little Colorado River (LCR) and near the LCR's confluence with the Colorado River, approximately 62 miles (~100 kilometers) downstream from Lees Ferry. Humpback chub readily reproduce in the LCR due to its warmer water temperatures than the Colorado River.
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.
Ongoing Research
Grand Canyon Monitoring and Research Center researchers use 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 estimates 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.
Understanding Research Results
Years of research have 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. Average 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.
Smallmouth bass expansion downstream of Glen Canyon Dam
Population Dynamics of Threatened Humpback Chub in Grand Canyon
Grand Canyon Monitoring and Research Projects
Bug Flows: Improving Food Web Health on the Colorado River
Rainbow trout are a desirable sport fish that have been introduced in many locations around the world, including the Colorado River. Although introductions of rainbow trout and other nonnative fishes provide recreational fishing opportunities, they can 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 the Lees Ferry Fishery, while minimizing impacts to downstream populations of native fish, especially threatened humpback chub.
Background & Importance
Rainbow trout were introduced to the Colorado River in 1964 as a sport fish below Glen Canyon Dam, following its closure. Most rainbow trout residing downstream of Glen Canyon Dam can be found in the segment of river between the dam and Lees Ferry called Glen Canyon, a 15.5 mile stretch of river (approximately 25 kilometers) below Glen Canyon Dam considered a Blue Ribbon trout fishery.
Rainbow trout were stocked until 1998. Stocking ceased after it became clear the population had become self-sustaining. In the 1990s, increases in rainbow trout abundance may have been caused by changes in how the dam was operated, with less daily variation in flows starting in late 1990 resulting in increased survival of trout eggs and juveniles. 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 since it provides desired recreational opportunities.
Interactions with Humpback Chub
Conditions upstream of Lees Ferry can have consequences for native fish populations which inhabit downstream sections of river. Glen Canyon Dam has impacted native fish species by restricting seasonal variation in flows, decreasing water temperature, and decreasing the amount of sediment in the water. A large proportion of threatened humpback chub, a fish species found only in the Colorado River Basin, resides in the Little Colorado River (LCR) and near the LCR's confluence with the Colorado River, approximately 62 miles (~100 kilometers) downstream from Lees Ferry. Humpback chub readily reproduce in the LCR due to its warmer water temperatures than the Colorado River.
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.
Ongoing Research
Grand Canyon Monitoring and Research Center researchers use 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 estimates 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.
Understanding Research Results
Years of research have 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. Average 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.