Nonnative Fish

Frenzied carp flee a predatory striper

NPS

 

Striped bass (Morone saxatilis)

Nonnative Fish in Lake Powell

Nonnative fish species were introduced or escaped to the Colorado River prior to the completion of the Glen Canyon Dam in 1963. Walleye (Stizostedion vitreum), bluegill (Lepomis macrochirus), green sunfish (L. cyanellus), European carp (Cyprinus carpio), and channel catfish (Ictalurus punctatus) are present in Lake Powell and developed from stock already present in the river system before the dam was completed.

Some nonnative fish were unable to survive the environmental changes caused by the filling of Lake Powell. These fish, which include the fathead minnow (Pimephales promelas), mosquitofish (Gambusia affinis), and plains killifish (Fundulus zebrinus), are still present in Glen Canyon NRA, but are now mainly found in the flowing rivers, inflows, and small perennial tributaries. Red shiner (Cyprinella lutrensis) established a marginal population in Lake Powell, but are more commonly found in flowing water.

Smallmouth bass (Morone dolomieu) and striped bass (M. saxatilis) were introduced to Lake Powell because of their preference for open habitat and now dominate the fish community. Other nonnative fish, such as largemouth bass (Micropterus salmoides) and black crappie (Poxomis nigromaculatus), were introduced to Lake Powell to provide additional recreational sport fishing opportunities. To provide high numbers of small forage fish for these sport fish species that live in the upper layers of open water, threadfin shad (Dorosoma petenense) were introduced in 1968. In recent years a new forage fish, gizzard shad, apparently migrated downriver into Lake Powell. They grow quickly and get larger than threadfin shad so may represent a different food source for the large sport fish. Another recent arrival in the very upper reaches of the lake are grass carp. Recent research has found that they are reproducing even though all stock in the U.S. is supposed to be triploid and sterile. The jury is still out on whether they will find enough forage to spread further into Lake Powell.

Today, Lake Powell is recognized as having one of the best sport fisheries in the country. Anglers travel great distances to enjoy the remarkable scenery and the unlimited take of striped bass. Angler harvest of striped bass helps keep their population, which is prone to booms and busts, healthy. Other species are targeted as well, including walleye, which can have negative impacts on native fish.

Though native fish species were declining prior to the construction of Glen Canyon Dam, nonnative fish compete with and predate on native species. With proper management and planning, Glen Canyon NRA and its many partner agencies and organizations can preserve the remaining unique native fish and provide excellent recreational sport fishing opportunities of nonnative fish.

 

Rainbow trout (Oncorhynchus mykiss)

Nonnative Fish in the Colorado River below Glen Canyon Dam

Rainbow trout (Oncorhynchus mykiss) were stocked in the water released from the dam in the Lees Ferry area in the 1970s. Native fish are unable to reproduce in the cold water released from the dam, but rainbow trout have flourished. Now, the 15-mile stretch from the Glen Canyon Dam to Lees Ferry is a world-famous recreational rainbow trout fishery.

Glen Canyon NRA manages the recreational rainbow trout fishery in partnership with the Arizona Game and Fish Department and the Bureau of Reclamation through the Glen Canyon Dam Adaptive Management Program (AMP) to keep the population healthy, with plentiful catch rates and larger sized fish. They also monitor for conditions that could lead to catastrophic events for the rainbow trout populations such as low dissolved oxygen levels in the water coming through the dam, disease outbreaks such as whirling disease which could decimate the population’s health and numbers, and the expansion of new predatory fish (including walleye and smallmouth bass) that could decimate rainbow trout numbers. All of this monitoring and management also has the goal to maintain rainbow trout numbers at levels that mass migrations do not occur downriver that could threaten endangered native fish populations in Grand Canyon.

In recent years, nonnative brown trout have expanded into the Lees Ferry reach to levels much higher than seen in the past and are now successfully reproducing every year from November to February. Brown trout are predatory on rainbow trout and are known to have negative impacts on native fish downriver, including the endangered humpback chub. More information on the management of brown trout to protect the recreational rainbow trout fishery is available on the Brown Trout Incentivized Harvest page.

 

Green Sunfish

Green sunfish thrive in Lake Powell and they survive passing through the penstocks and turbines of the dam. More appear to be passing through as the water levels of Lake Powell lower and get closer to the levels of the penstocks. Green sunfish are very prolific and they are highly predatory on young and small fish. They have been shown to impact native fish populations in other waters of the U.S. One small off-channel pond found in the Glen Canyon Reach is tied to a backwater area of the river. This pond warms to over 80 degrees F in the summer which allows green sunfish to reproduce. The NPS removes green sunfish populations from the small pond annually to ensure they do not move downriver during high flow experiment events. They have also developed long-term strategies to prevent green sunfish and other non-native warm-water fish from establishing below the dam and impacting native fish.

The river below the dam and into Grand Canyon is considered sacred by several Native American tribes. Fisheries management activities can be offensive, especially when many fish must be killed. To mitigate the negative tribal impact from taking fish in management actions, the NPS and partners identify beneficial uses for the removed fish. Most commonly the fish are provided to tribal aviaries to feed injured eagles. The NPS is also working on an incentivized harvest program to motivate anglers to remove more brown trout and enjoy eating them at home. There are also plans in the works for returning the large numbers of small green sunfish captured from the pond to Lake Powell where their parents originated from.

 

Brown Trout Management

Brown trout have existed in the Lees Ferry reach of the Colorado River in low numbers for many years. Brown trout are not native to this system, but were introduced into Bright Angel Creek in Grand Canyon National Park in the 1920's. Subadult and adult brown trout feed primarily on fish and have been shown to impact the endangered humpback chub in Grand Canyon. Brown trout numbers in the Lees Ferry reach have increased significantly in recent years and they appear to be spawning throughout the 15.5 mile reach down to Lees Ferry. Brown trout are not desired in the system due to potential impacts on native fish and on the recreational rainbow trout fishery.

The Comprehensive Fisheries Management Plan (2013) and Expanded Non-native Aquatic Species Management Plan (2019) for Glen Canyon NRA and Grand Canyon NP describe management actions and tools that can be taken in the Colorado River below Glen Canyon Dam to improve the recreational rainbow trout fishery in Glen Canyon National Recreation Area while protecting native fish in the Grand Canyon. These plans were developed in close cooperation with Arizona Game and Fish and other partners. They identify brown trout as high-risk non-native predators, targeted for removal. To learn more about fish monitoring and management in the river, especially brown trout research, visit the Fishery Management and Monitoring in the Glen Canyon Reach page. Anglers are encouraged to target and keep brown trout from the Colorado River below Glen Canyon Dam; Arizona Game and Fish has set no bag limit.

Scientists are uncertain why brown trout have increased in Glen Canyon NRA. The effects on the rainbow trout fishery are also uncertain. Subadult and adult brown trout prey heavily upon rainbow trout. Examples exist where brown trout displace a significant portion of the rainbow trout populations as their population increases. Competition, especially for spawning and rearing habitat, between the populations are additional threats to rainbow trout in Glen Canyon. The NPS is launching an incentivized harvest program to motivate anglers to remove more brown trout and enjoy eating them at home. Visit the Brown Trout Incentivized Harvest page to learn more about this program. It is difficult to predict how the unprecedented increase of brown trout in Glen Canyon will affect the fishery or if efforts to control brown trout can be successful.

 

Keep Reading

• Fishing Regulations
• Brown Trout Incentivized Harvest