Rainbow Trout Abundance and Movement in the Colorado River in Grand Canyon

Science Center Objects

Just below Glen Canyon Dam on the Colorado River is a very popular Blue Ribbon trout fishery known for its rainbow trout. However, approximately 78 miles downstream, near were the Little Colorado River flows into the Colorado River, is a population of endangered humpback chub. The introduced rainbow trout can negatively affect native humpback chub by competing with them for food (immature black flies and midges) and by preying on humpback chub. Therefore, it is important to understand rainbow trout movement and the abundance of these trout in the stretch of river directly below Glen Canyon Dam and the Little Colorado River. That information can help with maintaining the trout fishery while protecting humpback chub.

Background & Importance

Humpback chub fish

Endangered humpback chub caught near the confluence of the Colorado River and Little Colorado Rivers in Grand Canyon. (Credit: David Ward, USGS. Public domain.)

Rainbow trout were introduced to the Colorado River below Glen Canyon Dam in 1964 as a sport fish. Continued stocking of rainbow trout ceased in 1998 because of an increase in the natural reproduction of rainbow trout near Glen Canyon Dam. In fact, rainbow trout are now the dominant fish species in Glen Canyon and Marble Canyon, the stretches of river directly below Glen Canyon Dam. Additionally, Glen Canyon, the approximately 15 mile stretch of river below Glen Canyon Dam, is a Blue Ribbon trout fishery. The increases in rainbow trout abundance in the 1990s 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.

Humpback chub is a species only found in the Colorado River Basin. Now this species is endangered with only six populations in existence. It is thought that Glen Canyon Dam has negatively affected humpback chub by restricting seasonal variation in flows, decreasing water temperature, and decreasing the amount of sediment in the water. Additionally, nonnative fish species like rainbow trout and brown trout compete with humpback chub for food and can prey on the chub. Humpback chub readily reproduce in the Lower Colorado River because it is warmer than the Colorado River. However, some humpback chub find their way into the Colorado River making the area around the confluence of the Colorado and Little Colorado Rivers habitat for the largest population of humpback chub known today. 

Rainbow trout are prized as a sport fish yet they could be a threat to the endangered humpback chub. Although rainbow trout are most abundant in the 15-16 miles below Glen Canyon Dam, they can be found throughout the Colorado River below the dam, including 78 miles downstream at the confluence of the Colorado and Little Colorado Rivers where the largest population of humpback chub reside. It is important to understand rainbow trout abundance and movement to preserve both the Blue Ribbon fishery directly below the dam and the humpback chub downstream.